Thermodynamic and Dynamic Aspects of Ice Nucleation

NASA Technical Reports Server (NTRS)

Barahona, Donifan

2018-01-01

It is known that ice nucleating particles (INP) immersed within supercooled droplets promote the formation of ice. Common theoretical models used to represent this process assume that the immersed particle lowers the work of ice nucleation without significantly affecting the dynamics of water in the vicinity of the particle. This is contrary to evidence showing that immersed surfaces significantly affect the viscosity and diffusivity of vicinal water. To study how this may affect ice formation this work introduces a model linking the ice nucleation rate to the modification of the dynamics and thermodynamics of vicinal water by immersed particles. It is shown that INP that significantly reduce the work of ice nucleation also pose strong limitations to the growth of the nascent ice germs. This leads to the onset of a new ice nucleation regime, called spinodal ice nucleation, where the dynamics of ice germ growth instead of the ice germ size determines the nucleation rate. Nucleation in this regime is characterized by an enhanced sensitivity to particle area and cooling rate. Comparison of the predicted ice nucleation rate against experimental measurements for a diverse set of species relevant to cloud formation suggests that spinodal ice nucleation may be common in nature.

Ultrasound assisted nucleation and growth characteristics of glycine polymorphs--a combined experimental and analytical approach.

PubMed

Renuka Devi, K; Raja, A; Srinivasan, K

2015-05-01

For the first time, the effect of ultrasound in the diagnostic frequency range of 1-10 MHz on the nucleation and growth characteristics of glycine has been explored. The investigation employing the ultrasonic interferometer was carried out at a constant insonation time over a wide range of relative supersaturation from σ=-0.09 to 0.76 in the solution. Ultrasound promotes only α nucleation and completely inhibits both the β and γ nucleation in the system. The propagation of ultrasound assisted mass transport facilitates nucleation even at very low supersaturation levels in the solution. The presence of ultrasound exhibits a profound effect on nucleation and growth characteristics in terms of decrease in induction period, increase in nucleation rate and decrease in crystal size than its absence in the solution. With an increase in the frequency of ultrasound, a further decrease in induction period, increase in nucleation rate and decrease in the size of the crystal is noticed even at the same relative supersaturation levels. The increase in the nucleation rate explains the combined dominating effects of both the ultrasound frequency and the supersaturation in the solution. Analytically, the nucleation parameters of the nucleated polymorph have been deduced at different ultrasonic frequencies based on the classical nucleation theory and correlations with the experimental results have been obtained. Structural affirmation of the nucleated polymorph has been ascertained by powder X-ray diffraction. Copyright © 2014 Elsevier B.V. All rights reserved.

Kinetics of spontaneous filament nucleation via oligomers: Insights from theory and simulation

NASA Astrophysics Data System (ADS)

Šarić, Andela; Michaels, Thomas C. T.; Zaccone, Alessio; Knowles, Tuomas P. J.; Frenkel, Daan

2016-12-01

Nucleation processes are at the heart of a large number of phenomena, from cloud formation to protein crystallization. A recently emerging area where nucleation is highly relevant is the initiation of filamentous protein self-assembly, a process that has broad implications in many research areas ranging from medicine to nanotechnology. As such, spontaneous nucleation of protein fibrils has received much attention in recent years with many theoretical and experimental studies focussing on the underlying physical principles. In this paper we make a step forward in this direction and explore the early time behaviour of filamentous protein growth in the context of nucleation theory. We first provide an overview of the thermodynamics and kinetics of spontaneous nucleation of protein filaments in the presence of one relevant degree of freedom, namely the cluster size. In this case, we review how key kinetic observables, such as the reaction order of spontaneous nucleation, are directly related to the physical size of the critical nucleus. We then focus on the increasingly prominent case of filament nucleation that includes a conformational conversion of the nucleating building-block as an additional slow step in the nucleation process. Using computer simulations, we study the concentration dependence of the nucleation rate. We find that, under these circumstances, the reaction order of spontaneous nucleation with respect to the free monomer does no longer relate to the overall physical size of the nucleating aggregate but rather to the portion of the aggregate that actively participates in the conformational conversion. Our results thus provide a novel interpretation of the common kinetic descriptors of protein filament formation, including the reaction order of the nucleation step or the scaling exponent of lag times, and put into perspective current theoretical descriptions of protein aggregation.

On the Ice Nucleation Spectrum

NASA Technical Reports Server (NTRS)

Barahona, D.

2012-01-01

This work presents a novel formulation of the ice nucleation spectrum, i.e. the function relating the ice crystal concentration to cloud formation conditions and aerosol properties. The new formulation is physically-based and explicitly accounts for the dependency of the ice crystal concentration on temperature, supersaturation, cooling rate, and particle size, surface area and composition. This is achieved by introducing the concepts of ice nucleation coefficient (the number of ice germs present in a particle) and nucleation probability dispersion function (the distribution of ice nucleation coefficients within the aerosol population). The new formulation is used to generate ice nucleation parameterizations for the homogeneous freezing of cloud droplets and the heterogeneous deposition ice nucleation on dust and soot ice nuclei. For homogeneous freezing, it was found that by increasing the dispersion in the droplet volume distribution the fraction of supercooled droplets in the population increases. For heterogeneous ice nucleation the new formulation consistently describes singular and stochastic behavior within a single framework. Using a fundamentally stochastic approach, both cooling rate independence and constancy of the ice nucleation fraction over time, features typically associated with singular behavior, were reproduced. Analysis of the temporal dependency of the ice nucleation spectrum suggested that experimental methods that measure the ice nucleation fraction over few seconds would tend to underestimate the ice nuclei concentration. It is shown that inferring the aerosol heterogeneous ice nucleation properties from measurements of the onset supersaturation and temperature may carry significant error as the variability in ice nucleation properties within the aerosol population is not accounted for. This work provides a simple and rigorous ice nucleation framework where theoretical predictions, laboratory measurements and field campaign data can be reconciled, and that is suitable for application in atmospheric modeling studies.

Overview: Experimental studies of crystal nucleation: Metals and colloids.

PubMed

Herlach, Dieter M; Palberg, Thomas; Klassen, Ina; Klein, Stefan; Kobold, Raphael

2016-12-07

Crystallization is one of the most important phase transformations of first order. In the case of metals and alloys, the liquid phase is the parent phase of materials production. The conditions of the crystallization process control the as-solidified material in its chemical and physical properties. Nucleation initiates the crystallization of a liquid. It selects the crystallographic phase, stable or meta-stable. Its detailed knowledge is therefore mandatory for the design of materials. We present techniques of containerless processing for nucleation studies of metals and alloys. Experimental results demonstrate the power of these methods not only for crystal nucleation of stable solids but in particular also for investigations of crystal nucleation of metastable solids at extreme undercooling. This concerns the physical nature of heterogeneous versus homogeneous nucleation and nucleation of phases nucleated under non-equilibrium conditions. The results are analyzed within classical nucleation theory that defines the activation energy of homogeneous nucleation in terms of the interfacial energy and the difference of Gibbs free energies of solid and liquid. The interfacial energy acts as barrier for the nucleation process. Its experimental determination is difficult in the case of metals. In the second part of this work we therefore explore the potential of colloidal suspensions as model systems for the crystallization process. The nucleation process of colloids is observed in situ by optical observation and ultra-small angle X-ray diffraction using high intensity synchrotron radiation. It allows an unambiguous discrimination of homogeneous and heterogeneous nucleation as well as the determination of the interfacial free energy of the solid-liquid interface. Our results are used to construct Turnbull plots of colloids, which are discussed in relation to Turnbull plots of metals and support the hypothesis that colloids are useful model systems to investigate crystal nucleation.

Kinetics of spontaneous filament nucleation via oligomers: Insights from theory and simulation.

PubMed

Šarić, Anđela; Michaels, Thomas C T; Zaccone, Alessio; Knowles, Tuomas P J; Frenkel, Daan

2016-12-07

Nucleation processes are at the heart of a large number of phenomena, from cloud formation to protein crystallization. A recently emerging area where nucleation is highly relevant is the initiation of filamentous protein self-assembly, a process that has broad implications in many research areas ranging from medicine to nanotechnology. As such, spontaneous nucleation of protein fibrils has received much attention in recent years with many theoretical and experimental studies focussing on the underlying physical principles. In this paper we make a step forward in this direction and explore the early time behaviour of filamentous protein growth in the context of nucleation theory. We first provide an overview of the thermodynamics and kinetics of spontaneous nucleation of protein filaments in the presence of one relevant degree of freedom, namely the cluster size. In this case, we review how key kinetic observables, such as the reaction order of spontaneous nucleation, are directly related to the physical size of the critical nucleus. We then focus on the increasingly prominent case of filament nucleation that includes a conformational conversion of the nucleating building-block as an additional slow step in the nucleation process. Using computer simulations, we study the concentration dependence of the nucleation rate. We find that, under these circumstances, the reaction order of spontaneous nucleation with respect to the free monomer does no longer relate to the overall physical size of the nucleating aggregate but rather to the portion of the aggregate that actively participates in the conformational conversion. Our results thus provide a novel interpretation of the common kinetic descriptors of protein filament formation, including the reaction order of the nucleation step or the scaling exponent of lag times, and put into perspective current theoretical descriptions of protein aggregation.

Strategies to initiate and control the nucleation behavior of bimetallic nanoparticles.

PubMed

Krishnan, Gopi; de Graaf, Sytze; Ten Brink, Gert H; Persson, Per O Å; Kooi, Bart J; Palasantzas, George

2017-06-22

In this work we report strategies to nucleate bimetallic nanoparticles (NPs) made by gas phase synthesis of elements showing difficulty in homogeneous nucleation. It is shown that the nucleation assisted problem of bimetallic NP synthesis can be solved via the following pathways: (i) selecting an element which can itself nucleate and act as a nucleation center for the synthesis of bimetallic NPs; (ii) introducing H 2 or CH 4 as an impurity/trace gas to initiate nucleation during the synthesis of bimetallic NPs. The latter can solve the problem if none of the elements in a bimetallic NP can initiate nucleation. We illustrate the abovementioned strategies for the case of Mg based bimetallic NPs, which are interesting as hydrogen storage materials and exhibit both nucleation and oxidation issues even under ultra-high vacuum conditions. In particular, it is shown that adding H 2 in small proportions favors the formation of a solid solution/alloy structure even in the case of immiscible Mg and Ti, where normally phase separation occurs during synthesis. In addition, we illustrate the possibility of improving the nucleation rate, and controlling the structure and size distribution of bimetallic NPs using H 2 /CH 4 as a reactive/nucleating gas. This is shown to be associated with the dimer bond energies of the various formed species and the vapor pressures of the metals, which are key factors for NP nucleation.

Misfit paradox on nucleation potency of MgO and MgAl{sub 2}O{sub 4} for Al

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, D.; Wang, L.

MgO and MgAl{sub 2}O{sub 4} are believed to be effective heterogeneous nuclei for Al based alloys due to their small lattice misfits with Al. However, there is a strong evidence to suggest that liquid Al reacts with MgO and MgAl{sub 2}O{sub 4} phases but the heterogeneous nucleation behavior of such phases is rarely discussed. In order to identify the nucleation mechanism of Al, under the interference of the chemical reaction, the heterogeneous nucleation process is systematically investigated through thermal analysis and high resolution transmission electron microscopy (HRTEM). The observed multi-nucleation interfaces (Al/MgO, Al/MgAl{sub 2}O{sub 4} and Al/Al{sub 2}O{sub 3}) andmore » scattered experimental undercooling data indicate an independent multi-phase nucleation process in these systems. - Highlights: •Theoretical lattice misfit doesn’t always disclose nucleation potency. •The nucleation of liquid can be triggered by multi-nucleation interfaces. •An independent multi-agents nucleation was verified in the study.« less

Structural analysis of the role of TPX2 in branching microtubule nucleation

PubMed Central

Thawani, Akanksha

2017-01-01

The mitotic spindle consists of microtubules (MTs), which are nucleated by the γ-tubulin ring complex (γ-TuRC). How the γ-TuRC gets activated at the right time and location remains elusive. Recently, it was uncovered that MTs nucleate from preexisting MTs within the mitotic spindle, which requires the protein TPX2, but the mechanism basis for TPX2 action is unknown. Here, we investigate the role of TPX2 in branching MT nucleation. We establish the domain organization of Xenopus laevis TPX2 and define the minimal TPX2 version that stimulates branching MT nucleation, which we find is unrelated to TPX2’s ability to nucleate MTs in vitro. Several domains of TPX2 contribute to its MT-binding and bundling activities. However, the property necessary for TPX2 to induce branching MT nucleation is contained within newly identified γ-TuRC nucleation activator motifs. Separation-of-function mutations leave the binding of TPX2 to γ-TuRC intact, whereas branching MT nucleation is abolished, suggesting that TPX2 may activate γ-TuRC to promote branching MT nucleation. PMID:28264915

Complementary activities of TPX2 and chTOG constitute an efficient importin-regulated microtubule nucleation module

PubMed Central

Roostalu, Johanna; Cade, Nicholas I.; Surrey, Thomas

2016-01-01

Spindle assembly and function require precise control of microtubule nucleation and dynamics. The chromatin-driven spindle assembly pathway exerts such control locally in the vicinity of chromosomes. One of the key targets of this pathway is TPX2. The molecular mechanism of how TPX2 stimulates microtubule nucleation is not understood. Using microscopy-based dynamic in vitro reconstitution assays with purified proteins, we find that human TPX2 directly stabilises growing microtubule ends and stimulates microtubule nucleation by stabilising early microtubule nucleation intermediates. Human microtubule polymerase chTOG (XMAP215/Msps/Stu2p/Dis1/Alp14 homolog) only weakly promotes nucleation, but acts synergistically with TPX2. Hence, a combination of distinct and complementary activities is sufficient for efficient microtubule formation in vitro. Importins control the efficiency of the microtubule nucleation by selectively blocking TPX2’s interaction with microtubule nucleation intermediates. This in vitro reconstitution reveals the molecular mechanism of regulated microtubule formation by a minimal nucleation module essential for chromatin-dependent microtubule nucleation in cells. PMID:26414402

Probabilistic approach to lysozyme crystal nucleation kinetics.

PubMed

Dimitrov, Ivaylo L; Hodzhaoglu, Feyzim V; Koleva, Dobryana P

2015-09-01

Nucleation of lysozyme crystals in quiescent solutions at a regime of progressive nucleation is investigated under an optical microscope at conditions of constant supersaturation. A method based on the stochastic nature of crystal nucleation and using discrete time sampling of small solution volumes for the presence or absence of detectable crystals is developed. It allows probabilities for crystal detection to be experimentally estimated. One hundred single samplings were used for each probability determination for 18 time intervals and six lysozyme concentrations. Fitting of a particular probability function to experimentally obtained data made possible the direct evaluation of stationary rates for lysozyme crystal nucleation, the time for growth of supernuclei to a detectable size and probability distribution of nucleation times. Obtained stationary nucleation rates were then used for the calculation of other nucleation parameters, such as the kinetic nucleation factor, nucleus size, work for nucleus formation and effective specific surface energy of the nucleus. The experimental method itself is simple and adaptable and can be used for crystal nucleation studies of arbitrary soluble substances with known solubility at particular solution conditions.

Ice nucleation triggered by negative pressure.

PubMed

Marcolli, Claudia

2017-11-30

Homogeneous ice nucleation needs supercooling of more than 35 K to become effective. When pressure is applied to water, the melting and the freezing points both decrease. Conversely, melting and freezing temperatures increase under negative pressure, i.e. when water is stretched. This study presents an extrapolation of homogeneous ice nucleation temperatures from positive to negative pressures as a basis for further exploration of ice nucleation under negative pressure. It predicts that increasing negative pressure at temperatures below about 262 K eventually results in homogeneous ice nucleation while at warmer temperature homogeneous cavitation, i. e. bubble nucleation, dominates. Negative pressure occurs locally and briefly when water is stretched due to mechanical shock, sonic waves, or fragmentation. The occurrence of such transient negative pressure should suffice to trigger homogeneous ice nucleation at large supercooling in the absence of ice-nucleating surfaces. In addition, negative pressure can act together with ice-inducing surfaces to enhance their intrinsic ice nucleation efficiency. Dynamic ice nucleation can be used to improve properties and uniformity of frozen products by applying ultrasonic fields and might also be relevant for the freezing of large drops in rainclouds.

XMAP215 is a microtubule nucleation factor that functions synergistically with the γ-tubulin ring complex.

PubMed

Thawani, Akanksha; Kadzik, Rachel S; Petry, Sabine

2018-05-01

How microtubules (MTs) are generated in the cell is a major question in understanding how the cytoskeleton is assembled. For several decades, γ-tubulin has been accepted as the universal MT nucleator of the cell. Although there is evidence that γ-tubulin complexes are not the sole MT nucleators, identification of other nucleation factors has proven difficult. Here, we report that the well-characterized MT polymerase XMAP215 (chTOG/Msps/Stu2p/Alp14/Dis1 homologue) is essential for MT nucleation in Xenopus egg extracts. The concentration of XMAP215 determines the extent of MT nucleation. Even though XMAP215 and the γ-tubulin ring complex (γ-TuRC) possess minimal nucleation activity individually, together, these factors synergistically stimulate MT nucleation in vitro. The amino-terminal TOG domains 1-5 of XMAP215 bind to αβ-tubulin and promote MT polymerization, whereas the conserved carboxy terminus is required for efficient MT nucleation and directly binds to γ-tubulin. In summary, XMAP215 and γ-TuRC together function as the principal nucleation module that generates MTs in cells.

Complementary activities of TPX2 and chTOG constitute an efficient importin-regulated microtubule nucleation module.

PubMed

Roostalu, Johanna; Cade, Nicholas I; Surrey, Thomas

2015-11-01

Spindle assembly and function require precise control of microtubule nucleation and dynamics. The chromatin-driven spindle assembly pathway exerts such control locally in the vicinity of chromosomes. One of the key targets of this pathway is TPX2. The molecular mechanism of how TPX2 stimulates microtubule nucleation is not understood. Using microscopy-based dynamic in vitro reconstitution assays with purified proteins, we find that human TPX2 directly stabilizes growing microtubule ends and stimulates microtubule nucleation by stabilizing early microtubule nucleation intermediates. Human microtubule polymerase chTOG (XMAP215/Msps/Stu2p/Dis1/Alp14 homologue) only weakly promotes nucleation, but acts synergistically with TPX2. Hence, a combination of distinct and complementary activities is sufficient for efficient microtubule formation in vitro. Importins control the efficiency of the microtubule nucleation by selectively blocking the interaction of TPX2 with microtubule nucleation intermediates. This in vitro reconstitution reveals the molecular mechanism of regulated microtubule formation by a minimal nucleation module essential for chromatin-dependent microtubule nucleation in cells.

Mechanism of two-step vapour-crystal nucleation in a pore

NASA Astrophysics Data System (ADS)

van Meel, J. A.; Liu, Y.; Frenkel, D.

2015-09-01

We present a numerical study of the effect of hemispherical pores on the nucleation of Lennard-Jones crystals from the vapour phase. As predicted by Page and Sear, there is a narrow range of pore radii, where vapour-liquid nucleation can become a two-step process. A similar observation was made for different pore geometries by Giacomello et al. We find that the maximum nucleation rate depends on both the size and the adsorption strength of the pore. Moreover, a poe can be more effective than a planar wall with the same strength of attraction. Pore-induced vapour-liquid nucleation turns out to be the rate-limiting step for crystal nucleation. This implies that crystal nucleation can be enhanced by a judicious choice of the wetting properties of a microporous nucleating agent.

Deposition Nucleation or Pore Condensation and Freezing?

NASA Astrophysics Data System (ADS)

David, Robert O.; Mahrt, Fabian; Marcolli, Claudia; Fahrni, Jonas; Brühwiler, Dominik; Lohmann, Ulrike; Kanji, Zamin A.

2017-04-01

Ice nucleation plays an important role in moderating Earth's climate and precipitation formation. Over the last century of research, several mechanisms for the nucleation of ice have been identified. Of the known mechanisms for ice nucleation, only deposition nucleation occurs below water saturation. Deposition nucleation is defined as the formation of ice from supersaturated water vapor on an insoluble particle without the prior formation of liquid. However, recent work has found that the efficiency of so-called deposition nucleation shows a dependence on the homogeneous freezing temperature of water even though no liquid phase is presumed to be present. Additionally, the ability of certain particles to nucleate ice more efficiently after being pre-cooled (pre-activation) raises questions on the true mechanism when ice nucleation occurs below water saturation. In an attempt to explain the dependence of the efficiency of so-called deposition nucleation on the onset of homogeneous freezing of liquid water, pore condensation and freezing has been proposed. Pore condensation and freezing suggests that the liquid phase can exist under sub-saturated conditions with respect to liquid in narrow confinements or pores due to the inverse Kelvin effect. Once the liquid-phase condenses, it is capable of nucleating ice either homogeneously or heterogeneously. The role of pore condensation and freezing is assessed in the Zurich Ice Nucleation Chamber, a continuous flow diffusion chamber, using spherical nonporous and mesoporous silica particles. The mesoporous silica particles have a well-defined particle size range of 400 to 600nm with discreet pore sizes of 2.5, 2.8, 3.5 and 3.8nm. Experiments conducted between 218K and 238K show that so-called deposition nucleation only occurs below the homogenous freezing temperature of water and is highly dependent on the presence of pores and their size. The results strongly support pore condensation and freezing, questioning the role of deposition nucleation as an ice nucleation pathway.

Crystal nucleation in lithium borate glass

NASA Technical Reports Server (NTRS)

Smith, Gary L.; Neilson, George F.; Weinberg, Michael C.

1988-01-01

Crystal nucleation measurements were made on three lithium borate compositions in the vicinity of Li2O-2Br2O3. All nucleation measurements were performed at 500 C. Certain aspects of the nucleation behavior indicated (tentatively) that it proceeded by a homogeneous mechanism. The steady state nucleation rate was observed to have the largest value when the Li2O concentration was slightly in excess of the diborate composition. The change in nucleation rate with composition is controlled by the variation of viscosity as well as the change in free energy with composition. The variation of nucleation rate is explained qualitatively in these terms.

Duplex Heterogeneous Nucleation Behavior of Precipitates in C-Mn Steel Containing Sn

NASA Astrophysics Data System (ADS)

Sun, Guilin; Tao, Sufen

2018-04-01

The two successive heterogeneous nucleation behaviors of FeSn2-MnS-Al2O3 complex precipitates in ultrahigh Sn-bearing steel were investigated. First, Al2O3 was the nucleation site of the MnS at the end of solidification. Then, FeSn2 nucleated heterogeneously on the MnS particles that nucleated on the Al2O3 particles. The formation sequence of the precipitated phase caused the duplex heterogeneous nucleation to occur consecutively at most twice.

Attainment of unstable β nucleation of glycine in presence of L-tyrosine and its analytical interpretation-A combined approach

NASA Astrophysics Data System (ADS)

Renuka Devi, K.; Srinivasan, K.

2015-05-01

The ability of L-tyrosine molecules to act as a template and to facilitate the nucleation of unstable β polymorph in the solution has been revealed through in-situ nucleation study. This nucleation of β occurs along with the existing α nucleation at the critical concentration of additive in the solution. The presence of L-tyrosine molecules lowers the inherent barrier that exists for β nucleation in the solution. No nucleation of γ was observed over the entire range of concentrations studied. The molecular recognition capability and stereo selective inhibitory action of the added L-tyrosine molecules towards glycine molecule have been successfully revealed in terms of habit modification observed in the nucleated polymorphs. In the case of α polymorph, L-tyrosine induces a change in the morphology along the enantiopolar -b direction while in the case of β polymorph, habit modification from needle to plate like structure is observed. With the increase in time span, solution mediated phase transformation from β to α polymorph has been observed in the solution. Analytically the nucleation parameters of α and β polymorphs were estimated based on Classical Nucleation Theory. Form of crystallization of the nucleated polymorphs of glycine was confirmed by a powder x-ray diffraction analysis.

Rate of Homogenous Nucleation of Ice in Supercooled Water.

PubMed

Atkinson, James D; Murray, Benjamin J; O'Sullivan, Daniel

2016-08-25

The homogeneous freezing of water is of fundamental importance to a number of fields, including that of cloud formation. However, there is considerable scatter in homogeneous nucleation rate coefficients reported in the literature. Using a cold stage droplet system designed to minimize uncertainties in temperature measurements, we examined the freezing of over 1500 pure water droplets with diameters between 4 and 24 μm. Under the assumption that nucleation occurs within the bulk of the droplet, nucleation rate coefficients fall within the spread of literature data and are in good agreement with a subset of more recent measurements. To quantify the relative importance of surface and volume nucleation in our experiments, where droplets are supported by a hydrophobic surface and surrounded by oil, comparison of droplets with different surface area to volume ratios was performed. From our experiments it is shown that in droplets larger than 6 μm diameter (between 234.6 and 236.5 K), nucleation in the interior is more important than nucleation at the surface. At smaller sizes we cannot rule out a significant contribution of surface nucleation, and in order to further constrain surface nucleation, experiments with smaller droplets are necessary. Nevertheless, in our experiments, it is dominantly volume nucleation controlling the observed nucleation rate.

Controlling the crystal polymorph by exploiting the time dependence of nucleation rates.

PubMed

Little, Laurie J; King, Alice A K; Sear, Richard P; Keddie, Joseph L

2017-10-14

Most substances can crystallise into two or more different crystal lattices called polymorphs. Despite this, there are no systems in which we can quantitatively predict the probability of one competing polymorph forming instead of the other. We address this problem using large scale (hundreds of events) studies of the competing nucleation of the alpha and gamma polymorphs of glycine. In situ Raman spectroscopy is used to identify the polymorph of each crystal. We find that the nucleation kinetics of the two polymorphs is very different. Nucleation of the alpha polymorph starts off slowly but accelerates, while nucleation of the gamma polymorph starts off fast but then slows. We exploit this difference to increase the purity with which we obtain the gamma polymorph by a factor of ten. The statistics of the nucleation of crystals is analogous to that of human mortality, and using a result from medical statistics, we show that conventional nucleation data can say nothing about what, if any, are the correlations between competing nucleation processes. Thus we can show that with data of our form it is impossible to disentangle the competing nucleation processes. We also find that the growth rate and the shape of a crystal depend on it when nucleated. This is new evidence that nucleation and growth are linked.

New trends in the nucleation research

NASA Astrophysics Data System (ADS)

Anisimov, M. P.; Hopke, P. K.

2017-09-01

During the last half of century the most of efforts have been directed towards small molecule system modeling using intermolecular potentials. Summarizing the nucleation theory, it can be concluded that the nowadays theory is far from complete. The vapor-gas nucleation theory can produce values that deviate from the experimental results by several orders of magnitude currently. Experiments on the vapor-gas nucleation rate measurements using different devices show significant inconsistencies in the measured rates as well. Theoretical results generally are quite reasonable for sufficiently low vapor nucleation rates where the capillary approximation is applicable. In the present research the advantages and current problems of the vapor-gas nucleation experiments are discussed briefly and a view of the future studies is presented. Using the brake points of the first derivative for the nucleation rate surface as markers of the critical embryos phase change is fresh idea to show the gas-pressure effect for the nucleating vapor-gas systems. To test the accuracy of experimental techniques, it is important to have a standard system that can be measured over a range of nucleation conditions. Several results illustrate that high-pressure techniques are needed to study multi-channel nucleation. In practical applications, parametric theories can be used for the systems of interest. However, experimental measurements are still the best source of information on nucleation rates. Experiments are labor intensive and costly, and thus, it is useful to extend the value of limited experimental measurements to a broader range of nucleation conditions. Only limited experimental data one needs for use in normalizing the slopes of the linearized nucleation rate surfaces. The nucleation rate surface is described in terms of steady-state nucleation rates. It is supposed that several new measuring systems, such as High Pressure Flow Diffusion Chamber for pressure limit up to 150 bar will be created soon in the frame of the Russian Ministry of Science & Education project under Contract № 14.Z50.31.0041 issued by February 13th of 2017. The Project will provide the nucleation studies for basic problems of theoretical and practical applications. Published under licence in Journal Title by IOP Publishing Ltd.

The enhancement and suppression of immersion mode heterogeneous ice-nucleation by solutes.

PubMed

Whale, Thomas F; Holden, Mark A; Wilson, Theodore W; O'Sullivan, Daniel; Murray, Benjamin J

2018-05-07

Heterogeneous nucleation of ice from aqueous solutions is an important yet poorly understood process in multiple fields, not least the atmospheric sciences where it impacts the formation and properties of clouds. In the atmosphere ice-nucleating particles are usually, if not always, mixed with soluble material. However, the impact of this soluble material on ice nucleation is poorly understood. In the atmospheric community the current paradigm for freezing under mixed phase cloud conditions is that dilute solutions will not influence heterogeneous freezing. By testing combinations of nucleators and solute molecules we have demonstrated that 0.015 M solutions (predicted melting point depression <0.1 °C) of several ammonium salts can cause suspended particles of feldspars and quartz to nucleate ice up to around 3 °C warmer than they do in pure water. In contrast, dilute solutions of certain alkali metal halides can dramatically depress freezing points for the same nucleators. At 0.015 M, solutes can enhance or deactivate the ice-nucleating ability of a microcline feldspar across a range of more than 10 °C, which corresponds to a change in active site density of more than a factor of 10 5 . This concentration was chosen for a survey across multiple solutes-nucleant combinations since it had a minimal colligative impact on freezing and is relevant for activating cloud droplets. Other nucleators, for instance a silica gel, are unaffected by these 'solute effects', to within experimental uncertainty. This split in response to the presence of solutes indicates that different mechanisms of ice nucleation occur on the different nucleators or that surface modification of relevance to ice nucleation proceeds in different ways for different nucleators. These solute effects on immersion mode ice nucleation may be of importance in the atmosphere as sea salt and ammonium sulphate are common cloud condensation nuclei (CCN) for cloud droplets and are internally mixed with ice-nucleating particles in mixed-phase clouds. In addition, we propose a pathway dependence where activation of CCN at low temperatures might lead to enhanced ice formation relative to pathways where CCN activation occurs at higher temperatures prior to cooling to nucleation temperature.

The enhancement and suppression of immersion mode heterogeneous ice-nucleation by solutes

PubMed Central

Holden, Mark A.; Wilson, Theodore W.; O'Sullivan, Daniel; Murray, Benjamin J.

2018-01-01

Heterogeneous nucleation of ice from aqueous solutions is an important yet poorly understood process in multiple fields, not least the atmospheric sciences where it impacts the formation and properties of clouds. In the atmosphere ice-nucleating particles are usually, if not always, mixed with soluble material. However, the impact of this soluble material on ice nucleation is poorly understood. In the atmospheric community the current paradigm for freezing under mixed phase cloud conditions is that dilute solutions will not influence heterogeneous freezing. By testing combinations of nucleators and solute molecules we have demonstrated that 0.015 M solutions (predicted melting point depression <0.1 °C) of several ammonium salts can cause suspended particles of feldspars and quartz to nucleate ice up to around 3 °C warmer than they do in pure water. In contrast, dilute solutions of certain alkali metal halides can dramatically depress freezing points for the same nucleators. At 0.015 M, solutes can enhance or deactivate the ice-nucleating ability of a microcline feldspar across a range of more than 10 °C, which corresponds to a change in active site density of more than a factor of 105. This concentration was chosen for a survey across multiple solutes–nucleant combinations since it had a minimal colligative impact on freezing and is relevant for activating cloud droplets. Other nucleators, for instance a silica gel, are unaffected by these ‘solute effects’, to within experimental uncertainty. This split in response to the presence of solutes indicates that different mechanisms of ice nucleation occur on the different nucleators or that surface modification of relevance to ice nucleation proceeds in different ways for different nucleators. These solute effects on immersion mode ice nucleation may be of importance in the atmosphere as sea salt and ammonium sulphate are common cloud condensation nuclei (CCN) for cloud droplets and are internally mixed with ice-nucleating particles in mixed-phase clouds. In addition, we propose a pathway dependence where activation of CCN at low temperatures might lead to enhanced ice formation relative to pathways where CCN activation occurs at higher temperatures prior to cooling to nucleation temperature. PMID:29780544

A detailed study of ice nucleation by feldspar minerals

NASA Astrophysics Data System (ADS)

Whale, T. F.; Murray, B. J.; Wilson, T. W.; Carpenter, M. A.; Harrison, A.; Holden, M. A.; Vergara Temprado, J.; Morris, J.; O'Sullivan, D.

2015-12-01

Immersion mode heterogeneous ice nucleation plays a crucial role in controlling the composition of mixed phase clouds, which contain both supercooled liquid water and ice particles. The amount of ice in mixed phase clouds can affect cloud particle size, lifetime and extent and so affects radiative properties and precipitation. Feldspar minerals are probably the most important minerals for ice nucleation in mixed phase clouds because they nucleate ice more efficiently than other components of atmospheric mineral dust (Atkinson et al. 2013). The feldspar class of minerals is complex, containing numerous chemical compositions, several crystal polymorphs and wide variations in microscopic structure. Here we present the results of a study into ice nucleation by a wide range of different feldspars. We found that, in general, alkali feldspars nucleate ice more efficiently than plagioclase feldspars. However, we also found that particular alkali feldspars nucleate ice relatively inefficiently, suggesting that chemical composition is not the only important factor that dictates the ice nucleation efficiency of feldspar minerals. Ice nucleation by feldspar is described well by the singular model and is probably site specific in nature. The alkali feldspars that do not nucleate ice efficiently possess relatively homogenous structure on the micrometre scale suggesting that the important sites for nucleation are related to surface topography. Ice nucleation active site densities for the majority of tested alkali feldspars are similar to those found by Atkinson et al (2013), meaning that the validity of global aerosol modelling conducted in that study is not affected. Additionally, we have found that ice nucleation by feldspars is strongly influenced, both positively and negatively, by the solute content of droplets. Most other nucleants we have tested are unaffected by solutes. This provides insight into the mechanism of ice nucleation by feldspars and could be of importance when modelling ice nucleation in mixed phase clouds. Atkinson, J. D., Murray, B. J., Woodhouse, M. T., Carslaw, K. S., Whale, T. F., Baustian, K. J., Dobbie, S., O'Sullivan, D., and Malkin, T. L.: The importance of feldspar for ice nucleation by mineral dust in mixed-phase clouds, Nature, 10.1038/nature12278, (2013).

Kinetics of phase transformations in glass forming systems

NASA Technical Reports Server (NTRS)

Ray, Chandra S.

1994-01-01

A nucleation rate like curve for a glass can be determined from the functional dependence of the maximum height of its DTA crystallization peak, (delta T)(sub p), on the nucleation temperature, T(sub n). This nucleation rate curve provides information for the temperature range where nucleation for the glass can occur and the temperature where the nucleation rate is a maximum. However, this curve does not provide information for the nucleation rate, I, for the glass at different temperatures. A method for estimating I at different temperatures from (delta T)(sub p) was developed using a Li2O.2SiO2 (LS2) glass. Also, the dielectric constant (epsilon) and the loss factor (tan delta) of a glass-ceramic depend, in part, upon the amount of crystallinity which, in turn, depends upon the nucleation density in the starting glass. It is therefore expected that epsilon and tan delta should have a relationship with nucleation density and hence on the nucleation rate.

Correlation of Solubility with the Metastable Limit of Nucleation Using Gauge-Cell Monte Carlo Simulations.

PubMed

Clark, Michael D; Morris, Kenneth R; Tomassone, Maria Silvina

2017-09-12

We present a novel simulation-based investigation of the nucleation of nanodroplets from solution and from vapor. Nucleation is difficult to measure or model accurately, and predicting when nucleation should occur remains an open problem. Of specific interest is the "metastable limit", the observed concentration at which nucleation occurs spontaneously, which cannot currently be estimated a priori. To investigate the nucleation process, we employ gauge-cell Monte Carlo simulations to target spontaneous nucleation and measure thermodynamic properties of the system at nucleation. Our results reveal a widespread correlation over 5 orders of magnitude of solubilities, in which the metastable limit depends exclusively on solubility and the number density of generated nuclei. This three-way correlation is independent of other parameters, including intermolecular interactions, temperature, molecular structure, system composition, and the structure of the formed nuclei. Our results have great potential to further the prediction of nucleation events using easily measurable solute properties alone and to open new doors for further investigation.

Focus: Nucleation kinetics of shear bands in metallic glass.

PubMed

Wang, J Q; Perepezko, J H

2016-12-07

The development of shear bands is recognized as the primary mechanism in controlling the plastic deformability of metallic glasses. However, the kinetics of the nucleation of shear bands has received limited attention. The nucleation of shear bands in metallic glasses (MG) can be investigated using a nanoindentation method to monitor the development of the first pop-in event that is a signature of shear band nucleation. The analysis of a statistically significant number of first pop-in events demonstrates the stochastic behavior that is characteristic of nucleation and reveals a multimodal behavior associated with local spatial heterogeneities. The shear band nucleation rate of the two nucleation modes and the associated activation energy, activation volume, and site density were determined by loading rate experiments. The nucleation activation energy is very close to the value that is characteristic of the β relaxation in metallic glass. The identification of the rate controlling kinetics for shear band nucleation offers guidance for promoting plastic flow in metallic glass.

Kinetic model for binary homogeneous nucleation in the H2O-H2SO4 system: comparison with experiments and classical theory of nucleation.

PubMed

Sorokin, A; Vancassel, X; Mirabel, P

2005-12-22

A kinetic model to predict nucleation rates in the sulfuric acid-water system is presented. It allows calculating steady-state nucleation rates and the corresponding time lag, using a direct solution of a system of kinetic equations that describe the populations of sub- and near-critical clusters. This kinetic model takes into account cluster-cluster collisions and decay of clusters into smaller clusters. The model results are compared with some predictions obtained with the classical nucleation theory (CNT) and also with available measurement data obtained in smog chambers or flow tubes. It is shown that in the case of slow nucleation processes, the kinetic model and the CNT as used by Shugard et al. [J. Chem. Phys. 75, 5298 (1974)] give the same results. However, in the case of intensive nucleation, a large part of the nucleation flux is due to cluster-cluster collisions and the CNT underestimates the nucleation rates.

Argon nucleation in a cryogenic supersonic nozzle

NASA Astrophysics Data System (ADS)

Sinha, Somnath; Bhabhe, Ashutosh; Laksmono, Hartawan; Wölk, Judith; Strey, Reinhard; Wyslouzil, Barbara

2010-02-01

We have measured pressures p and temperatures T corresponding to the maximum nucleation rate of argon in a cryogenic supersonic nozzle apparatus where the estimated nucleation rates are J =1017±1 cm-3 s-1. As T increases from 34 to 53 K, p increases from 0.47 to 8 kPa. Under these conditions, classical nucleation theory predicts nucleation rates of 11-13 orders of magnitude lower than the observed rates while mean field kinetic nucleation theory predicts the observed rates within 1 order of magnitude. The current data set appears consistent with the measurements of Iland et al. [J. Chem. Phys. 127, 154506 (2007)] in the cryogenic nucleation pulse chamber. Combining the two data sets suggests that classical nucleation theory fails because it overestimates both the critical cluster size and the excess internal energy of the critical clusters.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kulkarni, Gourihar R.; Zhang, Kai; Zhao, Chun

Changes in the ice nucleation characteristics of atmospherically relevant mineral dust particles due to nitric acid coating are not well understood. Further, the atmospheric implications of dust coating on ice-cloud properties under different assumptions of primary ice nucleation mechanisms are unknown. We investigated ice nucleation ability of Arizona test dust, illite, K-feldspar and quartz as a function of temperature (-25 to -30°C) and relative humidity with respect to water (75 to 110%). Particles were size selected at 250 nm and transported (bare or coated) to the ice nucleation chamber to determine the fraction of particles nucleating ice at various temperaturemore » and water saturation conditions. All dust nucleated ice at water-subsaturated conditions, but the coated particles showed a reduction in their ice nucleation ability compared to bare particles. However, at water-supersaturated conditions, we observed that bare and coated particles had nearly similar ice nucleation characteristics. X-ray diffraction patterns indicated that structural properties of bare dust particles modified after acid treatment. We found that lattice parameters were slightly different, but crystallite sizes of the coated particles were reduced compared to bare particles. Next, single-column model results show that simulated ice crystal number concentrations mostly depends upon fraction of particles that are coated, primary ice nucleation mechanisms, and the competition between ice nucleation mechanisms to nucleate ice. In general, we observed that coating modify the ice-cloud properties and the picture of ice and mixed-phase cloud evolution is complex when different primary ice nucleation mechanisms are competing for fixed water vapor mass.« less

Ice-Nucleating Bacteria

NASA Astrophysics Data System (ADS)

Obata, Hitoshi

Since the discovery of ice-nucleating bacteria in 1974 by Maki et al., a large number of studies on the biological characteristics, ice-nucleating substance, ice nucleation gene and frost damage etc. of the bacteria have been carried out. Ice-nucleating bacteria can cause the freezing of water at relatively warm temperature (-2.3°C). Tween 20 was good substrates for ice-nucleating activity of Pseudomonas fluorescens KUIN-1. Major fatty acids of Isolate (Pseudomonas fluorescens) W-11 grown at 30°C were palmitic, cis-9-hexadecenoic and cis-11-octadecenoic which amounted to 90% of the total fatty acids. Sequence analysis shows that an ice nucleation gene from Pseudomonas fluorescens is related to the gene of Pseudomonas syringae.

Nucleation rates of Sn in undercooled Sn-Ag-Cu flip-chip solder joints

NASA Astrophysics Data System (ADS)

Arfaei, B.; Benedict, M.; Cotts, E. J.

2013-11-01

The nucleation of Sn from the melt in commercial SnAgCu flip chip solder joints was monitored at a number of different temperatures. Nucleation rates were estimated from measurements of nucleation times for 440 solder balls after one reflow and were found to be well epitomized by the expression I = 2 × 109 exp[(-1.6 × 105)/(T × (ΔT)2)] m-3 s-1, as per classical nucleation theory. After an additional reflow, the nucleation rates of the same 440 samples were observed to increase to I = 2 × 109 exp[(-8.9 × 104)/(T × (ΔT)2)] m-3 s-1. Thus it was shown that the expressions of classical nucleation theory well characterize nucleation kinetics for this system. These changes in nucleation kinetics were correlated with continued dissolution of Al and Ni in to the SnAgCu melt. Such increases in nucleation rates meant increases in the average solidification temperatures of the solder balls after reflow. Variations in the Sn grain morphology of the solder joints were correlated with these changes in solidification temperature, with larger Sn grains (beach ball Sn grain morphology) observed at higher solidification temperatures.

A Theory of Heterogeneous Ice Nucleation in the Immersion Mode

NASA Astrophysics Data System (ADS)

Barahona, D.

2017-12-01

Immersion ice nucleation is likely involved in the initiation of precipitation and determines to a large extent the phase partitioning in convective clouds. Theoretical models commonly used to describe immersion freezing in atmospheric models are based on the classical nucleation theory. CNT however neglects important interactions near the immersed particle that may affect nucleation rates. This work introduces a new theory of immersion freezing based on two premises. First, immersion ice nucleation is mediated by the modification of the properties of water near the particle-liquid interface rather than by the geometry of the ice germ. Second, the same mechanism that leads to the decrease in the work of germ formation also decreases the mobility of water molecules near the immersed particle. These two premises allow establishing general thermodynamic constraints to the ice nucleation rate. Analysis of the new theory shows that active sites likely trigger ice nucleation, but they do not control the overall nucleation rate nor the probability of freezing. It also suggests that materials with different ice nucleation efficiency may exhibit similar freezing temperatures under similar conditions but differ in their sensitivity to particle surface area and cooling rate. The theory suggests that many species are very efficient at nucleating ice and it is likely that highly effective INP are not uncommon in the atmosphere; however ice nucleation rates may be slower than currently believed. Predicted nucleation rates show good agreement with experimental results for a diverse set of atmospheric relevant materials including dust, black carbon and bacterial ice nucleating particles. The application of the new theory within the NASA Global Earth System Model (GEOS-5) is also discussed.

The XMAP215 Ortholog Alp14 Promotes Microtubule Nucleation in Fission Yeast.

PubMed

Flor-Parra, Ignacio; Iglesias-Romero, Ana Belén; Chang, Fred

2018-06-04

The organization and number of microtubules (MTs) in a cell depend on the proper regulation of MT nucleation. Currently, the mechanism of nucleation is the most poorly understood aspect of MT dynamics. XMAP215/chTOG/Alp14/Stu2 proteins are MT polymerases that stimulate MT polymerization at MT plus ends by binding and releasing tubulin dimers. Although these proteins also localize to MT organizing centers and have nucleating activity in vitro, it is not yet clear whether these proteins participate in MT nucleation in vivo. Here, we demonstrate that in the fission yeast Schizosaccharomyces pombe, the XMAP215 ortholog Alp14 is critical for efficient MT nucleation in vivo. In multiple assays, loss of Alp14 function led to reduced nucleation rate and numbers of interphase MT bundles. Conversely, activation of Alp14 led to increased nucleation frequency. Alp14 associated with Mto1 and γ-tubulin complex components, and artificially targeting Alp14 to the γ-tubulin ring complexes (γ-TuRCs) stimulated nucleation. In imaging individual nucleation events, we found that Alp14 transiently associated with a γ-tubulin particle shortly before the appearance of a new MT. The transforming acidic coiled-coil (TACC) ortholog Alp7 mediated the localization of Alp14 at nucleation sites but not plus ends, and was required for efficient nucleation but not for MT polymerization. Our findings provide the strongest evidence to date that Alp14 serves as a critical MT nucleation factor in vivo. We suggest a model in which Alp14 associates with the γ-tubulin complex in an Alp7-dependent manner to facilitate the assembly or stabilization of the nascent MT. Copyright © 2018 Elsevier Ltd. All rights reserved.

Review of nucleation and incipient boiling under pool and forced convection conditions

NASA Technical Reports Server (NTRS)

Merte, Herman, Jr.

1987-01-01

An overview of liquid-vapor nucleation is given. The result of thermodynamic equilibrium across curved liquid-vapor interfaces is presented. The extension of this to include the interaction with idealizations of surface cavities is made to demonstrate how superheat requirements for nucleation will be affected by surface roughness, flow velocity and buoyancy. Experimental measurements of high liquid superheats and nucleation delay times are presented as examples of homogeneous nucleation. Examples of nucleation and boiling on smooth glass substrates and on metal surfaces with various surface roughnesses are presented.

Change of supercooling capability in solutions containing different kinds of ice nucleators by flavonol glycosides from deep supercooling xylem parenchyma cells in trees.

PubMed

Kuwabara, Chikako; Kasuga, Jun; Wang, Donghui; Fukushi, Yukiharu; Arakawa, Keita; Koyama, Toshie; Inada, Takaaki; Fujikawa, Seizo

2011-12-01

Deep supercooling xylem parenchyma cells (XPCs) in Katsura tree contain flavonol glycosides with high supercooling-facilitating capability in solutions containing the ice nucleation bacterium (INB) Erwinia ananas, which is thought to have an important role in deep supercooling of XPCs. The present study, in order to further clarify the roles of these flavonol glycosides in deep supercooling of XPCs, the effects of these supercooling-facilitating (anti-ice nucleating) flavonol glycosides, kaempferol 3-O-β-D-glucopyranoside (K3Glc), kaempferol 7-O-β-D-glucopyranoside (K7Glc) and quercetin 3-O-β-D-glucopyranoside (Q3Glc), in buffered Milli-Q water (BMQW) containing different kinds of ice nucleators, including INB Xanthomonas campestris, silver iodide and phloroglucinol, were examined by a droplet freezing assay. The results showed that all of the flavonol glycosides promoted supercooling in all solutions containing different kinds of ice nucleators, although the magnitudes of supercooling capability of each flavonol glycoside changed in solutions containing different kinds of ice nucleators. On the other hand, these flavonol glycosides exhibited complicated nucleating reactions in BMQW, which did not contain identified ice nucleators but contained only unidentified airborne impurities. Q3Glc exhibited both supercooling-facilitating and ice nucleating capabilities depending on the concentrations in such water. Both K3Glc and K7Glc exhibited only ice nucleation capability in such water. It was also shown by an emulsion freezing assay in BMQW that K3Glc and Q3Glc had no effect on homogeneous ice nucleation temperature, whereas K7Glc increased ice nucleation temperature. The results indicated that each flavonol glycoside affected ice nucleation by very complicated and varied reactions. More studies are necessary to determine the exact roles of these flavonol glycosides in deep supercooling of XPCs in which unidentified heterogeneous ice nucleators may exist. Copyright © 2011 Elsevier Inc. All rights reserved.

Ice nucleation activity of agricultural soil dust aerosols from Mongolia, Argentina, and Germany

NASA Astrophysics Data System (ADS)

Steinke, I.; Funk, R.; Busse, J.; Iturri, A.; Kirchen, S.; Leue, M.; Möhler, O.; Schwartz, T.; Schnaiter, M.; Sierau, B.; Toprak, E.; Ullrich, R.; Ulrich, A.; Hoose, C.; Leisner, T.

2016-11-01

Soil dust particles emitted from agricultural areas contain considerable mass fractions of organic material. Also, soil dust particles may act as carriers for potentially ice-active biological particles. In this work, we present ice nucleation experiments conducted in the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) cloud chamber. We investigated the ice nucleation efficiency of four types of soil dust from different regions of the world. The results are expressed as ice nucleation active surface site (INAS) densities and presented for the immersion freezing and the deposition nucleation mode. For immersion freezing occurring at 254 K, samples from Argentina, China, and Germany show ice nucleation efficiencies which are by a factor of 10 higher than desert dusts. On average, the difference in ice nucleation efficiencies between agricultural and desert dusts becomes significantly smaller at temperatures below 247 K. In the deposition mode the soil dusts showed higher ice nucleation activity than Arizona Test Dust over a temperature range between 232 and 248 K and humidities RHice up to 125%. INAS densities varied between 109 and 1011 m-2 for these thermodynamic conditions. For one soil dust sample (Argentinian Soil), the effect of treatments with heat was investigated. Heat treatments (383 K) did not affect the ice nucleation efficiency observed at 249 K. This finding presumably excludes proteinaceous ice-nucleating entities as the only source of the increased ice nucleation efficiency.

Ice nucleation in the upper troposphere: Sensitivity to aerosol number density, temperature, and cooling rate

NASA Technical Reports Server (NTRS)

Jensen, E. J.; Toon, O. B.

1994-01-01

We have investigated the processes that control ice crystal nucleation in the upper troposphere using a numerical model. Nucleation of ice resulting from cooling was simulated for a range of aerosol number densities, initial temperatures, and cooling rates. In contrast to observations of stratus clouds, we find that the number of ice crystals that nucleate in cirrus is relatively insensitive to the number of aerosols present. The ice crystal size distribution at the end of the nucleation process is unaffected by the assumed initial aerosol number density. Essentially, nucleation continues until enough ice crystals are present such that their deposition growth rapidly depletes the vapor and shuts off any further nucleation. However, the number of ice crystals nucleated increases rapidly with decreasing initial temperature and increasing cooling rate. This temperature dependence alone could explain the large ice crystal number density observed in very cold tropical cirrus.

Molecular-dynamics simulations of urea nucleation from aqueous solution

PubMed Central

Salvalaglio, Matteo; Perego, Claudio; Giberti, Federico; Mazzotti, Marco; Parrinello, Michele

2015-01-01

Despite its ubiquitous character and relevance in many branches of science and engineering, nucleation from solution remains elusive. In this framework, molecular simulations represent a powerful tool to provide insight into nucleation at the molecular scale. In this work, we combine theory and molecular simulations to describe urea nucleation from aqueous solution. Taking advantage of well-tempered metadynamics, we compute the free-energy change associated to the phase transition. We find that such a free-energy profile is characterized by significant finite-size effects that can, however, be accounted for. The description of the nucleation process emerging from our analysis differs from classical nucleation theory. Nucleation of crystal-like clusters is in fact preceded by large concentration fluctuations, indicating a predominant two-step process, whereby embryonic crystal nuclei emerge from dense, disordered urea clusters. Furthermore, in the early stages of nucleation, two different polymorphs are seen to compete. PMID:25492932

Molecular-dynamics simulations of urea nucleation from aqueous solution.

PubMed

Salvalaglio, Matteo; Perego, Claudio; Giberti, Federico; Mazzotti, Marco; Parrinello, Michele

2015-01-06

Despite its ubiquitous character and relevance in many branches of science and engineering, nucleation from solution remains elusive. In this framework, molecular simulations represent a powerful tool to provide insight into nucleation at the molecular scale. In this work, we combine theory and molecular simulations to describe urea nucleation from aqueous solution. Taking advantage of well-tempered metadynamics, we compute the free-energy change associated to the phase transition. We find that such a free-energy profile is characterized by significant finite-size effects that can, however, be accounted for. The description of the nucleation process emerging from our analysis differs from classical nucleation theory. Nucleation of crystal-like clusters is in fact preceded by large concentration fluctuations, indicating a predominant two-step process, whereby embryonic crystal nuclei emerge from dense, disordered urea clusters. Furthermore, in the early stages of nucleation, two different polymorphs are seen to compete.

Acid–base chemical reaction model for nucleation rates in the polluted atmospheric boundary layer

PubMed Central

Chen, Modi; Titcombe, Mari; Jiang, Jingkun; Jen, Coty; Kuang, Chongai; Fischer, Marc L.; Eisele, Fred L.; Siepmann, J. Ilja; Hanson, David R.; Zhao, Jun; McMurry, Peter H.

2012-01-01

Climate models show that particles formed by nucleation can affect cloud cover and, therefore, the earth's radiation budget. Measurements worldwide show that nucleation rates in the atmospheric boundary layer are positively correlated with concentrations of sulfuric acid vapor. However, current nucleation theories do not correctly predict either the observed nucleation rates or their functional dependence on sulfuric acid concentrations. This paper develops an alternative approach for modeling nucleation rates, based on a sequence of acid–base reactions. The model uses empirical estimates of sulfuric acid evaporation rates obtained from new measurements of neutral molecular clusters. The model predicts that nucleation rates equal the sulfuric acid vapor collision rate times a prefactor that is less than unity and that depends on the concentrations of basic gaseous compounds and preexisting particles. Predicted nucleation rates and their dependence on sulfuric acid vapor concentrations are in reasonable agreement with measurements from Mexico City and Atlanta. PMID:23091030

Autocatalytic microtubule nucleation determines the size and mass of Xenopus laevis egg extract spindles

PubMed Central

Decker, Franziska; Oriola, David; Dalton, Benjamin

2018-01-01

Regulation of size and growth is a fundamental problem in biology. A prominent example is the formation of the mitotic spindle, where protein concentration gradients around chromosomes are thought to regulate spindle growth by controlling microtubule nucleation. Previous evidence suggests that microtubules nucleate throughout the spindle structure. However, the mechanisms underlying microtubule nucleation and its spatial regulation are still unclear. Here, we developed an assay based on laser ablation to directly probe microtubule nucleation events in Xenopus laevis egg extracts. Combining this method with theory and quantitative microscopy, we show that the size of a spindle is controlled by autocatalytic growth of microtubules, driven by microtubule-stimulated microtubule nucleation. The autocatalytic activity of this nucleation system is spatially regulated by the limiting amounts of active microtubule nucleators, which decrease with distance from the chromosomes. This mechanism provides an upper limit to spindle size even when resources are not limiting. PMID:29323637

Acid-base chemical reaction model for nucleation rates in the polluted atmospheric boundary layer.

PubMed

Chen, Modi; Titcombe, Mari; Jiang, Jingkun; Jen, Coty; Kuang, Chongai; Fischer, Marc L; Eisele, Fred L; Siepmann, J Ilja; Hanson, David R; Zhao, Jun; McMurry, Peter H

2012-11-13

Climate models show that particles formed by nucleation can affect cloud cover and, therefore, the earth's radiation budget. Measurements worldwide show that nucleation rates in the atmospheric boundary layer are positively correlated with concentrations of sulfuric acid vapor. However, current nucleation theories do not correctly predict either the observed nucleation rates or their functional dependence on sulfuric acid concentrations. This paper develops an alternative approach for modeling nucleation rates, based on a sequence of acid-base reactions. The model uses empirical estimates of sulfuric acid evaporation rates obtained from new measurements of neutral molecular clusters. The model predicts that nucleation rates equal the sulfuric acid vapor collision rate times a prefactor that is less than unity and that depends on the concentrations of basic gaseous compounds and preexisting particles. Predicted nucleation rates and their dependence on sulfuric acid vapor concentrations are in reasonable agreement with measurements from Mexico City and Atlanta.

A nucleator arms race: cellular control of actin assembly.

PubMed

Campellone, Kenneth G; Welch, Matthew D

2010-04-01

For over a decade, the actin-related protein 2/3 (ARP2/3) complex, a handful of nucleation-promoting factors and formins were the only molecules known to directly nucleate actin filament formation de novo. However, the past several years have seen a surge in the discovery of mammalian proteins with roles in actin nucleation and dynamics. Newly recognized nucleation-promoting factors, such as WASP and SCAR homologue (WASH), WASP homologue associated with actin, membranes and microtubules (WHAMM), and junction-mediating regulatory protein (JMY), stimulate ARP2/3 activity at distinct cellular locations. Formin nucleators with additional biochemical and cellular activities have also been uncovered. Finally, the Spire, cordon-bleu and leiomodin nucleators have revealed new ways of overcoming the kinetic barriers to actin polymerization.

Homogeneous ice nucleation from aqueous inorganic/organic particles representative of biomass burning: water activity, freezing temperatures, nucleation rates.

PubMed

Knopf, Daniel A; Rigg, Yannick J

2011-02-10

Homogeneous ice nucleation plays an important role in the formation of cirrus clouds with subsequent effects on the global radiative budget. Here we report on homogeneous ice nucleation temperatures and corresponding nucleation rate coefficients of aqueous droplets serving as surrogates of biomass burning aerosol. Micrometer-sized (NH(4))(2)SO(4)/levoglucosan droplets with mass ratios of 10:1, 1:1, 1:5, and 1:10 and aqueous multicomponent organic droplets with and without (NH(4))(2)SO(4) under typical tropospheric temperatures and relative humidities are investigated experimentally using a droplet conditioning and ice nucleation apparatus coupled to an optical microscope with image analysis. Homogeneous freezing was determined as a function of temperature and water activity, a(w), which was set at droplet preparation conditions. The ice nucleation data indicate that minor addition of (NH(4))(2)SO(4) to the aqueous organic droplets renders the temperature dependency of water activity negligible in contrast to the case of aqueous organic solution droplets. The mean homogeneous ice nucleation rate coefficient derived from 8 different aqueous droplet compositions with average diameters of ∼60 μm for temperatures as low as 195 K and a(w) of 0.82-1 is 2.18 × 10(6) cm(-3) s(-1). The experimentally derived freezing temperatures and homogeneous ice nucleation rate coefficients are in agreement with predictions of the water activity-based homogeneous ice nucleation theory when taking predictive uncertainties into account. However, the presented ice nucleation data indicate that the water activity-based homogeneous ice nucleation theory overpredicts the freezing temperatures by up to 3 K and corresponding ice nucleation rate coefficients by up to ∼2 orders of magnitude. A shift of 0.01 in a(w), which is well within the uncertainty of typical field and laboratory relative humidity measurements, brings experimental and predicted freezing temperatures and homogeneous ice nucleation rate coefficients into agreement. The experimentally derived ice nucleation data are applied to constrain the water activity-based homogeneous ice nucleation theory to smaller than ±1 order of magnitude compared to the predictive uncertainty of larger than ±6 orders of magnitude. The atmospheric implications of these findings are discussed.

Measuring and modeling polymer concentration profiles near spindle boundaries argues that spindle microtubules regulate their own nucleation

NASA Astrophysics Data System (ADS)

Kaye, Bryan; Stiehl, Olivia; Foster, Peter J.; Shelley, Michael J.; Needleman, Daniel J.; Fürthauer, Sebastian

2018-05-01

Spindles are self-organized microtubule-based structures that segregate chromosomes during cell division. The mass of the spindle is controlled by the balance between microtubule turnover and nucleation. The mechanisms that control the spatial regulation of microtubule nucleation remain poorly understood. While previous work found that microtubule nucleators bind to pre-existing microtubules in the spindle, it is still unclear whether this binding regulates the activity of those nucleators. Here we use a combination of experiments and mathematical modeling to investigate this issue. We measured the concentration of microtubules and soluble tubulin in and around the spindle. We found a very sharp decay in the concentration of microtubules at the spindle interface. This is inconsistent with a model in which the activity of nucleators is independent of their association with microtubules but consistent with a model in which microtubule nucleators are only active when bound to pre-existing microtubules. This argues that the activity of microtubule nucleators is greatly enhanced when bound to pre-existing microtubules. Thus, microtubule nucleators are both localized and activated by the microtubules they generate.

Improved Success of Sparse Matrix Protein Crystallization Screening with Heterogeneous Nucleating Agents

PubMed Central

Thakur, Anil S.; Robin, Gautier; Guncar, Gregor; Saunders, Neil F. W.; Newman, Janet; Martin, Jennifer L.; Kobe, Bostjan

2007-01-01

Background Crystallization is a major bottleneck in the process of macromolecular structure determination by X-ray crystallography. Successful crystallization requires the formation of nuclei and their subsequent growth to crystals of suitable size. Crystal growth generally occurs spontaneously in a supersaturated solution as a result of homogenous nucleation. However, in a typical sparse matrix screening experiment, precipitant and protein concentration are not sampled extensively, and supersaturation conditions suitable for nucleation are often missed. Methodology/Principal Findings We tested the effect of nine potential heterogenous nucleating agents on crystallization of ten test proteins in a sparse matrix screen. Several nucleating agents induced crystal formation under conditions where no crystallization occurred in the absence of the nucleating agent. Four nucleating agents: dried seaweed; horse hair; cellulose and hydroxyapatite, had a considerable overall positive effect on crystallization success. This effect was further enhanced when these nucleating agents were used in combination with each other. Conclusions/Significance Our results suggest that the addition of heterogeneous nucleating agents increases the chances of crystal formation when using sparse matrix screens. PMID:17971854

Forward flux sampling calculation of homogeneous nucleation rates from aqueous NaCl solutions.

PubMed

Jiang, Hao; Haji-Akbari, Amir; Debenedetti, Pablo G; Panagiotopoulos, Athanassios Z

2018-01-28

We used molecular dynamics simulations and the path sampling technique known as forward flux sampling to study homogeneous nucleation of NaCl crystals from supersaturated aqueous solutions at 298 K and 1 bar. Nucleation rates were obtained for a range of salt concentrations for the Joung-Cheatham NaCl force field combined with the Extended Simple Point Charge (SPC/E) water model. The calculated nucleation rates are significantly lower than the available experimental measurements. The estimates for the nucleation rates in this work do not rely on classical nucleation theory, but the pathways observed in the simulations suggest that the nucleation process is better described by classical nucleation theory than an alternative interpretation based on Ostwald's step rule, in contrast to some prior simulations of related models. In addition to the size of NaCl nucleus, we find that the crystallinity of a nascent cluster plays an important role in the nucleation process. Nuclei with high crystallinity were found to have higher growth probability and longer lifetimes, possibly because they are less exposed to hydration water.

The atomic-scale nucleation mechanism of NiTi metallic glasses upon isothermal annealing studied via molecular dynamics simulations.

PubMed

Li, Yang; Li, JiaHao; Liu, BaiXin

2015-10-28

Nucleation is one of the most essential transformation paths in phase transition and exerts a significant influence on the crystallization process. Molecular dynamics simulations were performed to investigate the atomic-scale nucleation mechanisms of NiTi metallic glasses upon devitrification at various temperatures (700 K, 750 K, 800 K, and 850 K). Our simulations reveal that at 700 K and 750 K, nucleation is polynuclear with high nucleation density, while at 800 K it is mononuclear. The underlying nucleation mechanisms have been clarified, manifesting that nucleation can be induced either by the initial ordered clusters (IOCs) or by the other precursors of nuclei evolved directly from the supercooled liquid. IOCs and other precursors stem from the thermal fluctuations of bond orientational order in supercooled liquids during the quenching process and during the annealing process, respectively. The simulation results not only elucidate the underlying nucleation mechanisms varied with temperature, but also unveil the origin of nucleation. These discoveries offer new insights into the devitrification mechanism of metallic glasses.

Forward flux sampling calculation of homogeneous nucleation rates from aqueous NaCl solutions

NASA Astrophysics Data System (ADS)

Jiang, Hao; Haji-Akbari, Amir; Debenedetti, Pablo G.; Panagiotopoulos, Athanassios Z.

2018-01-01

We used molecular dynamics simulations and the path sampling technique known as forward flux sampling to study homogeneous nucleation of NaCl crystals from supersaturated aqueous solutions at 298 K and 1 bar. Nucleation rates were obtained for a range of salt concentrations for the Joung-Cheatham NaCl force field combined with the Extended Simple Point Charge (SPC/E) water model. The calculated nucleation rates are significantly lower than the available experimental measurements. The estimates for the nucleation rates in this work do not rely on classical nucleation theory, but the pathways observed in the simulations suggest that the nucleation process is better described by classical nucleation theory than an alternative interpretation based on Ostwald's step rule, in contrast to some prior simulations of related models. In addition to the size of NaCl nucleus, we find that the crystallinity of a nascent cluster plays an important role in the nucleation process. Nuclei with high crystallinity were found to have higher growth probability and longer lifetimes, possibly because they are less exposed to hydration water.

Interpretation of the microwave effect on induction time during CaSO4 primary nucleation by a cluster coagulation model

NASA Astrophysics Data System (ADS)

Guo, Zhichao; Li, Liye; Han, Wenxiang; Li, Jiawei; Wang, Baodong; Xiao, Yongfeng

2017-10-01

The effects of microwave on the induction time of CaSO4 are studied experimentally and theoretically. In the experiments, calcium sulfate is precipitated by mixing aqueous CaCl2 solution and Na2SO4 solution. The induction time is measured by recording the change of turbidity in solution. Various energy inputs are used to investigate the effect of energy input on nucleation. The results show that the induction time decreases with increasing supersaturation and increasing energy input. Employing the classical nucleation theory, the interfacial tension is estimated. In addition, the microwave effects on nucleation order (n) and nucleation coefficient (kN) are also investigated, and the corresponding values of homogeneous nucleation are compared with the values of heterogeneous nucleation in the microwave field. A cluster coagulation model, which brings together the classic nucleation models and the theories describing the behavior of colloidal suspension, was applied to estimate the induction time under various energy inputs. It is found that when nucleation is prominently homogeneous, the microwave energy input does not change the number of monomers in dominating clusters. And when nucleation is prominently heterogeneous, although the dominating cluster size increases with supersaturation increasing, at the same supersaturation level, the dominating cluster size remains constant in the microwave field.

Cytoplasmic Nucleation and Atypical Branching Nucleation Generate Endoplasmic Microtubules in Physcomitrella patens[OPEN

PubMed Central

Nakaoka, Yuki; Kimura, Akatsuki; Tani, Tomomi; Goshima, Gohta

2015-01-01

The mechanism underlying microtubule (MT) generation in plants has been primarily studied using the cortical MT array, in which fixed-angled branching nucleation and katanin-dependent MT severing predominate. However, little is known about MT generation in the endoplasm. Here, we explored the mechanism of endoplasmic MT generation in protonemal cells of Physcomitrella patens. We developed an assay that utilizes flow cell and oblique illumination fluorescence microscopy, which allowed visualization and quantification of individual MT dynamics. MT severing was infrequently observed, and disruption of katanin did not severely affect MT generation. Branching nucleation was observed, but it showed markedly variable branch angles and was occasionally accompanied by the transport of nucleated MTs. Cytoplasmic nucleation at seemingly random locations was most frequently observed and predominated when depolymerized MTs were regrown. The MT nucleator γ-tubulin was detected at the majority of the nucleation sites, at which a single MT was generated in random directions. When γ-tubulin was knocked down, MT generation was significantly delayed in the regrowth assay. However, nucleation occurred at a normal frequency in steady state, suggesting the presence of a γ-tubulin-independent backup mechanism. Thus, endoplasmic MTs in this cell type are generated in a less ordered manner, showing a broader spectrum of nucleation mechanisms in plants. PMID:25616870

A review of phosphate mineral nucleation in biology and geobiology.

PubMed

Omelon, Sidney; Ariganello, Marianne; Bonucci, Ermanno; Grynpas, Marc; Nanci, Antonio

2013-10-01

Relationships between geological phosphorite deposition and biological apatite nucleation have often been overlooked. However, similarities in biological apatite and phosphorite mineralogy suggest that their chemical formation mechanisms may be similar. This review serves to draw parallels between two newly described phosphorite mineralization processes, and proposes a similar novel mechanism for biologically controlled apatite mineral nucleation. This mechanism integrates polyphosphate biochemistry with crystal nucleation theory. Recently, the roles of polyphosphates in the nucleation of marine phosphorites were discovered. Marine bacteria and diatoms have been shown to store and concentrate inorganic phosphate (Pi) as amorphous, polyphosphate granules. Subsequent release of these P reserves into the local marine environment as Pi results in biologically induced phosphorite nucleation. Pi storage and release through an intracellular polyphosphate intermediate may also occur in mineralizing oral bacteria. Polyphosphates may be associated with biologically controlled apatite nucleation within vertebrates and invertebrates. Historically, biological apatite nucleation has been attributed to either a biochemical increase in local Pi concentration or matrix-mediated apatite nucleation control. This review proposes a mechanism that integrates both theories. Intracellular and extracellular amorphous granules, rich in both calcium and phosphorus, have been observed in apatite-biomineralizing vertebrates, protists, and atremate brachiopods. These granules may represent stores of calcium-polyphosphate. Not unlike phosphorite nucleation by bacteria and diatoms, polyphosphate depolymerization to Pi would be controlled by phosphatase activity. Enzymatic polyphosphate depolymerization would increase apatite saturation to the level required for mineral nucleation, while matrix proteins would simultaneously control the progression of new biological apatite formation.

Partitioning of ice nucleating particles: Which modes matter?

NASA Astrophysics Data System (ADS)

Hande, Luke; Hoose, Corinna

2017-04-01

Ice particles in clouds have a large impact on cloud lifetime, precipitation amount, and cloud radiative properties through the indirect aerosol effect. Thus, correctly modelling ice formation processes is important for simulations preformed on all spatial and temporal scales. Ice forms on aerosol particles through several different mechanisms, namely deposition nucleation, immersion freezing, and contact freezing. However there is conflicting evidence as to which mode dominates, and the relative importance of the three heterogeneous ice nucleation mechanisms, as well as homogeneous nucleation, remains an open question. The environmental conditions, and hence the cloud type, have a large impact on determining which nucleation mode dominates. In order to understand this, simulations were performed with the COSMO-LES model, utilising state of the art parameterisations to describe the different nucleation mechanisms for several semi-idealised cloud types commonly occurring over central Europe. The cloud types investigated include a semi-idealised, and an idealised convective cloud, an orographic cloud, and a stratiform cloud. Results show that immersion and contact freezing dominate at warmer temperatures, and under most conditions, deposition nucleation plays only a minor role. In clouds where sufficiently high levels of water vapour are present at colder temperatures, deposition nucleation can play a role, however in general homogeneous nucleation dominates at colder temperatures. Since contact nucleation depends on the environmental relative humidity, enhancements in this nucleation mode can be seen in areas of dry air entrainment. The results indicate that ice microphysical processes are somewhat sensitve to the environmental conditions and therefore the cloud type.

Probing Individual Ice Nucleation Events with Environmental Scanning Electron Microscopy

NASA Astrophysics Data System (ADS)

Wang, Bingbing; China, Swarup; Knopf, Daniel; Gilles, Mary; Laskin, Alexander

2016-04-01

Heterogeneous ice nucleation is one of the processes of critical relevance to a range of topics in the fundamental and the applied science and technologies. Heterogeneous ice nucleation initiated by particles proceeds where microscopic properties of particle surfaces essentially control nucleation mechanisms. Ice nucleation in the atmosphere on particles governs the formation of ice and mixed phase clouds, which in turn influence the Earth's radiative budget and climate. Heterogeneous ice nucleation is still insufficiently understood and poses significant challenges in predictive understanding of climate change. We present a novel microscopy platform allowing observation of individual ice nucleation events at temperature range of 193-273 K and relative humidity relevant for ice formation in the atmospheric clouds. The approach utilizes a home built novel ice nucleation cell interfaced with Environmental Scanning Electron Microscope (IN-ESEM system). The IN-ESEM system is applied for direct observation of individual ice formation events, determining ice nucleation mechanisms, freezing temperatures, and relative humidity onsets. Reported microanalysis of the ice nucleating particles (INP) include elemental composition detected by the energy dispersed analysis of X-rays (EDX), and advanced speciation of the organic content in particles using scanning transmission x-ray microscopy with near edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS). The performance of the IN-ESEM system is validated through a set of experiments with kaolinite particles with known ice nucleation propensity. We demonstrate an application of the IN-ESEM system to identify and characterize individual INP within a complex mixture of ambient particles.

Ice nucleation on nanotextured surfaces: the influence of surface fraction, pillar height and wetting states.

PubMed

Metya, Atanu K; Singh, Jayant K; Müller-Plathe, Florian

2016-09-29

In this work, we address the nucleation behavior of a supercooled monatomic cylindrical water droplet on nanoscale textured surfaces using molecular dynamics simulations. The ice nucleation rate at 203 K on graphite based textured surfaces with nanoscale roughness is evaluated using the mean fast-passage time method. The simulation results show that the nucleation rate depends on the surface fraction as well as the wetting states. The nucleation rate enhances with increasing surface fraction for water in the Cassie-Baxter state, while contrary behavior is observed for the case of Wenzel state. Based on the spatial histogram distribution of ice formation, we observed two pathways for ice nucleation. Heterogeneous nucleation is observed at a high surface fraction. However, the probability of homogeneous ice nucleation events increases with decreasing surface fraction. We further investigate the role of the nanopillar height in ice nucleation. The nucleation rate is enhanced with increasing nanopillar height. This is attributed to the enhanced contact area with increasing nanopillar height and the shift in nucleation events towards the three-phase contact line associated with the nanotextured surface. The ice-surface work of adhesion for the Wenzel state is found to be 1-2 times higher than that in the Cassie-Baxter state. Furthermore, the work of adhesion of ice in the Wenzel state is found to be linearly dependent on the contour length of the droplet, which is in line with that reported for liquid droplets.

Insight into Nucleation Mechanisms of Tetrahedral Materials from Advanced Molecular Dynamics Simulations

NASA Astrophysics Data System (ADS)

Bi, Yuanfei

This dissertation studies the nucleation mechanisms of ice, clathrate hydrate and silicon clathrate which all belong to tetrahedral materials and carry significant importance to modern society. Because of the stochastic nature and the ultra-fine scale of nucleation, the mechanisms through which these important tetrahedral materials form from liquid remain poorly understood. Our goal is to address the current knowledge gap between experiment and theory on the nucleation mechanisms by conducting molecular dynamics (MD) studies. To overcome the rare event nature of nucleation, an advanced sampling method Forward Flux Sampling (FFS) is integrated with classical MD simulations. This integration allows obtaining not only nucleation rate explicitly but also an ensemble of nucleation trajectories with their correct statistical weights. By analyzing the ensemble of trajectories obtained from FFS, we reveal the important details of nucleation at the molecular level, particular at the early stage of nucleation. By combining Backward Flux Sampling (BFS) with FFS, we can also compute the free energy profile of nucleation explicitly, which allows a comparison against the classical nucleation theory (CNT). We began our investigation by studying heterogeneous ice nucleation, which is the most relevant form of ice formation. In this part of study, we aim to understand the key microscopic factors that control ice formation, including surface hydrophilicity, surface crystallinity, and surface geometry. Our simulations reveal that heterogeneous ice nucleation on graphitic surfaces is controlled by the coupling of surface crystallinity and surface hydrophilicity. In particular, our analysis shows that the crystalline graphitic lattice with an appropriate hydrophilicity may indeed template ice basal plane by forming a strained ice layer, thus significantly enhance its ice nucleation efficiency. The templating effect is further found to transit from within the first contact layer of water to the second as the surface hydrophilicity increases, which yields an oscillating distinction between the crystalline and amorphous graphitic surfaces in their ice nucleation efficiency. With this understanding, we then shifted our focus on the role of surface geometry, where we find that an atomically sharp, concave wedge can further promote ice nucleation. Remarkably, our molecular analysis shows a significant enhancement of ice nucleation can emerge both when the geometry of a wedge matches the ice lattice and when such lattice match does not exist. We then investigated the nucleation of gas hydrate, a binary compound composed of water and natural gas, e.g., methane. To facilitate the application of FFS in studying gas hydrate nucleation, we proposed a new order parameter: Half-Cage Order Parameter (H-COP), based on the topological signature of hydrate structure, and conducted a pB histogram analysis to verify the effectiveness of the H-COP order parameter. Our analyses show that the nucleation of M-hydrate (M can be methane or carbon dioxide) starts in the vicinity of water-M interface, and gradually transit from amorphous to crystalline structures. This is the direct support to the proposed two-step nucleation mechanism of methane hydrate. However, within the ensemble of nucleation trajectories, we also identified some direct crystallization pathways without going through a amorphous phase in the nucleation of both M-hydrate and L-hydrate. Remarkably the free energy profile of L-hydrate nucleation, which is obtained independent of nucleation theory, is found to fit well against the CNT, albeit with noticeable differences when the nucleus is small. Therefore, the CNT-like free energy profile and the existence of multiple nucleation pathways indicate the near degeneracy of these pathways in their free energy profiles and highlight the complexity of hydrate nucleation. On the basis of these results, we further proposed a new perspective of gas hydrate nucleation, which can be considered to be an entropically driven, kinetic process that proceeds via multiple pathways that have similar free energy profiles. Finally, inspired by our insight gained in studying gas hydrate nucleation, we proposed a novel synthesis route to obtain inert gas silicon clathrate, which has an attractive opto-electronic property for energy application, but remains as an experimental challenge for synthesis. We thoroughly examined this proposal through high-throughput computational studies that show the novel phases of silicon could form spontaneously from liquid silicon in the presence of noble gases under high pressure and high temperature. In particular, our results show that a medium size of noble gas, e.g., Ar, can trigger the nucleation and growth of inert-gas silicon clathrate, whereas a small noble gas such as He is able to induce the formation of an unconventional, inclusion-type compound Si2He. Our findings, along with the gained molecular insights, thus strongly suggest it is viable to experimentally synthesize novel silicon phases with noble gas through high pressure and high temperature. (Abstract shortened by ProQuest.).

Promotion of Homogeneous Ice Nucleation by Soluble Molecules.

PubMed

Mochizuki, Kenji; Qiu, Yuqing; Molinero, Valeria

2017-11-29

Atmospheric aerosols nucleate ice in clouds, strongly impacting precipitation and climate. The prevailing consensus is that ice nucleation is promoted heterogeneously by the surface of ice nucleating particles in the aerosols. However, recent experiments indicate that water-soluble molecules, such as polysaccharides of pollen and poly(vinyl alcohol) (PVA), increase the ice freezing temperature. This poses the question of how do flexible soluble molecules promote the formation of water crystals, as they do not expose a well-defined surface to ice. Here we use molecular simulations to demonstrate that PVA promotes ice nucleation through a homogeneous mechanism: PVA increases the nucleation rate by destabilizing water in the solution. This work demonstrates a novel paradigm for understanding ice nucleation by soluble molecules and provides a new handle to design additives that promote crystallization.

Three separate classes of bacterial ice nucleation structures

DOE Office of Scientific and Technical Information (OSTI.GOV)

Turner, M.A.; Arellano, F.; Kozloff, L.M.

1990-05-01

Studies of the properties of the ice nucleation structure exposed on the surfaces of various bacteria such as Pseudomonas syringae, Erwinia herbicola, or various strains of Ice+ recombinant Escherichia coli have shown that there are clearly three major related but chemically distinct types of structures on these cells. First, the ability of Ice+ cells to nucleate super-cooled D2O has been examined, and it has been found that this ability (relative to the ability of the same cells to nucleate super-cooled H2O) exhibited three characteristic nucleating patterns. The rarest structure, called class A, is found on only a small fraction ofmore » cells in a culture, nucleates H2O at temperatures above -4.4 degrees C, and is an effective nucleator of super-cooled D2O. A second class of structure, called class B, is found on a larger portion of the cells, nucleates H2O between -4.8 and -5.7 degrees C, and is a relatively poor nucleator of super-cooled D2O. The class C structure is found on almost all cells and nucleates at -7.6 degrees C or colder. These three classes of structures were also differentiated by their sensitivities to low concentrations of water-miscible organic solvents such as dioxane or dimethyl sulfoxide. Depending on the specific bacterial strain, the addition of these solvents to bacterial suspensions lowered the nucleation activity of the class A structure by 1,000-fold or more. The nucleation activities of class B structures in the same culture were highly resistant to these compounds and were lowered only by 20 to 40%.« less

The effects of ice on methane hydrate nucleation: a microcanonical molecular dynamics study.

PubMed

Zhang, Zhengcai; Guo, Guang-Jun

2017-07-26

Although ice powders are widely used in gas hydrate formation experiments, the effects of ice on hydrate nucleation and what happens in the quasi-liquid layer of ice are still not well understood. Here, we used high-precision constant energy molecular dynamics simulations to study methane hydrate nucleation from vapor-liquid mixtures exposed to the basal, prismatic, and secondary prismatic planes of hexagonal ice (ice Ih). Although no significant difference is observed in hydrate nucleation processes for these different crystal planes, it is found, more interestingly, that methane hydrate can nucleate either on the ice surface heterogeneously or in the bulk solution phase homogeneously. Several factors are mentioned to be able to promote the heterogeneous nucleation of hydrates, including the adsorption of methane molecules at the solid-liquid interface, hydrogen bonding between hydrate cages and the ice structure, the stronger ability of ice to transfer heat than that of the aqueous solution, and the higher occurrence probability of hydrate cages in the vicinity of the ice surface than in the bulk solution. Meanwhile, however, the other factors including the hydrophilicity of ice and the ice lattice mismatch with clathrate hydrates can inhibit heterogeneous nucleation on the ice surface and virtually promote homogeneous nucleation in the bulk solution. Certainly, the efficiency of ice as a promoter and as an inhibitor for heterogeneous nucleation is different. We estimate that the former is larger than the latter under the working conditions. Additionally, utilizing the benefit of ice to absorb heat, the NVE simulation of hydrate formation with ice can mimic the phenomenon of ice shrinking during the heterogeneous nucleation of hydrates and lower the overly large temperature increase during homogeneous nucleation. These results are helpful in understanding the nucleation mechanism of methane hydrate in the presence of ice.

Direct measurement of free-energy barrier to nucleation of crystallites in amorphous silicon thin films

NASA Technical Reports Server (NTRS)

Shi, Frank G.

1994-01-01

A method is introduced to measure the free-energy barrier W(sup *), the activation energy, and activation entropy to nucleation of crystallites in amorphous solids, independent of the energy barrier to growth. The method allows one to determine the temperature dependence of W(sup *), and the effect of the preparation conditions of the initial amorphous phase, the dopants, and the crystallization methds on W(sup *). The method is applied to determine the free-energy barrier to nucleation of crystallites in amorphous silicon (a-Si) thin films. For thermally induced nucleation in a-Si thin films with annealing temperatures in the range of from 824 to 983 K, the free-energy barrier W(sup *) to nucleation of silicon crystals is about 2.0 - 2.1 eV regardless of the preparation conditions of the films. The observation supports the idea that a-Si transforms into an intermediate amorphous state through the structural relaxation prior to the onset of nucleation of crystallites in a-Si. The observation also indicates that the activation entropy may be an insignificant part of the free-energy barrier for the nucleation of crystallites in a-Si. Compared with the free-energy barrier to nucleation of crystallites in undoped a-Si films, a significant reduction is observed in the free-energy barrier to nucleation in Cu-doped a-Si films. For a-Si under irradiation of Xe(2+) at 10(exp 5) eV, the free-energy barrier to ion-induced nucleation of crystallites is shown to be about half of the value associated with thermal-induced nucleation of crystallites in a-Si under the otherwise same conditions, which is much more significant than previously expected. The present method has a general kinetic basis; it thus should be equally applicable to nucleation of crystallites in any amorphous elemental semiconductors and semiconductor alloys, metallic and polymeric glasses, and to nucleation of crystallites in melts and solutions.

The ice nucleation temperature determines the primary drying rate of lyophilization for samples frozen on a temperature-controlled shelf.

PubMed

Searles, J A; Carpenter, J F; Randolph, T W

2001-07-01

The objective of this study was to determine the influence of ice nucleation temperature on the primary drying rate during lyophilization for samples in vials that were frozen on a lyophilizer shelf. Aqueous solutions of 10% (w/v) hydroxyethyl starch were frozen in vials with externally mounted thermocouples and then partially lyophilized to determine the primary drying rate. Low- and high-particulate-containing samples, ice-nucleating additives silver iodide and Pseudomonas syringae, and other methods were used to obtain a wide range of nucleation temperatures. In cases where the supercooling exceeded 5 degrees C, freezing took place in the following three steps: (1) primary nucleation, (2) secondary nucleation encompassing the entire liquid volume, and (3) final solidification. The primary drying rate was dependent on the ice nucleation temperature, which is stochastic in nature but is affected by particulate content and the presence of ice nucleators. Sample cooling rates of 0.05 to 1 degrees C/min had no effect on nucleation temperatures and drying rate. We found that the ice nucleation temperature is the primary determinant of the primary drying rate. However, the nucleation temperature is not under direct control, and its stochastic nature and sensitivity to difficult-to-control parameters result in drying rate heterogeneity. Nucleation temperature heterogeneity may also result in variation in other morphology-related parameters such as surface area and secondary drying rate. Overall, these results document that factors such as particulate content and vial condition, which influence ice nucleation temperature, must be carefully controlled to avoid, for example, lot-to-lot variability during cGMP production. In addition, if these factors are not controlled and/or are inadvertently changed during process development and scaleup, a lyophilization cycle that was successful on the research scale may fail during large-scale production.

Experiments on Nucleation in Different Flow Regimes

NASA Technical Reports Server (NTRS)

Bayuzick, R. J.; Hofmeister, W. H.; Morton, C. M.; Robinson, M. B.

1999-01-01

The vast majority of metallic engineering materials are solidified from the liquid phase. Understanding the solidification process is essential to control microstructure, which in turn, determines the properties of materials. The genesis of solidification is nucleation, where the first stable solid forms from the liquid phase. Nucleation kinetics determine the degree of undercooling and phase selection. As such, it is important to understand nucleation phenomena in order to control solidification or glass formation in metals and alloys. Early experiments in nucleation kinetics were accomplished by droplet dispersion methods. Dilatometry was used by Turnbull and others, and more recently differential thermal analysis and differential scanning calorimetry have been used for kinetic studies. These techniques have enjoyed success; however, there are difficulties with these experiments. Since materials are dispersed in a medium, the character of the emulsion/metal interface affects the nucleation behavior. Statistics are derived from the large number of particles observed in a single experiment, but dispersions have a finite size distribution which adds to the uncertainty of the kinetic determinations. Even though temperature can be controlled quite well before the onset of nucleation, the release of the latent heat of fusion during nucleation of particles complicates the assumption of isothermality during these experiments. Containerless processing has enabled another approach to the study of nucleation kinetics. With levitation techniques it is possible to undercool one sample to nucleation repeatedly in a controlled manner, such that the statistics of the nucleation process can be derived from multiple experiments on a single sample. The authors have fully developed the analysis of nucleation experiments on single samples following the suggestions of Skripov. The advantage of these experiments is that the samples are directly observable. The nucleation temperature can be measured by noncontact optical pyrometry, the mass of the sample is known, and post processing analysis can be conducted on the sample. The disadvantages are that temperature measurement must have exceptionally high precision, and it is not possible to isolate specific heterogeneous sites as in droplet dispersions.

A Geometric Approach to Modeling Microstructurally Small Fatigue Crack Formation. 2; Simulation and Prediction of Crack Nucleation in AA 7075-T651

NASA Technical Reports Server (NTRS)

Hochhalter, Jake D.; Littlewood, David J.; Christ, Robert J., Jr.; Veilleux, M. G.; Bozek, J. E.; Ingraffea, A. R.; Maniatty, Antionette M.

2010-01-01

The objective of this paper is to develop further a framework for computationally modeling microstructurally small fatigue crack growth in AA 7075-T651 [1]. The focus is on the nucleation event, when a crack extends from within a second-phase particle into a surrounding grain, since this has been observed to be an initiating mechanism for fatigue crack growth in this alloy. It is hypothesized that nucleation can be predicted by computing a non-local nucleation metric near the crack front. The hypothesis is tested by employing a combination of experimentation and nite element modeling in which various slip-based and energy-based nucleation metrics are tested for validity, where each metric is derived from a continuum crystal plasticity formulation. To investigate each metric, a non-local procedure is developed for the calculation of nucleation metrics in the neighborhood of a crack front. Initially, an idealized baseline model consisting of a single grain containing a semi-ellipsoidal surface particle is studied to investigate the dependence of each nucleation metric on lattice orientation, number of load cycles, and non-local regularization method. This is followed by a comparison of experimental observations and computational results for microstructural models constructed by replicating the observed microstructural geometry near second-phase particles in fatigue specimens. It is found that orientation strongly influences the direction of slip localization and, as a result, in uences the nucleation mechanism. Also, the baseline models, replication models, and past experimental observation consistently suggest that a set of particular grain orientations is most likely to nucleate fatigue cracks. It is found that a continuum crystal plasticity model and a non-local nucleation metric can be used to predict the nucleation event in AA 7075-T651. However, nucleation metric threshold values that correspond to various nucleation governing mechanisms must be calibrated.

Silica-Assisted Nucleation of Polymer Foam Cells with Nanoscopic Dimensions: Impact of Particle Size, Line Tension, and Surface Functionality

PubMed Central

2017-01-01

Core–shell nanoparticles consisting of silica as core and surface-grafted poly(dimethylsiloxane) (PDMS) as shell with different diameters were prepared and used as heterogeneous nucleation agents to obtain CO2-blown poly(methyl methacrylate) (PMMA) nanocomposite foams. PDMS was selected as the shell material as it possesses a low surface energy and high CO2-philicity. The successful synthesis of core–shell nanoparticles was confirmed by Fourier transform infrared spectroscopy, thermogravimetric analysis, and transmission electron microscopy. The cell size and cell density of the PMMA micro- and nanocellular materials were determined by scanning electron microscopy. The cell nucleation efficiency using core–shell nanoparticles was significantly enhanced when compared to that of unmodified silica. The highest nucleation efficiency observed had a value of ∼0.5 for nanoparticles with a core diameter of 80 nm. The particle size dependence of cell nucleation efficiency is discussed taking into account line tension effects. Complete engulfment by the polymer matrix of particles with a core diameter below 40 nm at the cell wall interface was observed corresponding to line tension values of approximately 0.42 nN. This line tension significantly increases the energy barrier of heterogeneous nucleation and thus reduces the nucleation efficiency. The increase of the CO2 saturation pressure to 300 bar prior to batch foaming resulted in an increased line tension length. We observed a decrease of the heterogeneous nucleation efficiency for foaming after saturation with CO2 at 300 bar, which we attribute to homogenous nucleation becoming more favorable at the expense of heterogeneous nucleation in this case. Overall, it is shown that the contribution of line tension to the free energy barrier of heterogeneous foam cell nucleation must be considered to understand foaming of viscoelastic materials. This finding emphasizes the need for new strategies including the use of designer nucleating particles to enhance the foam cell nucleation efficiency. PMID:28980799

Silica-Assisted Nucleation of Polymer Foam Cells with Nanoscopic Dimensions: Impact of Particle Size, Line Tension, and Surface Functionality.

PubMed

Liu, Shanqiu; Eijkelenkamp, Rik; Duvigneau, Joost; Vancso, G Julius

2017-11-01

Core-shell nanoparticles consisting of silica as core and surface-grafted poly(dimethylsiloxane) (PDMS) as shell with different diameters were prepared and used as heterogeneous nucleation agents to obtain CO 2 -blown poly(methyl methacrylate) (PMMA) nanocomposite foams. PDMS was selected as the shell material as it possesses a low surface energy and high CO 2 -philicity. The successful synthesis of core-shell nanoparticles was confirmed by Fourier transform infrared spectroscopy, thermogravimetric analysis, and transmission electron microscopy. The cell size and cell density of the PMMA micro- and nanocellular materials were determined by scanning electron microscopy. The cell nucleation efficiency using core-shell nanoparticles was significantly enhanced when compared to that of unmodified silica. The highest nucleation efficiency observed had a value of ∼0.5 for nanoparticles with a core diameter of 80 nm. The particle size dependence of cell nucleation efficiency is discussed taking into account line tension effects. Complete engulfment by the polymer matrix of particles with a core diameter below 40 nm at the cell wall interface was observed corresponding to line tension values of approximately 0.42 nN. This line tension significantly increases the energy barrier of heterogeneous nucleation and thus reduces the nucleation efficiency. The increase of the CO 2 saturation pressure to 300 bar prior to batch foaming resulted in an increased line tension length. We observed a decrease of the heterogeneous nucleation efficiency for foaming after saturation with CO 2 at 300 bar, which we attribute to homogenous nucleation becoming more favorable at the expense of heterogeneous nucleation in this case. Overall, it is shown that the contribution of line tension to the free energy barrier of heterogeneous foam cell nucleation must be considered to understand foaming of viscoelastic materials. This finding emphasizes the need for new strategies including the use of designer nucleating particles to enhance the foam cell nucleation efficiency.

Toward a mechanistic understanding of patterns in biomineralization and new insights for old dogmas in geological settings (Invited)

NASA Astrophysics Data System (ADS)

Dove, P. M.; Hamm, L.; Giuffre, A. J.; Han, N.; De Yoreo, J. J.

2013-12-01

The ability of organisms to mineralize tissues into skeletons and other functional structures is a remarkable achievement of biology. Yet, the physical basis for how macromolecules regulate the placement and onset of mineral formation is not well established. Efforts to understand nucleation onto organic substrates have produced two, seemingly contradictory, lines of thought: The biomineralization community widely assumes the organic matrix promotes nucleation through stereochemical matching to guide the organization of solute ions, while materials synthesis groups use simple binding assays to correlate high binding strength with good promoters of nucleation. This study reconciles the two views and provides a mechanistic explanation for template-directed nucleation by correlating heterogeneous nucleation barriers with crystal-substrate binding free energies. Using surface assembled monolayers (SAM) as simple model systems, we first measure the kinetics of calcite nucleation onto model substrates that present different functional group chemistries (carboxyl, thiol, phosphate, hydroxyl) and conformations (C11, C16 chain lengths). We find rates are substrate-specific and obey predictions of classical nucleation theory at supersaturations that extend above the solubility of amorphous calcium carbonate (ACC). Analysis of the kinetic data shows the thermodynamic barrier to nucleation is reduced by minimizing the interfacial free energy of the system, γ. We then use dynamic force spectroscopy to independently measure calcite-substrate binding free energies, ΔGb. Moreover, we show that within the classical theory of nucleation, γ and ΔGb should be linearly related. The results bear out this prediction and demonstrate that low energy barriers to nucleation correlate with strong crystal-substrate binding. This relationship is general to all functional group chemistries and conformations. These findings reconcile the long-standing concept of templated nucleation through stereochemical matching with the conventional wisdom that ';good binders are good nucleators'. Alternative perspectives become internally consistent when viewed through the lens of crystal-substrate binding and provide a physical basis for how organic chemistry can direct temporal and spatial patterns of carbonate nucleation.

Statistical theory of nucleation in the presence of uncharacterized impurities

NASA Astrophysics Data System (ADS)

Sear, Richard P.

2004-08-01

First order phase transitions proceed via nucleation. The rate of nucleation varies exponentially with the free-energy barrier to nucleation, and so is highly sensitive to variations in this barrier. In practice, very few systems are absolutely pure, there are typically some impurities present which are rather poorly characterized. These interact with the nucleus, causing the barrier to vary, and so must be taken into account. Here the impurity-nucleus interactions are modelled by random variables. The rate then has the same form as the partition function of Derrida’s random energy model, and as in this model there is a regime in which the behavior is non-self-averaging. Non-self-averaging nucleation is nucleation with a rate that varies significantly from one realization of the random variables to another. In experiment this corresponds to variation in the nucleation rate from one sample to another. General analytic expressions are obtained for the crossover from a self-averaging to a non-self-averaging rate of nucleation.

Structural Basis of Actin Filament Nucleation by Tandem W Domains

PubMed Central

Chen, Xiaorui; Ni, Fengyun; Tian, Xia; Kondrashkina, Elena; Wang, Qinghua; Ma, Jianpeng

2013-01-01

SUMMARY Spontaneous nucleation of actin is very inefficient in cells. To overcome this barrier, cells have evolved a set of actin filament nucleators to promote rapid nucleation and polymerization in response to specific stimuli. However, the molecular mechanism of actin nucleation remains poorly understood. This is hindered largely by the fact that actin nucleus, once formed, rapidly polymerizes into filament, thus making it impossible to capture stable multisubunit actin nucleus. Here, we report an effective double-mutant strategy to stabilize actin nucleus by preventing further polymerization. Employing this strategy, we solved the crystal structure of AMPPNP-actin in complex with the first two tandem W domains of Cordon-bleu (Cobl), a potent actin filament nucleator. Further sequence comparison and functional studies suggest that the nucleation mechanism of Cobl is probably shared by the p53 cofactor JMY, but not Spire. Moreover, the double-mutant strategy opens the way for atomic mechanistic study of actin nucleation and polymerization. PMID:23727244

The adsorption of fungal ice-nucleating proteins on mineral dusts: a terrestrial reservoir of atmospheric ice-nucleating particles

NASA Astrophysics Data System (ADS)

O'Sullivan, Daniel; Murray, Benjamin J.; Ross, James F.; Webb, Michael E.

2016-06-01

The occurrence of ice-nucleating particles (INPs) in our atmosphere has a profound impact on the properties and lifetime of supercooled clouds. To date, the identities, sources and abundances of particles capable of nucleating ice at relatively low supercoolings (T > -15 °C) remain enigmatic. While biomolecules such as proteins and carbohydrates have been implicated as important high-temperature INPs, the lack of knowledge on the environmental fates of these species makes it difficult to assess their potential atmospheric impacts. Here we show that such nanoscale ice-nucleating proteins from a common soil-borne fungus (Fusarium avenaceum) preferentially bind to and confer their ice-nucleating properties to kaolinite. The ice-nucleating activity of the proteinaceous INPs is unaffected by adsorption to the clay, and once bound the proteins do not readily desorb, retaining much of the activity even after multiple washings with pure water. The atmospheric implications of the finding that biological residues can confer their ice-nucleating ability to dust particles are discussed.

Heterogeneous Nucleation of Protein Crystals on Fluorinated Layered Silicate

PubMed Central

Ino, Keita; Udagawa, Itsumi; Iwabata, Kazuki; Takakusagi, Yoichi; Kubota, Munehiro; Kurosaka, Keiichi; Arai, Kazuhito; Seki, Yasutaka; Nogawa, Masaya; Tsunoda, Tatsuo; Mizukami, Fujio; Taguchi, Hayao; Sakaguchi, Kengo

2011-01-01

Here, we describe an improved system for protein crystallization based on heterogeneous nucleation using fluorinated layered silicate. In addition, we also investigated the mechanism of nucleation on the silicate surface. Crystallization of lysozyme using silicates with different chemical compositions indicated that fluorosilicates promoted nucleation whereas the silicates without fluorine did not. The use of synthesized saponites for lysozyme crystallization confirmed that the substitution of hydroxyl groups contained in the lamellae structure for fluorine atoms is responsible for the nucleation-inducing property of the nucleant. Crystallization of twelve proteins with a wide range of pI values revealed that the nucleation promoting effect of the saponites tended to increase with increased substitution rate. Furthermore, the saponite with the highest fluorine content promoted nucleation in all the test proteins regardless of their overall net charge. Adsorption experiments of proteins on the saponites confirmed that the density of adsorbed molecules increased according to the substitution rate, thereby explaining the heterogeneous nucleation on the silicate surface. PMID:21818343

Enhanced Hydrate Nucleation Near the Limit of Stability.

PubMed

Jimenez-Angeles, Felipe; Firoozabadi, Abbas

2015-03-30

Clathrate hydrates are crystalline structures composed of small guest molecules trapped into cages formed by hydrogen-bonded water molecules. In hydrate nucleation, water and the guest molecules may stay in a metastable fluid mixture for a long period. Metastability is broken if the concentration of the guest is above certain limit. We perform molecular dynamics (MD) simulations of supersaturated water-propane solutions close to the limit of stability. We show that hydrate nucleation can be very fast in a very narrow range of composition at moderate temperatures. Propane density fluctuations near the fluid-fluid demixing are coupled with crystallization producing en- hanced nucleation rates. This is the first report of propane-hydrate nucleation by MD simulations. We observe motifs of the crystalline structure II in line with experiments and new hydrate cages not reported in the literature. Our study relates nucleation to the fluid-fluid spinodal decomposition and demonstration that the enhanced nucleation phenomenon is more general than short range attractive interactions as suggested in nucleation of proteins.

Ice nucleation activity of diesel soot particles at Cirrus relevant conditions: Effects of hydration, secondary organics coating, hydration, soot morphology, and coagulation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kulkarni, Gourihar R.; China, Swarup; Liu, Shang

The role of atmospheric relevant soot particles that are processed in the atmosphere toward ice nucleation at cirrus cloud condition is poorly understood. In this study, the ice nucleating properties of diesel soot particles subjected to various physical and chemical aging treatments were investigated at temperatures ranging from -40 to -50 °C. We show that bare soot particles nucleate ice in deposition mode, but coating with secondary organics suppresses the heterogeneous ice nucleation potential of soot particles requiring homogeneous freezing threshold conditions. However, the ice nucleation efficiency of soot particles coated with an aqueous organic layer was similar to baremore » soot particles. Hydration of bare soot particles slightly enhanced the ice nucleation efficiency, and the IN abilities of compact soot particles (roundness = ~ 0.6) were similar to bare lacey soot particles (roundness = ~ 0.4). These results indicate that ice nucleation properties are sensitive to the various aging treatments.« less

Autocatalytic microtubule nucleation determines the size and mass of Xenopus laevis egg extract spindles.

PubMed

Decker, Franziska; Oriola, David; Dalton, Benjamin; Brugués, Jan

2018-01-11

Regulation of size and growth is a fundamental problem in biology. A prominent example is the formation of the mitotic spindle, where protein concentration gradients around chromosomes are thought to regulate spindle growth by controlling microtubule nucleation. Previous evidence suggests that microtubules nucleate throughout the spindle structure. However, the mechanisms underlying microtubule nucleation and its spatial regulation are still unclear. Here, we developed an assay based on laser ablation to directly probe microtubule nucleation events in Xenopus laevis egg extracts. Combining this method with theory and quantitative microscopy, we show that the size of a spindle is controlled by autocatalytic growth of microtubules, driven by microtubule-stimulated microtubule nucleation. The autocatalytic activity of this nucleation system is spatially regulated by the limiting amounts of active microtubule nucleators, which decrease with distance from the chromosomes. This mechanism provides an upper limit to spindle size even when resources are not limiting. © 2018, Decker et al.

Protein crystal nucleation in pores.

PubMed

Nanev, Christo N; Saridakis, Emmanuel; Chayen, Naomi E

2017-01-16

The most powerful method for protein structure determination is X-ray crystallography which relies on the availability of high quality crystals. Obtaining protein crystals is a major bottleneck, and inducing their nucleation is of crucial importance in this field. An effective method to form crystals is to introduce nucleation-inducing heterologous materials into the crystallization solution. Porous materials are exceptionally effective at inducing nucleation. It is shown here that a combined diffusion-adsorption effect can increase protein concentration inside pores, which enables crystal nucleation even under conditions where heterogeneous nucleation on flat surfaces is absent. Provided the pore is sufficiently narrow, protein molecules approach its walls and adsorb more frequently than they can escape. The decrease in the nucleation energy barrier is calculated, exhibiting its quantitative dependence on the confinement space and the energy of interaction with the pore walls. These results provide a detailed explanation of the effectiveness of porous materials for nucleation of protein crystals, and will be useful for optimal design of such materials.

An aggregation-volume-bias Monte Carlo investigation on the condensation of a Lennard-Jones vapor below the triple point and crystal nucleation in cluster systems: an in-depth evaluation of the classical nucleation theory.

PubMed

Chen, Bin; Kim, Hyunmi; Keasler, Samuel J; Nellas, Ricky B

2008-04-03

The aggregation-volume-bias Monte Carlo based simulation technique, which has led to our recent success in vapor-liquid nucleation research, was extended to the study of crystal nucleation processes. In contrast to conventional bulk-phase techniques, this method deals with crystal nucleation events in cluster systems. This approach was applied to the crystal nucleation of Lennard-Jonesium under a wide range of undercooling conditions from 35% to 13% below the triple point. It was found that crystal nucleation in these model clusters proceeds initially via a vapor-liquid like aggregation followed by the formation of crystals inside the aggregates. The separation of these two stages of nucleation is distinct except at deeper undercooling conditions where the crystal nucleation barrier was found to diminish. The simulation results obtained for these two nucleation steps are separately compared to the classical nucleation theory (CNT). For the vapor-liquid nucleation step, the CNT was shown to provide a reasonable description of the critical cluster size but overestimate the barrier heights, consistent with previous simulation studies. On the contrary, for the crystal nucleation step, nearly perfect agreement with the barrier heights was found between the simulations and the CNT. For the critical cluster size, the comparison is more difficult as the simulation data were found to be sensitive to the definition of the solid cluster, but a stringent criterion and lower undercooling conditions generally lead to results closer with the CNT. Additional simulations at undercooling conditions of 40% or above indicate a nearly barrierless transition from the liquid to crystalline-like structure for sufficiently large clusters, which leads to further departure of the barrier height predicted by the CNT from the simulation data for the aggregation step. This is consistent with the latest experimental results on argon that show an unusually large underestimation of the nucleation rate by the CNT toward deep undercooling conditions.

Classical nucleation theory in the phase-field crystal model

NASA Astrophysics Data System (ADS)

Jreidini, Paul; Kocher, Gabriel; Provatas, Nikolas

2018-04-01

A full understanding of polycrystalline materials requires studying the process of nucleation, a thermally activated phase transition that typically occurs at atomistic scales. The numerical modeling of this process is problematic for traditional numerical techniques: commonly used phase-field methods' resolution does not extend to the atomic scales at which nucleation takes places, while atomistic methods such as molecular dynamics are incapable of scaling to the mesoscale regime where late-stage growth and structure formation takes place following earlier nucleation. Consequently, it is of interest to examine nucleation in the more recently proposed phase-field crystal (PFC) model, which attempts to bridge the atomic and mesoscale regimes in microstructure simulations. In this work, we numerically calculate homogeneous liquid-to-solid nucleation rates and incubation times in the simplest version of the PFC model, for various parameter choices. We show that the model naturally exhibits qualitative agreement with the predictions of classical nucleation theory (CNT) despite a lack of some explicit atomistic features presumed in CNT. We also examine the early appearance of lattice structure in nucleating grains, finding disagreement with some basic assumptions of CNT. We then argue that a quantitatively correct nucleation theory for the PFC model would require extending CNT to a multivariable theory.

Bubble nucleation in simple and molecular liquids via the largest spherical cavity method

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gonzalez, Miguel A., E-mail: m.gonzalez12@imperial.ac.uk; Department of Chemistry, Imperial College London, London SW7 2AZ; Abascal, José L. F.

2015-04-21

In this work, we propose a methodology to compute bubble nucleation free energy barriers using trajectories generated via molecular dynamics simulations. We follow the bubble nucleation process by means of a local order parameter, defined by the volume of the largest spherical cavity (LSC) formed in the nucleating trajectories. This order parameter simplifies considerably the monitoring of the nucleation events, as compared with the previous approaches which require ad hoc criteria to classify the atoms and molecules as liquid or vapor. The combination of the LSC and the mean first passage time technique can then be used to obtain themore » free energy curves. Upon computation of the cavity distribution function the nucleation rate and free-energy barrier can then be computed. We test our method against recent computations of bubble nucleation in simple liquids and water at negative pressures. We obtain free-energy barriers in good agreement with the previous works. The LSC method provides a versatile and computationally efficient route to estimate the volume of critical bubbles the nucleation rate and to compute bubble nucleation free-energies in both simple and molecular liquids.« less

Sulfur driven nucleation mode formation in diesel exhaust under transient driving conditions.

PubMed

Karjalainen, Panu; Rönkkö, Topi; Pirjola, Liisa; Heikkilä, Juha; Happonen, Matti; Arnold, Frank; Rothe, Dieter; Bielaczyc, Piotr; Keskinen, Jorma

2014-02-18

Sulfur driven diesel exhaust nucleation particle formation processes were studied in an aerosol laboratory, on engine dynamometers, and on the road. All test engines were equipped with a combination of a diesel oxidation catalyst (DOC) and a partial diesel particulate filter (pDPF). At steady operating conditions, the formation of semivolatile nucleation particles directly depended on SO2 conversion in the catalyst. The nucleation particle emission was most significant after a rapid increase in engine load and exhaust gas temperature. Results indicate that the nucleation particle formation at transient driving conditions does not require compounds such as hydrocarbons or sulfated hydrocarbons, however, it cannot be explained only by the nucleation of sulfuric acid. A real-world exhaust study with a heavy duty diesel truck showed that the nucleation particle formation occurs even with ultralow sulfur diesel fuel, even at downhill driving conditions, and that nucleation particles can contribute 60% of total particle number emissions. In general, due to sulfur storage and release within the exhaust aftertreatment systems and transients in driving, emissions of nucleation particles can even be the dominant part of modern diesel vehicle exhaust particulate number emissions.

Rapid syntheses of a metal-organic framework material Cu3(BTC)2(H2O)3 under microwave: a quantitative analysis of accelerated syntheses.

PubMed

Khan, Nazmul Abedin; Haque, Enamul; Jhung, Sung Hwa

2010-03-20

A typical MOF material, Cu-BTC has been synthesized with microwave and conventional electric heating in various conditions to elucidate, for the first time, the quantitative acceleration in the synthesis of a MOF by microwaves. The acceleration by microwaves is mainly due to rapid nucleation rather than rapid crystal growth, even though both stages are accelerated. The acceleration in the nucleation stage by microwaves is due to the very large pre-exponential factor (about 1.4 x 10(10) times that of conventional synthesis) in the Arrhenius plot. However, the activation energy for the nucleation in the case of microwave synthesis is higher than the activation energy of conventional synthesis. The large acceleration in the nucleation, compared with that in the crystal growth, is observed once again by the syntheses in two-steps (changing heating methods from microwave into conventional heating or from conventional heating into microwave heating just after the nucleation is completed). The crystal size of Cu-BTC obtained by microwave-nucleation is generally smaller than the Cu-BTC made by conventional-nucleation, probably due to rapid nucleation and the small size of nuclei with microwave-nucleation.

Electrochemical Nucleation and Growth of Uranium and Plutonium from Molten Salts

DOE PAGES

Tylka, M. M.; Willit, J. L.; Williamson, M. A.

2017-07-18

This work examines the nucleation and growth behavior of uranium and plutonium from molten LiCl-KCl eutectic on inert electrodes using electrochemical techniques. Current-time transients obtained from chronoamperometric experiments were compared with theoretical models to characterize the type of nucleation (progressive or instantaneous) for deposition of U and Pu, and co-deposition of U-Pu, from molten LiCl-KCl at inert electrodes. It was established that the nucleation mode of actinides present as chlorides in molten chloride salts changes from progressive to instantaneous with an increasing concentration of the trivalent actinide ions in the salt. The effect of the material of the working electrodemore » was investigated, and it was found that changing the material from tungsten to silver improves resolvability of the nucleation peaks and allows more accurate analysis of the experimental measurements. Using the nucleation data, diffusion coefficients were obtained for U 3+ and Pu 3+, and were found to be in very good agreement with the values obtained from other studies. Furthermore, the density of nuclei produced during instantaneous nucleation, the rate of nucleation for progressive nucleation, and the radius of the deposited nuclei were evaluated and examined at different overpotentials.« less

Direct in situ observation of ZnO nucleation and growth via transmission X-ray microscopy

NASA Astrophysics Data System (ADS)

Tay, S. E. R.; Goode, A. E.; Nelson Weker, J.; Cruickshank, A. A.; Heutz, S.; Porter, A. E.; Ryan, M. P.; Toney, M. F.

2016-01-01

The nucleation and growth of a nanostructure controls its size and morphology, and ultimately its functional properties. Hence it is crucial to investigate growth mechanisms under relevant growth conditions at the nanometer length scale. Here we image the nucleation and growth of electrodeposited ZnO nanostructures in situ, using a transmission X-ray microscope and specially designed electrochemical cell. We show that this imaging technique leads to new insights into the nucleation and growth mechanisms in electrodeposited ZnO including direct, in situ observations of instantaneous versus delayed nucleation.The nucleation and growth of a nanostructure controls its size and morphology, and ultimately its functional properties. Hence it is crucial to investigate growth mechanisms under relevant growth conditions at the nanometer length scale. Here we image the nucleation and growth of electrodeposited ZnO nanostructures in situ, using a transmission X-ray microscope and specially designed electrochemical cell. We show that this imaging technique leads to new insights into the nucleation and growth mechanisms in electrodeposited ZnO including direct, in situ observations of instantaneous versus delayed nucleation. Electronic supplementary information (ESI) available: Methods and videos of nanoparticle growth. See DOI: 10.1039/c5nr07019h

Thermokinetics of heterogeneous droplet nucleation on conically textured substrates.

PubMed

Singha, Sanat K; Das, Prasanta K; Maiti, Biswajit

2015-11-28

Within the framework of the classical theory of heterogeneous nucleation, a thermokinetic model is developed for line-tension-associated droplet nucleation on conical textures considering growth or shrinkage of the formed cluster due to both interfacial and peripheral monomer exchange and by considering different geometric configurations. Along with the principle of free energy extremization, Katz kinetic approach has been employed to study the effect of substrate conicity and wettability on the thermokinetics of heterogeneous water droplet nucleation. Not only the peripheral tension is found to have a considerable effect on the free energy barrier but also the substrate hydrophobicity and hydrophilicity are observed to switch over their roles between conical crest and trough for different growth rates of the droplet. Besides, the rate of nucleation increases and further promotes nucleation for negative peripheral tension as it diminishes the free energy barrier appreciably. Moreover, nucleation inhibition can be achievable for positive peripheral tension due to the enhancement of the free energy barrier. Analyzing all possible geometric configurations, the hydrophilic narrower conical cavity is found to be the most preferred nucleation site. These findings suggest a physical insight into the context of surface engineering for the promotion or the suppression of nucleation on real or engineered substrates.

Classical nucleation theory in the phase-field crystal model.

PubMed

Jreidini, Paul; Kocher, Gabriel; Provatas, Nikolas

2018-04-01

A full understanding of polycrystalline materials requires studying the process of nucleation, a thermally activated phase transition that typically occurs at atomistic scales. The numerical modeling of this process is problematic for traditional numerical techniques: commonly used phase-field methods' resolution does not extend to the atomic scales at which nucleation takes places, while atomistic methods such as molecular dynamics are incapable of scaling to the mesoscale regime where late-stage growth and structure formation takes place following earlier nucleation. Consequently, it is of interest to examine nucleation in the more recently proposed phase-field crystal (PFC) model, which attempts to bridge the atomic and mesoscale regimes in microstructure simulations. In this work, we numerically calculate homogeneous liquid-to-solid nucleation rates and incubation times in the simplest version of the PFC model, for various parameter choices. We show that the model naturally exhibits qualitative agreement with the predictions of classical nucleation theory (CNT) despite a lack of some explicit atomistic features presumed in CNT. We also examine the early appearance of lattice structure in nucleating grains, finding disagreement with some basic assumptions of CNT. We then argue that a quantitatively correct nucleation theory for the PFC model would require extending CNT to a multivariable theory.

A simple model for heterogeneous nucleation of isotactic polypropylene

NASA Astrophysics Data System (ADS)

Howard, Michael; Milner, Scott

2013-03-01

Flow-induced crystallization (FIC) is of interest because of its relevance to processes such as injection molding. It has been suggested that flow increases the homogeneous nucleation rate by reducing the melt state entropy. However, commercial polypropylene (iPP) exhibits quiescent nucleation rates that are much too high to be consistent with homogeneous nucleation in carefully purified samples. This suggests that heterogeneous nucleation is dominant for typical samples used in FIC experiments. We describe a simple model for heterogeneous nucleation of iPP, in terms of a cylindrical nucleus on a flat surface with the critical size and barrier set by the contact angle. Analysis of quiescent crystallization data with this model gives reasonable values for the contact angle. We have also employed atomistic simulations of iPP crystals to determine surface energies with vacuum and with Hamaker-matched substrates, and find values consistent with the contact angles inferred from heterogeneous nucleation experiments. In future work, these results combined with calculations from melt rheology of entropy reduction due to flow can be used to estimate the heterogeneous nucleation barrier reduction due to flow, and hence the increase in nucleation rate due to FIC for commecial iPP.

Aerosol nucleation and its role for clouds and Earth's radiative forcing in the aerosol-climate model ECHAM5-HAM

NASA Astrophysics Data System (ADS)

Kazil, J.; Stier, P.; Zhang, K.; Quaas, J.; Kinne, S.; O'Donnell, D.; Rast, S.; Esch, M.; Ferrachat, S.; Lohmann, U.; Feichter, J.

2010-05-01

Nucleation from the gas phase is an important source of aerosol particles in the Earth's atmosphere, contributing to the number of cloud condensation nuclei, which form cloud droplets. We have implemented in the aerosol-climate model ECHAM5-HAM a new scheme for neutral and charged nucleation of sulfuric acid and water based on laboratory data, and nucleation of an organic compound and sulfuric acid using a parametrization of cluster activation based on field measurements. We give details of the implementation, compare results with observations, and investigate the role of the individual aerosol nucleation mechanisms for clouds and the Earth's radiative budget. The results of our simulations are most consistent with observations when neutral and charged nucleation of sulfuric acid proceed throughout the troposphere and nucleation due to cluster activation is limited to the forested boundary layer. The globally averaged annual mean contributions of the individual nucleation processes to total absorbed solar short-wave radiation via the direct, semi-direct, indirect cloud-albedo and cloud-lifetime effects in our simulations are -1.15 W/m2 for charged H2SO4/H2O nucleation, -0.235 W/m2 for cluster activation, and -0.05 W/m2 for neutral H2SO4/H2O nucleation. The overall effect of nucleation is -2.55 W/m2, which exceeds the sum of the individual terms due to feedbacks and interactions in the model. Aerosol nucleation contributes over the oceans with -2.18 W/m2 to total absorbed solar short-wave radiation, compared to -0.37 W/m2 over land. We explain the higher effect of aerosol nucleation on Earth's radiative budget over the oceans with the larger area covered by ocean clouds, due to the larger contrast in albedo between clouds and the ocean surface compared to continents, and the larger susceptibility of pristine clouds owing to the saturation of effects. The large effect of charged nucleation in our simulations is not in contradiction with small effects seen in local measurements: over southern Finland, where cluster activation proceeds efficiently, we find that charged nucleation of sulfuric acid and water contributes on average less than 10% to ultrafine aerosol concentrations, in good agreement with observations.

Aerosol nucleation and its role for clouds and Earth's radiative forcing in the aerosol-climate model ECHAM5-HAM

NASA Astrophysics Data System (ADS)

Kazil, J.; Stier, P.; Zhang, K.; Quaas, J.; Kinne, S.; O'Donnell, D.; Rast, S.; Esch, M.; Ferrachat, S.; Lohmann, U.; Feichter, J.

2010-11-01

Nucleation from the gas phase is an important source of aerosol particles in the Earth's atmosphere, contributing to the number of cloud condensation nuclei, which form cloud droplets. We have implemented in the aerosol-climate model ECHAM5-HAM a new scheme for neutral and charged nucleation of sulfuric acid and water based on laboratory data, and nucleation of an organic compound and sulfuric acid using a parametrization of cluster activation based on field measurements. We give details of the implementation, compare results with observations, and investigate the role of the individual aerosol nucleation mechanisms for clouds and the Earth's radiative forcing. The results of our simulations are most consistent with observations when neutral and charged nucleation of sulfuric acid proceed throughout the troposphere and nucleation due to cluster activation is limited to the forested boundary layer. The globally averaged annual mean contributions of the individual nucleation processes to total absorbed solar short-wave radiation via the direct, semi-direct, indirect cloud-albedo and cloud-lifetime effects in our simulations are -1.15 W/m2 for charged H2SO4/H2O nucleation, -0.235 W/m2 for cluster activation, and -0.05 W/m2 for neutral H2SO4/H2O nucleation. The overall effect of nucleation is -2.55 W/m2, which exceeds the sum of the individual terms due to feedbacks and interactions in the model. Aerosol nucleation contributes over the oceans with -2.18 W/m2 to total absorbed solar short-wave radiation, compared to -0.37 W/m2 over land. We explain the higher effect of aerosol nucleation on Earth's radiative forcing over the oceans with the larger area covered by ocean clouds, due to the larger contrast in albedo between clouds and the ocean surface compared to continents, and the larger susceptibility of pristine clouds owing to the saturation of effects. The large effect of charged nucleation in our simulations is not in contradiction with small effects seen in local measurements: over southern Finland, where cluster activation proceeds efficiently, we find that charged nucleation of sulfuric acid and water contributes on average less than 10% to ultrafine aerosol concentrations, in good agreement with observations.

Nucleation in Polymers and Soft Matter

NASA Astrophysics Data System (ADS)

Xu, Xiaofei; Ting, Christina L.; Kusaka, Isamu; Wang, Zhen-Gang

2014-04-01

Nucleation is a ubiquitous phenomenon in many physical, chemical, and biological processes. In this review, we describe recent progress on the theoretical study of nucleation in polymeric fluids and soft matter, including binary mixtures (polymer blends, polymers in poor solvents, compressible polymer-small molecule mixtures), block copolymer melts, and lipid membranes. We discuss the methodological development for studying nucleation as well as novel insights and new physics obtained in the study of the nucleation behavior in these systems.

Filament assembly by Spire: key residues and concerted actin binding.

PubMed

Rasson, Amy S; Bois, Justin S; Pham, Duy Stephen L; Yoo, Haneul; Quinlan, Margot E

2015-02-27

The most recently identified class of actin nucleators, WASp homology domain 2 (WH2) nucleators, use tandem repeats of monomeric actin-binding WH2 domains to facilitate actin nucleation. WH2 domains are involved in a wide variety of actin regulatory activities. Structurally, they are expected to clash with interprotomer contacts within the actin filament. Thus, the discovery of their role in nucleation was surprising. Here we use Drosophila Spire (Spir) as a model system to investigate both how tandem WH2 domains can nucleate actin and what differentiates nucleating WH2-containing proteins from their non-nucleating counterparts. We found that the third WH2 domain in Spir (Spir-C or SC) plays a unique role. In the context of a short nucleation construct (containing only two WH2 domains), placement of SC in the N-terminal position was required for the most potent nucleation. We found that the native organization of the WH2 domains with respect to each other is necessary for binding to actin with positive cooperativity. We identified two residues within SC that are critical for its activity. Using this information, we were able to convert a weak synthetic nucleator into one with activity equal to a native Spir construct. Lastly, we found evidence that SC binds actin filaments, in addition to monomers. Copyright © 2014 Elsevier Ltd. All rights reserved.

Nonrandom γ-TuNA-dependent spatial pattern of microtubule nucleation at the Golgi

PubMed Central

Sanders, Anna A. W. M.; Chang, Kevin; Zhu, Xiaodong; Thoppil, Roslin J.; Holmes, William R.; Kaverina, Irina

2017-01-01

Noncentrosomal microtubule (MT) nucleation at the Golgi generates MT network asymmetry in motile vertebrate cells. Investigating the Golgi-derived MT (GDMT) distribution, we find that MT asymmetry arises from nonrandom nucleation sites at the Golgi (hotspots). Using computational simulations, we propose two plausible mechanistic models of GDMT nucleation leading to this phenotype. In the “cooperativity” model, formation of a single GDMT promotes further nucleation at the same site. In the “heterogeneous Golgi” model, MT nucleation is dramatically up-regulated at discrete and sparse locations within the Golgi. While MT clustering in hotspots is equally well described by both models, simulating MT length distributions within the cooperativity model fits the data better. Investigating the molecular mechanism underlying hotspot formation, we have found that hotspots are significantly smaller than a Golgi subdomain positive for scaffolding protein AKAP450, which is thought to recruit GDMT nucleation factors. We have further probed potential roles of known GDMT-promoting molecules, including γ-TuRC-mediated nucleation activator (γ-TuNA) domain-containing proteins and MT stabilizer CLASPs. While both γ-TuNA inhibition and lack of CLASPs resulted in drastically decreased GDMT nucleation, computational modeling revealed that only γ-TuNA inhibition suppressed hotspot formation. We conclude that hotspots require γ-TuNA activity, which facilitates clustered GDMT nucleation at distinct Golgi sites. PMID:28931596

TONNEAU2/FASS Regulates the Geometry of Microtubule Nucleation and Cortical Array Organization in Interphase Arabidopsis Cells[C][W

PubMed Central

Kirik, Angela; Ehrhardt, David W.; Kirik, Viktor

2012-01-01

Organization of microtubules into ordered arrays involves spatial and temporal regulation of microtubule nucleation. Here, we show that acentrosomal microtubule nucleation in plant cells involves a previously unknown regulatory step that determines the geometry of microtubule nucleation. Dynamic imaging of interphase cortical microtubules revealed that the ratio of branching to in-bundle microtubule nucleation on cortical microtubules is regulated by the Arabidopsis thaliana B′′ subunit of protein phosphatase 2A, which is encoded by the TONNEAU2/FASS (TON2) gene. The probability of nucleation from γ-tubulin complexes localized at the cell cortex was not affected by a loss of TON2 function, suggesting a specific role of TON2 in regulating the nucleation geometry. Both loss of TON2 function and ectopic targeting of TON2 to the plasma membrane resulted in defects in cell shape, suggesting the importance of TON2-mediated regulation of the microtubule cytoskeleton in cell morphogenesis. Loss of TON2 function also resulted in an inability for cortical arrays to reorient in response to light stimulus, suggesting an essential role for TON2 and microtubule branching nucleation in reorganization of microtubule arrays. Our data establish TON2 as a regulator of interphase microtubule nucleation and provide experimental evidence for a novel regulatory step in the process of microtubule-dependent nucleation. PMID:22395485

Properties of the seismic nucleation phase

USGS Publications Warehouse

Beroza, G.C.; Ellsworth, W.L.

1996-01-01

Near-source observations show that earthquakes begin abruptly at the P-wave arrival, but that this beginning is weak, with a low moment rate relative to the rest of the main shock. We term this initial phase of low moment rate the seismic nucleation phase. We have observed the seismic nucleation phase for a set of 48 earthquakes ranging in magnitude from 1.1-8.1. The size and duration of the seismic nucleation phase scale with the total seismic moment of the earthquake, suggesting that the process responsible for the seismic nucleation phase carries information about the eventual size of the earthquake. The seismic nucleation phase is characteristically followed by quadratic growth in the moment rate, consistent with self-similar rupture at constant stress drop. In this paper we quantify the properties of the seismic nucleation phase and offer several possible explanations for it.

Crystal nucleation of colloidal hard dumbbells

NASA Astrophysics Data System (ADS)

Ni, Ran; Dijkstra, Marjolein

2011-01-01

Using computer simulations, we investigate the homogeneous crystal nucleation in suspensions of colloidal hard dumbbells. The free energy barriers are determined by Monte Carlo simulations using the umbrella sampling technique. We calculate the nucleation rates for the plastic crystal and the aperiodic crystal phase using the kinetic prefactor as determined from event driven molecular dynamics simulations. We find good agreement with the nucleation rates determined from spontaneous nucleation events observed in event driven molecular dynamics simulations within error bars of one order of magnitude. We study the effect of aspect ratio of the dumbbells on the nucleation of plastic and aperiodic crystal phases, and we also determine the structure of the critical nuclei. Moreover, we find that the nucleation of the aligned close-packed crystal structure is strongly suppressed by a high free energy barrier at low supersaturations and slow dynamics at high supersaturations.

Phase transformations in steel studied by 3DXRD microscopy

NASA Astrophysics Data System (ADS)

Offerman, S. E.; van Dijk, N. H.; Sietsma, J.; Lauridsen, E. M.; Margulies, L.; Grigull, S.; Poulsen, H. F.; van der Zwaag, S.

2006-05-01

The ferrite grain nucleation mechanism during the austenite/ferrite phase transformation is studied in situ in the bulk of three different steel grades by three-dimensional X-ray diffraction (3DXRD) microscopy. The main difference between the three steel grades is the carbon concentration. For each steel grade the ferrite fraction, nucleus density and nucleation rate are measured simultaneously during continuous cooling. By comparing the measured nucleation rate to the classical nucleation theory it is concluded that the activation energy for ferrite nucleation is about two orders of magnitude smaller than the current models predict for the three steel grades. During slow cooling the same nucleation behaviour is found for the three steel grades concerning the balance between the energy that is released by the elimination of interfaces and the energy that is required for the formation new interfaces during ferrite nucleation.

Solute Nucleation and Growth in Supercritical Fluid Mixtures

NASA Technical Reports Server (NTRS)

Smedley, Gregory T.; Wilemski, Gerald; Rawlins, W. Terry; Joshi, Prakash; Oakes, David B.; Durgin, William W.

1996-01-01

This research effort is directed toward two primary scientific objectives: (1) to determine the gravitational effect on the measurement of nucleation and growth rates near a critical point and (2) to investigate the nucleation process in supercritical fluids to aid in the evaluation and development of existing theoretical models and practical applications. A nucleation pulse method will be employed for this investigation using a rapid expansion to a supersaturated state that is maintained for approximately 1 ms followed by a rapid recompression to a less supersaturated state that effectively terminates nucleation while permitting growth to continue. Nucleation, which occurs during the initial supersaturated state, is decoupled from growth by producing rapid pressure changes. Thermodynamic analysis, condensation modeling, apparatus design, and optical diagnostic design necessary for the initiation of a theoretical and experimental investigation of naphthalene nucleation from supercritical CO2 have been completed.

Novel polypropylene/inorganic fullerene-like WS2 nanocomposites containing a β-nucleating agent: dynamic crystallization and melting behavior.

PubMed

Naffakh, Mohammed; Marco, Carlos; Ellis, Gary

2011-09-22

The dynamic crystallization and melting behavior of isotactic polypropylene-tungsten disulfide (iPP/IF-WS(2)) nanocomposites incorporating a β-nucleating agent is investigated by X-ray diffraction and differential scanning calorimetry. A conventional melt-processing strategy is employed to generate new materials that exhibit variable α and β polymorphism under the appropriate kinetic conditions. The results show that when the dual additive system is employed the nucleation ability on isotactic polypropylene not only depends on the nucleation efficiency (NE) and relative content of the individual α and β-nucleating agents, but also on the cooling rates employed. The nucleating behavior of the additives is explained by competitive nucleation, and the correlation between crystallization and melting temperatures and relative content of α and β-crystals of iPP in the nanocomposites is discussed.

Heterogeneous Ice Nucleation Ability of NaCl and Sea Salt Aerosol Particles at Cirrus Temperatures

NASA Astrophysics Data System (ADS)

Wagner, Robert; Kaufmann, Julia; Möhler, Ottmar; Saathoff, Harald; Schnaiter, Martin; Ullrich, Romy; Leisner, Thomas

2018-03-01

In situ measurements of the composition of heterogeneous cirrus ice cloud residuals have indicated a substantial contribution of sea salt in sampling regions above the ocean. We have investigated the heterogeneous ice nucleation ability of sodium chloride (NaCl) and sea salt aerosol (SSA) particles at cirrus cloud temperatures between 235 and 200 K in the Aerosol Interaction and Dynamics in the Atmosphere aerosol and cloud chamber. Effloresced NaCl particles were found to act as ice nucleating particles in the deposition nucleation mode at temperatures below about 225 K, with freezing onsets in terms of the ice saturation ratio, Sice, between 1.28 and 1.40. Above 225 K, the crystalline NaCl particles deliquesced and nucleated ice homogeneously. The heterogeneous ice nucleation efficiency was rather similar for the two crystalline forms of NaCl (anhydrous NaCl and NaCl dihydrate). Mixed-phase (solid/liquid) SSA particles were found to act as ice nucleating particles in the immersion freezing mode at temperatures below about 220 K, with freezing onsets in terms of Sice between 1.24 and 1.42. Above 220 K, the SSA particles fully deliquesced and nucleated ice homogeneously. Ice nucleation active surface site densities of the SSA particles were found to be in the range between 1.0 · 1010 and 1.0 · 1011 m-2 at T < 220 K. These values are of the same order of magnitude as ice nucleation active surface site densities recently determined for desert dust, suggesting a potential contribution of SSA particles to low-temperature heterogeneous ice nucleation in the atmosphere.

Probing methane hydrate nucleation through the forward flux sampling method.

PubMed

Bi, Yuanfei; Li, Tianshu

2014-11-26

Understanding the nucleation of hydrate is the key to developing effective strategies for controlling methane hydrate formation. Here we present a computational study of methane hydrate nucleation, by combining the forward flux sampling (FFS) method and the coarse-grained water model mW. To facilitate the application of FFS in studying the formation of methane hydrate, we developed an effective order parameter λ on the basis of the topological analysis of the tetrahedral network. The order parameter capitalizes the signature of hydrate structure, i.e., polyhedral cages, and is capable of efficiently distinguishing hydrate from ice and liquid water while allowing the formation of different hydrate phases, i.e., sI, sII, and amorphous. Integration of the order parameter λ with FFS allows explicitly computing hydrate nucleation rates and obtaining an ensemble of nucleation trajectories under conditions where spontaneous hydrate nucleation becomes too slow to occur in direct simulation. The convergence of the obtained hydrate nucleation rate was found to depend crucially on the convergence of the spatial distribution for the spontaneously formed hydrate seeds obtained from the initial sampling of FFS. The validity of the approach is also verified by the agreement between the calculated nucleation rate and that inferred from the direct simulation. Analyzing the obtained large ensemble of hydrate nucleation trajectories, we show hydrate formation at 220 K and 500 bar is initiated by the nucleation events occurring in the vicinity of water-methane interface, and facilitated by a gradual transition from amorphous to crystalline structure. The latter provides the direct support to the proposed two-step nucleation mechanism of methane hydrate.

Molecular simulations of heterogeneous ice nucleation. II. Peeling back the layers.

PubMed

Cox, Stephen J; Kathmann, Shawn M; Slater, Ben; Michaelides, Angelos

2015-05-14

Coarse grained molecular dynamics simulations are presented in which the sensitivity of the ice nucleation rate to the hydrophilicity of a graphene nanoflake is investigated. We find that an optimal interaction strength for promoting ice nucleation exists, which coincides with that found previously for a face centered cubic (111) surface. We further investigate the role that the layering of interfacial water plays in heterogeneous ice nucleation and demonstrate that the extent of layering is not a good indicator of ice nucleating ability for all surfaces. Our results suggest that to be an efficient ice nucleating agent, a surface should not bind water too strongly if it is able to accommodate high coverages of water.

The Lack of Chemical Equilibrium does not Preclude the Use of the Classical Nucleation Theory in Circumstellar Outflows

NASA Technical Reports Server (NTRS)

Paquette, John A.; Nuth, Joseph A., III

2011-01-01

Classical nucleation theory has been used in models of dust nucleation in circumstellar outflows around oxygen-rich asymptotic giant branch stars. One objection to the application of classical nucleation theory (CNT) to astrophysical systems of this sort is that an equilibrium distribution of clusters (assumed by CNT) is unlikely to exist in such conditions due to a low collision rate of condensable species. A model of silicate grain nucleation and growth was modified to evaluate the effect of a nucleation flux orders of magnitUde below the equilibrium value. The results show that a lack of chemical equilibrium has only a small effect on the ultimate grain distribution.

Nucleation of holin domains and holes optimizes lysis timing of E. coli by phage λ

NASA Astrophysics Data System (ADS)

Ryan, Gillian; Rutenberg, Andrew

2007-03-01

Holin proteins regulate the precise scheduling of Escherichia coli lysis during infection by bacteriophage λ. Inserted into the host bacterium's inner membrane during infection, holins aggregate to form rafts and then holes within those rafts. We present a two-stage nucleation model of holin action, with the nucleation of condensed holin domains followed by the nucleation of holes within these domains. Late nucleation of holin rafts leads to a weak dependence of lysis timing on host cell size, though both nucleation events contribute equally to timing errors. Our simulations recover the accurate scheduling observed experimentally, and also suggest that phage-λ lysis of E.coli is optimized.

Nucleation theory in Langevin's approach and lifetime of a Brownian particle in potential wells.

PubMed

Alekseechkin, N V

2008-07-14

The multivariable theory of nucleation suggested by Alekseechkin [J. Chem. Phys. 124, 124512 (2006)] is further developed in the context of Langevin's approach. The use of this approach essentially enhances the capability of the nucleation theory, because it makes possible to consider the cases of small friction which are not taken into account by the classical Zel'dovich-Frenkel theory and its multivariable extensions. The procedure for the phenomenological determination of the nucleation parameters is described. Using the similarity of the Kramers model with that of nucleation, the lifetime of a Brownian particle in potential wells in various dimensionalities is calculated with the help of the expression for the steady state nucleation rate.

The effect of ultrasonic waves on the nucleation of pure water and degassed water.

PubMed

Yu, Deyang; Liu, Baolin; Wang, Bochun

2012-05-01

In order to clarify the mechanism of nucleation of ice induced by ultrasound, ultrasonic waves have been applied to supercooled pure water and degassed water, respectively. For each experiment, water sample is cooled at a constant cooling rate of 0.15 °C/min and the ultrasonic waves are applied from the water temperature of 0 °C until the water in a sample vessel nucleates. This nucleation temperature is measured. The use of ultrasound increased the nucleation temperature of both degassed water and pure water. However, the undercooling temperature for pure water to nucleate is less than that of degassed water. It is concluded that cavitation and fluctuations of density, energy and temperature induced by ultrasound are factors that affect the nucleation of water. Cavitation is a major factor for sonocrystallisation of ice. Crown Copyright © 2011. Published by Elsevier B.V. All rights reserved.

A computationally efficient description of heterogeneous freezing: A simplified version of the Soccer ball model

NASA Astrophysics Data System (ADS)

Niedermeier, Dennis; Ervens, Barbara; Clauss, Tina; Voigtländer, Jens; Wex, Heike; Hartmann, Susan; Stratmann, Frank

2014-01-01

In a recent study, the Soccer ball model (SBM) was introduced for modeling and/or parameterizing heterogeneous ice nucleation processes. The model applies classical nucleation theory. It allows for a consistent description of both apparently singular and stochastic ice nucleation behavior, by distributing contact angles over the nucleation sites of a particle population assuming a Gaussian probability density function. The original SBM utilizes the Monte Carlo technique, which hampers its usage in atmospheric models, as fairly time-consuming calculations must be performed to obtain statistically significant results. Thus, we have developed a simplified and computationally more efficient version of the SBM. We successfully used the new SBM to parameterize experimental nucleation data of, e.g., bacterial ice nucleation. Both SBMs give identical results; however, the new model is computationally less expensive as confirmed by cloud parcel simulations. Therefore, it is a suitable tool for describing heterogeneous ice nucleation processes in atmospheric models.

Crystallization of hard spheres revisited. II. Thermodynamic modeling, nucleation work, and the surface of tension.

PubMed

Richard, David; Speck, Thomas

2018-06-14

Combining three numerical methods (forward flux sampling, seeding of droplets, and finite-size droplets), we probe the crystallization of hard spheres over the full range from close to coexistence to the spinodal regime. We show that all three methods allow us to sample different regimes and agree perfectly in the ranges where they overlap. By combining the nucleation work calculated from forward flux sampling of small droplets and the nucleation theorem, we show how to compute the nucleation work spanning three orders of magnitude. Using a variation of the nucleation theorem, we show how to extract the pressure difference between the solid droplet and ambient liquid. Moreover, combining the nucleation work with the pressure difference allows us to calculate the interfacial tension of small droplets. Our results demonstrate that employing bulk quantities yields inaccurate results for the nucleation rate.

Branching microtubule nucleation in Xenopus egg extracts mediated by augmin and TPX2

PubMed Central

Petry, Sabine; Groen, Aaron C.; Ishihara, Keisuke; Mitchison, Timothy J.; Vale, Ronald D.

2013-01-01

Summary The microtubules that comprise mitotic spindles in animal cells are nucleated at centrosomes and by spindle assembly factors that are activated in the vicinity of chromatin. Indirect evidence also has suggested that microtubules might be nucleated from pre-existing microtubules throughout the spindle, but this process has not been observed directly. Here, we demonstrate microtubule nucleation from the sides of existing microtubules in meiotic Xenopus egg extracts. Daughter microtubules grow at a low branch angle and with the same polarity as mother filaments. Branching microtubule nucleation requires gamma-tubulin and augmin and is stimulated by GTP-bound Ran and its effector TPX2, factors previously implicated in chromatin-stimulated nucleation. Because of the rapid amplification of microtubule numbers and the preservation of microtubule polarity, microtubule-dependent microtubule nucleation is well suited for spindle assembly and maintenance. PMID:23415226

The role of confined collagen geometry in decreasing nucleation energy barriers to intrafibrillar mineralization.

PubMed

Kim, Doyoon; Lee, Byeongdu; Thomopoulos, Stavros; Jun, Young-Shin

2018-03-06

Mineralization of collagen is critical for the mechanical functions of bones and teeth. Calcium phosphate nucleation in collagenous structures follows distinctly different patterns in highly confined gap regions (nanoscale confinement) than in less confined extrafibrillar spaces (microscale confinement). Although the mechanism(s) driving these differences are still largely unknown, differences in the free energy for nucleation may explain these two mineralization behaviors. Here, we report on experimentally obtained nucleation energy barriers to intra- and extrafibrillar mineralization, using in situ X-ray scattering observations and classical nucleation theory. Polyaspartic acid, an extrafibrillar nucleation inhibitor, increases interfacial energies between nuclei and mineralization fluids. In contrast, the confined gap spaces inside collagen fibrils lower the energy barrier by reducing the reactive surface area of nuclei, decreasing the surface energy penalty. The confined gap geometry, therefore, guides the two-dimensional morphology and structure of bioapatite and changes the nucleation pathway by reducing the total energy barrier.

Microtubule array reorientation in response to hormones does not involve changes in microtubule nucleation modes at the periclinal cell surface

PubMed Central

Atkinson, Samantha; Kirik, Angela; Kirik, Viktor

2014-01-01

Aligned microtubule arrays spatially organize cell division, trafficking, and determine the direction of cell expansion in plant cells. In response to changes in environmental and developmental signals, cells reorganize their microtubule arrays into new configurations. Here, we tested the role of microtubule nucleation during hormone-induced microtubule array reorientation. We have found that in the process of microtubule array reorientation the ratios between branching, parallel, and de-novo nucleations remained constant, suggesting that the microtubule reorientation mechanism does not involve changes in nucleation modes. In the ton2/fass mutant, which has reduced microtubule branching nucleation frequency and decreased nucleation activity of the γ-tubulin complexes, microtubule arrays were able to reorient. Presented data suggest that reorientation of microtubules into transverse arrays in response to hormones does not involve changes in microtubule nucleation at the periclinal cell surface PMID:25135522

The impact on UT/LS cirrus clouds in the CAM/CARMA model using a new interactive aerosol parameterization.

NASA Astrophysics Data System (ADS)

Maloney, C.; Toon, B.; Bardeen, C.

2017-12-01

Recent studies indicate that heterogeneous nucleation may play a large role in cirrus cloud formation in the UT/LS, a region previously thought to be primarily dominated by homogeneous nucleation. As a result, it is beneficial to ensure that general circulation models properly represent heterogeneous nucleation in ice cloud simulations. Our work strives towards addressing this issue in the NSF/DOE Community Earth System Model's atmospheric model, CAM. More specifically we are addressing the role of heterogeneous nucleation in the coupled sectional microphysics cloud model, CARMA. Currently, our CAM/CARMA cirrus model only performs homogenous ice nucleation while ignoring heterogeneous nucleation. In our work, we couple the CAM/CARMA cirrus model with the Modal Aerosol Model (MAM). By combining the aerosol model with CAM/CARMA we can both account for heterogeneous nucleation, as well as directly link the sulfates used for homogeneous nucleation to computed fields instead of the current static field being utilized. Here we present our initial results and compare our findings to observations from the long running CALIPSO and MODIS satellite missions.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhukhovitskii, D. I., E-mail: dmr@ihed.ras.ru

The vapor–liquid nucleation in a dense Lennard-Jones system is studied analytically and numerically. A solution of the nucleation kinetic equations, which includes the elementary processes of condensation/evaporation involving the lightest clusters, is obtained, and the nucleation rate is calculated. Based on the equation of state for the cluster vapor, the pre-exponential factor is obtained. The latter diverges as a spinodal is reached, which results in the nucleation enhancement. The work of critical cluster formation is calculated using the previously developed two-parameter model (TPM) of small clusters. A simple expression for the nucleation rate is deduced and it is shown thatmore » the work of cluster formation is reduced for a dense vapor. This results in the nucleation enhancement as well. To verify the TPM, a simulation is performed that mimics a steady-state nucleation experiments in the thermal diffusion cloud chamber. The nucleating vapor with and without a carrier gas is simulated using two different thermostats for the monomers and clusters. The TPM proves to match the simulation results of this work and of other studies.« less

Understanding homogeneous nucleation in solidification of aluminum by molecular dynamics simulations

NASA Astrophysics Data System (ADS)

Mahata, Avik; Asle Zaeem, Mohsen; Baskes, Michael I.

2018-02-01

Homogeneous nucleation from aluminum (Al) melt was investigated by million-atom molecular dynamics simulations utilizing the second nearest neighbor modified embedded atom method potentials. The natural spontaneous homogenous nucleation from the Al melt was produced without any influence of pressure, free surface effects and impurities. Initially isothermal crystal nucleation from undercooled melt was studied at different constant temperatures, and later superheated Al melt was quenched with different cooling rates. The crystal structure of nuclei, critical nucleus size, critical temperature for homogenous nucleation, induction time, and nucleation rate were determined. The quenching simulations clearly revealed three temperature regimes: sub-critical nucleation, super-critical nucleation, and solid-state grain growth regimes. The main crystalline phase was identified as face-centered cubic, but a hexagonal close-packed (hcp) and an amorphous solid phase were also detected. The hcp phase was created due to the formation of stacking faults during solidification of Al melt. By slowing down the cooling rate, the volume fraction of hcp and amorphous phases decreased. After the box was completely solid, grain growth was simulated and the grain growth exponent was determined for different annealing temperatures.

Texture control and seeded nucleation of nanosize structures of ferroelectric thin films

NASA Astrophysics Data System (ADS)

Muralt, Paul

2006-09-01

An overview is given on nucleation phenomena of Pb(Zr ,Ti)O3 (PZT) thin films on Pt(111)-based substrates. Emphasis is given on in situ growth methods, particularly in situ reactive sputtering from three metallic targets. Growth of PZT thin films is discussed from the point of view of the PbOx-TiO2 phase diagram, PbO vapor pressure, and classical nucleation theory. The role of thin TiO2 affinity layers and spots is explained in the frame of this theory. Activation energies for desorption and chemisorption are adapted to comply with the fact that nucleation rates on TiO2 are much larger than the ones on bare Pt(111). The model reproduces well the PbO surface flux from bare Pt(111) to the affinity spots in the case of PbTiO3 nucleation and the reversed tendency in the case of PZT 40/60 nucleation, explaining experimental observations. The critical size of nuclei was calculated to contain 8-10unit cells for PbTiO3/Pt nucleation and 14-17 for PZT/Pt nucleation.

Heterogeneous nucleation of ice on carbon surfaces.

PubMed

Lupi, Laura; Hudait, Arpa; Molinero, Valeria

2014-02-26

Atmospheric aerosols can promote the heterogeneous nucleation of ice, impacting the radiative properties of clouds and Earth's climate. The experimental investigation of heterogeneous freezing of water droplets by carbonaceous particles reveals widespread ice freezing temperatures. It is not known which structural and chemical characteristics of soot account for the variability in ice nucleation efficiency. Here we use molecular dynamics simulations to investigate the nucleation of ice from liquid water in contact with graphitic surfaces. We find that atomically flat carbon surfaces promote heterogeneous nucleation of ice, while molecularly rough surfaces with the same hydrophobicity do not. Graphitic surfaces and other surfaces that promote ice nucleation induce layering in the interfacial water, suggesting that the order imposed by the surface on liquid water may play an important role in the heterogeneous nucleation mechanism. We investigate a large set of graphitic surfaces of various dimensions and radii of curvature and find that variations in nanostructures alone could account for the spread in the freezing temperatures of ice on soot in experiments. We conclude that a characterization of the nanostructure of soot is needed to predict its ice nucleation efficiency.

Sensitivity of liquid clouds to homogenous freezing parameterizations.

PubMed

Herbert, Ross J; Murray, Benjamin J; Dobbie, Steven J; Koop, Thomas

2015-03-16

Water droplets in some clouds can supercool to temperatures where homogeneous ice nucleation becomes the dominant freezing mechanism. In many cloud resolving and mesoscale models, it is assumed that homogeneous ice nucleation in water droplets only occurs below some threshold temperature typically set at -40°C. However, laboratory measurements show that there is a finite rate of nucleation at warmer temperatures. In this study we use a parcel model with detailed microphysics to show that cloud properties can be sensitive to homogeneous ice nucleation as warm as -30°C. Thus, homogeneous ice nucleation may be more important for cloud development, precipitation rates, and key cloud radiative parameters than is often assumed. Furthermore, we show that cloud development is particularly sensitive to the temperature dependence of the nucleation rate. In order to better constrain the parameterization of homogeneous ice nucleation laboratory measurements are needed at both high (>-35°C) and low (<-38°C) temperatures. Homogeneous freezing may be significant as warm as -30°CHomogeneous freezing should not be represented by a threshold approximationThere is a need for an improved parameterization of homogeneous ice nucleation.

Microstructurally-sensitive fatigue crack nucleation in Ni-based single and oligo crystals

NASA Astrophysics Data System (ADS)

Chen, Bo; Jiang, Jun; Dunne, Fionn P. E.

2017-09-01

An integrated experimental, characterisation and computational crystal plasticity study of cyclic plastic beam loading has been carried out for nickel single crystal (CMSX4) and oligocrystal (MAR002) alloys in order to assess quantitatively the mechanistic drivers for fatigue crack nucleation. The experimentally validated modelling provides knowledge of key microstructural quantities (accumulated slip, stress and GND density) at experimentally observed fatigue crack nucleation sites and it is shown that while each of these quantities is potentially important in crack nucleation, none of them in its own right is sufficient to be predictive. However, the local (elastic) stored energy density, measured over a length scale determined by the density of SSDs and GNDs, has been shown to predict crack nucleation sites in the single and oligocrystals tests. In addition, once primary nucleated cracks develop and are represented in the crystal model using XFEM, the stored energy correctly identifies where secondary fatigue cracks are observed to nucleate in experiments. This (Griffith-Stroh type) quantity also correctly differentiates and explains intergranular and transgranular fatigue crack nucleation.

A computational fluid dynamics approach to nucleation in the water-sulfuric acid system.

PubMed

Herrmann, E; Brus, D; Hyvärinen, A-P; Stratmann, F; Wilck, M; Lihavainen, H; Kulmala, M

2010-08-12

This study presents a computational fluid dynamics modeling approach to investigate the nucleation in the water-sulfuric acid system in a flow tube. On the basis of an existing experimental setup (Brus, D.; Hyvärinen, A.-P.; Viisanen, Y.; Kulmala, M.; Lihavainen, H. Atmos. Chem. Phys. 2010, 10, 2631-2641), we first establish the effect of convection on the flow profile. We then proceed to simulate nucleation for relative humidities of 10, 30, and 50% and for sulfuric acid concentration between 10(9) to 3 x 10(10) cm(-3). We describe the nucleation zone in detail and determine how flow rate and relative humidity affect its characteristics. Experimental nucleation rates are compared to rates gained from classical binary and kinetic nucleation theory as well as cluster activation theory. For low RH values, kinetic theory yields the best agreement with experimental results while binary nucleation best reproduces the experimental nucleation behavior at 50% relative humidity. Particle growth is modeled for an example case at 50% relative humidity. The final simulated diameter is very close to the experimental result.

The influence of ion hydration on nucleation and growth of LiF crystals in aqueous solution.

PubMed

Lanaro, G; Patey, G N

2018-01-14

Molecular dynamics (MD) simulations are employed to investigate crystal nucleation and growth in oversaturated aqueous LiF solutions. Results obtained for a range of temperatures provide evidence that the rate of crystal growth is determined by a substantial energy barrier (∼49 kJ mol -1 ) related to the loss of water from the ion hydration shells. Employing direct MD simulations, we do not observe spontaneous nucleation of LiF crystals at 300 K, but nucleation is easily observable in NVT simulations at 500 K. This contrasts with the NaCl case, where crystal nucleation is directly observed in similar simulations at 300 K. Based on these observations, together with a detailed analysis of ion clustering in metastable LiF solutions, we argue that the ion dehydration barrier also plays a key role in crystal nucleation. The hydration of the relatively small Li + and F - ions strongly influences the probability of forming large, crystal-like ion clusters, which are a necessary precursor to nucleation. This important factor is not accounted for in classical nucleation theory.

Heterogeneous Nucleation of Colloidal Crystals on a Glass Substrate with Depletion Attraction.

PubMed

Guo, Suxia; Nozawa, Jun; Hu, Sumeng; Koizumi, Haruhiko; Okada, Junpei; Uda, Satoshi

2017-10-10

The heterogeneous nucleation of colloidal crystals with attractive interactions has been investigated via in situ observations. We have found two types of nucleation processes: a cluster that overcomes the critical size for nucleation with a monolayer, and a method that occurs with two layers. The Gibbs free energy changes (ΔG) for these two types of nucleation processes are evaluated by taking into account the effect of various interfacial energies. In contrast to homogeneous nucleation, the change in interfacial free energy, Δσ, is generated for colloidal nucleation on a foreign substrate such as a cover glass in the present study. The Δσ and step free energy of the first layer, γ 1 , are obtained experimentally based on the equation deduced from classical nucleation theory (CNT). It is concluded that the ΔG of q-2D nuclei is smaller than of monolayer nuclei, provided that the same number of particles are used, which explains the experimental result that the critical size in q-2D nuclei is smaller than that in monolayer nuclei.

An improved model of homogeneous nucleation for high supersaturation conditions: aluminum vapor.

PubMed

Savel'ev, A M; Starik, A M

2016-12-21

A novel model of stationary nucleation, treating the thermodynamic functions of small clusters, has been built. The model is validated against the experimental data on the nucleation rate of water vapor obtained in a broad range of supersaturation values (S = 10-120), and, at high supersaturation values, it reproduces the experimental data much better than the traditional classical nucleation model. A comprehensive analysis of the nucleation of aluminum vapor with the usage of developed stationary and non-stationary nucleation models has been performed. It has been shown that, at some value of supersaturation, there exists a double potential nucleation barrier. It has been revealed that the existence of this barrier notably delayed the establishment of a stationary distribution of subcritical clusters. It has also been demonstrated that the non-stationary model of the present work and the model of liquid-droplet approximation predict different values of nucleation delay time, τ s . In doing so, the liquid-droplet model can underestimate notably (by more than an order of magnitude) the value of τ s .

Heterogeneity in homogeneous nucleation from billion-atom molecular dynamics simulation of solidification of pure metal.

PubMed

Shibuta, Yasushi; Sakane, Shinji; Miyoshi, Eisuke; Okita, Shin; Takaki, Tomohiro; Ohno, Munekazu

2017-04-05

Can completely homogeneous nucleation occur? Large scale molecular dynamics simulations performed on a graphics-processing-unit rich supercomputer can shed light on this long-standing issue. Here, a billion-atom molecular dynamics simulation of homogeneous nucleation from an undercooled iron melt reveals that some satellite-like small grains surrounding previously formed large grains exist in the middle of the nucleation process, which are not distributed uniformly. At the same time, grains with a twin boundary are formed by heterogeneous nucleation from the surface of the previously formed grains. The local heterogeneity in the distribution of grains is caused by the local accumulation of the icosahedral structure in the undercooled melt near the previously formed grains. This insight is mainly attributable to the multi-graphics processing unit parallel computation combined with the rapid progress in high-performance computational environments.Nucleation is a fundamental physical process, however it is a long-standing issue whether completely homogeneous nucleation can occur. Here the authors reveal, via a billion-atom molecular dynamics simulation, that local heterogeneity exists during homogeneous nucleation in an undercooled iron melt.

Thermodynamics and Kinetics of Prenucleation Clusters, Classical and Non-Classical Nucleation.

PubMed

Zahn, Dirk

2015-07-20

Recent observations of prenucleation species and multi-stage crystal nucleation processes challenge the long-established view on the thermodynamics of crystal formation. Here, we review and generalize extensions to classical nucleation theory. Going beyond the conventional implementation as has been used for more than a century now, nucleation inhibitors, precursor clusters and non-classical nucleation processes are rationalized as well by analogous concepts based on competing interface and bulk energy terms. This is illustrated by recent examples of species formed prior to/instead of crystal nucleation and multi-step nucleation processes. Much of the discussed insights were obtained from molecular simulation using advanced sampling techniques, briefly summarized herein for both nucleation-controlled and diffusion-controlled aggregate formation. © 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This is an open access article under the terms of the Creative Commons Attribution Non-Commercial NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

The influence of ion hydration on nucleation and growth of LiF crystals in aqueous solution

NASA Astrophysics Data System (ADS)

Lanaro, G.; Patey, G. N.

2018-01-01

Molecular dynamics (MD) simulations are employed to investigate crystal nucleation and growth in oversaturated aqueous LiF solutions. Results obtained for a range of temperatures provide evidence that the rate of crystal growth is determined by a substantial energy barrier (˜49 kJ mol-1) related to the loss of water from the ion hydration shells. Employing direct MD simulations, we do not observe spontaneous nucleation of LiF crystals at 300 K, but nucleation is easily observable in NVT simulations at 500 K. This contrasts with the NaCl case, where crystal nucleation is directly observed in similar simulations at 300 K. Based on these observations, together with a detailed analysis of ion clustering in metastable LiF solutions, we argue that the ion dehydration barrier also plays a key role in crystal nucleation. The hydration of the relatively small Li+ and F- ions strongly influences the probability of forming large, crystal-like ion clusters, which are a necessary precursor to nucleation. This important factor is not accounted for in classical nucleation theory.

Boreal pollen contain ice-nucleating as well as ice-binding ‘antifreeze’ polysaccharides

NASA Astrophysics Data System (ADS)

Dreischmeier, Katharina; Budke, Carsten; Wiehemeier, Lars; Kottke, Tilman; Koop, Thomas

2017-02-01

Ice nucleation and growth is an important and widespread environmental process. Accordingly, nature has developed means to either promote or inhibit ice crystal formation, for example ice-nucleating proteins in bacteria or ice-binding antifreeze proteins in polar fish. Recently, it was found that birch pollen release ice-nucleating macromolecules when suspended in water. Here we show that birch pollen washing water exhibits also ice-binding properties such as ice shaping and ice recrystallization inhibition, similar to antifreeze proteins. We present spectroscopic evidence that both the ice-nucleating as well as the ice-binding molecules are polysaccharides bearing carboxylate groups. The spectra suggest that both polysaccharides consist of very similar chemical moieties, but centrifugal filtration indicates differences in molecular size: ice nucleation occurs only in the supernatant of a 100 kDa filter, while ice shaping is strongly enhanced in the filtrate. This finding may suggest that the larger ice-nucleating polysaccharides consist of clusters of the smaller ice-binding polysaccharides, or that the latter are fragments of the ice-nucleating polysaccharides. Finally, similar polysaccharides released from pine and alder pollen also display both ice-nucleating as well as ice-binding ability, suggesting a common mechanism of interaction with ice among several boreal pollen with implications for atmospheric processes and antifreeze protection.

Stochastic kinetics reveal imperative role of anisotropic interfacial tension to determine morphology and evolution of nucleated droplets in nematogenic films

NASA Astrophysics Data System (ADS)

Bhattacharjee, Amit Kumar

2017-01-01

For isotropic fluids, classical nucleation theory predicts the nucleation rate, barrier height and critical droplet size by ac- counting for the competition between bulk energy and interfacial tension. The nucleation process in liquid crystals is less understood. We numerically investigate nucleation in monolayered nematogenic films using a mesoscopic framework, in par- ticular, we study the morphology and kinetic pathway in spontaneous formation and growth of droplets of the stable phase in the metastable background. The parameter κ that quantifies the anisotropic elastic energy plays a central role in determining the geometric structure of the droplets. Noncircular nematic droplets with homogeneous director orientation are nucleated in a background of supercooled isotropic phase for small κ. For large κ, noncircular droplets with integer topological charge, accompanied by a biaxial ring at the outer surface, are nucleated. The isotropic droplet shape in a superheated nematic background is found to depend on κ in a similar way. Identical growth laws are found in the two cases, although an unusual two-stage mechanism is observed in the nucleation of isotropic droplets. Temporal distributions of successive events indi- cate the relevance of long-ranged elasticity-mediated interactions within the isotropic domains. Implications for a theoretical description of nucleation in anisotropic fluids are discussed.

Simple improvements to classical bubble nucleation models.

PubMed

Tanaka, Kyoko K; Tanaka, Hidekazu; Angélil, Raymond; Diemand, Jürg

2015-08-01

We revisit classical nucleation theory (CNT) for the homogeneous bubble nucleation rate and improve the classical formula using a correct prefactor in the nucleation rate. Most of the previous theoretical studies have used the constant prefactor determined by the bubble growth due to the evaporation process from the bubble surface. However, the growth of bubbles is also regulated by the thermal conduction, the viscosity, and the inertia of liquid motion. These effects can decrease the prefactor significantly, especially when the liquid pressure is much smaller than the equilibrium one. The deviation in the nucleation rate between the improved formula and the CNT can be as large as several orders of magnitude. Our improved, accurate prefactor and recent advances in molecular dynamics simulations and laboratory experiments for argon bubble nucleation enable us to precisely constrain the free energy barrier for bubble nucleation. Assuming the correction to the CNT free energy is of the functional form suggested by Tolman, the precise evaluations of the free energy barriers suggest the Tolman length is ≃0.3σ independently of the temperature for argon bubble nucleation, where σ is the unit length of the Lennard-Jones potential. With this Tolman correction and our prefactor one gets accurate bubble nucleation rate predictions in the parameter range probed by current experiments and molecular dynamics simulations.

Homogeneous SPC/E water nucleation in large molecular dynamics simulations.

PubMed

Angélil, Raymond; Diemand, Jürg; Tanaka, Kyoko K; Tanaka, Hidekazu

2015-08-14

We perform direct large molecular dynamics simulations of homogeneous SPC/E water nucleation, using up to ∼ 4 ⋅ 10(6) molecules. Our large system sizes allow us to measure extremely low and accurate nucleation rates, down to ∼ 10(19) cm(-3) s(-1), helping close the gap between experimentally measured rates ∼ 10(17) cm(-3) s(-1). We are also able to precisely measure size distributions, sticking efficiencies, cluster temperatures, and cluster internal densities. We introduce a new functional form to implement the Yasuoka-Matsumoto nucleation rate measurement technique (threshold method). Comparison to nucleation models shows that classical nucleation theory over-estimates nucleation rates by a few orders of magnitude. The semi-phenomenological nucleation model does better, under-predicting rates by at worst a factor of 24. Unlike what has been observed in Lennard-Jones simulations, post-critical clusters have temperatures consistent with the run average temperature. Also, we observe that post-critical clusters have densities very slightly higher, ∼ 5%, than bulk liquid. We re-calibrate a Hale-type J vs. S scaling relation using both experimental and simulation data, finding remarkable consistency in over 30 orders of magnitude in the nucleation rate range and 180 K in the temperature range.

Protein Polymerization into Fibrils from the Viewpoint of Nucleation Theory.

PubMed

Kashchiev, Dimo

2015-11-17

The assembly of various proteins into fibrillar aggregates is an important phenomenon with wide implications ranging from human disease to nanoscience. Using general kinetic results of nucleation theory, we analyze the polymerization of protein into linear or helical fibrils in the framework of the Oosawa-Kasai (OK) model. We show that while within the original OK model of linear polymerization the process does not involve nucleation, within a modified OK model it is nucleation-mediated. Expressions are derived for the size of the fibril nucleus, the work for fibril formation, the nucleation barrier, the equilibrium and stationary fibril size distributions, and the stationary fibril nucleation rate. Under otherwise equal conditions, this rate decreases considerably when the short (subnucleus) fibrils lose monomers much more frequently than the long (supernucleus) fibrils, a feature that should be born in mind when designing a strategy for stymying or stimulating fibril nucleation. The obtained dependence of the nucleation rate on the concentration of monomeric protein is convenient for experimental verification and for use in rate equations accounting for nucleation-mediated fibril formation. The analysis and the results obtained for linear fibrils are fully applicable to helical fibrils whose formation is describable by a simplified OK model. Copyright © 2015 Biophysical Society. Published by Elsevier Inc. All rights reserved.

Boreal pollen contain ice-nucleating as well as ice-binding ‘antifreeze’ polysaccharides

PubMed Central

Dreischmeier, Katharina; Budke, Carsten; Wiehemeier, Lars; Kottke, Tilman; Koop, Thomas

2017-01-01

Ice nucleation and growth is an important and widespread environmental process. Accordingly, nature has developed means to either promote or inhibit ice crystal formation, for example ice-nucleating proteins in bacteria or ice-binding antifreeze proteins in polar fish. Recently, it was found that birch pollen release ice-nucleating macromolecules when suspended in water. Here we show that birch pollen washing water exhibits also ice-binding properties such as ice shaping and ice recrystallization inhibition, similar to antifreeze proteins. We present spectroscopic evidence that both the ice-nucleating as well as the ice-binding molecules are polysaccharides bearing carboxylate groups. The spectra suggest that both polysaccharides consist of very similar chemical moieties, but centrifugal filtration indicates differences in molecular size: ice nucleation occurs only in the supernatant of a 100 kDa filter, while ice shaping is strongly enhanced in the filtrate. This finding may suggest that the larger ice-nucleating polysaccharides consist of clusters of the smaller ice-binding polysaccharides, or that the latter are fragments of the ice-nucleating polysaccharides. Finally, similar polysaccharides released from pine and alder pollen also display both ice-nucleating as well as ice-binding ability, suggesting a common mechanism of interaction with ice among several boreal pollen with implications for atmospheric processes and antifreeze protection. PMID:28157236

Role of stacking disorder in ice nucleation

NASA Astrophysics Data System (ADS)

Lupi, Laura; Hudait, Arpa; Peters, Baron; Grünwald, Michael; Gotchy Mullen, Ryan; Nguyen, Andrew H.; Molinero, Valeria

2017-11-01

The freezing of water affects the processes that determine Earth’s climate. Therefore, accurate weather and climate forecasts hinge on good predictions of ice nucleation rates. Such rate predictions are based on extrapolations using classical nucleation theory, which assumes that the structure of nanometre-sized ice crystallites corresponds to that of hexagonal ice, the thermodynamically stable form of bulk ice. However, simulations with various water models find that ice nucleated and grown under atmospheric temperatures is at all sizes stacking-disordered, consisting of random sequences of cubic and hexagonal ice layers. This implies that stacking-disordered ice crystallites either are more stable than hexagonal ice crystallites or form because of non-equilibrium dynamical effects. Both scenarios challenge central tenets of classical nucleation theory. Here we use rare-event sampling and free energy calculations with the mW water model to show that the entropy of mixing cubic and hexagonal layers makes stacking-disordered ice the stable phase for crystallites up to a size of at least 100,000 molecules. We find that stacking-disordered critical crystallites at 230 kelvin are about 14 kilojoules per mole of crystallite more stable than hexagonal crystallites, making their ice nucleation rates more than three orders of magnitude higher than predicted by classical nucleation theory. This effect on nucleation rates is temperature dependent, being the most pronounced at the warmest conditions, and should affect the modelling of cloud formation and ice particle numbers, which are very sensitive to the temperature dependence of ice nucleation rates. We conclude that classical nucleation theory needs to be corrected to include the dependence of the crystallization driving force on the size of the ice crystallite when interpreting and extrapolating ice nucleation rates from experimental laboratory conditions to the temperatures that occur in clouds.

Do protein crystals nucleate within dense liquid clusters?

DOE Office of Scientific and Technical Information (OSTI.GOV)

Maes, Dominique, E-mail: dommaes@vub.ac.be; Vorontsova, Maria A.; Potenza, Marco A. C.

2015-06-27

The evolution of protein-rich clusters and nucleating crystals were characterized by dynamic light scattering (DLS), confocal depolarized dynamic light scattering (cDDLS) and depolarized oblique illumination dark-field microscopy. Newly nucleated crystals within protein-rich clusters were detected directly. These observations indicate that the protein-rich clusters are locations for crystal nucleation. Protein-dense liquid clusters are regions of high protein concentration that have been observed in solutions of several proteins. The typical cluster size varies from several tens to several hundreds of nanometres and their volume fraction remains below 10{sup −3} of the solution. According to the two-step mechanism of nucleation, the protein-rich clustersmore » serve as locations for and precursors to the nucleation of protein crystals. While the two-step mechanism explained several unusual features of protein crystal nucleation kinetics, a direct observation of its validity for protein crystals has been lacking. Here, two independent observations of crystal nucleation with the proteins lysozyme and glucose isomerase are discussed. Firstly, the evolutions of the protein-rich clusters and nucleating crystals were characterized simultaneously by dynamic light scattering (DLS) and confocal depolarized dynamic light scattering (cDDLS), respectively. It is demonstrated that protein crystals appear following a significant delay after cluster formation. The cDDLS correlation functions follow a Gaussian decay, indicative of nondiffusive motion. A possible explanation is that the crystals are contained inside large clusters and are driven by the elasticity of the cluster surface. Secondly, depolarized oblique illumination dark-field microscopy reveals the evolution from liquid clusters without crystals to newly nucleated crystals contained in the clusters to grown crystals freely diffusing in the solution. Collectively, the observations indicate that the protein-rich clusters in lysozyme and glucose isomerase solutions are locations for crystal nucleation.« less

Free energy landscape and molecular pathways of gas hydrate nucleation.

PubMed

Bi, Yuanfei; Porras, Anna; Li, Tianshu

2016-12-07

Despite the significance of gas hydrates in diverse areas, a quantitative knowledge of hydrate formation at a molecular level is missing. The impediment to acquiring this understanding is primarily attributed to the stochastic nature and ultra-fine scales of nucleation events, posing a great challenge for both experiment and simulation to explore hydrate nucleation. Here we employ advanced molecular simulation methods, including forward flux sampling (FFS), p B histogram analysis, and backward flux sampling, to overcome the limit of direct molecular simulation for exploring both the free energy landscape and molecular pathways of hydrate nucleation. First we test the half-cage order parameter (H-COP) which we developed for driving FFS, through conducting the p B histogram analysis. Our results indeed show that H-COP describes well the reaction coordinates of hydrate nucleation. Through the verified order parameter, we then directly compute the free energy landscape for hydrate nucleation by combining both forward and backward flux sampling. The calculated stationary distribution density, which is obtained independently of nucleation theory, is found to fit well against the classical nucleation theory (CNT). Subsequent analysis of the obtained large ensemble of hydrate nucleation trajectories show that although on average, hydrate formation is facilitated by a two-step like mechanism involving a gradual transition from an amorphous to a crystalline structure, there also exist nucleation pathways where hydrate crystallizes directly, without going through the amorphous stage. The CNT-like free energy profile and the structural diversity suggest the existence of multiple active transition pathways for hydrate nucleation, and possibly also imply the near degeneracy in their free energy profiles among different pathways. Our results thus bring a new perspective to the long standing question of how hydrates crystallize.

Free energy landscape and molecular pathways of gas hydrate nucleation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bi, Yuanfei; Porras, Anna; Li, Tianshu, E-mail: tsli@gwu.edu

Despite the significance of gas hydrates in diverse areas, a quantitative knowledge of hydrate formation at a molecular level is missing. The impediment to acquiring this understanding is primarily attributed to the stochastic nature and ultra-fine scales of nucleation events, posing a great challenge for both experiment and simulation to explore hydrate nucleation. Here we employ advanced molecular simulation methods, including forward flux sampling (FFS), p{sub B} histogram analysis, and backward flux sampling, to overcome the limit of direct molecular simulation for exploring both the free energy landscape and molecular pathways of hydrate nucleation. First we test the half-cage ordermore » parameter (H-COP) which we developed for driving FFS, through conducting the p{sub B} histogram analysis. Our results indeed show that H-COP describes well the reaction coordinates of hydrate nucleation. Through the verified order parameter, we then directly compute the free energy landscape for hydrate nucleation by combining both forward and backward flux sampling. The calculated stationary distribution density, which is obtained independently of nucleation theory, is found to fit well against the classical nucleation theory (CNT). Subsequent analysis of the obtained large ensemble of hydrate nucleation trajectories show that although on average, hydrate formation is facilitated by a two-step like mechanism involving a gradual transition from an amorphous to a crystalline structure, there also exist nucleation pathways where hydrate crystallizes directly, without going through the amorphous stage. The CNT-like free energy profile and the structural diversity suggest the existence of multiple active transition pathways for hydrate nucleation, and possibly also imply the near degeneracy in their free energy profiles among different pathways. Our results thus bring a new perspective to the long standing question of how hydrates crystallize.« less

Nucleation Rate Analysis of Methane Hydrate from Molecular Dynamics Simulations

DOE PAGES

Yuhara, Daisuke; Barnes, Brian C.; Suh, Donguk; ...

2015-01-06

Clathrate hydrates are solid crystalline structures most commonly formed from solutions that have nucleated to form a mixed solid composed of water and gas. Understanding the mechanism of clathrate hydrate nucleation is essential to grasp the fundamental chemistry of these complex structures and their applications. Molecular dynamics (MD) simulation is an ideal method to study nucleation at the molecular level because the size of the critical nucleus and formation rate occur on the nano scale. Moreover, various analysis methods for nucleation have been developed through MD to analyze nucleation. In particular, the mean first-passage time (MFPT) and survival probability (SP)more » methods have proven to be effective in procuring the nucleation rate and critical nucleus size for monatomic systems. This study assesses the MFPT and SP methods, previously used for monatomic systems, when applied to analyzing clathrate hydrate nucleation. Because clathrate hydrate nucleation is relatively difficult to observe in MD simulations (due to its high free energy barrier), these methods have yet to be applied to clathrate hydrate systems. In this study, we have analyzed the nucleation rate and critical nucleus size of methane hydrate using MFPT and SP methods from data generated by MD simulations at 255 K and 50 MPa. MFPT was modified for clathrate hydrate from the original version by adding the maximum likelihood estimate and growth effect term. The nucleation rates were calculated by MFPT and SP methods and are within 5%; the critical nucleus size estimated by the MFPT method was 50% higher, than values obtained through other more rigorous but computationally expensive estimates. These methods can also be extended to the analysis of other clathrate hydrates.« less

Nucleation Rate Analysis of Methane Hydrate from Molecular Dynamics Simulations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yuhara, Daisuke; Barnes, Brian C.; Suh, Donguk

Clathrate hydrates are solid crystalline structures most commonly formed from solutions that have nucleated to form a mixed solid composed of water and gas. Understanding the mechanism of clathrate hydrate nucleation is essential to grasp the fundamental chemistry of these complex structures and their applications. Molecular dynamics (MD) simulation is an ideal method to study nucleation at the molecular level because the size of the critical nucleus and formation rate occur on the nano scale. Moreover, various analysis methods for nucleation have been developed through MD to analyze nucleation. In particular, the mean first-passage time (MFPT) and survival probability (SP)more » methods have proven to be effective in procuring the nucleation rate and critical nucleus size for monatomic systems. This study assesses the MFPT and SP methods, previously used for monatomic systems, when applied to analyzing clathrate hydrate nucleation. Because clathrate hydrate nucleation is relatively difficult to observe in MD simulations (due to its high free energy barrier), these methods have yet to be applied to clathrate hydrate systems. In this study, we have analyzed the nucleation rate and critical nucleus size of methane hydrate using MFPT and SP methods from data generated by MD simulations at 255 K and 50 MPa. MFPT was modified for clathrate hydrate from the original version by adding the maximum likelihood estimate and growth effect term. The nucleation rates were calculated by MFPT and SP methods and are within 5%; the critical nucleus size estimated by the MFPT method was 50% higher, than values obtained through other more rigorous but computationally expensive estimates. These methods can also be extended to the analysis of other clathrate hydrates.« less

Pre-ordering of interfacial water in the pathway of heterogeneous ice nucleation does not lead to a two-step crystallization mechanism.

PubMed

Lupi, Laura; Peters, Baron; Molinero, Valeria

2016-12-07

According to Classical Nucleation Theory (CNT), the transition from liquid to crystal occurs in a single activated step with a transition state controlled by the size of the crystal embryo. This picture has been challenged in the last two decades by several reports of two-step crystallization processes in which the liquid first produces pre-ordered or dense domains, within which the crystal nucleates in a second step. Pre-ordering preceding crystal nucleation has been recently reported in simulations of ice crystallization, raising the question of whether the mechanism of ice nucleation involves two steps. In this paper, we investigate the heterogeneous nucleation of ice on carbon surfaces. We use molecular simulations with efficient coarse-grained models combined with rare event sampling methods and free energy calculations to elucidate the role of pre-ordering of liquid water at the carbon surface in the reaction coordinate for heterogeneous nucleation. We find that ice nucleation proceeds through a classical mechanism, with a single barrier between liquid and crystal. The reaction coordinate that determines the crossing of the nucleation barrier is the size of the crystal nucleus, as predicted by CNT. Wetting of the critical ice nuclei within pre-ordered domains decreases the nucleation barrier, increasing the nucleation rates. The preferential pathway for crystallization involves the early creation of pre-ordered domains that are the birthplace of the ice crystallites but do not represent a minimum in the free energy pathway from liquid to ice. We conclude that a preferential pathway through an intermediate-order precursor does not necessarily result in a two-step mechanism.

Role of stacking disorder in ice nucleation.

PubMed

Lupi, Laura; Hudait, Arpa; Peters, Baron; Grünwald, Michael; Gotchy Mullen, Ryan; Nguyen, Andrew H; Molinero, Valeria

2017-11-08

The freezing of water affects the processes that determine Earth's climate. Therefore, accurate weather and climate forecasts hinge on good predictions of ice nucleation rates. Such rate predictions are based on extrapolations using classical nucleation theory, which assumes that the structure of nanometre-sized ice crystallites corresponds to that of hexagonal ice, the thermodynamically stable form of bulk ice. However, simulations with various water models find that ice nucleated and grown under atmospheric temperatures is at all sizes stacking-disordered, consisting of random sequences of cubic and hexagonal ice layers. This implies that stacking-disordered ice crystallites either are more stable than hexagonal ice crystallites or form because of non-equilibrium dynamical effects. Both scenarios challenge central tenets of classical nucleation theory. Here we use rare-event sampling and free energy calculations with the mW water model to show that the entropy of mixing cubic and hexagonal layers makes stacking-disordered ice the stable phase for crystallites up to a size of at least 100,000 molecules. We find that stacking-disordered critical crystallites at 230 kelvin are about 14 kilojoules per mole of crystallite more stable than hexagonal crystallites, making their ice nucleation rates more than three orders of magnitude higher than predicted by classical nucleation theory. This effect on nucleation rates is temperature dependent, being the most pronounced at the warmest conditions, and should affect the modelling of cloud formation and ice particle numbers, which are very sensitive to the temperature dependence of ice nucleation rates. We conclude that classical nucleation theory needs to be corrected to include the dependence of the crystallization driving force on the size of the ice crystallite when interpreting and extrapolating ice nucleation rates from experimental laboratory conditions to the temperatures that occur in clouds.

Thermostability analysis of line-tension-associated nucleation at a gas-liquid interface.

PubMed

Singha, Sanat Kumar; Das, Prasanta Kumar; Maiti, Biswajit

2017-01-01

The influence of line tension on the thermostability of a droplet nucleated from an oversaturated vapor at the interface of the vapor and another immiscible liquid is investigated. Along with the condition of mechanical equilibrium, the notion of extremization of the reversible work of formation is considered to obtain the critical parameters related to heterogeneous nucleation. From the energetic formulation, the critical reversible work of formation is found to be greater than that of homogeneous nucleation for high value of the positive line tension. On the other hand, for high value of the negative line tension, the critical reversible work of formation becomes negative. Therefore, these thermodynamic instabilities under certain substrate wettability situations necessitate a free-energetics-based stability of the nucleated droplet, because the system energy is not minimized under these conditions. This thermostability is analogous to the transition-based stability proposed by Widom [B. Widom, J. Phys. Chem. 99, 2803 (1995)]10.1021/j100009a041 in the case of partial wetting phenomena along with the positive line tension. The thermostability analysis limits the domain of the solution space of the present critical-value problem as the thermodynamic transformation in connection with homogeneous and workless nucleation is considered. Within the stability range of the geometry-based wetting parameters, three limiting modes of nucleation, i.e., total-dewetting-related homogeneous nucleation, and total-wetting-associated and total-submergence-associated workless nucleation scenarios, are identified. Either of the two related limiting wetting scenarios of workless nucleation, namely, total wetting and total submergence, is found to be favorable depending on the geometry-based wetting conditions. The line-tension-associated nucleation on a liquid surface can be differentiated from that on a rigid substrate, as in the former, the stability based on mechanical equilibrium and a typical case of workless nucleation with complete submergence are observed.

Experiments on Nucleation in Different Flow Regimes

NASA Technical Reports Server (NTRS)

Bayuzick, Robert J.

1999-01-01

The vast majority of metallic engineering materials are solidified from the liquid phase. Understanding the solidification process is essential to control microstructure, which in turn, determines the properties of materials. The genesis of solidification is nucleation, where the first stable solid forms from the liquid phase. Nucleation kinetics determine the degree of undercooling and phase selection. As such, it is important to understand nucleation phenomena in order to control solidification or glass formation in metals and alloys. Early experiments in nucleation kinetics were accomplished by droplet dispersion methods [1-6]. Dilitometry was used by Turnbull and others, and more recently differential thermal analysis and differential scanning calorimetry have been used for kinetic studies. These techniques have enjoyed success; however, there are difficulties with these experiments. Since materials are dispersed in a medium, the character of the emulsion/metal interface affects the nucleation behavior. Statistics are derived from the large number of particles observed in a single experiment, but dispersions have a finite size distribution which adds to the uncertainty of the kinetic determinations. Even though temperature can be controlled quite well before the onset of nucleation, the release of the latent heat of fusion during nucleation of particles complicates the assumption of isothermality during these experiments. Containerless processing has enabled another approach to the study of nucleation kinetics [7]. With levitation techniques it is possible to undercool one sample to nucleation repeatedly in a controlled manner, such that the statistics of the nucleation process can be derived from multiple experiments on a single sample. The authors have fully developed the analysis of nucleation experiments on single samples following the suggestions of Skripov [8]. The advantage of these experiments is that the samples are directly observable. The nucleation temperature can be measured by noncontact optical pyrometry, the mass of the sample is known, and post processing analysis can be conducted on the sample. The disadvantages are that temperature measurement must have exceptionally high precision, and it is not possible to isolate specific heterogeneous sites as in droplet dispersions.

Orientation dependence of heterogeneous nucleation at the Cu-Pb solid-liquid interface.

PubMed

Palafox-Hernandez, J Pablo; Laird, Brian B

2016-12-07

In this work, we examine the effect of surface structure on the heterogeneous nucleation of Pb crystals from the melt at a Cu substrate using molecular-dynamics (MD) simulation. In a previous work [Palafox-Hernandez et al., Acta Mater. 59, 3137 (2011)] studying the Cu/Pb solid-liquid interface with MD simulation, we observed that the structure of the Cu(111) and Cu(100) interfaces was significantly different at 625 K, just above the Pb melting temperature (618 K for the model). The Cu(100) interface exhibited significant surface alloying in the crystal plane in contact with the melt. In contrast, no surface alloying was seen at the Cu(111) interface; however, a prefreezing layer of crystalline Pb, 2-3 atomic planes thick and slightly compressed relative to bulk Pb crystal, was observed to form at the interface. We observe that at the Cu(111) interface the prefreezing layer is no longer present at 750 K, but surface alloying in the Cu(100) interface persists. In a series of undercooling MD simulations, heterogeneous nucleation of fcc Pb is observed at the Cu(111) interface within the simulation time (5 ns) at 592 K-a 26 K undercooling. Nucleation and growth at Cu(111) proceeded layerwise with a nearly planar critical nucleus. Quantitative analysis yielded heterogeneous nucleation barriers that are more than two orders of magnitude smaller than the predicted homogeneous nucleation barriers from classical nucleation theory. Nucleation was considerably more difficult on the Cu(100) surface-alloyed substrate. An undercooling of approximately 170 K was necessary to observe nucleation at this interface within the simulation time. From qualitative observation, the critical nucleus showed a contact angle with the Cu(100) surface of over 90°, indicating poor wetting of the Cu(100) surface by the nucleating phase, which according to classical heterogeneous nucleation theory provides an explanation of the large undercooling necessary to nucleate on the Cu(100) surface, relative to Cu(111), whose surface is more similar to the nucleating phase due to the presence of the prefreezing layer.

Nacre biomineralisation: A review on the mechanisms of crystal nucleation.

PubMed

Nudelman, Fabio

2015-10-01

The wide diversity of biogenic minerals that is found in nature, each with its own morphology, mechanical properties and composition, is remarkable. In order to produce minerals that are optimally adapted for their function, biomineralisation usually occurs under strict cellular control. This control is exerted by specialised proteins and polysaccharides that assemble into a 3-dimensional organic matrix framework, forming a microenvironment where mineral deposition takes place. Molluscs are unique in that they use a striking variety of structural motifs to build their shells, each made of crystals with different morphologies and different calcium carbonate polymorphs. Much of want is known about mollusc shell formation comes from studies on the nacreous layer, or mother-of-pearl. In this review, we discuss two existing models on the nucleation of aragonite crystals during nacre formation: heteroepitaxial nucleation and mineral bridges. The heteroepitaxial nucleation model is based on the identification of chemical functional groups and aragonite-nucleating proteins at the centre of crystal imprints. It proposes that during nacre formation, each aragonite tablet nucleates independently on a nucleation site that is formed by acidic proteins and/or glycoproteins adsorbed on the chitin scaffold. The mineral bridges model is based on the identification of physical connections between the crystals in a stack, which results in a large number of crystals across several layers sharing the same crystallographic orientation. These observations suggest that there is one nucleation event per stack of tablets. Once the first crystal nucleates and reaches the top interlamellar matrix, it continues growing through pores, giving rise to the next layer of nacre, subsequently propagating into a stack. We compare both models and propose that they work in concert to control crystal nucleation in nacre. De novo crystal nucleation has to occur at least once per stack of aligned crystals, and is induced by nucleation sites. We suggest that further growth is controlled both by mineral bridges and nucleation sites. Finally, we discuss the role of amorphous calcium carbonate precursor in nacre formation. Copyright © 2015 Elsevier Ltd. All rights reserved.

GCSS Cirrus Parcel Model Comparison Project

NASA Technical Reports Server (NTRS)

Lin, Ruei-Fong; Starr, David OC.; DeMott, Paul J.; Cotton, Richard; Jensen, Eric; Sassen, Kenneth; Einaudi, Franco (Technical Monitor)

2000-01-01

The Cirrus Parcel Model Comparison Project, a project of GCSS Working Group on Cirrus Cloud Systems (WG2), involves the systematic comparison of current models of ice crystal nucleation and growth for specified, typical, cirrus cloud environments. The goal of this project is to document and understand the factors resulting in significant inter-model differences. The intent is to foment research leading to model improvement and validation. In Phase 1 of the project reported here, simulated cirrus cloud microphysical properties are compared for situations of "warm" (-40 C) and "cold" (-60 C) cirrus subject to updrafts of 4, 20 and 100 cm/s, respectively. Five models participated. These models employ explicit microphysical schemes wherein the size distribution of each class of particles (aerosols and ice crystals) is resolved into bins. Simulations are made including both homogeneous and heterogeneous ice nucleation mechanisms. A single initial aerosol population of sulfuric acid particles is prescribed for all simulations. To isolate the treatment of the homogeneous freezing (of haze drops) nucleation process, the heterogeneous nucleation mechanism is disabled for a second parallel set of simulations. Qualitative agreement is found for the homogeneous-nucleation-only simulations, e.g., the number density of nucleated ice crystals increases with the strength of the prescribed updraft. However, non-negligible quantitative differences are found. Detailed analysis reveals that the homogeneous nucleation formulation, aerosol size, ice crystal growth rate (particularly the deposition coefficient), and water vapor uptake rate are critical components that lead to differences in predicted microphysics. Systematic bias exists between results based on a modified classical theory approach and models using an effective freezing temperature approach to the treatment of nucleation. Each approach is constrained by critical freezing data from laboratory studies, but each includes assumptions that can only be justified by further laboratory data. Consequently, it is not yet clear if the two approaches can be made consistent. Large haze particles may deviate considerably from equilibrium size in moderate to strong updrafts (20-100 cm/s) at -60 C when the commonly invoked equilibrium assumption is lifted. The resulting difference in particle-size-dependent solution concentration of haze particles may significantly affect the ice nucleation rate during the initial nucleation interval. The uptake rate for water vapor excess by ice crystals is another key component regulating the total number of nucleated ice crystals. This rate, the product of ice number concentration and ice crystal diffusional growth rate, which is sensitive to the deposition coefficient when ice particles are small, partially controls the peak nucleation rate achieved in an air parcel and the duration of the active nucleation time period. The effects of heterogeneous nucleation are most pronounced in weak updraft situations. Vapor competition by the nucleated (heterogeneous) ice crystals limits the achieved ice supersaturation and thus suppresses the contribution of homogeneous nucleation. Correspondingly, ice crystal number density is markedly reduced. Definitive laboratory and atmospheric benchmark data are needed for the heterogeneous nucleation process. Inter-model differences are correspondingly greater than in the case of the homogeneous nucleation process acting alone.

AEROSOL NUCLEATION AND GROWTH DURING LAMINAR TUBE FLOW: MAXIMUM SATURATIONS AND NUCLEATION RATES. (R827354C008)

EPA Science Inventory

An approximate method of estimating the maximum saturation, the nucleation rate, and the total number nucleated per second during the laminar flow of a hot vapour–gas mixture along a tube with cold walls is described. The basis of the approach is that the temperature an...

Spatial distribution of nuclei in progressive nucleation: Modeling and application

NASA Astrophysics Data System (ADS)

Tomellini, Massimo

2018-04-01

Phase transformations ruled by non-simultaneous nucleation and growth do not lead to random distribution of nuclei. Since nucleation is only allowed in the untransformed portion of space, positions of nuclei are correlated. In this article an analytical approach is presented for computing pair-correlation function of nuclei in progressive nucleation. This quantity is further employed for characterizing the spatial distribution of nuclei through the nearest neighbor distribution function. The modeling is developed for nucleation in 2D space with power growth law and it is applied to describe electrochemical nucleation where correlation effects are significant. Comparison with both computer simulations and experimental data lends support to the model which gives insights into the transition from Poissonian to correlated nearest neighbor probability density.

Nitrogen nucleation in a cryogenic supersonic nozzle

NASA Astrophysics Data System (ADS)

Bhabhe, Ashutosh; Wyslouzil, Barbara

2011-12-01

We follow the vapor-liquid phase transition of N2 in a cryogenic supersonic nozzle apparatus using static pressure measurements. Under our operating conditions, condensation always occurs well below the triple point. Mean field kinetic nucleation theory (MKNT) does a better job of predicting the conditions corresponding to the estimated maximum nucleation rates, Jmax = 1017±1 cm-3 s-1, than two variants of classical nucleation theory. Combining the current results with the nucleation pulse chamber measurements of Iland et al. [J. Chem. Phys. 130, 114508-1 (2009)], we use nucleation theorems to estimate the critical cluster properties. Both the theories overestimate the size of the critical cluster, but MKNT does a good job of estimating the excess internal energy of the clusters.

Effects of temperature and pressure on the nucleation and growth of silver clusters from supersaturated vapor: A molecular dynamics analysis

NASA Astrophysics Data System (ADS)

Wang, Qin; Xie, Hui; Chen, Yongshi; Liu, Chao

2017-04-01

The nucleation and growth of silver nanoparticles in the supersaturated system are investigated by molecular dynamics simulation at different temperatures and pressures. The variety of the atoms in the biggest cluster and the size of average clusters in the system versus the time are estimated to reveal the relationship between the nucleation as well as cluster growth. The nucleation rates in different situations are calculated with the threshold method. The effect of temperature and pressure on the nucleation rate is identified as obeying a linear function. Finally, the development of basal elements, such as monomers, dimers and trimmers, is revealed how the temperature and pressure affect the nucleation and growth of the silver cluster.

Climate Impacts of Ice Nucleation

NASA Technical Reports Server (NTRS)

Gettelman, Andrew; Liu, Xiaohong; Barahona, Donifan; Lohmann, Ulrike; Chen, Celia

2012-01-01

Several different ice nucleation parameterizations in two different General Circulation Models (GCMs) are used to understand the effects of ice nucleation on the mean climate state, and the Aerosol Indirect Effects (AIE) of cirrus clouds on climate. Simulations have a range of ice microphysical states that are consistent with the spread of observations, but many simulations have higher present-day ice crystal number concentrations than in-situ observations. These different states result from different parameterizations of ice cloud nucleation processes, and feature different balances of homogeneous and heterogeneous nucleation. Black carbon aerosols have a small (0.06 Wm(exp-2) and not statistically significant AIE when included as ice nuclei, for nucleation efficiencies within the range of laboratory measurements. Indirect effects of anthropogenic aerosols on cirrus clouds occur as a consequence of increasing anthropogenic sulfur emissions with different mechanisms important in different models. In one model this is due to increases in homogeneous nucleation fraction, and in the other due to increases in heterogeneous nucleation with coated dust. The magnitude of the effect is the same however. The resulting ice AIE does not seem strongly dependent on the balance between homogeneous and heterogeneous ice nucleation. Regional effects can reach several Wm2. Indirect effects are slightly larger for those states with less homogeneous nucleation and lower ice number concentration in the base state. The total ice AIE is estimated at 0.27 +/- 0.10 Wm(exp-2) (1 sigma uncertainty). This represents a 20% offset of the simulated total shortwave AIE for ice and liquid clouds of 1.6 Wm(sup-2).

A new temperature and humidity dependent surface site density approach for deposition ice nucleation

NASA Astrophysics Data System (ADS)

Steinke, I.; Hoose, C.; Möhler, O.; Connolly, P.; Leisner, T.

2014-07-01

Deposition nucleation experiments with Arizona Test Dust (ATD) as a surrogate for mineral dusts were conducted at the AIDA cloud chamber at temperatures between 220 and 250 K. The influence of the aerosol size distribution and the cooling rate on the ice nucleation efficiencies was investigated. Ice nucleation active surface site (INAS) densities were calculated to quantify the ice nucleation efficiency as a function of temperature, humidity and the aerosol surface area concentration. Additionally, a contact angle parameterization according to classical nucleation theory was fitted to the experimental data in order to relate the ice nucleation efficiencies to contact angle distributions. From this study it can be concluded that the INAS density formulation is a very useful tool to decribe the temperature and humidity dependent ice nucleation efficiency of ATD particles. Deposition nucleation on ATD particles can be described by a temperature and relative humidity dependent INAS density function ns(T, Sice) with ns(xtherm) = 1.88 × 105 \\centerdot exp(0.2659 \\centerdot xtherm) [m-2] (1) where the thermodynamic variable xtherm is defined as xtherm = -(T - 273.2) + (Sice-1) × 100 (2) with Sice>1 and within a temperature range between 226 and 250 K. For lower temperatures, xtherm deviates from a linear behavior with temperature and relative humidity over ice. Two different approaches for describing the time dependence of deposition nucleation initiated by ATD particles are proposed. Box model estimates suggest that the time dependent contribution is only relevant for small cooling rates and low number fractions of ice-active particles.

Twin nucleation and migration in FeCr single crystals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Patriarca, L.; Abuzaid, Wael; Sehitoglu, Huseyin, E-mail: huseyin@illinois.edu

2013-01-15

Tension and compression experiments were conducted on body-centered cubic Fe -47.8 at pct. Cr single crystals. The critical resolved shear stress (CRSS) magnitudes for slip nucleation, twin nucleation and twin migration were established. We show that the nucleation of slip occurs at a CRSS of about 88 MPa, while twinning nucleates at a CRSS of about 191 MPa with an associated load drop. Following twin nucleation, twin migration proceeds at a CRSS that is lower than the initiation stress ( Almost-Equal-To 114-153 MPa). The experimental results of the nucleation stresses indicate that the Schmid law holds to a first approximationmore » for the slip and twin nucleation cases, but to a lesser extent for twin migration particularly when considerable slip strains preceded twinning. The CRSSs were determined experimentally using digital image correlation (DIC) in conjunction with electron back scattering diffraction (EBSD). The DIC measurements enabled pinpointing the precise stress on the stress-strain curves where twins or slip were activated. The crystal orientations were obtained using EBSD and used to determine the activated twin and slip systems through trace analysis. - Highlights: Black-Right-Pointing-Pointer Digital image correlation allows to capture slip/twin initiation for bcc FeCr. Black-Right-Pointing-Pointer Crystal orientations from EBSD allow slip/twin system indexing. Black-Right-Pointing-Pointer Nucleation of slip always precedes twinning. Black-Right-Pointing-Pointer Twin growth is sustained with a lower stress than required for nucleation. Black-Right-Pointing-Pointer Twin-slip interactions provide high hardening at the onset of plasticity.« less

Crystal Nucleation of Tolbutamide in Solution: Relationship to Solvent, Solute Conformation, and Solution Structure.

PubMed

Zeglinski, Jacek; Kuhs, Manuel; Khamar, Dikshitkumar; Hegarty, Avril C; Devi, Renuka K; Rasmuson, Åke C

2018-04-03

The influence of the solvent in nucleation of tolbutamide, a medium-sized, flexible and polymorphic organic molecule, has been explored by measuring nucleation induction times, estimating solvent-solute interaction enthalpies using molecular modelling and calorimetric data, probing interactions and clustering with spectroscopy, and modelling solvent-dependence of molecular conformation in solution. The nucleation driving force required to reach the same induction time is strongly solvent-dependent, increasing in the order: acetonitrile

A Theory of Immersion Freezing

NASA Technical Reports Server (NTRS)

Barahona, Donifan

2017-01-01

Immersion freezing is likely involved in the initiation of precipitation and determines to large extent the phase partitioning in convective clouds. Theoretical models commonly used to describe immersion freezing in atmospheric models are based on the classical nucleation theory which however neglects important interactions near the immersed particle that may affect nucleation rates. This work introduces a new theory of immersion freezing based on two premises. First, immersion ice nucleation is mediated by the modification of the properties of water near the particle-liquid interface, rather than by the geometry of the ice germ. Second, the same mechanism that leads to the decrease in the work of germ formation also decreases the mobility of water molecules near the immersed particle. These two premises allow establishing general thermodynamic constraints to the ice nucleation rate. Analysis of the new theory shows that active sites likely trigger ice nucleation, but they do not control the overall nucleation rate nor the probability of freezing. It also suggests that materials with different ice nucleation efficiency may exhibit similar freezing temperatures under similar conditions but differ in their sensitivity to particle surface area and cooling rate. Predicted nucleation rates show good agreement with observations for a diverse set of materials including dust, black carbon and bacterial ice nucleating particles. The application of the new theory within the NASA Global Earth System Model (GEOS-5) is also discussed.

The adsorption of fungal ice-nucleating proteins on mineral dusts: a terrestrial reservoir of atmospheric ice-nucleating particles

NASA Astrophysics Data System (ADS)

O'Sullivan, Daniel; Murray, Benjamin J.; Ross, James; Webb, Michael E.

2016-04-01

The occurrence of ice-nucleating particles (INPs) in our atmosphere has a profound impact on the properties and lifetime of supercooled clouds. However, the identities, sources and abundances of airborne particles capable of efficiently nucleating ice at relatively low supercoolings (T > -15 °C) remain enigmatic. Recently, several studies have suggested that unidentified biogenic residues in soil dusts are likely to be an important source of these efficient atmospheric INPs. While it has been shown that cell-free proteins produced by common soil-borne fungi are exceptional INPs, whether these fungi are a source of ice-nucleating biogenic residues in soils has yet to be shown. In particular, it is unclear whether upon adsorption to soil mineral particles, the activity of fungal ice-nucleating proteins is retained or is reduced, as observed for other soil enzymes. Here we show that proteins from a common soil fungus (Fusarium avenaceum) do in fact preferentially bind to and impart their ice-nucleating properties to the common clay mineral kaolinite. The ice-nucleating activity of the proteinaceous INPs is found to be unaffected by adsorption to the clay, and once bound the proteins do not readily desorb, retaining much of their activity even after multiple washings with pure water. The atmospheric implications of the finding that nanoscale fungal INPs can effectively determine the nucleating abilities of lofted soil dusts are discussed.

Dynamics of ice nucleation on water repellent surfaces.

PubMed

Alizadeh, Azar; Yamada, Masako; Li, Ri; Shang, Wen; Otta, Shourya; Zhong, Sheng; Ge, Liehui; Dhinojwala, Ali; Conway, Ken R; Bahadur, Vaibhav; Vinciquerra, A Joseph; Stephens, Brian; Blohm, Margaret L

2012-02-14

Prevention of ice accretion and adhesion on surfaces is relevant to many applications, leading to improved operation safety, increased energy efficiency, and cost reduction. Development of passive nonicing coatings is highly desirable, since current antiicing strategies are energy and cost intensive. Superhydrophobicity has been proposed as a lead passive nonicing strategy, yet the exact mechanism of delayed icing on these surfaces is not clearly understood. In this work, we present an in-depth analysis of ice formation dynamics upon water droplet impact on surfaces with different wettabilities. We experimentally demonstrate that ice nucleation under low-humidity conditions can be delayed through control of surface chemistry and texture. Combining infrared (IR) thermometry and high-speed photography, we observe that the reduction of water-surface contact area on superhydrophobic surfaces plays a dual role in delaying nucleation: first by reducing heat transfer and second by reducing the probability of heterogeneous nucleation at the water-substrate interface. This work also includes an analysis (based on classical nucleation theory) to estimate various homogeneous and heterogeneous nucleation rates in icing situations. The key finding is that ice nucleation delay on superhydrophobic surfaces is more prominent at moderate degrees of supercooling, while closer to the homogeneous nucleation temperature, bulk and air-water interface nucleation effects become equally important. The study presented here offers a comprehensive perspective on the efficacy of textured surfaces for nonicing applications.

Effect of Controlled Ice Nucleation on Stability of Lactate Dehydrogenase During Freeze-Drying.

PubMed

Fang, Rui; Tanaka, Kazunari; Mudhivarthi, Vamsi; Bogner, Robin H; Pikal, Michael J

2018-03-01

Several controlled ice nucleation techniques have been developed to increase the efficiency of the freeze-drying process as well as to improve the quality of pharmaceutical products. Owing to the reduction in ice surface area, these techniques have the potential to reduce the degradation of proteins labile during freezing. The objective of this study was to evaluate the effect of ice nucleation temperature on the in-process stability of lactate dehydrogenase (LDH). LDH in potassium phosphate buffer was nucleated at -4°C, -8°C, and -12°C using ControLyo™ or allowed to nucleate spontaneously. Both the enzymatic activity and tetramer recovery after freeze-thawing linearly correlated with product ice nucleation temperature (n = 24). Controlled nucleation also significantly improved batch homogeneity as reflected by reduced inter-vial variation in activity and tetramer recovery. With the correlation established in the laboratory, the degradation of protein in manufacturing arising from ice nucleation temperature differences can be quantitatively predicted. The results show that controlled nucleation reduced the degradation of LDH during the freezing process, but this does not necessarily translate to vastly superior stability during the entire freeze-drying process. The capability of improving batch homogeneity provides potential advantages in scaling-up from lab to manufacturing scale. Copyright © 2018 American Pharmacists Association®. Published by Elsevier Inc. All rights reserved.

Homogeneous nucleation of water in argon. Nucleation rate computation from molecular simulations of TIP4P and TIP4P/2005 water model.

PubMed

Dumitrescu, Lucia R; Smeulders, David M J; Dam, Jacques A M; Gaastra-Nedea, Silvia V

2017-02-28

Molecular dynamics (MD) simulations were conducted to study nucleation of water at 350 K in argon using TIP4P and TIP4P/2005 water models. We found that the stability of any cluster, even if large, strongly depends on the energetic interactions with its vicinity, while the stable clusters change their composition almost entirely during nucleation. Using the threshold method, direct nucleation rates are obtained. Our nucleation rates are found to be 1.08×10 27 cm -3 s -1 for TIP4P and 2.30×10 27 cm -3 s -1 for TIP4P/2005. The latter model prescribes a faster dynamics than the former, with a nucleation rate two times larger due to its higher electrostatic charges. The non-equilibrium water densities derived from simulations and state-of-art equilibrium parameters from Vega and de Miguel [J. Chem. Phys. 126, 154707 (2007)] are used for the classical nucleation theory (CNT) prediction. The CNT overestimates our results for both water models, where TIP4P/2005 shows largest discrepancy. Our results complement earlier data at high nucleation rates and supersaturations in the Hale plot [Phys. Rev. A 33, 4156 (1986)], and are consistent with MD data on the SPC/E and the TIP4P/2005 model.

Estimation of the nucleation kinetics for the anti-solvent crystallisation of paracetamol in methanol/water solutions

NASA Astrophysics Data System (ADS)

Ó'Ciardhá, Clifford T.; Frawley, Patrick J.; Mitchell, Niall A.

2011-08-01

In this work the primary nucleation kinetics have been estimated for the anti-solvent crystallisation of paracetamol in methanol-water solutions from metastable zone widths (MSZW) and induction times at 25 °C. Laser back-scattering via a focused beam reflectance Measurement (FBRM ®) is utilised to detect the onset of nucleation. The theoretical approach of Kubota was employed to estimate the nucleation kinetics, which accounts for the sensitivity of the nucleation detection technique. This approach is expanded in this work to analyse the induction time for an anti-solvent crystallisation process. Solvent composition is known to have a significant impact on the measured induction times and MSZW. The induction time in this paper was measured from 40% to 70% mass water and the MSZW is measured from 40% to 60% mass water. The primary focus of the paper was to gauge the extent of how solvent composition affects nucleation kinetics so that this effect may be incorporated into a population balance model. Furthermore, the effects of solvent composition on the estimated nucleation rates are investigated. The primary nucleation rates were found to decrease with dynamic solvent composition, with the extent of their reduction linked to the gradient of the solubility curve. Finally, both MSZW and induction time methods have been found to produce similar estimates for the nucleation parameters.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tylka, M. M.; Willit, J. L.; Williamson, M. A.

This work examines the nucleation and growth behavior of uranium and plutonium from molten LiCl-KCl eutectic on inert electrodes using electrochemical techniques. Current-time transients obtained from chronoamperometric experiments were compared with theoretical models to characterize the type of nucleation (progressive or instantaneous) for deposition of U and Pu, and co-deposition of U-Pu, from molten LiCl-KCl at inert electrodes. It was established that the nucleation mode of actinides present as chlorides in molten chloride salts changes from progressive to instantaneous with an increasing concentration of the trivalent actinide ions in the salt. The effect of the material of the working electrodemore » was investigated, and it was found that changing the material from tungsten to silver improves resolvability of the nucleation peaks and allows more accurate analysis of the experimental measurements. Using the nucleation data, diffusion coefficients were obtained for U 3+ and Pu 3+, and were found to be in very good agreement with the values obtained from other studies. Furthermore, the density of nuclei produced during instantaneous nucleation, the rate of nucleation for progressive nucleation, and the radius of the deposited nuclei were evaluated and examined at different overpotentials.« less

Performance of some nucleation theories with a nonsharp droplet-vapor interface.

PubMed

Napari, Ismo; Julin, Jan; Vehkamäki, Hanna

2010-10-21

Nucleation theories involving the concept of nonsharp boundary between the droplet and vapor are compared to recent molecular dynamics (MD) simulation data of Lennard-Jones vapors at temperatures above the triple point. The theories are diffuse interface theory (DIT), extended modified liquid drop-dynamical nucleation theory (EMLD-DNT), square gradient theory (SGT), and density functional theory (DFT). Particular attention is paid to thermodynamic consistency in the comparison: the applied theories either use or, with a proper parameter adjustment, result in the same values of equilibrium vapor pressure, bulk liquid density, and surface tension as the MD simulations. Realistic pressure-density correlations are also used. The best agreement between the simulated nucleation rates and calculations is obtained from DFT, SGT, and EMLD-DNT, all of which, in the studied temperature range, show deviations of less than one order of magnitude in the nucleation rate. DIT underestimates the nucleation rate by up to two orders of magnitude. DFT and SGT give the best estimate of the molecular content of the critical nuclei. Overall, at the vapor conditions of this study, all the investigated theories perform better than classical nucleation theory in predicting nucleation rates.

Nucleation of ordered solid phases of proteins via a disordered high-density state: Phenomenological approach

NASA Astrophysics Data System (ADS)

Pan, Weichun; Kolomeisky, Anatoly B.; Vekilov, Peter G.

2005-05-01

Nucleation of ordered solid phases of proteins triggers numerous phenomena in laboratory, industry, and in healthy and sick organisms. Recent simulations and experiments with protein crystals suggest that the formation of an ordered crystalline nucleus is preceded by a disordered high-density cluster, akin to a droplet of high-density liquid that has been observed with some proteins; this mechanism allowed a qualitative explanation of recorded complex nucleation kinetics curves. Here, we present a simple phenomenological theory that takes into account intermediate high-density metastable states in the nucleation process. Nucleation rate data at varying temperature and protein concentration are reproduced with high fidelity using literature values of the thermodynamic and kinetic parameters of the system. Our calculations show that the growth rate of the near-critical and supercritical ordered clusters within the dense intermediate is a major factor for the overall nucleation rate. This highlights the role of viscosity within the dense intermediate for the formation of the ordered nucleus. The model provides an understanding of the action of additives that delay or accelerate nucleation and presents a framework within which the nucleation of other ordered protein solid phases, e.g., the sickle cell hemoglobin polymers, can be analyzed.

Nucleation of shear bands in amorphous alloys

PubMed Central

Perepezko, John H.; Imhoff, Seth D.; Chen, Ming-Wei; Wang, Jun-Qiang; Gonzalez, Sergio

2014-01-01

The initiation and propagation of shear bands is an important mode of localized inhomogeneous deformation that occurs in a wide range of materials. In metallic glasses, shear band development is considered to center on a structural heterogeneity, a shear transformation zone that evolves into a rapidly propagating shear band under a shear stress above a threshold. Deformation by shear bands is a nucleation-controlled process, but the initiation process is unclear. Here we use nanoindentation to probe shear band nucleation during loading by measuring the first pop-in event in the load–depth curve which is demonstrated to be associated with shear band formation. We analyze a large number of independent measurements on four different bulk metallic glasses (BMGs) alloys and reveal the operation of a bimodal distribution of the first pop-in loads that are associated with different shear band nucleation sites that operate at different stress levels below the glass transition temperature, Tg. The nucleation kinetics, the nucleation barriers, and the density for each site type have been determined. The discovery of multiple shear band nucleation sites challenges the current view of nucleation at a single type of site and offers opportunities for controlling the ductility of BMG alloys. PMID:24594599

Nucleation and hysteresis of vapor-liquid phase transitions in confined spaces: effects of fluid-wall interaction.

PubMed

Men, Yumei; Yan, Qingzhao; Jiang, Guangfeng; Zhang, Xianren; Wang, Wenchuan

2009-05-01

In this work, we propose a method to stabilize a nucleus in the framework of lattice density-functional theory (LDFT) by imposing a suitable constraint. Using this method, the shape of critical nucleus and height of the nucleation barrier can be determined without using a predefined nucleus as input. As an application of this method, we study the nucleation behavior of vapor-liquid transition in nanosquare pores with infinite length and relate the observed hysteresis loop on an adsorption isotherm to the nucleation mechanism. According to the dependence of hysteresis and the nucleation mechanism on the fluid-wall interaction, w , in this work, we have classified w into three regions ( w>0.9 , 0.1< or =w< or =0.9 , and w<0.1 ), which are denoted as strongly, moderately, and weakly attractive fluid-wall interaction, respectively. The dependence of hysteresis on the fluid-wall interaction is interpreted by the different nucleation mechanisms. Our constrained LDFT calculations also show that the different transition paths may induce different nucleation behaviors. The transition path dependence should be considered if morphological transition of nuclei exists during a nucleation process.

Partitioning the primary ice formation modes in large eddy simulations of mixed-phase clouds

NASA Astrophysics Data System (ADS)

Hande, Luke B.; Hoose, Corinna

2017-11-01

State-of-the-art aerosol-dependent parameterisations describing each heterogeneous ice nucleation mode (contact, immersion, and deposition ice nucleation), as well as homogeneous nucleation, were incorporated into a large eddy simulation model. Several cases representing commonly occurring cloud types were simulated in an effort to understand which ice nucleation modes contribute the most to total concentrations of ice crystals. The cases include a completely idealised warm bubble, semi-idealised deep convection, an orographic cloud, and a stratiform case. Despite clear differences in thermodynamic conditions between the cases, the results are remarkably consistent between the different cloud types. In all the investigated cloud types and under normal aerosol conditions, immersion freezing dominates and contact freezing also contributes significantly. At colder temperatures, deposition nucleation plays only a small role, and homogeneous freezing is important. To some extent, the temporal evolution of the cloud determines the dominant freezing mechanism and hence the subsequent microphysical processes. Precipitation is not correlated with any one ice nucleation mode, instead occurring simultaneously when several nucleation modes are active. Furthermore, large variations in the aerosol concentration do affect the dominant ice nucleation mode; however, they have only a minor influence on the precipitation amount.

Heterogeneous Ice Nucleation: Interplay of Surface Properties and Their Impact on Water Orientations.

PubMed

Glatz, Brittany; Sarupria, Sapna

2018-01-23

Ice is ubiquitous in nature, and heterogeneous ice nucleation is the most common pathway of ice formation. How surface properties affect the propensity to observe ice nucleation on that surface remains an open question. We present results of molecular dynamics studies of heterogeneous ice nucleation on model surfaces. The models surfaces considered emulate the chemistry of kaolinite, an abundant component of mineral dust. We investigate the interplay of surface lattice and hydrogen bonding properties in affecting ice nucleation. We find that lattice matching and hydrogen bonding are necessary but not sufficient conditions for observing ice nucleation at these surfaces. We correlate this behavior to the orientations sampled by the metastable supercooled water in contact with the surfaces. We find that ice is observed in cases where water molecules not only sample orientations favorable for bilayer formation but also do not sample unfavorable orientations. This distribution depends on both surface-water and water-water interactions and can change with subtle modifications to the surface properties. Our results provide insights into the diverse behavior of ice nucleation observed at different surfaces and highlight the complexity in elucidating heterogeneous ice nucleation.

Nucleation and Growth Kinetics from LaMer Burst Data.

PubMed

Chu, Daniel B K; Owen, Jonathan S; Peters, Baron

2017-10-12

In LaMer burst nucleation, the individual nucleation events happen en masse, quasi-simultaneously, and at nearly identical homogeneous conditions. These properties make LaMer burst nucleation important for applications that require monodispersed particles and also for theoretical analyses. Sugimoto and co-workers predicted that the number of nuclei generated during a LaMer burst depends only on the solute supply rate and the growth rate, independent of the nucleation kinetics. Some experiments confirm that solute supply kinetics control the number of nuclei, but flaws in the original theoretical analysis raise questions about the predicted roles of growth and nucleation kinetics. We provide a rigorous analysis of the coupled equations that govern concentrations of nuclei and solutes. Our analysis confirms that the number of nuclei is largely determined by the solute supply and growth rates, but our predicted relationship differs from that of Sugimoto et al. Moreover, we find that additional nucleus size dependent corrections should emerge in systems with slow growth kinetics. Finally, we show how the nucleation kinetics determine the particle size distribution. We suggest that measured particle size distributions might therefore provide ways to test theoretical models of homogeneous nucleation kinetics.

The role of confined collagen geometry in decreasing nucleation energy barriers to intrafibrillar mineralization

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kim, Doyoon; Lee, Byeongdu; Thomopoulos, Stavros

Mineralization of collagen is critical for the mechanical functions of bones and teeth. Calcium phosphate nucleation in collagenous structures follows distinctly different patterns in highly confined gap regions (nanoscale confinement) than in less confined extrafibrillar spaces (microscale confinement). Although the mechanism(s) driving these differences are still largely unknown, differences in the free energy for nucleation may explain these two mineralization behaviors. Here, we report on experimentally obtained nucleation energy barriers to intra- and extrafibrillar mineralization, using in situ X-ray scattering observations and classical nucleation theory. Polyaspartic acid, an extrafibrillar nucleation inhibitor, increases interfacial energies between nuclei and mineralization fluids. Inmore » contrast, the confined gap spaces inside collagen fibrils lower the energy barrier by reducing the reactive surface area of nuclei, decreasing the surface energy penalty. The confined gap geometry, therefore, guides the two-dimensional morphology and structure of bioapatite and changes the nucleation pathway by reducing the total energy barrier.« less

The role of confined collagen geometry in decreasing nucleation energy barriers to intrafibrillar mineralization

DOE PAGES

Kim, Doyoon; Lee, Byeongdu; Thomopoulos, Stavros; ...

2018-03-06

Mineralization of collagen is critical for the mechanical functions of bones and teeth. Calcium phosphate nucleation in collagenous structures follows distinctly different patterns in highly confined gap regions (nanoscale confinement) than in less confined extrafibrillar spaces (microscale confinement). Although the mechanism(s) driving these differences are still largely unknown, differences in the free energy for nucleation may explain these two mineralization behaviors. Here, we report on experimentally obtained nucleation energy barriers to intra- and extrafibrillar mineralization, using in situ X-ray scattering observations and classical nucleation theory. Polyaspartic acid, an extrafibrillar nucleation inhibitor, increases interfacial energies between nuclei and mineralization fluids. Inmore » contrast, the confined gap spaces inside collagen fibrils lower the energy barrier by reducing the reactive surface area of nuclei, decreasing the surface energy penalty. The confined gap geometry, therefore, guides the two-dimensional morphology and structure of bioapatite and changes the nucleation pathway by reducing the total energy barrier.« less

Effect of nucleation time on bending response of ionic polymer–metal composite actuators

DOE PAGES

Kim, Suran; Hong, Seungbum; Choi, Yoon-Young; ...

2013-07-02

We attempted an autocatalytic electro-less plating of nickel in order to replace an electroless impregnation-reduction (IR) method in ionic polymer–metal composite (IPMC) actuators to reduce cost and processing time. Because nucleation time of Pd–Sn colloids is the determining factor of overall processing time, we used the nucleation time as our control parameter. In order to optimize nucleation time and investigate its effect on the performance of IPMC actuators, we analyzed the relationship between the nucleation time, interface morphology and electrical properties. The optimized nucleation time was 10 h. Furthermore, the trends of the performance and electrical properties as a functionmore » of nucleation time were attributed to the fact that the Ni penetration depth was determined by the minimum diffusion length of either Pd–Sn colloids or reducing agent ions. The Ni-IPMC actuators can be fabricated less than 14 h processing time without deteriorating performance of the actuators, which is comparable to Pt-IPMC prepared by IR method.« less

Nucleation and microstructure development in Cr-Mo-V tool steel during gas atomization

NASA Astrophysics Data System (ADS)

Behúlová, M.; Grgač, P.; Čička, R.

2017-11-01

Nucleation studies of undercooled metallic melts are of essential interest for the understanding of phase selection, growth kinetics and microstructure development during their rapid non-equilibrium solidification. The paper deals with the modelling of nucleation processes and microstructure development in the hypoeutectic tool steel Ch12MF4 with the chemical composition of 2.37% C, 12.06 % Cr, 1.2% Mo, 4.0% V and balance Fe [wt. %] in the process of nitrogen gas atomization. Based on the classical theory of homogeneous nucleation, the nucleation temperature of molten rapidly cooled spherical particles from this alloy with diameter from 40 μm to 600 μm in the gas atomization process is calculated using various estimations of parameters influencing the nucleation process - the Gibbs free energy difference between solid and liquid phases and the solid/liquid interfacial energy. Results of numerical calculations are compared with experimentally measured nucleation temperatures during levitation experiments and microstructures developed in rapidly solidified powder particles from the investigated alloy.

Overcoming the Time Limitation in Molecular Dynamics Simulation of Crystal Nucleation: A Persistent-Embryo Approach

NASA Astrophysics Data System (ADS)

Sun, Yang; Song, Huajing; Zhang, Feng; Yang, Lin; Ye, Zhuo; Mendelev, Mikhail I.; Wang, Cai-Zhuang; Ho, Kai-Ming

2018-02-01

The crystal nucleation from liquid in most cases is too rare to be accessed within the limited time scales of the conventional molecular dynamics (MD) simulation. Here, we developed a "persistent embryo" method to facilitate crystal nucleation in MD simulations by preventing small crystal embryos from melting using external spring forces. We applied this method to the pure Ni case for a moderate undercooling where no nucleation can be observed in the conventional MD simulation, and obtained nucleation rate in good agreement with the experimental data. Moreover, the method is applied to simulate an even more sluggish event: the nucleation of the B 2 phase in a strong glass-forming Cu-Zr alloy. The nucleation rate was found to be 8 orders of magnitude smaller than Ni at the same undercooling, which well explains the good glass formability of the alloy. Thus, our work opens a new avenue to study solidification under realistic experimental conditions via atomistic computer simulation.

Janus effect of antifreeze proteins on ice nucleation.

PubMed

Liu, Kai; Wang, Chunlei; Ma, Ji; Shi, Guosheng; Yao, Xi; Fang, Haiping; Song, Yanlin; Wang, Jianjun

2016-12-20

The mechanism of ice nucleation at the molecular level remains largely unknown. Nature endows antifreeze proteins (AFPs) with the unique capability of controlling ice formation. However, the effect of AFPs on ice nucleation has been under debate. Here we report the observation of both depression and promotion effects of AFPs on ice nucleation via selectively binding the ice-binding face (IBF) and the non-ice-binding face (NIBF) of AFPs to solid substrates. Freezing temperature and delay time assays show that ice nucleation is depressed with the NIBF exposed to liquid water, whereas ice nucleation is facilitated with the IBF exposed to liquid water. The generality of this Janus effect is verified by investigating three representative AFPs. Molecular dynamics simulation analysis shows that the Janus effect can be established by the distinct structures of the hydration layer around IBF and NIBF. Our work greatly enhances the understanding of the mechanism of AFPs at the molecular level and brings insights to the fundamentals of heterogeneous ice nucleation.

Janus effect of antifreeze proteins on ice nucleation

PubMed Central

Liu, Kai; Wang, Chunlei; Ma, Ji; Shi, Guosheng; Yao, Xi; Fang, Haiping; Song, Yanlin; Wang, Jianjun

2016-01-01

The mechanism of ice nucleation at the molecular level remains largely unknown. Nature endows antifreeze proteins (AFPs) with the unique capability of controlling ice formation. However, the effect of AFPs on ice nucleation has been under debate. Here we report the observation of both depression and promotion effects of AFPs on ice nucleation via selectively binding the ice-binding face (IBF) and the non–ice-binding face (NIBF) of AFPs to solid substrates. Freezing temperature and delay time assays show that ice nucleation is depressed with the NIBF exposed to liquid water, whereas ice nucleation is facilitated with the IBF exposed to liquid water. The generality of this Janus effect is verified by investigating three representative AFPs. Molecular dynamics simulation analysis shows that the Janus effect can be established by the distinct structures of the hydration layer around IBF and NIBF. Our work greatly enhances the understanding of the mechanism of AFPs at the molecular level and brings insights to the fundamentals of heterogeneous ice nucleation. PMID:27930318

Overcoming the Time Limitation in Molecular Dynamics Simulation of Crystal Nucleation: A Persistent-Embryo Approach.

PubMed

Sun, Yang; Song, Huajing; Zhang, Feng; Yang, Lin; Ye, Zhuo; Mendelev, Mikhail I; Wang, Cai-Zhuang; Ho, Kai-Ming

2018-02-23

The crystal nucleation from liquid in most cases is too rare to be accessed within the limited time scales of the conventional molecular dynamics (MD) simulation. Here, we developed a "persistent embryo" method to facilitate crystal nucleation in MD simulations by preventing small crystal embryos from melting using external spring forces. We applied this method to the pure Ni case for a moderate undercooling where no nucleation can be observed in the conventional MD simulation, and obtained nucleation rate in good agreement with the experimental data. Moreover, the method is applied to simulate an even more sluggish event: the nucleation of the B2 phase in a strong glass-forming Cu-Zr alloy. The nucleation rate was found to be 8 orders of magnitude smaller than Ni at the same undercooling, which well explains the good glass formability of the alloy. Thus, our work opens a new avenue to study solidification under realistic experimental conditions via atomistic computer simulation.

Monomer-dependent secondary nucleation in amyloid formation.

PubMed

Linse, Sara

2017-08-01

Secondary nucleation of monomers on the surface of an already existing aggregate that is formed from the same kind of monomers may lead to autocatalytic amplification of a self-assembly process. Such monomer-dependent secondary nucleation occurs during the crystallization of small molecules or proteins and self-assembled materials, as well as in protein self-assembly into fibrous structures. Indications of secondary nucleation may come from analyses of kinetic experiments starting from pure monomers or monomers supplemented with a low concentration of pre-formed aggregates (seeds). More firm evidence requires additional experiments, for example those employing isotope labels to distinguish new aggregates arising from the monomer from those resulting from fragmentation of the seed. In cases of amyloid formation, secondary nucleation leads to the formation of toxic oligomers, and inhibitors of secondary nucleation may serve as starting points for therapeutic developments. Secondary nucleation displays a high degree of structural specificity and may be enhanced by mutations or screening of electrostatic repulsion.

Two types of amorphous protein particles facilitate crystal nucleation.

PubMed

Yamazaki, Tomoya; Kimura, Yuki; Vekilov, Peter G; Furukawa, Erika; Shirai, Manabu; Matsumoto, Hiroaki; Van Driessche, Alexander E S; Tsukamoto, Katsuo

2017-02-28

Nucleation, the primary step in crystallization, dictates the number of crystals, the distribution of their sizes, the polymorph selection, and other crucial properties of the crystal population. We used time-resolved liquid-cell transmission electron microscopy (TEM) to perform an in situ examination of the nucleation of lysozyme crystals. Our TEM images revealed that mesoscopic clusters, which are similar to those previously assumed to consist of a dense liquid and serve as nucleation precursors, are actually amorphous solid particles (ASPs) and act only as heterogeneous nucleation sites. Crystalline phases never form inside them. We demonstrate that a crystal appears within a noncrystalline particle assembling lysozyme on an ASP or a container wall, highlighting the role of heterogeneous nucleation. These findings represent a significant departure from the existing formulation of the two-step nucleation mechanism while reaffirming the role of noncrystalline particles. The insights gained may have significant implications in areas that rely on the production of protein crystals, such as structural biology, pharmacy, and biophysics, and for the fundamental understanding of crystallization mechanisms.

Numerical investigation of homogeneous cavitation nucleation in a microchannel

NASA Astrophysics Data System (ADS)

Lyu, Xiuxiu; Pan, Shucheng; Hu, Xiangyu; Adams, Nikolaus A.

2018-06-01

The physics of nucleation in water is an important issue for many areas, ranging from biomedical to engineering applications. Within the present study, we investigate numerically homogeneous nucleation in a microchannel induced by shock reflection to gain a better understanding of the mechanism of homogeneous nucleation. The liquid expands due to the reflected shock and homogeneous cavitation nuclei are generated. An Eulerian-Lagrangian approach is employed for modeling this process in a microchanel. Two-dimensional axisymmetric Euler equations are solved for obtaining the time evolution of shock, gas bubble, and the ambient fluid. The dynamics of dispersed vapor bubbles is coupled with the surrounding fluid in a Lagrangian framework, describing bubble location and bubble size variation. Our results reproduce nuclei distributions at different stages of homogeneous nucleation and are in good agreement with experimental results. We obtain numerical data for the negative pressure that water can sustain under the process of homogeneous nucleation. An energy transformation description for the homogeneous nucleation inside a microchannel flow is derived and analyzed in detail.

Homogeneous crystal nucleation in Ni droplets

NASA Astrophysics Data System (ADS)

Kožíšek, Zdeněk; Demo, Pavel

2017-10-01

Crystal nucleation kinetics is often represented by induction times or metastable zone widths (Kulkarni et al., 2013; Bokeloh et al., 2011). Repeating measurements of supercooling or time delay, at which phase transition is detected, are statistically processed to determine the so-called survivorship function, from which nucleation rate is computed. The size distribution of nuclei is difficult to measure near the critical size directly, and it is not clear which amount of nuclei is formed at the moment when the phase transition is detected. In the present paper, kinetic nucleation equations are solved for the crystal nucleation in Ni liquid droplet to determine the number of nuclei formed within a considered system. Analysis of supercooling experimental data, based on the classical nucleation theory CNT), computes appropriate values of the nucleation rate. However, CNT underestimates the number of nuclei F (F ≪ 1 for supercritical sizes). Taking into account the dependence of the surface energy on nucleus size to data analysis overcomes this discrepancy and leads to reasonable values of the size distribution of nuclei.

Observation of ice nucleation in acoustically levitated water drops

NASA Astrophysics Data System (ADS)

Lü, Y. J.; Xie, W. J.; Wei, B.

2005-10-01

The supercooling and nucleation of acoustically levitated water drops were investigated at two different sound pressure levels (SPL). These water drops were supercooled by 13to16K at the low SPL of 160.6dB, whereas their supercoolings varied from 5to11K at the high SPL of 164.4dB. The maximum supercooling obtained in the experiments is 32K. Statistical analyses based on the classical nucleation theory reveal that the occurrence of ice nucleation in water drops is mainly confined to the surface region under acoustic levitation conditions and the enlargement of drop surface area caused by the acoustic radiation pressure reduces water supercoolability remarkably. A comparison of the nucleation rates at the two SPLs indicates that the sound pressure can strengthen the surface-dominated nucleation of water drops. The acoustic stream around levitated water drops and the cavitation effect associated with ultrasonic field are the main factors that induce surface-dominated nucleation.

Effect of AlB2 on the P-threshold in Al-Si alloy

NASA Astrophysics Data System (ADS)

Wu, Yuying; Liu, Xiangfa

2018-06-01

The nucleation of primary Si in Al-Si alloys has been investigated in this work. It was found that there was a threshold concentration of P, below which AlP can not heterogeneous nucleate primary Si in Al-12 wt%Si alloy. AlB2 can not nucleate primary Si directly, but the presence of AlB2 may assist the nucleation of AlP leading to the nucleation of primary Si particles. In addition, with addition of AlB2, the nucleation efficiency of AlP can be improved in Al-18 wt%Si alloy. The orientation relationship between AlB2 and AlP has been calculated, and the adsorption model for AlB2 and AlP was proposed in this work.

Experiments on Nucleation in Different Flow Regimes

NASA Technical Reports Server (NTRS)

Bayuzick, R. J.; Hofmeister, W. H.; Morton, C. M.; Robinson, M. B.

1998-01-01

The vast majority of metallic engineering materials are solidified from the liquid phase. Understanding the solidification process is essential to control microstructure, which in turn, determines the properties of materials. The genesis of solidification is nucleation, where the first stable solid forms from the liquid phase. Nucleation kinetics determine the degree of undercooling and phase selection. As such, it is important to understand nucleation phenomena in order to control solidification or glass formation in metals and alloys. Early experiments in nucleation kinetics were accomplished by droplet dispersion methods. Dilitometry was used by Turnbull and others, and more recently differential thermal analysis and differential scanning calorimetry have been used for kinetic studies. These techniques have enjoyed success; however, there are difficulties with these experiments. Since materials are dispersed in a medium, the character of the emulsion/metal interface affects the nucleation behavior. Statistics are derived from the large number of particles observed in a single experiment, but dispersions have a finite size distribution which adds to the uncertainty of the kinetic determinations. Even though temperature can be controlled quite well before the onset of nucleation, the release of the latent heat of fusion during nucleation of particles complicates the assumption of isothermality during these experiments. Containerless processing has enabled another approach to the study of nucleation kinetics. With levitation techniques it is possible to undercool one sample to nucleation repeatedly in a controlled manner, such that the statistics of the nucleation process can be derived from multiple experiments on a single sample. The authors have fully developed the analysis of nucleation experiments on single samples following the suggestions of Skripov. The advantage of these experiments is that the samples are directly observable. The nucleation temperature can be measured by noncontact optical pyrometry, the mass of the sample is known, and post-processing analysis can be conducted on the sample. The disadvantages are that temperature measurement must have exceptionally high precision, and it is not possible to isolate specific heterogeneous sites as in droplet dispersions. Levitation processing of refractory materials in ultra high vacuum provides an avenue to conduct these kinetic studies on single samples. Two experimental methods have been identified where ultra high vacuum experiments are possible; electrostatic levitation in ground-based experiments and electromagnetic processing in low earth orbit on TEMPUS. Such experiments, reported here, were conducted on zirconium. Liquid zirconium is an excellent solvent and has a high solubility for contaminants contained in the bulk material as well as those contaminants found in the vacuum environment. Oxides, nitrides, and carbides do not exist in the melt, and do not form on the surface of molten zirconium, for the materials and vacuum levels used in this study. Ground-based experiments with electrostatic levitation have shown that the statistical nucleation kinetic experiments are viable and yield results which are consistent with classical nucleation theory. The advantage of low earth orbit experiments is the ability to vary the flow conditions in the liquid prior to nucleation. The put-pose of nucleation experiments in TEMPUS was to examine.

Immersion freezing of ice nucleation active protein complexes

NASA Astrophysics Data System (ADS)

Hartmann, S.; Augustin, S.; Clauss, T.; Wex, H.; Šantl-Temkiv, T.; Voigtländer, J.; Niedermeier, D.; Stratmann, F.

2013-06-01

Utilising the Leipzig Aerosol Cloud Interaction Simulator (LACIS), the immersion freezing behaviour of droplet ensembles containing monodisperse particles, generated from a Snomax™ solution/suspension, was investigated. Thereto ice fractions were measured in the temperature range between -5 °C to -38 °C. Snomax™ is an industrial product applied for artificial snow production and contains Pseudomonas syringae} bacteria which have long been used as model organism for atmospheric relevant ice nucleation active (INA) bacteria. The ice nucleation activity of such bacteria is controlled by INA protein complexes in their outer membrane. In our experiments, ice fractions increased steeply in the temperature range from about -6 °C to about -10 °C and then levelled off at ice fractions smaller than one. The plateau implies that not all examined droplets contained an INA protein complex. Assuming the INA protein complexes to be Poisson distributed over the investigated droplet populations, we developed the CHESS model (stoCHastic modEl of similar and poiSSon distributed ice nuclei) which allows for the calculation of ice fractions as function of temperature and time for a given nucleation rate. Matching calculated and measured ice fractions, we determined and parameterised the nucleation rate of INA protein complexes exhibiting class III ice nucleation behaviour. Utilising the CHESS model, together with the determined nucleation rate, we compared predictions from the model to experimental data from the literature and found good agreement. We found that (a) the heterogeneous ice nucleation rate expression quantifying the ice nucleation behaviour of the INA protein complex is capable of describing the ice nucleation behaviour observed in various experiments for both, Snomax™ and P. syringae bacteria, (b) the ice nucleation rate, and its temperature dependence, seem to be very similar regardless of whether the INA protein complexes inducing ice nucleation are attached to the outer membrane of intact bacteria or membrane fragments, (c) the temperature range in which heterogeneous droplet freezing occurs, and the fraction of droplets being able to freeze, both depend on the actual number of INA protein complexes present in the droplet ensemble, and (d) possible artifacts suspected to occur in connection with the drop freezing method, i.e., the method frequently used by biologist for quantifying ice nucleation behaviour, are of minor importance, at least for substances such as P. syringae, which induce freezing at comparably high temperatures. The last statement implies that for single ice nucleation entities such as INA protein complexes, it is the number of entities present in the droplet population, and the entities' nucleation rate, which control the freezing behaviour of the droplet population. Quantities such as ice active surface site density are not suitable in this context. The results obtained in this study allow a different perspective on the quantification of the immersion freezing behaviour of bacterial ice nucleation.

Kinetic Monte Carlo simulations of nucleation and growth in electrodeposition.

PubMed

Guo, Lian; Radisic, Aleksandar; Searson, Peter C

2005-12-22

Nucleation and growth during bulk electrodeposition is studied using kinetic Monte Carlo (KMC) simulations. Ion transport in solution is modeled using Brownian dynamics, and the kinetics of nucleation and growth are dependent on the probabilities of metal-on-substrate and metal-on-metal deposition. Using this approach, we make no assumptions about the nucleation rate, island density, or island distribution. The influence of the attachment probabilities and concentration on the time-dependent island density and current transients is reported. Various models have been assessed by recovering the nucleation rate and island density from the current-time transients.

Identification and quantification of ice nucleation active microorganisms by digital droplet PCR (ddPCR)

NASA Astrophysics Data System (ADS)

Linden, Martin; Pöschl, Ulrich; Fröhlich-Nowoisky, Janine

2015-04-01

Several bioaerosol types, including bacteria, fungi, pollen and lichen, have been identified as sources of biological ice nucleators (IN) which induce ice formation already at temperatures as high as -10 °C or above. Accordingly, they potentially contribute widely to environmental ice nucleation in the atmosphere and are of great interest in the study of natural heterogenous ice nucleation processes. Ice nucleation active microorganisms have been found and studied among bacteria (Proteobacteria) and fungi (phyla Basidiomycota and Ascomycota). The mechanisms enabling the microorganisms to ice nucleation are subject to ongoing research. While it has been demonstrated that whole cells can act as ice nucleators in the case of bacteria due to the presence of specific membrane proteins, cell-free ice nucleation active particles seem to be responsible for this phenomenon in fungi and lichen. The identification and quantification of these ice nucleation active microorganisms and their IN in atmospheric samples is crucial to understand their contribution to the pool of atmospheric IN. This is not a trivial task since the respective microorganisms are often prevalent in lowest concentrations and a variety of states, be it viable cells, spores or cell debris from dead cells. Molecular biology provides tools to identify and quantify ice nucleation active microorganisms independent of their state by detecting genetic markers specific for the organism of interest. Those methods are not without their drawbacks in terms of sample material concentration required or reliable standardization. Digital Droplet Polymerase Chain Reaction (ddPCR) was chosen for our demands as a more elegant, quick and specific method in the investigation of ice nucleation active microorganisms in atmospheric samples. The advantages of ddPCR lie in the simultaneous detection and quantification of genetic markers and their original copy numbers in a sample. This is facilitated by the fractionation of the PCR reaction volumes containing template DNA of ice nucleation active microorganisms from atmospheric samples in thousands of identical droplets. Each droplet encapsulates the reagents necessary for DNA amplification. With template DNA concentrations low enough, the droplets will statistically contain either no template molecules or one molecule. A molecule of template DNA corresponds to exactly one cell of an ice nucleation active microorganism in the original sample provided the genetic marker on the template is present in a single copy. Successful amplification in the presence of template DNA is coupled to a measurable fluorescence signal. The original template DNA concentration is automatically derived from the fraction of fluorescence positive droplets to total droplet number. This far, molecular probes against single-copy genetic markers for ice nucleation active fungi Mortierella alpina, Acremonium implicatum, Isaria farinosa and the ice nucleation active bacterium Pseudomonas syringae have been successfully designed and tested by our group.

Quantification of an atmospheric nucleation and growth process as a single source of aerosol particles in a city

NASA Astrophysics Data System (ADS)

Salma, Imre; Varga, Veronika; Németh, Zoltán

2017-12-01

Effects of a new aerosol particle formation (NPF) and particle diameter growth process as a single source of atmospheric particle number concentrations were evaluated and quantified on the basis of experimental data sets obtained from particle number size distribution measurements in the city centre and near-city background of Budapest for 5 years. Nucleation strength factors for a nucleation day (NSFNUC) and for a general day (NSFGEN) were derived separately for seasons and full years. The former characteristic represents the concentration increment of ultrafine (UF) particles specifically on nucleation days with respect to accumulation-mode (regional background) concentrations (particles with equivalent diameters of 100-1000 nm; N100-1000) due solely to the nucleation process. The latter factor expresses the contribution of nucleation to particle numbers on general days; thus, it represents a longer time interval such as season or year. The nucleation source had the largest effect on the concentrations around noon and early afternoon, as expected. During this time interval, it became the major source of particles in the near-city background. Nucleation increased the daily mean concentrations on nucleation days by mean factors of 2.3 and 1.58 in the near-city background and city centre, respectively. Its effect was largest in winter, which was explained by the substantially lower N100-1000 levels on nucleation days than those on non-nucleation days. On an annual timescale, 37 % of the UF particles were generated by nucleation in the near-city background, while NPF produced 13 % of UF particles in the city centre. The differences among the annual mean values, and among the corresponding seasonal mean values, were likely caused by the variability in controlling factors from year to year. The values obtained represent the lower limits of the contributions. The shares determined imply that NPF is a non-negligible or substantial source of particles in near-city background environments and even in city centres, where the vehicular road emissions usually prevail. Atmospheric residence time of nucleation-mode particles was assessed by a decay curve analysis, and a mean of 02:30 was obtained. The present study suggests that the health-related consequences of the atmospheric NPF and growth process in cities should also be considered in addition to its urban climate implications.

Laboratory, Computational and Theoretical Investigations of Ice Nucleation and its Implications for Mixed Phase Clouds

NASA Astrophysics Data System (ADS)

Yang, Fan

Ice particles in atmospheric clouds play an important role in determining cloud lifetime, precipitation and radiation. It is therefore important to understand the whole life cycle of ice particles in the atmosphere, e.g., where they come from (nucleation), how they evolve (growth), and where they go (precipitation). Ice nucleation is the crucial step for ice formation, and in this study, we will mainly focus on ice nucleation in the lab and its effect on mixed-phase stratiform clouds. In the first half of this study, we investigate the relevance of moving contact lines (i.e., the region where three or more phases meet) on the phenomenon of contact nucleation. High speed video is used to investigate heterogeneous ice nucleation in supercooled droplets resting on cold substrates under two different dynamic conditions: droplet electrowetting and droplet vibration. The results show that contact-line motion is not a sufficient condition to trigger ice nucleation, while locally curved contact lines that can result from contact-line motion are strongly related to ice nucleation. We propose that pressure perturbations due to locally curved contact lines can strongly enhance the ice nucleation rate, which gives another interpretation for the mechanism for contact nucleation. Corresponding theoretical results provide a quantitative connection between pressure perturbations and temperature, providing a useful tool for ice nucleation calculations in atmospheric models. In this second half of the study, we build a minimalist model for long lifetime mixed-phase stratiform clouds based on stochastic ice nucleation. Our result shows that there is a non-linear relationship between ice water contact and ice number concentration in the mixed-phase cloud, as long as the volume ice nucleation rate is constant. This statistical property may help identify the source of ice nuclei in mixed-phase clouds. In addition, results from Lagrangian ice particle tracking in time dependent fields show that long lifetime ice particles exist in mixed-phase stratiform clouds. We find that small ice particle can be trapped in eddy-like structures. Whether ice particles grow or sublimate depends on the thermodynamic field in the trapping region. This dynamic-thermodynamic coupling effect on the lifetime of ice particles might explain the fast phase-partition change observed in the mixed phase cloud.

Alloy substantially free of dendrites and method of forming the same

DOE Office of Scientific and Technical Information (OSTI.GOV)

de Figueredo, Anacleto M.; Apelian, Diran; Findon, Matt M.

2009-04-07

Described herein are alloys substantially free of dendrites. A method includes forming an alloy substantially free of dendrites. A superheated alloy is cooled to form a nucleated alloy. The temperature of the nucleated alloy is controlled to prevent the nuclei from melting. The nucleated alloy is mixed to distribute the nuclei throughout the alloy. The nucleated alloy is cooled with nuclei distributed throughout.

Classical nucleation theory of homogeneous freezing of water: thermodynamic and kinetic parameters.

PubMed

Ickes, Luisa; Welti, André; Hoose, Corinna; Lohmann, Ulrike

2015-02-28

The probability of homogeneous ice nucleation under a set of ambient conditions can be described by nucleation rates using the theoretical framework of Classical Nucleation Theory (CNT). This framework consists of kinetic and thermodynamic parameters, of which three are not well-defined (namely the interfacial tension between ice and water, the activation energy and the prefactor), so that any CNT-based parameterization of homogeneous ice formation is less well-constrained than desired for modeling applications. Different approaches to estimate the thermodynamic and kinetic parameters of CNT are reviewed in this paper and the sensitivity of the calculated nucleation rate to the choice of parameters is investigated. We show that nucleation rates are very sensitive to this choice. The sensitivity is governed by one parameter - the interfacial tension between ice and water, which determines the energetic barrier of the nucleation process. The calculated nucleation rate can differ by more than 25 orders of magnitude depending on the choice of parameterization for this parameter. The second most important parameter is the activation energy of the nucleation process. It can lead to a variation of 16 orders of magnitude. By estimating the nucleation rate from a collection of droplet freezing experiments from the literature, the dependence of these two parameters on temperature is narrowed down. It can be seen that the temperature behavior of these two parameters assumed in the literature does not match with the predicted nucleation rates from the fit in most cases. Moreover a comparison of all possible combinations of theoretical parameterizations of the dominant two free parameters shows that one combination fits the fitted nucleation rates best, which is a description of the interfacial tension coming from a molecular model [Reinhardt and Doye, J. Chem. Phys., 2013, 139, 096102] in combination with the activation energy derived from self-diffusion measurements [Zobrist et al., J. Phys. Chem. C, 2007, 111, 2149]. However, some fundamental understanding of the processes is still missing. Further research in future might help to tackle this problem. The most important questions, which need to be answered to constrain CNT, are raised in this study.

The Cirrus Parcel Model Comparison Project. Phase 1

NASA Technical Reports Server (NTRS)

Lin, Ruei-Fong; Starr, D.; DeMott, P.; Cotten, R.; Jensen, E.; Sassen, K.

2000-01-01

The cirrus Parcel Model Comparison Project involves the systematic comparison of current models of ice crystal nucleation and growth for specified, typical, cirrus cloud environments. In Phase 1 of the project reported here, simulated cirrus cloud microphysical properties are compared for situations of "warm" (-40 C) and "cold" (-60 C) cirrus subject to updrafts of 4, 20 and 100 centimeters per second, respectively. Five models are participating in the project. These models employ explicit microphysical schemes wherein the size distribution of each class of particles (aerosols and ice crystals) is resolved into bins. Simulations are made including both homogeneous and heterogeneous ice nucleation mechanisms. A single initial aerosol population of sulfuric acid particles is prescribed for all simulations. To isolate the treatment of the homogeneous freezing (of haze drops) nucleation process, the heterogeneous nucleation mechanism is disabled for a second parallel set of simulations. Qualitative agreement is found amongst the models for the homogeneous-nucleation-only simulations, e.g., the number density of nucleated ice crystals increases with the strength of the prescribed updraft. However, non-negligible quantitative differences are found. Systematic bias exists between results of a model based on a modified classical theory approach and models using an effective freezing temperature approach to the treatment of nucleation. Each approach is constrained by critical freezing data from laboratory studies. This information is necessary, but not sufficient, to construct consistent formulae for the two approaches. Large haze particles may deviate considerably from equilibrium size in moderate to strong updrafts (20-100 centimeters per second) at -60 C when the commonly invoked equilibrium assumption is lifted. The resulting difference in particle-size-dependent solution concentration of haze particles may significantly affect the ice nucleation rate during the initial nucleation interval. The uptake rate for water vapor excess by ice crystals is another key component regulating the total number of nucleated ice crystals. This rate, the product of ice number concentration and ice crystal diffusional growth rate, partially controls the peak nucleation rate achieved in an air parcel and the duration of the active nucleation time period.

Activity of different proteinaceous ice nucleating particles

NASA Astrophysics Data System (ADS)

Hartmann, Susann; Augustin-Bauditz, Stefanie; Grawe, Sarah; Ling, Meilee; Hellner, Lisa; Zapf, Jean-Michel; Šantl-Temkiv, Tina; Pummer, Bernhard; Boesen, Thomas; Wex, Heike; Finster, Kai; Stratmann, Frank

2017-04-01

A variety of microorganisms (bacteria, fungi, lichen) from land produce protein structures, which act as a template for ice nucleation [1]. Also marine sources of ice nucleating particles (INPs) came in focus in the recent years. The atmospheric spatio-temporal distribution of INPs from microorganisms is still not well known. However, it is often assumed that the observed onset of atmospheric ice nucleation (T>-20°C) is due to the existence of ice-nucleation active biological particles. In this study we compare the ice nucleation activity of different proteinaceous particles produced by bacteria and fungi. For bacteria we investigate (i) cells and fragments of Pseudomonas syringae from commercially available SnomaxTM and (ii) the Pseudomonas syringae INA protein expressed in living Escherichia coli bacteria. We also analyzed freeze-dried leaves [2] where we assume that proteinaceous particles are responsible for the ice nucleation activity. For fungi the widespread soil fungus Mortierella alpina was investigated which had been extracted from natural soil [3]. Immersion freezing experiments are performed at the cold stage LINA (Leipzig Ice Nucleation Array). We attempt to describe the activity of a single proteinaceous ice nucleating particle [4] in order to achieve direct comparability. Further, the results are compared with complex natural systems e.g. soil dust. The objectives of this study are to clarify potential differences in the ice nucleation potential of proteinaceous particles and to draw conclusions concerning the need to differentiate them for modelling purposes. 1. Szyrmer, W. and I. Zawadzki, Biogenic and anthropogenic sources of ice-forming nuclei: A review, Bull. Amer. Meteorol. Soc., 1997. 2. Schnell, R.C. and G. Vali, Biogenic ice nucleai .1: Terrestrial and marine sources, doi: 10.1175/1520-0469(1976)033<1554:binpit>2.0.co;2, 1976. 3. Froehlich-Nowoisky, J. et al., Ice nucleation activity in the widespread soil fungus Mortierella alpina, doi: 10.5194/bg-12-1057-2015, 2015. 4. Hartmann, S. et al., Immersion freezing of ice nucleation active protein complexes, Atmos. Chem. Phys., doi: 10.5194/acp-13-5751-2013, 2013.

Nucleation of protein crystals under the influence of solution shear flow.

PubMed

Penkova, Anita; Pan, Weichun; Hodjaoglu, Feyzim; Vekilov, Peter G

2006-09-01

Several recent theories and simulations have predicted that shear flow could enhance, or, conversely, suppress the nucleation of crystals from solution. Such modulations would offer a pathway for nucleation control and provide a novel explanation for numerous mysteries in nucleation research. For experimental tests of the effects of shear flow on protein crystal nucleation, we found that if a protein solution droplet of approximately 5 microL (2-3 mm diameter at base) is held on a hydrophobic substrate in an enclosed environment and in a quasi-uniform constant electric field of 2 to 6 kV cm(-1), a rotational flow with a maximum rate at the droplet top of approximately 10 microm s(-1) is induced. The shear rate varies from 10(-3) to 10(-1) s(-1). The likely mechanism of the rotational flow involves adsorption of the protein and amphiphylic buffer molecules on the air-water interface and their redistribution in the electric field, leading to nonuniform surface tension of the droplet and surface tension-driven flow. Observations of the number of nucleated crystals in 24- and 72-h experiments with the proteins ferritin, apoferritin, and lysozyme revealed that the crystals are typically nucleated at a certain radius of the droplet, that is, at a preferred shear rate. Variations of the rotational flow velocity resulted in suppression or enhancement of the total number of nucleated crystals of ferritin and apoferritin, while all solution flow rates were found to enhance lysozyme crystal nucleation. These observations show that shear flow may strongly affect nucleation, and that for some systems, an optimal flow velocity, leading to fastest nucleation, exists. Comparison with the predictions of theories and simulations suggest that the formation of ordered nuclei in a "normal" protein solution cannot be affected by such low shear rates. We conclude that the flow acts by helping or suppressing the formation of ordered nuclei within mesoscopic metastable dense liquid clusters. Such clusters were recently shown to exist in protein solutions and to constitute the first step in the nucleation mechanism of many protein and nonproteinsystems.

A physically constrained classical description of the homogeneous nucleation of ice in water.

PubMed

Koop, Thomas; Murray, Benjamin J

2016-12-07

Liquid water can persist in a supercooled state to below 238 K in the Earth's atmosphere, a temperature range where homogeneous nucleation becomes increasingly probable. However, the rate of homogeneous ice nucleation in supercooled water is poorly constrained, in part, because supercooled water eludes experimental scrutiny in the region of the homogeneous nucleation regime where it can exist only fleetingly. Here we present a new parameterization of the rate of homogeneous ice nucleation based on classical nucleation theory. In our approach, we constrain the key terms in classical theory, i.e., the diffusion activation energy and the ice-liquid interfacial energy, with physically consistent parameterizations of the pertinent quantities. The diffusion activation energy is related to the translational self-diffusion coefficient of water for which we assess a range of descriptions and conclude that the most physically consistent fit is provided by a power law. The other key term is the interfacial energy between the ice embryo and supercooled water whose temperature dependence we constrain using the Turnbull correlation, which relates the interfacial energy to the difference in enthalpy between the solid and liquid phases. The only adjustable parameter in our model is the absolute value of the interfacial energy at one reference temperature. That value is determined by fitting this classical model to a selection of laboratory homogeneous ice nucleation data sets between 233.6 K and 238.5 K. On extrapolation to temperatures below 233 K, into a range not accessible to standard techniques, we predict that the homogeneous nucleation rate peaks between about 227 and 231 K at a maximum nucleation rate many orders of magnitude lower than previous parameterizations suggest. This extrapolation to temperatures below 233 K is consistent with the most recent measurement of the ice nucleation rate in micrometer-sized droplets at temperatures of 227-232 K on very short time scales using an X-ray laser technique. In summary, we present a new physically constrained parameterization for homogeneous ice nucleation which is consistent with the latest literature nucleation data and our physical understanding of the properties of supercooled water.

Comparing the ice nucleation efficiencies of ice nucleating substrates to natural mineral dusts

NASA Astrophysics Data System (ADS)

Steinke, Isabelle; Funk, Roger; Höhler, Kristina; Haarig, Moritz; Hoffmann, Nadine; Hoose, Corinna; Kiselev, Alexei; Möhler, Ottmar; Leisner, Thomas

2014-05-01

Mineral dust particles in the atmosphere may act as efficient ice nuclei over a wide range of temperature and relative humidity conditions. The ice nucleation capability of dust particles mostly depends on the particle surface area and the associated physico-chemical surface properties. It has been observed that the surface-related ice nucleation efficiency of different dust particles and mineral species can vary by several orders of magnitude. However, the relation between aerosol surface properties and observed ice nucleation efficiency is still not completely understood due to the large variability of chemical compositions and morphological features. In order to gain a better understanding of small scale freezing processes, we investigated the freezing of several hundreds of small droplets (V=0.4 nl) deposited on materials with reasonably well defined surfaces such as crystalline silicon wafers, graphite and freshly cleaved mica sheets under atmospherically relevant conditions. These substrates are intended to serve as simple model structures compared to the surface of natural aerosol particles. To learn more about the impact of particle morphology on ice nucleation processes, we also investigated micro-structured silicon wafers with prescribed trenches. The ice nucleation efficiencies deduced from these experiments are expressed as ice nucleation active surface site density values. With this approach, the freezing properties of the above-described substrates could be compared to those of natural mineral dusts such as agricultural soil dusts, volcanic ash and fossil diatoms, which have been investigated in AIDA cloud chamber experiments. All tested ice nucleating substrates were consistently less efficient at nucleating ice than the natural mineral dusts. Crystalline silicon only had a negligible influence on the freezing of small droplets, leading to freezing near the homogeneous freezing temperature threshold. Applying surface structures to silicon led to a shift towards heterogeneous freezing. However, the measured ice nucleation active surface site densities were still smaller than those of mineral dusts.

Microbial ice nucleators scavenged from the atmosphere during simulated rain events

NASA Astrophysics Data System (ADS)

Hanlon, Regina; Powers, Craig; Failor, Kevin; Monteil, Caroline L.; Vinatzer, Boris A.; Schmale, David G.

2017-08-01

Rain and snow collected at ground level have been found to contain biological ice nucleators. These ice nucleators have been proposed to have originated in clouds, where they may have participated in the formation of precipitation via ice phase nucleation. We conducted a series of field experiments to test the hypothesis that at least some of the microbial ice nucleators (prokaryotes and eukaryotes) present in rain may not originate in clouds but instead be scavenged from the lower atmosphere by rainfall. Thirty-three simulated rain events were conducted over four months off the side of the Smart Road Bridge in Blacksburg, VA, USA. In each event, sterile water was dispensed over the side of the bridge and recovered in sterile containers in an open fallow agricultural field below (a distance of ∼55 m). Microbes scavenged from the simulated rain events were cultured and their ice nucleation activity was examined. Putative microbial ice nucleators were cultured from 94% (31/33) of the simulated rain events, and represented 1.5% (121/8331) of the total colonies assayed. Putative ice nucleators were subjected to additional droplet freezing assays, and those confirmed through these repeated assays represented 0.4% (34/8331) of the total. Mean CFUs scavenged by simulated rain ranged from 2 to 267 CFUs/mL. Scavenged ice nucleators belong to a number of taxa including the bacterial genera Pseudomonas, Pantoea, and Xanthomonas, and the fungal genera Fusarium, Humicola, and Mortierella. An ice-nucleating strain of the fungal genus Penicillium was also recovered from a volumetric air sampler at the study site. This work expands our knowledge of the scavenging properties of rainfall, and suggests that at least some ice nucleators in natural precipitation events may have been scrubbed from the atmosphere during rainfall, and thus are not likely to be involved in precipitation.

Marine sources of ice nucleating particles: results from phytoplankton cultures and samples collected at sea

NASA Astrophysics Data System (ADS)

Wilbourn, E.; Thornton, D.; Brooks, S. D.; Graff, J.

2016-12-01

The role of marine aerosols as ice nucleating particles is currently poorly understood. Despite growing interest, there are remarkably few ice nucleation measurements on representative marine samples. Here we present results of heterogeneous ice nucleation from laboratory studies and in-situ air and sea water samples collected during NAAMES (North Atlantic Aerosol and Marine Ecosystems Study). Thalassiosira weissflogii (CCMP 1051) was grown under controlled conditions in batch cultures and the ice nucleating activity depended on the growth phase of the cultures. Immersion freezing temperatures of the lab-grown diatoms were determined daily using a custom ice nucleation apparatus cooled at a set rate. Our results show that the age of the culture had a significant impact on ice nucleation temperature, with samples in stationary phase causing nucleation at -19.9 °C, approximately nine degrees warmer than the freezing temperature during exponential growth phase. Field samples gathered during the NAAMES II cruise in May 2016 were also tested for ice nucleating ability. Two types of samples were gathered. Firstly, whole cells were fractionated by size from surface seawater using a BD Biosciences Influx Cell Sorter (BD BS ISD). Secondly, aerosols were generated using the SeaSweep and subsequently size-selected using a PIXE Cascade Impactor. Samples were tested for the presence of ice nucleating particles (INP) using the technique described above. There were significant differences in the freezing temperature of the different samples; of the three sample types the lab-grown cultures tested during stationary phase froze at the warmest temperatures, followed by the SeaSweep samples (-25.6 °C) and the size-fractionated cell samples (-31.3 °C). Differences in ice nucleation ability may be due to size differences between the INP, differences in chemical composition of the sample, or some combination of these two factors. Results will be presented and atmospheric implications discussed.

Nucleation Behavior of Oxygen-Acetylene Torch-Produced Diamond Films

NASA Technical Reports Server (NTRS)

Roberts, F. E.

2003-01-01

A mechanism is presented for the nucleation of diamond in the combustion flame environment. A series of six experiments and two associated simulations provide results from which the mechanism was derived. A substantial portion of the prior literature was reviewed and the data and conclusions from the previous experimenters were found to support the proposed mechanism. The nucleation mechanism builds on the work of previous researchers but presents an approach to nucleation in a detail and direction not fully presented heretofore. This work identifies the gas phase as the controlling environment for the initial formation steps leading to nucleation. The developed mechanism explains some of the difficulty which has been found in producing single crystal epitaxial films. An experiment which modified the initial gas phase precursor using methane and carbon monoxide is presented. Addition of methane into the precursor gases was found to be responsible for pillaring of the films. Atomic force microscopy surface roughness data provides a reasonable look at suppression of nucleation by carbon monoxide. Surface finish data was taken on crystals which were open to the nucleation environment and generally parallel to the substrate surface. The test surfaces were measured as an independent measure of the instantaneous nucleation environent. A gas flow and substrate experiment changed the conditions on the surface of the sample by increasing the gas flow rate while remaining on a consistent point of the atomic constituent diagram, and by changing the carbide potential of the substrate. Two tip modification experiments looked at the behavior of gas phase nucleation by modifying the shape and behavior of the flame plasma in which the diamond nucleation is suspected to occur. Diamond nucleation and growth was additionally examined using a high-velocity oxygen fuel gun and C3H6 as the fuel gas phase precursor with addition of carbon monoxide gas 01 addition of liquid toluene.

Phosphorylation of Ser136 is critical for potent bone sialoprotein-mediated nucleation of hydroxyapatite crystals.

PubMed

Baht, Gurpreet S; O'Young, Jason; Borovina, Antonia; Chen, Hong; Tye, Coralee E; Karttunen, Mikko; Lajoie, Gilles A; Hunter, Graeme K; Goldberg, Harvey A

2010-05-27

Acidic phosphoproteins of mineralized tissues such as bone and dentin are believed to play important roles in HA (hydroxyapatite) nucleation and growth. BSP (bone sialoprotein) is the most potent known nucleator of HA, an activity that is thought to be dependent on phosphorylation of the protein. The present study identifies the role phosphate groups play in mineral formation. Recombinant BSP and peptides corresponding to residues 1-100 and 133-205 of the rat sequence were phosphorylated with CK2 (protein kinase CK2). Phosphorylation increased the nucleating activity of BSP and BSP-(133-205), but not BSP-(1-100). MS analysis revealed that the major site phosphorylated within BSP-(133-205) was Ser136, a site adjacent to the series of contiguous glutamate residues previously implicated in HA nucleation. The critical role of phosphorylated Ser136 in HA nucleation was confirmed by site-directed mutagenesis and functional analyses. Furthermore, peptides corresponding to the 133-148 sequence of rat BSP were synthesized with or without a phosphate group on Ser136. As expected, the phosphopeptide was a more potent nucleator. The mechanism of nucleation was investigated using molecular-dynamics simulations analysing BSP-(133-148) interacting with the {100} crystal face of HA. Both phosphorylated and non-phosphorylated sequences adsorbed to HA in extended conformations with alternating residues in contact with and facing away from the crystal face. However, this alternating-residue pattern was more pronounced when Ser136 was phosphorylated. These studies demonstrate a critical role for Ser136 phosphorylation in BSP-mediated HA nucleation and identify a unique mode of interaction between the nucleating site of the protein and the {100} face of HA.

A new temperature- and humidity-dependent surface site density approach for deposition ice nucleation

NASA Astrophysics Data System (ADS)

Steinke, I.; Hoose, C.; Möhler, O.; Connolly, P.; Leisner, T.

2015-04-01

Deposition nucleation experiments with Arizona Test Dust (ATD) as a surrogate for mineral dusts were conducted at the AIDA cloud chamber at temperatures between 220 and 250 K. The influence of the aerosol size distribution and the cooling rate on the ice nucleation efficiencies was investigated. Ice nucleation active surface site (INAS) densities were calculated to quantify the ice nucleation efficiency as a function of temperature, humidity and the aerosol surface area concentration. Additionally, a contact angle parameterization according to classical nucleation theory was fitted to the experimental data in order to relate the ice nucleation efficiencies to contact angle distributions. From this study it can be concluded that the INAS density formulation is a very useful tool to describe the temperature- and humidity-dependent ice nucleation efficiency of ATD particles. Deposition nucleation on ATD particles can be described by a temperature- and relative-humidity-dependent INAS density function ns(T, Sice) with ns(xtherm) = 1.88 ×105 · exp(0.2659 · xtherm) [m-2] , (1) where the temperature- and saturation-dependent function xtherm is defined as xtherm = -(T-273.2)+(Sice-1) ×100, (2) with the saturation ratio with respect to ice Sice >1 and within a temperature range between 226 and 250 K. For lower temperatures, xtherm deviates from a linear behavior with temperature and relative humidity over ice. Also, two different approaches for describing the time dependence of deposition nucleation initiated by ATD particles are proposed. Box model estimates suggest that the time-dependent contribution is only relevant for small cooling rates and low number fractions of ice-active particles.

Nano scale dynamics of bubble nucleation in confined liquid subjected to rapid cooling: Effect of solid-liquid interfacial wettability

NASA Astrophysics Data System (ADS)

Hasan, Mohammad Nasim; Rabbi, Kazi Fazle; Mukut, K. M.; Tamim, Saiful Islam; Faisal, A. H. M.

2017-06-01

This study focuses on the occurrence of bubble nucleation in a liquid confined in a nano scale confinement and subjected to rapid cooling at one of its wall. Due to the very small size scale of the present problem, we adopt the molecular dynamics (MD) approach. The liquid (Argon) is confined within two solid (Platinum) walls. The temperature of the upper wall of the confinement is maintained at 90 K while the lower wall is being cooled rapidly to 50 K from initial equilibrium temperature of 90 K within 0.1 ns. This results in the nucleation and formation of nanobubbles in the liquid. The pattern of bubble nucleation has been studied for three different conditions of solid-liquid interfacial wettability such as hydrophilic, hydrophobic and neutral. Behavior of bubble nucleation is significantly different in the three case of solid-liquid interfacial wettability. In case of the hydrophobic confinement (weakly adsorbing), the liquid cannot achieve deeper metastability; vapor layers appear immediately on the walls. In case of the neutral confinement (moderately adsorbing), bubble nucleation is promoted by the walls where the nucleation is heterogeneous. In case of the hydrophilic walls (strongly adsorbing) bubbles are developed inside the liquid; that is the nucleation process is homogeneous. The variation in bubble nucleation under different conditions of surface wettability has been studied by the analysis of number density distribution, spatial temperature distribution, spatial number density distribution and heat flux through the upper and lower walls of the confinement. The present study indicates that the variation of heat transfer efficiency due to different surface wettability has significant effect on the size, shape and location of bubble nucleation in case rapid cooling of liquid in nano confinement.

Calcium carbonate nucleation in an alkaline lake surface water, Pyramid Lake, Nevada, USA

USGS Publications Warehouse

Reddy, Michael M.; Hoch, Anthony

2012-01-01

Calcium concentration and calcite supersaturation (Ω) needed for calcium carbonate nucleation and crystal growth in Pyramid Lake (PL) surface water were determined during August of 1997, 2000, and 2001. PL surface water has Ω values of 10-16. Notwithstanding high Ω, calcium carbonate growth did not occur on aragonite single crystals suspended PL surface water for several months. However, calcium solution addition to PL surface-water samples caused reproducible calcium carbonate mineral nucleation and crystal growth. Mean PL surface-water calcium concentration at nucleation was 2.33 mM (n = 10), a value about nine times higher than the ambient PL surface-water calcium concentration (0.26 mM); mean Ω at nucleation (109 with a standard deviation of 8) is about eight times the PL surface-water Ω. Calcium concentration and Ω regulated the calcium carbonate formation in PL nucleation experiments and surface water. Unfiltered samples nucleated at lower Ω than filtered samples. Calcium concentration and Ω at nucleation for experiments in the presence of added particles were within one standard deviation of the mean for all samples. Calcium carbonate formation rates followed a simple rate expression of the form, rate (mM/min) = A (Ω) + B. The best fit rate equation "Rate (Δ mM/Δ min) = -0.0026 Ω + 0.0175 (r = 0.904, n = 10)" was statistically significant at greater than the 0.01 confidence level and gives, after rearrangement, Ω at zero rate of 6.7. Nucleation in PL surface water and morphology of calcium carbonate particles formed in PL nucleation experiments and in PL surface-water samples suggest crystal growth inhibition by multiple substances present in PL surface water mediates PL calcium carbonate formation, but there is insufficient information to determine the chemical nature of all inhibitors.

Calcium Carbonate Nucleation in an Alkaline Lake Surface Water, Pyramid Lake, Nevada, USA

USGS Publications Warehouse

Reddy, M.M.; Hoch, A.

2012-01-01

Calcium concentration and calcite supersaturation (??) needed for calcium carbonate nucleation and crystal growth in Pyramid Lake (PL) surface water were determined during August of 1997, 2000, and 2001. PL surface water has ?? values of 10-16. Notwithstanding high ??, calcium carbonate growth did not occur on aragonite single crystals suspended PL surface water for several months. However, calcium solution addition to PL surface-water samples caused reproducible calcium carbonate mineral nucleation and crystal growth. Mean PL surface-water calcium concentration at nucleation was 2.33 mM (n = 10), a value about nine times higher than the ambient PL surface-water calcium concentration (0.26 mM); mean ?? at nucleation (109 with a standard deviation of 8) is about eight times the PL surface-water ??. Calcium concentration and ?? regulated the calcium carbonate formation in PL nucleation experiments and surface water. Unfiltered samples nucleated at lower ?? than filtered samples. Calcium concentration and ?? at nucleation for experiments in the presence of added particles were within one standard deviation of the mean for all samples. Calcium carbonate formation rates followed a simple rate expression of the form, rate (mM/min) = A (??) + B. The best fit rate equation "Rate (?? mM/?? min) = -0.0026 ?? + 0.0175 (r = 0.904, n = 10)" was statistically significant at greater than the 0.01 confidence level and gives, after rearrangement, ?? at zero rate of 6.7. Nucleation in PL surface water and morphology of calcium carbonate particles formed in PL nucleation experiments and in PL surface-water samples suggest crystal growth inhibition by multiple substances present in PL surface water mediates PL calcium carbonate formation, but there is insufficient information to determine the chemical nature of all inhibitors. ?? 2011 U.S. Government.

Bacterial Ice Nucleation in Monodisperse D2O and H2O-in-Oil Emulsions.

PubMed

Weng, Lindong; Tessier, Shannon N; Smith, Kyle; Edd, Jon F; Stott, Shannon L; Toner, Mehmet

2016-09-13

Ice nucleation is of fundamental significance in many areas, including atmospheric science, food technology, and cryobiology. In this study, we investigated the ice-nucleation characteristics of picoliter-sized drops consisting of different D2O and H2O mixtures with and without the ice-nucleating bacteria Pseudomonas syringae. We also studied the effects of commonly used cryoprotectants such as ethylene glycol, propylene glycol, and trehalose on the nucleation characteristics of D2O and H2O mixtures. The results show that the median freezing temperature of the suspension containing 1 mg/mL of a lyophilized preparation of P. syringae is as high as -4.6 °C for 100% D2O, compared to -8.9 °C for 100% H2O. As the D2O concentration increases every 25% (v/v), the profile of the ice-nucleation kinetics of D2O + H2O mixtures containing 1 mg/mL Snomax shifts by about 1 °C, suggesting an ideal mixing behavior of D2O and H2O. Furthermore, all of the cryoprotectants investigated in this study are found to depress the freezing phenomenon. Both the homogeneous and heterogeneous freezing temperatures of these aqueous solutions depend on the water activity and are independent of the nature of the solute. These findings enrich our fundamental knowledge of D2O-related ice nucleation and suggest that the combination of D2O and ice-nucleating agents could be a potential self-ice-nucleating formulation. The implications of self-nucleation include a higher, precisely controlled ice seeding temperature for slow freezing that would significantly improve the viability of many ice-assisted cryopreservation protocols.

Ice nucleation efficiency of AgI: review and new insights

NASA Astrophysics Data System (ADS)

Marcolli, Claudia; Nagare, Baban; Welti, André; Lohmann, Ulrike

2016-07-01

AgI is one of the best-investigated ice-nucleating substances. It has relevance for the atmosphere since it is used for glaciogenic cloud seeding. Theoretical and experimental studies over the last 60 years provide a complex picture of silver iodide as an ice-nucleating agent with conflicting and inconsistent results. This review compares experimental ice nucleation studies in order to analyze the factors that influence the ice nucleation ability of AgI. The following picture emerges from this analysis: the ice nucleation ability of AgI seems to be enhanced when the AgI particle is on the surface of a droplet, which is indeed the position that a particle takes when it can freely move in a droplet. The ice nucleation by particles with surfaces exposed to air depends on water adsorption. AgI surfaces seem to be most efficient at nucleating ice when they are exposed to relative humidity at or even above water saturation. For AgI particles that are completely immersed in water, the freezing temperature increases with increasing AgI surface area. Higher threshold freezing temperatures seem to correlate with improved lattice matches as can be seen for AgI-AgCl solid solutions and 3AgI·NH4I·6H2O, which have slightly better lattice matches with ice than AgI and also higher threshold freezing temperatures. However, the effect of a good lattice match is annihilated when the surfaces have charges. Also, the ice nucleation ability seems to decrease during dissolution of AgI particles. This introduces an additional history and time dependence for ice nucleation in cloud chambers with short residence times.

Critical Nuclei Size, Rate, and Activation Energy of H2 Gas Nucleation.

PubMed

German, Sean R; Edwards, Martin A; Ren, Hang; White, Henry S

2018-03-21

Electrochemical measurements of the nucleation rate of individual H 2 bubbles at the surface of Pt nanoelectrodes (radius = 7-41 nm) are used to determine the critical size and geometry of H 2 nuclei leading to stable bubbles. Precise knowledge of the H 2 concentration at the electrode surface, C H 2 surf , is obtained by controlled current reduction of H + in a H 2 SO 4 solution. Induction times of single-bubble nucleation events are measured by stepping the current, to control C H 2 surf , while monitoring the voltage. We find that gas nucleation follows a first-order rate process; a bubble spontaneously nucleates after a stochastic time delay, as indicated by a sudden voltage spike that results from impeded transport of H + to the electrode. Hundreds of individual induction times, at different applied currents and using different Pt nanoelectrodes, are used to characterize the kinetics of phase nucleation. The rate of bubble nucleation increases by four orders of magnitude (0.3-2000 s -1 ) over a very small relative change in C H 2 surf (0.21-0.26 M, corresponding to a ∼0.025 V increase in driving force). Classical nucleation theory yields thermodynamic radii of curvature for critical nuclei of 4.4 to 5.3 nm, corresponding to internal pressures of 330 to 270 atm, and activation energies for nuclei formation of 14 to 26 kT, respectively. The dependence of nucleation rate on H 2 concentration indicates that nucleation occurs by a heterogeneous mechanism, where the nuclei have a contact angle of ∼150° with the electrode surface and contain between 35 and 55 H 2 molecules.

Nonrandom γ-TuNA-dependent spatial pattern of microtubule nucleation at the Golgi.

PubMed

Sanders, Anna A W M; Chang, Kevin; Zhu, Xiaodong; Thoppil, Roslin J; Holmes, William R; Kaverina, Irina

2017-11-07

Noncentrosomal microtubule (MT) nucleation at the Golgi generates MT network asymmetry in motile vertebrate cells. Investigating the Golgi-derived MT (GDMT) distribution, we find that MT asymmetry arises from nonrandom nucleation sites at the Golgi (hotspots). Using computational simulations, we propose two plausible mechanistic models of GDMT nucleation leading to this phenotype. In the "cooperativity" model, formation of a single GDMT promotes further nucleation at the same site. In the "heterogeneous Golgi" model, MT nucleation is dramatically up-regulated at discrete and sparse locations within the Golgi. While MT clustering in hotspots is equally well described by both models, simulating MT length distributions within the cooperativity model fits the data better. Investigating the molecular mechanism underlying hotspot formation, we have found that hotspots are significantly smaller than a Golgi subdomain positive for scaffolding protein AKAP450, which is thought to recruit GDMT nucleation factors. We have further probed potential roles of known GDMT-promoting molecules, including γ-TuRC-mediated nucleation activator (γ-TuNA) domain-containing proteins and MT stabilizer CLASPs. While both γ-TuNA inhibition and lack of CLASPs resulted in drastically decreased GDMT nucleation, computational modeling revealed that only γ-TuNA inhibition suppressed hotspot formation. We conclude that hotspots require γ-TuNA activity, which facilitates clustered GDMT nucleation at distinct Golgi sites. © 2017 Sanders et al. This article is distributed by The American Society for Cell Biology under license from the author(s). Two months after publication it is available to the public under an Attribution–Noncommercial–Share Alike 3.0 Unported Creative Commons License (http://creativecommons.org/licenses/by-nc-sa/3.0).

The carrier gas pressure effect in a laminar flow diffusion chamber, homogeneous nucleation of n-butanol in helium.

PubMed

Hyvärinen, Antti-Pekka; Brus, David; Zdímal, Vladimír; Smolík, Jiri; Kulmala, Markku; Viisanen, Yrjö; Lihavainen, Heikki

2006-06-14

Homogeneous nucleation rate isotherms of n-butanol+helium were measured in a laminar flow diffusion chamber at total pressures ranging from 50 to 210 kPa to investigate the effect of carrier gas pressure on nucleation. Nucleation temperatures ranged from 265 to 280 K and the measured nucleation rates were between 10(2) and 10(6) cm(-3) s(-1). The measured nucleation rates decreased as a function of increasing pressure. The pressure effect was strongest at pressures below 100 kPa. This negative carrier gas effect was also temperature dependent. At nucleation temperature of 280 K and at the same saturation ratio, the maximum deviation between nucleation rates measured at 50 and 210 kPa was about three orders of magnitude. At nucleation temperature of 265 K, the effect was negligible. Qualitatively the results resemble those measured in a thermal diffusion cloud chamber. Also the slopes of the isothermal nucleation rates as a function of saturation ratio were different as a function of total pressure, 50 kPa isotherms yielded the steepest slopes, and 210 kPa isotherms the shallowest slopes. Several sources of inaccuracies were considered in the interpretation of the results: uncertainties in the transport properties, nonideal behavior of the vapor-carrier gas mixture, and shortcomings of the used mathematical model. Operation characteristics of the laminar flow diffusion chamber at both under-and over-pressure were determined to verify a correct and stable operation of the device. We conclude that a negative carrier gas pressure effect is seen in the laminar flow diffusion chamber and it cannot be totally explained with the aforementioned reasons.

Observations on the nucleation of ice VII in compressed water

NASA Astrophysics Data System (ADS)

Stafford, Samuel J. P.; Chapman, David J.; Bland, Simon N.; Eakins, Daniel E.

2017-01-01

Water can freeze upon multiple shock compression, but the window material determines the pressure of the phase transition. Several plate impact experiments were conducted with liquid targets on a single-stage gas gun, diagnosed simultaneously using photonic doppler velocimetry (PDV) and high speed imaging through the water. The experiments investigated why silica windows instigate freezing above 2.5 GPa whilst sapphire windows do not until 7 GPa. We find that the nucleation of ice occurs on the surfaces of windows and can be affected by the surface coating suggesting the surface energy of fused silica, likely due to hydroxyl groups, encourages nucleation of ice VII crystallites. Aluminium coatings prevent nucleation and sapphire surfaces do not nucleate until approximately 6.5 GPa. This is believed to be the threshold pressure for the homogeneous nucleation of water.

Collaborative Research: failure of RockMasses from Nucleation and Growth of Microscopic Defects and Disorder

DOE Office of Scientific and Technical Information (OSTI.GOV)

Klein, William

Over the 21 years of funding we have pursued several projects related to earthquakes, damage and nucleation. We developed simple models of earthquake faults which we studied to understand Gutenburg-Richter scaling, foreshocks and aftershocks, the effect of spatial structure of the faults and its interaction with underlying self organization and phase transitions. In addition we studied the formation of amorphous solids via the glass transition. We have also studied nucleation with a particular concentration on transitions in systems with a spatial symmetry change. In addition we investigated the nucleation process in models that mimic rock masses. We obtained the structuremore » of the droplet in both homogeneous and heterogeneous nucleation. We also investigated the effect of defects or asperities on the nucleation of failure in simple models of earthquake faults.« less

Interfacial Free Energy as the Key to the Pressure-Induced Deceleration of Ice Nucleation

NASA Astrophysics Data System (ADS)

Espinosa, Jorge R.; Zaragoza, Alberto; Rosales-Pelaez, Pablo; Navarro, Caridad; Valeriani, Chantal; Vega, Carlos; Sanz, Eduardo

2016-09-01

The avoidance of water freezing is the holy grail in the cryopreservation of biological samples, food, and organs. Fast cooling rates are used to beat ice nucleation and avoid cell damage. This strategy can be enhanced by applying high pressures to decrease the nucleation rate, but the physics behind this procedure has not been fully understood yet. We perform computer experiments to investigate ice nucleation at high pressures consisting in embedding ice seeds in supercooled water. We find that the slowing down of the nucleation rate is mainly due to an increase of the ice I -water interfacial free energy with pressure. Our work also clarifies the molecular mechanism of ice nucleation for a wide pressure range. This study is not only relevant to cryopreservation, but also to water amorphization and climate change modeling.

Ion-induced nucleation in solution: promotion of solute nucleation in charged levitated droplets.

PubMed

Draper, Neil D; Bakhoum, Samuel F; Haddrell, Allen E; Agnes, George R

2007-09-19

We have investigated the nucleation and growth of sodium chloride in both single quiescent charged droplets and charged droplet populations that were levitated in an electrodynamic levitation trap (EDLT). In both cases, the magnitude of a droplet's net excess charge (ions(DNEC)) influenced NaCl nucleation and growth, albeit in different capacities. We have termed the phenomenon ion-induced nucleation in solution. For single quiescent levitated droplets, an increase in ions(DNEC) resulted in a significant promotion of NaCl nucleation, as determined by the number of crystals observed. For levitated droplet populations, a change in NaCl crystal habit, from regular cubic shapes to dome-shaped dendrites, was observed once a surface charge density threshold of -9 x 10(-4) e.nm(-2) was surpassed. Although promotion of NaCl nucleation was observed for droplet population experiments, this can be attributed in part to the increased rate of solvent evaporation observed for levitated droplet populations having a high net charge. Promotion of nucleation was also observed for two organic acids, 2,4,6-trihydroxyacetophenone monohydrate (THAP) and alpha-cyano-4-hydroxycinnamic acid (CHCA). These results are of direct relevance to processes that occur in both soft-ionization techniques for mass spectrometry and to a variety of industrial processes. To this end, we have demonstrated the use of ion-induced nucleation in solution to form ammonium nitrate particles from levitated droplets to be used in in vitro toxicology studies of ambient particle types.

On the usage of classical nucleation theory in quantification of the impact of bacterial INP on weather and climate

NASA Astrophysics Data System (ADS)

Sahyoun, Maher; Wex, Heike; Gosewinkel, Ulrich; Šantl-Temkiv, Tina; Nielsen, Niels W.; Finster, Kai; Sørensen, Jens H.; Stratmann, Frank; Korsholm, Ulrik S.

2016-08-01

Bacterial ice-nucleating particles (INP) are present in the atmosphere and efficient in heterogeneous ice-nucleation at temperatures up to -2 °C in mixed-phase clouds. However, due to their low emission rates, their climatic impact was considered insignificant in previous modeling studies. In view of uncertainties about the actual atmospheric emission rates and concentrations of bacterial INP, it is important to re-investigate the threshold fraction of cloud droplets containing bacterial INP for a pronounced effect on ice-nucleation, by using a suitable parameterization that describes the ice-nucleation process by bacterial INP properly. Therefore, we compared two heterogeneous ice-nucleation rate parameterizations, denoted CH08 and HOO10 herein, both of which are based on classical-nucleation-theory and measurements, and use similar equations, but different parameters, to an empirical parameterization, denoted HAR13 herein, which considers implicitly the number of bacterial INP. All parameterizations were used to calculate the ice-nucleation probability offline. HAR13 and HOO10 were implemented and tested in a one-dimensional version of a weather-forecast-model in two meteorological cases. Ice-nucleation-probabilities based on HAR13 and CH08 were similar, in spite of their different derivation, and were higher than those based on HOO10. This study shows the importance of the method of parameterization and of the input variable, number of bacterial INP, for accurately assessing their role in meteorological and climatic processes.

How capping protein enhances actin filament growth and nucleation on biomimetic beads.

PubMed

Wang, Ruizhe; Carlsson, Anders E

2015-11-25

Capping protein (CP), which caps the growing ends of actin filaments, accelerates actin-based motility. Recent experiments on biomimetic beads have shown that CP also enhances the rate of actin filament nucleation. Proposed explanations for these phenomena include (i) the actin funneling hypothesis (AFH), in which the presence of CP increases the free-actin concentration, and (ii) the monomer gating model, in which CP binding to actin filament barbed ends makes more monomers available for filament nucleation. To establish how CP increases the rates of filament elongation and nucleation on biomimetic beads, we perform a quantitative modeling analysis of actin polymerization, using rate equations that include actin filament nucleation, polymerization and capping, as modified by monomer depletion near the surface of the bead. With one adjustable parameter, our simulation results match previously measured time courses of polymerized actin and filament number. The results support a version of the AFH where CP increases the local actin monomer concentration at the bead surface, but leaves the global free-actin concentration nearly constant. Because the rate of filament nucleation increases with the monomer concentration, the increased local monomer concentration enhances actin filament nucleation. We derive a closed-form formula for the characteristic CP concentration where the local free-actin concentration reaches half the bulk value, and find it to be comparable to the global Arp2/3 complex concentration. We also propose an experimental protocol for distinguishing branching nucleation of filaments from spontaneous nucleation.

The influence of solution environment on the nucleation kinetics and crystallisability of para-aminobenzoic acid.

PubMed

Turner, T D; Corzo, D M C; Toroz, D; Curtis, A; Dos Santos, M M; Hammond, R B; Lai, X; Roberts, K J

2016-10-05

The influence of solvent type on the solution thermodynamics, nucleation-kinetics and crystal growth of alpha para-aminobenzoic acid (PABA) crystallising from supersaturated ethanol, acetonitrile and water solutions, is examined using poly-thermal analysis of the metastable zone width. Application of a recently proposed model for analysis of crystallisation kinetics (J. Cryst. Growth, 2010, 312, 698-704) indicates a solvent and concentration dependence of the nucleation mechanism and key nucleation parameters for the alpha form of PABA. The mechanism of nucleation is found to change from instantaneous to progressive with decreasing concentration and also when changing the solvent from ethanol to acetonitrile to water. The dependence of the nucleation mechanism is correlated to the kinetic component of the nucleation rate through calculated values of instantaneously nucleated crystallites, which increase from 1.40 × 10 9 m -3 in ethanol to 1.08 × 10 10 m -3 in acetonitrile to 2.58 × 10 10 m -3 in water. This in combination with low calculated number concentrations of interfacial tension between 1.13 and 2.71 mJ m -2 , supports the conclusion that the kinetic component of the nucleation rate is more limiting when crystallising PABA from ethanol solutions in comparison to water solutions. This finding is further supported by molecular dynamics simulations of the solvation free energy of PABA, which is found to be greatest in water, -42.4 kJ mol -1 and lowest in ethanol, -58.5 kJ mol -1 .

Truncated Dual-Cap Nucleation Site Development

NASA Technical Reports Server (NTRS)

Matson, Douglas M.; Sander, Paul J.

2012-01-01

During heterogeneous nucleation within a metastable mushy-zone, several geometries for nucleation site development must be considered. Traditional spherical dual cap and crevice models are compared to a truncated dual cap to determine the activation energy and critical cluster growth kinetics in ternary Fe-Cr-Ni steel alloys. Results of activation energy results indicate that nucleation is more probable at grain boundaries within the solid than at the solid-liquid interface.

Crystal Nucleation in Liquids: Open Questions and Future Challenges in Molecular Dynamics Simulations

PubMed Central

2016-01-01

The nucleation of crystals in liquids is one of nature’s most ubiquitous phenomena, playing an important role in areas such as climate change and the production of drugs. As the early stages of nucleation involve exceedingly small time and length scales, atomistic computer simulations can provide unique insights into the microscopic aspects of crystallization. In this review, we take stock of the numerous molecular dynamics simulations that, in the past few decades, have unraveled crucial aspects of crystal nucleation in liquids. We put into context the theoretical framework of classical nucleation theory and the state-of-the-art computational methods by reviewing simulations of such processes as ice nucleation and the crystallization of molecules in solutions. We shall see that molecular dynamics simulations have provided key insights into diverse nucleation scenarios, ranging from colloidal particles to natural gas hydrates, and that, as a result, the general applicability of classical nucleation theory has been repeatedly called into question. We have attempted to identify the most pressing open questions in the field. We believe that, by improving (i) existing interatomic potentials and (ii) currently available enhanced sampling methods, the community can move toward accurate investigations of realistic systems of practical interest, thus bringing simulations a step closer to experiments. PMID:27228560

Ice nucleation in nature: supercooling point (SCP) measurements and the role of heterogeneous nucleation.

PubMed

Wilson, P W; Heneghan, A F; Haymet, A D J

2003-02-01

In biological systems, nucleation of ice from a supercooled aqueous solution is a stochastic process and always heterogeneous. The average time any solution may remain supercooled is determined only by the degree of supercooling and heterogeneous nucleation sites it encounters. Here we summarize the many and varied definitions of the so-called "supercooling point," also called the "temperature of crystallization" and the "nucleation temperature," and exhibit the natural, inherent width associated with this quantity. We describe a new method for accurate determination of the supercooling point, which takes into account the inherent statistical fluctuations of the value. We show further that many measurements on a single unchanging sample are required to make a statistically valid measure of the supercooling point. This raises an interesting difference in circumstances where such repeat measurements are inconvenient, or impossible, for example for live organism experiments. We also discuss the effect of solutes on this temperature of nucleation. Existing data appear to show that various solute species decrease the nucleation temperature somewhat more than the equivalent melting point depression. For non-ionic solutes the species appears not to be a significant factor whereas for ions the species does affect the level of decrease of the nucleation temperature.

Nucleation in food colloids

NASA Astrophysics Data System (ADS)

Povey, Malcolm J. W.

2016-12-01

Nucleation in food colloids has been studied in detail using ultrasound spectroscopy. Our data show that classical nucleation theory (CNT) remains a sound basis from which to understand nucleation in food colloids and analogous model systems using n-alkanes. Various interpretations and modifications of CNT are discussed with regard to their relevance to food colloids. Much of the evidence presented is based on the ultrasound velocity spectrometry measurements which has many advantages for the study of nucleating systems compared to light scattering and NMR due to its sensitivity at low solid contents and its ability to measure true solid contents in the nucleation and early crystal growth stages. Ultrasound attenuation spectroscopy also responds to critical fluctuations in the induction region. We show, however, that a periodic pressure fluctuation such as a quasi-continuous (as opposed to a pulse comprising only a few pressure cycles) ultrasound field can alter the nucleation process, even at very low acoustic intensity. Thus care must be taken when using ultrasound techniques that the measurements do not alter the studied processes. Quasi-continuous ultrasound fields may enhance or suppress nucleation and the criteria to determine such effects are derived. The conclusions of this paper are relevant to colloidal systems in foods, pharmaceuticals, agro-chemicals, cosmetics, and personal products.

Effect of nucleation on instability of step meandering during step-flow growth on vicinal 3C-SiC (0001) surfaces

NASA Astrophysics Data System (ADS)

Li, Yuan; Chen, Xuejiang; Su, Juan

2017-06-01

A three-dimensional kinetic Monte Carlo (KMC) model has been developed to study the step instability caused by nucleation during the step-flow growth of 3C-SiC. In the model, a lattice mesh was established to fix the position of atoms and bond partners based on the crystal lattice of 3C-SiC. The events considered in the model were adsorption and diffusion of adatoms on the terraces, attachment, detachment and interlayer transport of adatoms at the step edges, and nucleation of adatoms. Then the effects of nucleation on the instability of step meandering and the coalescence of both islands and steps were simulated by the model. The results showed that the instability of step meandering caused by nucleation was affected by the growth temperature. And the effects of nucleation on the instability was also analyzed. Moreover, the surface roughness as a function of time for different temperatures was discussed. Finally, a phase diagram was presented to predict in which conditions the effects of nucleation on step meandering become significant and the three different regimes, the step-flow (SF), 2D nucleation (2DN), and 3D layer by layer (3DLBL) were determined.

The influence of dopants on the nucleation of semiconductor nanocrystals from homogeneous solution.

PubMed

Bryan, J Daniel; Schwartz, Dana A; Gamelin, Daniel R

2005-09-01

The influence of Co2+ ions on the homogeneous nucleation of ZnO is examined. Using electronic absorption spectroscopy as a dopant-specific in-situ spectroscopic probe, Co2+ ions are found to be quantitatively excluded from the ZnO critical nuclei but incorporated nearly statistically in the subsequent growth layers, resulting in crystallites with pure ZnO cores and Zn(1-x)Co(x)O shells. Strong inhibition of ZnO nucleation by Co2+ ions is also observed. These results are explained using the classical nucleation model. Statistical analysis of nucleation inhibition data allows estimation of the critical nucleus size as 25 +/- 4 Zn2+ ions. Bulk calorimetric data allow the activation barrier for ZnO nucleation containing a single Co2+ impurity to be estimated as 5.75 kcal/mol cluster greater than that of pure ZnO, corresponding to a 1.5 x 10(4)-fold reduction in the ZnO nucleation rate constant upon introduction of a single Co2+ impurity. These data and analysis offer a rare view into the role of composition in homogeneous nucleation processes, and specifically address recent experiments targeting formation of semiconductor quantum dots containing single magnetic impurity ions at their precise centers.

Nucleation mode particles with a nonvolatile core in the exhaust of a heavy duty diesel vehicle.

PubMed

Rönkkö, Topi; Virtanen, Annele; Kannosto, Jonna; Keskinen, Jorma; Lappi, Maija; Pirjola, Liisa

2007-09-15

The characteristics of the nucleation mode particles of a Euro IV heavy-duty diesel vehicle exhaust were studied. The NOx and PM emissions of the vehicle were controlled through the use of cooled EGR and high-pressure fuel injection techniques; no exhaust gas after-treatment was used. Particle measurements were performed in vehicle laboratory and on road. Nucleation mode dominated the particle number size distribution in all the tested driving conditions. According to the on-road measurements, the nucleation mode was already formed after 0.7 s residence time in the atmosphere and no significant changes were observed for longer residence times. The nucleation mode was insensitive to the fuel sulfur content, dilution air temperature, and relative humidity. An increase in the dilution ratio decreased the size of the nucleation mode particles. This behavior was observed to be linked to the total hydrocarbon concentration in the diluted sample. In volatility measurements, the nucleation mode particles were observed to have a nonvolatile core with volatile species condensed on it. The results indicate that the nucleation mode particles have a nonvolatile core formed before the dilution process. The core particles have grown because of the condensation of semivolatile material, mainly hydrocarbons, during the dilution.

Non-monotonic variations of the nucleation free energy in a glass-forming ultra-soft particles fluid.

PubMed

Desgranges, Caroline; Delhommelle, Jerome

2018-06-18

Using molecular dynamics simulation, we study the impact of the degree of supercooling on the crystal nucleation of ultra-soft particles, modeled with the Gaussian core potential. Focusing on systems with a high number density, our simulations reveal dramatically different behaviors as the degree of supercooling is varied. In the moderate supercooling regime, crystal nucleation proceeds as expected from classical nucleation theory, with a decrease in the free energy of nucleation, as well as in the size of the critical nucleus, as supercooling is increased. On the other hand, in the large supercooling regime, we observe an unusual reversal of behavior with an increase in the free energy of nucleation and in the critical size, as supercooling is increased. This unexpected result is analyzed in terms of the interplay between the glass transition and the crystal nucleation process. Specifically, medium range order crystal-like domains, with structural features different from that of the crystal nucleus, are found to form throughout the system when the supercooling is very large. These, in turn, play a pivotal role in the increase in the free energy of nucleation, as well as in the critical size, as the temperature gets closer to the glass transition.

Nucleation barrier reconstruction via the seeding method in a lattice model with competing nucleation pathways.

PubMed

Lifanov, Yuri; Vorselaars, Bart; Quigley, David

2016-12-07

We study a three-species analogue of the Potts lattice gas model of nucleation from solution in a regime where partially disordered solute is a viable thermodynamic phase. Using a multicanonical sampling protocol, we compute phase diagrams for the system, from which we determine a parameter regime where the partially disordered phase is metastable almost everywhere in the temperature-fugacity plane. The resulting model shows non-trivial nucleation and growth behaviour, which we examine via multidimensional free energy calculations. We consider the applicability of the model in capturing the multi-stage nucleation mechanisms of polymorphic biominerals (e.g., CaCO 3 ). We then quantitatively explore the kinetics of nucleation in our model using the increasingly popular "seeding" method. We compare the resulting free energy barrier heights to those obtained via explicit free energy calculations over a wide range of temperatures and fugacities, carefully considering the propagation of statistical error. We find that the ability of the "seeding" method to reproduce accurate free energy barriers is dependent on the degree of supersaturation, and severely limited by the use of a nucleation driving force Δμ computed for bulk phases. We discuss possible reasons for this in terms of underlying kinetic assumptions, and those of classical nucleation theory.

Crystal Nucleation in Liquids: Open Questions and Future Challenges in Molecular Dynamics Simulations.

PubMed

Sosso, Gabriele C; Chen, Ji; Cox, Stephen J; Fitzner, Martin; Pedevilla, Philipp; Zen, Andrea; Michaelides, Angelos

2016-06-22

The nucleation of crystals in liquids is one of nature's most ubiquitous phenomena, playing an important role in areas such as climate change and the production of drugs. As the early stages of nucleation involve exceedingly small time and length scales, atomistic computer simulations can provide unique insights into the microscopic aspects of crystallization. In this review, we take stock of the numerous molecular dynamics simulations that, in the past few decades, have unraveled crucial aspects of crystal nucleation in liquids. We put into context the theoretical framework of classical nucleation theory and the state-of-the-art computational methods by reviewing simulations of such processes as ice nucleation and the crystallization of molecules in solutions. We shall see that molecular dynamics simulations have provided key insights into diverse nucleation scenarios, ranging from colloidal particles to natural gas hydrates, and that, as a result, the general applicability of classical nucleation theory has been repeatedly called into question. We have attempted to identify the most pressing open questions in the field. We believe that, by improving (i) existing interatomic potentials and (ii) currently available enhanced sampling methods, the community can move toward accurate investigations of realistic systems of practical interest, thus bringing simulations a step closer to experiments.

Preparation of Hybrid Nanoparticle Nucleating Agents and Their Effects on the Crystallization Behavior of Poly(ethylene terephthalate)

PubMed Central

Han, Zhenzhen; Wang, Yao; Wang, Jiuxing; Wang, Shichao; Zhuang, Hongwei; Liu, Jixian; Huang, Linjun; Wang, Yanxin; Wang, Wei; Belfiore, Laurence A.; Tang, Jianguo

2018-01-01

In this research contribution, the primary objective was to enhance the crystallization behavior of poly(ethylene terephthalate) (PET). To accomplish this tack, three kinds of new nucleating agents SiO2-diethylene glycol-LMPET (PET-3), SiO2-triethylene glycol–LMPET(PET-4) and SiO2-tetraethylene glycol-LMPET (PET-5) nucleating agents were prepared via grafting different oligomers (diethylene glycol; triethylene glycol and tetraethylene glycol) to the surface of nano-SiO2 and then linking to the low molecular weight poly(ethylene terephthalate) (LMPET). These nano-particle nucleating agents facilitated the crystallization of PET. Differential scanning calorimetry (DSC) studies of the composites that pure PET blended with PET-3, PET-4 and PET-5 indicated that the longer ethoxy segment in the nucleating agents exhibited (i) higher degrees of crystallinity; (ii) faster rates of crystallization; and (iii) higher crystallization temperatures. The Jeziorny method was employed to analyze the non-isothermal crystallization kinetics of the composites. These works demonstrated that the PET-3, PET-4 and PET-5 were attractive nucleating agents for poly(ethylene terephthalate), and the longer the chain length of the ethoxy segment in the nucleating agents, the more efficient the nucleation effect. PMID:29641456

Theory of anomalous critical-cluster content in high-pressure binary nucleation.

PubMed

Kalikmanov, V I; Labetski, D G

2007-02-23

Nucleation experiments in binary (a-b) mixtures, when component a is supersaturated and b (carrier gas) is undersaturated, reveal that for some mixtures at high pressures the a content of the critical cluster dramatically decreases with pressure contrary to expectations based on classical nucleation theory. We show that this phenomenon is a manifestation of the dominant role of the unlike interactions at high pressures resulting in the negative partial molar volume of component a in the vapor phase beyond the compensation pressure. The analysis is based on the pressure nucleation theorem for multicomponent systems which is invariant to a nucleation model.

Verification of an Analytical Method for Measuring Crystal Nucleation Rates in Glasses from DTA Data

NASA Technical Reports Server (NTRS)

Ranasinghe, K. S.; Wei, P. F.; Kelton, K. F.; Ray, C. S.; Day, D. E.

2004-01-01

A recently proposed analytical (DTA) method for estimating the nucleation rates in glasses has been evaluated by comparing experimental data with numerically computed nucleation rates for a model lithium disilicate glass. The time and temperature dependent nucleation rates were predicted using the model and compared with those values from an analysis of numerically calculated DTA curves. The validity of the numerical approach was demonstrated earlier by a comparison with experimental data. The excellent agreement between the nucleation rates from the model calculations and fiom the computer generated DTA data demonstrates the validity of the proposed analytical DTA method.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Radhakrishnan, Bala; Gorti, Sarma; Babu, Suresh Sudharsanam

Here, we present phase field simulations incorporating energy contributions due to thermodynamics, and anisotropic interfacial and strain energies, to demonstrate the nucleation and growth of multiple variants of alpha from beta in Ti-6Al-4V under isothermal conditions. The simulations focused on the effect of thermodynamic driving force and nucleation rate on the morphology of the transformed alpha assuming that the partitioning of V between beta and alpha is negligible for short isothermal holds. The results indicate that a high nucleation rate favors the formation of the basket-weave structure. However, at a lower nucleation rate the simulations show the intragranular nucleation ofmore » a colony structure by an autocatalytic nucleation mechanism adjacent to a pre-existing alpha variant. New side-plates of the same variant appear to nucleate progressively and grow to form the colony. The isothermal simulation results are used to offer a possible explanation for the transition from a largely basket weave structure to a colony structure inside narrow layer bands occurring during continuous heating and cooling conditions encountered during laser additive manufacturing of Ti-6Al-4V.« less

Understanding the Growth Mechanism of GaN Epitaxial Layers on Mechanically Exfoliated Graphite

NASA Astrophysics Data System (ADS)

Li, Tianbao; Liu, Chenyang; Zhang, Zhe; Yu, Bin; Dong, Hailiang; Jia, Wei; Jia, Zhigang; Yu, Chunyan; Gan, Lin; Xu, Bingshe; Jiang, Haiwei

2018-04-01

The growth mechanism of GaN epitaxial layers on mechanically exfoliated graphite is explained in detail based on classic nucleation theory. The number of defects on the graphite surface can be increased via O-plasma treatment, leading to increased nucleation density on the graphite surface. The addition of elemental Al can effectively improve the nucleation rate, which can promote the formation of dense nucleation layers and the lateral growth of GaN epitaxial layers. The surface morphologies of the nucleation layers, annealed layers and epitaxial layers were characterized by field-emission scanning electron microscopy, where the evolution of the surface morphology coincided with a 3D-to-2D growth mechanism. High-resolution transmission electron microscopy was used to characterize the microstructure of GaN. Fast Fourier transform diffraction patterns showed that cubic phase (zinc-blend structure) GaN grains were obtained using conventional GaN nucleation layers, while the hexagonal phase (wurtzite structure) GaN films were formed using AlGaN nucleation layers. Our work opens new avenues for using highly oriented pyrolytic graphite as a substrate to fabricate transferable optoelectronic devices.

Modelling heterogeneous ice nucleation on mineral dust and soot with parameterizations based on laboratory experiments

NASA Astrophysics Data System (ADS)

Hoose, C.; Hande, L. B.; Mohler, O.; Niemand, M.; Paukert, M.; Reichardt, I.; Ullrich, R.

2016-12-01

Between 0 and -37°C, ice formation in clouds is triggered by aerosol particles acting as heterogeneous ice nuclei. At lower temperatures, heterogeneous ice nucleation on aerosols can occur at lower supersaturations than homogeneous freezing of solutes. In laboratory experiments, the ability of different aerosol species (e.g. desert dusts, soot, biological particles) has been studied in detail and quantified via various theoretical or empirical parameterization approaches. For experiments in the AIDA cloud chamber, we have quantified the ice nucleation efficiency via a temperature- and supersaturation dependent ice nucleation active site density. Here we present a new empirical parameterization scheme for immersion and deposition ice nucleation on desert dust and soot based on these experimental data. The application of this parameterization to the simulation of cirrus clouds, deep convective clouds and orographic clouds will be shown, including the extension of the scheme to the treatment of freezing of rain drops. The results are compared to other heterogeneous ice nucleation schemes. Furthermore, an aerosol-dependent parameterization of contact ice nucleation is presented.

Experimental investigation on the phenomena around the onset nucleate boiling during the impacting of a droplet on the hot surface

NASA Astrophysics Data System (ADS)

Mitrakusuma, Windy H.; Deendarlianto, Kamal, Samsul; Indarto, Nuriyadi, M.

2016-06-01

Onset of nucleate boiling of a droplet when impacted onto hot surface was investigated. Three kinds of surfaces, normal stainless steel (NSS), stainless steel with TiO2 coating (UVN), and stainless steel with TiO2 coating and radiated by ultraviolet ray were employed to examine the effect of wettability. The droplet size was 2.4 mm diameter, and dropped under different We number. The image is generated by high speed camera with the frame speed of 1000 fps. The boiling conditions are identified as natural convection, nucleate boiling, critical heat flux, transition, and film boiling. In the present report, the discussion will be focused on the beginning of nucleate boiling on the droplet. Nucleate boiling occurs when bubbles are generated. These bubbles are probably caused by nucleation on the impurities within the liquid rather than at nucleation sites on the heated surface because the bubbles appear to be in the bulk of the liquid instead of at the liquid-solid interface. In addition, the smaller the contact angle, the fastest the boiling.

Geometrical and Mechanical Properties Control Actin Filament Organization

PubMed Central

Ennomani, Hajer; Théry, Manuel; Nedelec, Francois; Blanchoin, Laurent

2015-01-01

The different actin structures governing eukaryotic cell shape and movement are not only determined by the properties of the actin filaments and associated proteins, but also by geometrical constraints. We recently demonstrated that limiting nucleation to specific regions was sufficient to obtain actin networks with different organization. To further investigate how spatially constrained actin nucleation determines the emergent actin organization, we performed detailed simulations of the actin filament system using Cytosim. We first calibrated the steric interaction between filaments, by matching, in simulations and experiments, the bundled actin organization observed with a rectangular bar of nucleating factor. We then studied the overall organization of actin filaments generated by more complex pattern geometries used experimentally. We found that the fraction of parallel versus antiparallel bundles is determined by the mechanical properties of actin filament or bundles and the efficiency of nucleation. Thus nucleation geometry, actin filaments local interactions, bundle rigidity, and nucleation efficiency are the key parameters controlling the emergent actin architecture. We finally simulated more complex nucleation patterns and performed the corresponding experiments to confirm the predictive capabilities of the model. PMID:26016478

Understanding the Growth Mechanism of GaN Epitaxial Layers on Mechanically Exfoliated Graphite.

PubMed

Li, Tianbao; Liu, Chenyang; Zhang, Zhe; Yu, Bin; Dong, Hailiang; Jia, Wei; Jia, Zhigang; Yu, Chunyan; Gan, Lin; Xu, Bingshe; Jiang, Haiwei

2018-04-27

The growth mechanism of GaN epitaxial layers on mechanically exfoliated graphite is explained in detail based on classic nucleation theory. The number of defects on the graphite surface can be increased via O-plasma treatment, leading to increased nucleation density on the graphite surface. The addition of elemental Al can effectively improve the nucleation rate, which can promote the formation of dense nucleation layers and the lateral growth of GaN epitaxial layers. The surface morphologies of the nucleation layers, annealed layers and epitaxial layers were characterized by field-emission scanning electron microscopy, where the evolution of the surface morphology coincided with a 3D-to-2D growth mechanism. High-resolution transmission electron microscopy was used to characterize the microstructure of GaN. Fast Fourier transform diffraction patterns showed that cubic phase (zinc-blend structure) GaN grains were obtained using conventional GaN nucleation layers, while the hexagonal phase (wurtzite structure) GaN films were formed using AlGaN nucleation layers. Our work opens new avenues for using highly oriented pyrolytic graphite as a substrate to fabricate transferable optoelectronic devices.

Control of Protein Crystal Nucleation and Growth Using Stirring Solution

NASA Astrophysics Data System (ADS)

Niino, Ai; Adachi, Hiroaki; Takano, Kazufumi; Matsumura, Hiroyoshi; Kinoshita, Takayoshi; Warizaya, Masaichi; Inoue, Tsuyoshi; Mori, Yusuke; Sasaki, Takatomo

2004-11-01

We have previously developed a protein crystallization technique using a stirring protein solution and revealed that (i) continuous stirring prevents excess spontaneous nucleation and accelerates the growth of protein crystals and (ii) prestirring (solution stirring in advance) promotes the crystal nucleation of hen egg-white lysozyme. In bovine adenosine deaminase (ADA) crystallization, continuous stirring improves the crystal quality but elongates the nucleation time. In this paper, in order to control both the crystal nucleation and growth of ADA using a Micro-Stirring technique, we carried out five different stirring patterns such as (i) no stirring, (ii) continuous stirring, (iii) prestirring, (iv) poststirring (stirring late in the growth period) and (v) restirring (combined pre- and poststirring). The results showed that high-quality well-shaped crystals were obtained under the continuous stirring and restirring conditions and the nucleation time under the prestirring and restirring conditions was shorter than that under the continuous stirring and poststirring conditions. Consequently, high-quality crystals were promptly obtained under the restirring condition. These results suggest that we are able to control both the nucleation and growth of protein crystals with the stirring techniques.

Novel polypropylene/inorganic fullerene-like WS2 nanocomposites containing a β-nucleating agent: isothermal crystallization and melting behavior.

PubMed

Naffakh, Mohammed; Marco, Carlos; Ellis, Gary

2012-02-16

The isothermal crystallization and subsequent melting behavior of isotactic polypropylene (iPP) nucleated with different nucleating agents (NAs) are investigated. Tungsten disulfide (IF-WS(2)) and N,N'-dicyclohexyl-2,6-naphthalene (NJ) and dual-additive mixtures are introduced into an iPP matrix to generate new materials that exhibit variable α- and β-polymorphism. As shown in previous work, small amounts of IF-WS(2) or NJ have a nucleating effect during the crystallization of iPP. However, the isothermal crystallization and melting behavior of iPP nucleated by dual α(IF-WS(2))/β(NJ) additive systems are dependent on both the NA composition balance and the crystallization temperature. In particular, our results demonstrate that it is possible to obtain any α-phase to β-phase content ratio by controlling the composition of NAs under appropriate isothermal crystallization conditions. The nucleating behavior of the additives can be illustrated by competitive nucleation, and the correlation between crystallization and melting temperatures and relative α- and β-crystals content in iPP in the nanocomposites is discussed.

Mto2 multisite phosphorylation inactivates non-spindle microtubule nucleation complexes during mitosis

PubMed Central

Borek, Weronika E.; Groocock, Lynda M.; Samejima, Itaru; Zou, Juan; de Lima Alves, Flavia; Rappsilber, Juri; Sawin, Kenneth E.

2015-01-01

Microtubule nucleation is highly regulated during the eukaryotic cell cycle, but the underlying molecular mechanisms are largely unknown. During mitosis in fission yeast Schizosaccharomyces pombe, cytoplasmic microtubule nucleation ceases simultaneously with intranuclear mitotic spindle assembly. Cytoplasmic nucleation depends on the Mto1/2 complex, which binds and activates the γ-tubulin complex and also recruits the γ-tubulin complex to both centrosomal (spindle pole body) and non-centrosomal sites. Here we show that the Mto1/2 complex disassembles during mitosis, coincident with hyperphosphorylation of Mto2 protein. By mapping and mutating multiple Mto2 phosphorylation sites, we generate mto2-phosphomutant strains with enhanced Mto1/2 complex stability, interaction with the γ-tubulin complex and microtubule nucleation activity. A mutant with 24 phosphorylation sites mutated to alanine, mto2[24A], retains interphase-like behaviour even in mitotic cells. This provides a molecular-level understanding of how phosphorylation ‘switches off' microtubule nucleation complexes during the cell cycle and, more broadly, illuminates mechanisms regulating non-centrosomal microtubule nucleation. PMID:26243668

Process and Formulation Effects on Protein Structure in Lyophilized Solids using Mass Spectrometric Methods

PubMed Central

Iyer, Lavanya K.; Sacha, Gregory A.; Moorthy, Balakrishnan S.; Nail, Steven L.; Topp, Elizabeth M.

2016-01-01

Myoglobin (Mb) was lyophilized in the absence (Mb-A) and presence (Mb-B) of sucrose in a pilot-scale lyophilizer with or without controlled ice nucleation. Cake morphology was characterized using scanning electron microscopy (SEM) and changes in protein structure were monitored using solid-state Fourier-transform infrared spectroscopy (ssFTIR), solid-state hydrogen-deuterium exchange-mass spectrometry (ssHDX-MS) and solid-state photolytic labeling-mass spectrometry (ssPL-MS). The results showed greater variability in nucleation temperature and irregular cake structure for formulations lyophilized without controlled nucleation. Controlled nucleation resulted in nucleation at ~ −5 °C and uniform cake structure. Formulations containing sucrose showed better retention of protein structure by all measures than formulations without sucrose. Samples lyophilized with and without controlled nucleation were similar by most measures of protein structure. However, ssPL-MS showed the greatest pLeu incorporation and more labeled regions for Mb-B lyophilized with controlled nucleation. The data support the use of ssHDX-MS and ssPL-MS to study formulation and process-induced conformational changes in lyophilized proteins. PMID:27044943

Reduced Ultrafine Particle Concentration in Urban Air: Changes in Nucleation and Anthropogenic Emissions.

PubMed

Saha, Provat K; Robinson, Ellis S; Shah, Rishabh U; Zimmerman, Naomi; Apte, Joshua S; Robinson, Allen L; Presto, Albert A

2018-06-19

Nucleation is an important source of ambient ultrafine particles (UFP). We present observational evidence of the changes in the frequency and intensity of nucleation events in urban air by analyzing long-term particle size distribution measurements at an urban background site in Pittsburgh, Pennsylvania during 2001-2002 and 2016-2017. We find that both frequency and intensity of nucleation events have been reduced by 40-50% over the past 15 years, resulting in a 70% reduction in UFP concentrations from nucleation. On average, the particle growth rates are 30% slower than 15 years ago. We attribute these changes to dramatic reductions in SO 2 (more than 90%) and other pollutant concentrations. Overall, UFP concentrations in Pittsburgh have been reduced by ∼48% in the past 15 years, with a ∼70% reduction in nucleation, ∼27% in weekday local sources (e.g., weekday traffic), and 49% in the regional background. Our results highlight that a reduction in anthropogenic emissions can considerably reduce nucleation events and UFP concentrations in a polluted urban environment.

Current state of aerosol nucleation parameterizations for air-quality and climate modeling

NASA Astrophysics Data System (ADS)

Semeniuk, Kirill; Dastoor, Ashu

2018-04-01

Aerosol nucleation parameterization models commonly used in 3-D air quality and climate models have serious limitations. This includes classical nucleation theory based variants, empirical models and other formulations. Recent work based on detailed and extensive laboratory measurements and improved quantum chemistry computation has substantially advanced the state of nucleation parameterizations. In terms of inorganic nucleation involving BHN and THN including ion effects these new models should be considered as worthwhile replacements for the old models. However, the contribution of organic species to nucleation remains poorly quantified. New particle formation consists of a distinct post-nucleation growth regime which is characterized by a strong Kelvin curvature effect and is thus dependent on availability of very low volatility organic species or sulfuric acid. There have been advances in the understanding of the multiphase chemistry of biogenic and anthropogenic organic compounds which facilitate to overcome the initial aerosol growth barrier. Implementation of processes influencing new particle formation is challenging in 3-D models and there is a lack of comprehensive parameterizations. This review considers the existing models and recent innovations.

Cirrus Parcel Model Comparison Project. Phase 1; The Critical Components to Simulate Cirrus Initiation Explicitly

NASA Technical Reports Server (NTRS)

Lin, Ruei-Fong; Starr, David OC; DeMott, Paul J.; Cotton, Richard; Sassen, Kenneth; Jensen, Eric; Einaudi, Franco (Technical Monitor)

2001-01-01

The Cirrus Parcel Model Comparison Project, a project of the GCSS (GEWEX Cloud System Studies) Working Group on Cirrus Cloud Systems, involves the systematic comparison of current models of ice crystal nucleation and growth for specified, typical, cirrus cloud environments. In Phase I of the project reported here, simulated cirrus cloud microphysical properties are compared for situations of "warm" (40 C) and "cold" (-60 C) cirrus, both subject to updrafts of 4, 20 and 100 centimeters per second. Five models participated. The various models employ explicit microphysical schemes wherein the size distribution of each class of particles (aerosols and ice crystals) is resolved into bins or treated separately. Simulations are made including both the homogeneous and heterogeneous ice nucleation mechanisms. A single initial aerosol population of sulfuric acid particles is prescribed for all simulations. To isolate the treatment of the homogeneous freezing (of haze droplets) nucleation process, the heterogeneous nucleation mechanism is disabled for a second parallel set of simulations. Qualitative agreement is found for the homogeneous-nucleation- only simulations, e.g., the number density of nucleated ice crystals increases with the strength of the prescribed updraft. However, significant quantitative differences are found. Detailed analysis reveals that the homogeneous nucleation rate, haze particle solution concentration, and water vapor uptake rate by ice crystal growth (particularly as controlled by the deposition coefficient) are critical components that lead to differences in predicted microphysics. Systematic bias exists between results based on a modified classical theory approach and models using an effective freezing temperature approach to the treatment of nucleation. Each approach is constrained by critical freezing data from laboratory studies, but each includes assumptions that can only be justified by further laboratory research. Consequently, it is not yet clear if the two approaches can be made consistent. Large haze particles may deviate considerably from equilibrium size in moderate to strong updrafts (20-100 centimeters per second) at -60 C when the commonly invoked equilibrium assumption is lifted. The resulting difference in particle-size- dependent solution concentration of haze particles may significantly affect the ice particle formation rate during the initial nucleation interval. The uptake rate for water vapor excess by ice crystals is another key component regulating the total number of nucleated ice crystals. This rate, the product of particle number concentration and ice crystal diffusional growth rate, which is particularly sensitive to the deposition coefficient when ice particles are small, modulates the peak particle formation rate achieved in an air parcel and the duration of the active nucleation time period. The effects of heterogeneous nucleation are most pronounced in weak updraft situations. Vapor competition by the heterogeneously nucleated ice crystals may limit the achieved ice supersaturation and thus suppresses the contribution of homogeneous nucleation. Correspondingly, ice crystal number density is markedly reduced. Definitive laboratory and atmospheric benchmark data are needed for the heterogeneous nucleation process. Inter-model differences are correspondingly greater than in the case of the homogeneous nucleation process acting alone.

Diamond Nucleation Using Polyethene

NASA Technical Reports Server (NTRS)

Morell, Gerardo (Inventor); Makarov, Vladimir (Inventor); Varshney, Deepak (Inventor); Weiner, Brad (Inventor)

2013-01-01

The invention presents a simple, non-destructive and non-abrasive method of diamond nucleation using polyethene. It particularly describes the nucleation of diamond on an electrically viable substrate surface using polyethene via chemical vapor deposition (CVD) technique in a gaseous environment.

Diamond nucleation using polyethene

DOEpatents

Morell, Gerardo; Makarov, Vladimir; Varshney, Deepak; Weiner, Brad

2013-07-23

The invention presents a simple, non-destructive and non-abrasive method of diamond nucleation using polyethene. It particularly describes the nucleation of diamond on an electrically viable substrate surface using polyethene via chemical vapor deposition (CVD) technique in a gaseous environment.

Fracture of Rolled Homogeneous Steel Armor (Nucleation Threshold Stress).

DTIC Science & Technology

1980-01-01

AD-AO81 618 ARMY ARMAMENT RESEARCH AND DEVELOPMENT COMMAND ABERD--ETC F/B 19/4 FRACTURE OF ROLLED HOMOGENEOUS STEEL ARMOR (NUCLEATION THRESHOL--ETC(U...ARBRL-MR-02984A QQ FRACTURE OF ROLLED HOMOGENEOUS STEEL ARMOR (NUCLEATION THRESHOLD STRESS) Gerald L Moss Lynn SeamanLy~ S, ,.DTIC S ELECTED January...nucleation stress, Crack threshold stress, Fracture, Fracture stress, Spallation, Armor, Rolled homogeneous steel armor M~ AS$TRACr (Vita ssf -- ebb

Improved performance of GaN based light emitting diodes with ex-situ sputtered AlN nucleation layers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chen, Shuo-Wei; Epistar Corporation, Hsinchu 300, Taiwan; Li, Heng

The crystal quality, electrical and optical properties of GaN based light emitting diodes (LEDs) with ex-situ sputtered physical vapor deposition (PVD) aluminum nitride (AlN) nucleation layers were investigated. It was found that the crystal quality in terms of defect density and x-ray diffraction linewidth was greatly improved in comparison to LEDs with in-situ low temperature GaN nucleation layer. The light output power was 3.7% increased and the reverse bias voltage of leakage current was twice on LEDs with ex-situ PVD AlN nucleation layers. However, larger compressive strain was discovered in LEDs with ex-situ PVD AlN nucleation layers. The study showsmore » the potential and constrain in applying ex-situ PVD AlN nucleation layers to fabricate high quality GaN crystals in various optoelectronics.« less

Nanometer-scale mapping of irreversible electrochemical nucleation processes on solid Li-ion electrolytes

NASA Astrophysics Data System (ADS)

Kumar, Amit; Arruda, Thomas M.; Tselev, Alexander; Ivanov, Ilia N.; Lawton, Jamie S.; Zawodzinski, Thomas A.; Butyaev, Oleg; Zayats, Sergey; Jesse, Stephen; Kalinin, Sergei V.

2013-04-01

Electrochemical processes associated with changes in structure, connectivity or composition typically proceed via new phase nucleation with subsequent growth of nuclei. Understanding and controlling reactions requires the elucidation and control of nucleation mechanisms. However, factors controlling nucleation kinetics, including the interplay between local mechanical conditions, microstructure and local ionic profile remain inaccessible. Furthermore, the tendency of current probing techniques to interfere with the original microstructure prevents a systematic evaluation of the correlation between the microstructure and local electrochemical reactivity. In this work, the spatial variability of irreversible nucleation processes of Li on a Li-ion conductive glass-ceramics surface is studied with ~30 nm resolution. An increased nucleation rate at the boundaries between the crystalline AlPO4 phase and amorphous matrix is observed and attributed to Li segregation. This study opens a pathway for probing mechanisms at the level of single structural defects and elucidation of electrochemical activities in nanoscale volumes.

How does spallation microdamage nucleate in bulk amorphous alloys under shock loading?

NASA Astrophysics Data System (ADS)

Huang, X.; Ling, Z.; Zhang, H. S.; Ma, J.; Dai, L. H.

2011-11-01

Specially designed plate-impact experiments have been conducted on a Zr-based amorphous alloy using a single-stage light gas gun. To understand the microdamage nucleation process in the material, the samples are subjected to dynamic tensile loadings of identical amplitude (˜ 3.18 GPa) but with different durations (83-201 ns). A cellular pattern with an equiaxed shape is observed on the spallation surface, which shows that spallation in the tested amorphous alloy is a typical ductile fracture and that microvoids have been nucleated during the process. Based on the observed fracture morphologies of the spallation surface and free-volume theory, we propose a microvoid nucleation model of bulk amorphous alloys. It is found that nucleation of microvoids at the early stage of spallation in amorphous alloys results from diffusion and coalescence of free volume, and that high mean tensile stress plays a dominant role in microvoid nucleation.

Nanometer-scale mapping of irreversible electrochemical nucleation processes on solid Li-ion electrolytes.

PubMed

Kumar, Amit; Arruda, Thomas M; Tselev, Alexander; Ivanov, Ilia N; Lawton, Jamie S; Zawodzinski, Thomas A; Butyaev, Oleg; Zayats, Sergey; Jesse, Stephen; Kalinin, Sergei V

2013-01-01

Electrochemical processes associated with changes in structure, connectivity or composition typically proceed via new phase nucleation with subsequent growth of nuclei. Understanding and controlling reactions requires the elucidation and control of nucleation mechanisms. However, factors controlling nucleation kinetics, including the interplay between local mechanical conditions, microstructure and local ionic profile remain inaccessible. Furthermore, the tendency of current probing techniques to interfere with the original microstructure prevents a systematic evaluation of the correlation between the microstructure and local electrochemical reactivity. In this work, the spatial variability of irreversible nucleation processes of Li on a Li-ion conductive glass-ceramics surface is studied with ~30 nm resolution. An increased nucleation rate at the boundaries between the crystalline AlPO4 phase and amorphous matrix is observed and attributed to Li segregation. This study opens a pathway for probing mechanisms at the level of single structural defects and elucidation of electrochemical activities in nanoscale volumes.

Nanometer-scale mapping of irreversible electrochemical nucleation processes on solid Li-ion electrolytes

PubMed Central

Kumar, Amit; Arruda, Thomas M.; Tselev, Alexander; Ivanov, Ilia N.; Lawton, Jamie S.; Zawodzinski, Thomas A.; Butyaev, Oleg; Zayats, Sergey; Jesse, Stephen; Kalinin, Sergei V.

2013-01-01

Electrochemical processes associated with changes in structure, connectivity or composition typically proceed via new phase nucleation with subsequent growth of nuclei. Understanding and controlling reactions requires the elucidation and control of nucleation mechanisms. However, factors controlling nucleation kinetics, including the interplay between local mechanical conditions, microstructure and local ionic profile remain inaccessible. Furthermore, the tendency of current probing techniques to interfere with the original microstructure prevents a systematic evaluation of the correlation between the microstructure and local electrochemical reactivity. In this work, the spatial variability of irreversible nucleation processes of Li on a Li-ion conductive glass-ceramics surface is studied with ~30 nm resolution. An increased nucleation rate at the boundaries between the crystalline AlPO4 phase and amorphous matrix is observed and attributed to Li segregation. This study opens a pathway for probing mechanisms at the level of single structural defects and elucidation of electrochemical activities in nanoscale volumes. PMID:23563856

Seismogenic width controls aspect ratios of earthquake ruptures

NASA Astrophysics Data System (ADS)

Weng, Huihui; Yang, Hongfeng

2017-03-01

We investigate the effect of seismogenic width on aspect ratios of earthquake ruptures by using numerical simulations of strike-slip faulting and an energy balance criterion near rupture tips. If the seismogenic width is smaller than a critical value, then ruptures cannot break the entire fault, regardless of the size of the nucleation zone. The seismic moments of these self-arresting ruptures increase with the nucleation size, forming nucleation-related events. The aspect ratios increase with the seismogenic width but are smaller than 8. In contrast, ruptures become breakaway and tend to have high aspect ratios (>8) if the seismogenic width is sufficiently large. But the critical nucleation size is larger than the theoretical estimate for an unbounded fault. The eventual seismic moments of breakaway ruptures do not depend on the nucleation size. Our results suggest that estimating final earthquake magnitude from the nucleation phase may only be plausible on faults with small seismogenic width.

Regulation of microtubule nucleation mediated by γ-tubulin complexes.

PubMed

Sulimenko, Vadym; Hájková, Zuzana; Klebanovych, Anastasiya; Dráber, Pavel

2017-05-01

The microtubule cytoskeleton is critically important for spatio-temporal organization of eukaryotic cells. The nucleation of new microtubules is typically restricted to microtubule organizing centers (MTOCs) and requires γ-tubulin that assembles into multisubunit complexes of various sizes. γ-Tubulin ring complexes (TuRCs) are efficient microtubule nucleators and are associated with large number of targeting, activating and modulating proteins. γ-Tubulin-dependent nucleation of microtubules occurs both from canonical MTOCs, such as spindle pole bodies and centrosomes, and additional sites such as Golgi apparatus, nuclear envelope, plasma membrane-associated sites, chromatin and surface of pre-existing microtubules. Despite many advances in structure of γ-tubulin complexes and characterization of γTuRC interacting factors, regulatory mechanisms of microtubule nucleation are not fully understood. Here, we review recent work on the factors and regulatory mechanisms that are involved in centrosomal and non-centrosomal microtubule nucleation.

Time-Resolved In Situ Liquid-Phase Atomic Force Microscopy and Infrared Nanospectroscopy during the Formation of Metal-Organic Framework Thin Films.

PubMed

Mandemaker, Laurens D B; Filez, Matthias; Delen, Guusje; Tan, Huanshu; Zhang, Xuehua; Lohse, Detlef; Weckhuysen, Bert M

2018-04-19

Metal-organic framework (MOF) thin films show unmatched promise as smart membranes and photocatalytic coatings. However, their nucleation and growth resulting from intricate molecular assembly processes are not well understood yet are crucial to control the thin film properties. Here, we directly observe the nucleation and growth behavior of HKUST-1 thin films by real-time in situ AFM at different temperatures in a Cu-BTC solution. In combination with ex situ infrared (nano)spectroscopy, synthesis at 25 °C reveals initial nucleation of rapidly growing HKUST-1 islands surrounded by a continuously nucleating but slowly growing HKUST-1 carpet. Monitoring at 13 and 50 °C shows the strong impact of temperature on thin film formation, resulting in (partial) nucleation and growth inhibition. The nucleation and growth mechanisms as well as their kinetics provide insights to aid in future rational design of MOF thin films.

Formation of Nitrogen Bubbles During Solidification of Duplex Stainless Steels

NASA Astrophysics Data System (ADS)

Dai, Kaiju; Wang, Bo; Xue, Fei; Liu, Shanshan; Huang, Junkai; Zhang, Jieyu

2018-04-01

The nucleation and growth of nitrogen bubbles for duplex stainless steels are of great significance for the formation mechanism of bubbles during solidification. In the current study, numerical method and theoretical analysis of formula derivation were used to study the formation of nitrogen bubbles during solidification. The critical sizes of the bubble for homogeneous nucleation and heterogeneous nucleation at the solid-liquid interface during solidification were derived theoretically by the classical nucleation theory. The results show that the calculated values for the solubility of nitrogen in duplex stainless steel are in good agreement with the experimental values which are quoted by references: for example, when the temperature T = 1823 K and the nitrogen partial pressure P_{{N2 }} = 40P^{Θ} , the calculated value (0.8042 wt pct) for the solubility of Fe-12Cr alloy nitrogen in molten steel is close to the experimental value (0.780 wt pct). Moreover, the critical radii for homogeneous nucleation and heterogeneous nucleation are identical during solidification. On the one hand, with the increasing temperature or the melt depth, the critical nucleation radius of bubbles at the solid-liquid interface increases, but the bubble growth rate decreases. On the other hand, with the decreasing initial content of nitrogen or the cooling rate, the critical nucleation radius of bubbles at the solid-liquid interface increases, but the bubble growth rate decreases. Furthermore, when the melt depth is greater than the critical depth, which is determined by the technological conditions, the change in the Gibbs free energy for the nucleation is not conducive enough to form new bubbles.

Nucleation versus instability race in strained films

NASA Astrophysics Data System (ADS)

Liu, Kailang; Berbezier, Isabelle; David, Thomas; Favre, Luc; Ronda, Antoine; Abbarchi, Marco; Voorhees, Peter; Aqua, Jean-Noël

2017-10-01

Under the generic term "Stranski-Krastanov" are grouped two different growth mechanisms of SiGe quantum dots. They result from the self-organized Asaro-Tiller-Grinfel'd (ATG) instability at low strain, while at high strain, from a stochastic nucleation. While these regimes are well known, we elucidate here the origin of the transition between these two pathways thanks to a joint theoretical and experimental work. Nucleation is described within the master equation framework. By comparing the time scales for ATG instability development and three-dimensional (3D) nucleation onset, we demonstrate that the transition between these two regimes is simply explained by the crossover between their divergent evolutions. Nucleation exhibits a strong exponential deviation at low strain while ATG behaves only algebraically. The associated time scale varies with exp(1 /x4) for nucleation, while it only behaves as 1 /x8 for the ATG instability. Consequently, at high (low) strain, nucleation (instability) occurs faster and inhibits the alternate evolution. It is then this different kinetic evolution which explains the transition from one regime to the other. Such a kinetic view of the transition between these two 3D growth regimes was not provided before. The crossover between nucleation and ATG instability is found to occur both experimentally and theoretically at a Ge composition around 50% in the experimental conditions used here. Varying the experimental conditions and/or the system parameters does not allow us to suppress the transition. This means that the SiGe quantum dots always grow via ATG instability at low strain and nucleation at high strain. This result is important for the self-organization of quantum dots.

Heterogeneous ice nucleation of α-pinene SOA particles before and after ice cloud processing

NASA Astrophysics Data System (ADS)

Wagner, Robert; Höhler, Kristina; Huang, Wei; Kiselev, Alexei; Möhler, Ottmar; Mohr, Claudia; Pajunoja, Aki; Saathoff, Harald; Schiebel, Thea; Shen, Xiaoli; Virtanen, Annele

2017-05-01

The ice nucleation ability of α-pinene secondary organic aerosol (SOA) particles was investigated at temperatures between 253 and 205 K in the Aerosol Interaction and Dynamics in the Atmosphere cloud simulation chamber. Pristine SOA particles were nucleated and grown from pure gas precursors and then subjected to repeated expansion cooling cycles to compare their intrinsic ice nucleation ability during the first nucleation event with that observed after ice cloud processing. The unprocessed α-pinene SOA particles were found to be inefficient ice-nucleating particles at cirrus temperatures, with nucleation onsets (for an activated fraction of 0.1%) as high as for the homogeneous freezing of aqueous solution droplets. Ice cloud processing at temperatures below 235 K only marginally improved the particles' ice nucleation ability and did not significantly alter their morphology. In contrast, the particles' morphology and ice nucleation ability was substantially modified upon ice cloud processing in a simulated convective cloud system, where the α-pinene SOA particles were first activated to supercooled cloud droplets and then froze homogeneously at about 235 K. As evidenced by electron microscopy, the α-pinene SOA particles adopted a highly porous morphology during such a freeze-drying cycle. When probing the freeze-dried particles in succeeding expansion cooling runs in the mixed-phase cloud regime up to 253 K, the increase in relative humidity led to a collapse of the porous structure. Heterogeneous ice formation was observed after the droplet activation of the collapsed, freeze-dried SOA particles, presumably caused by ice remnants in the highly viscous material or the larger surface area of the particles.

Controlled ice nucleation using freeze-dried Pseudomonas syringae encapsulated in alginate beads.

PubMed

Weng, Lindong; Tessier, Shannon N; Swei, Anisa; Stott, Shannon L; Toner, Mehmet

2017-04-01

The control of ice nucleation is of fundamental significance in many process technologies related to food and pharmaceutical science and cryobiology. Mechanical perturbation, electromagnetic fields and ice-nucleating agents (INAs) have been known to induce ice nucleation in a controlled manner. But these ice-nucleating methods may suffer from cumbersome manual operations, safety concerns of external fields, and biocompatibility and recovery issues of INA particles, especially when used in living systems. Given the automatic ice-seeding nature of INAs, a promising solution to overcome some of the above limitations is to engineer a biocomposite that accommodates the INA particles but minimizes their interactions with biologics, as well as enabling the recovery of used particles. In this study, freeze-dried Pseudomonas syringae, a model ice-nucleating agent, was encapsulated into microliter-sized alginate beads. We evaluated the performance of the bacterial hydrogel beads to initiate ice nucleation in water and aqueous glycerol solution by investigating factors including the size and number of the beads and the local concentration of INA particles. In the aqueous sample of a fixed volume, the total mass of the INA particles (m) was found to be the governing parameter that is solely responsible for determining the ice nucleation performance of the bacterial hydrogel beads. The freezing temperature has a strong positive linear correlation with log 10 m. The findings in this study provide an effective, predictable approach to control ice nucleation, which can improve the outcome and standardization of many ice-assisted process technologies. Copyright © 2017 Elsevier Inc. All rights reserved.

What experiments on pinned nanobubbles can tell about the critical nucleus for bubble nucleation.

PubMed

Xiao, Qianxiang; Liu, Yawei; Guo, Zhenjiang; Liu, Zhiping; Frenkel, Daan; Dobnikar, Jure; Zhang, Xianren

2017-12-22

The process of homogeneous bubble nucleation is almost impossible to probe experimentally, except near the critical point or for liquids under large negative tension. Elsewhere in the phase diagram, the bubble nucleation barrier is so high as to be effectively insurmountable. Consequently, there is a severe lack of experimental studies of homogenous bubble nucleation under conditions of practical importance (e.g., cavitation). Here we use a simple geometric relation to show that we can obtain information about the homogeneous nucleation process from Molecular Dynamics studies of bubble formation in solvophobic nanopores on a solid surface. The free energy of pinned nanobubbles has two extrema as a function of volume: one state corresponds to a free-energy maximum ("the critical nucleus"), the other corresponds to a free-energy minimum (the metastable, pinned nanobubble). Provided that the surface tension does not depend on nanobubble curvature, the radius of the curvature of the metastable surface nanobubble is independent of the radius of the pore and is equal to the radius of the critical nucleus in homogenous bubble nucleation. This observation opens the way to probe the parameters that determine homogeneous bubble nucleation under experimentally accessible conditions, e.g. with AFM studies of metastable nanobubbles. Our theoretical analysis also indicates that a surface with pores of different sizes can be used to determine the curvature corrections to the surface tension. Our conclusions are not limited to bubble nucleation but suggest that a similar approach could be used to probe the structure of critical nuclei in crystal nucleation.

Mammalian amyloidogenic proteins promote prion nucleation in yeast.

PubMed

Chandramowlishwaran, Pavithra; Sun, Meng; Casey, Kristin L; Romanyuk, Andrey V; Grizel, Anastasiya V; Sopova, Julia V; Rubel, Aleksandr A; Nussbaum-Krammer, Carmen; Vorberg, Ina M; Chernoff, Yury O

2018-03-02

Fibrous cross-β aggregates (amyloids) and their transmissible forms (prions) cause diseases in mammals (including humans) and control heritable traits in yeast. Initial nucleation of a yeast prion by transiently overproduced prion-forming protein or its (typically, QN-rich) prion domain is efficient only in the presence of another aggregated (in most cases, QN-rich) protein. Here, we demonstrate that a fusion of the prion domain of yeast protein Sup35 to some non-QN-rich mammalian proteins, associated with amyloid diseases, promotes nucleation of Sup35 prions in the absence of pre-existing aggregates. In contrast, both a fusion of the Sup35 prion domain to a multimeric non-amyloidogenic protein and the expression of a mammalian amyloidogenic protein that is not fused to the Sup35 prion domain failed to promote prion nucleation, further indicating that physical linkage of a mammalian amyloidogenic protein to the prion domain of a yeast protein is required for the nucleation of a yeast prion. Biochemical and cytological approaches confirmed the nucleation of protein aggregates in the yeast cell. Sequence alterations antagonizing or enhancing amyloidogenicity of human amyloid-β (associated with Alzheimer's disease) and mouse prion protein (associated with prion diseases), respectively, antagonized or enhanced nucleation of a yeast prion by these proteins. The yeast-based prion nucleation assay, developed in our work, can be employed for mutational dissection of amyloidogenic proteins. We anticipate that it will aid in the identification of chemicals that influence initial amyloid nucleation and in searching for new amyloidogenic proteins in a variety of proteomes. © 2018 by The American Society for Biochemistry and Molecular Biology, Inc.

Heterogeneous Nucleation of Methane Hydrate in a Water-Decane-Methane Emulsion

NASA Astrophysics Data System (ADS)

Shestakov, V. A.; Kosyakov, V. I.; Manakov, A. Yu.; Stoporev, A. S.; Grachev, E. V.

2018-07-01

Heterogeneous nucleation in disperse systems with metastable disperse phases plays an important role in the mechanisms of environmental and technological processes. The effect the concentration and activity of particles that initiate the formation of a new phase have on nucleation processes in such systems is considered. An approach is proposed that allows construction of a spectrum of particle activity characterizing the features of nucleation in a sample, based on the fraction of crystallized droplets depending on the level of supercooling and the use of Weibull's distribution. The proposed method is used to describe experimental data on the heterogeneous nucleation of methane hydrate in an emulsion in a water-decane-methane system.

Surface nanobubble nucleation dynamics during water-ethanol exchange

NASA Astrophysics Data System (ADS)

Chan, Chon U.; Ohl, Claus-Dieter

2015-11-01

Water-ethanol exchange has been a promising nucleation method for surface attached nanobubbles since their discovery. In this process, water and ethanol displace each other sequentially on a substrate. As the gas solubility is 36 times higher in ethanol than water, it was suggested that the exchange process leads to transient supersaturation and is responsible for the nanobubble nucleation. In this work, we visualize the nucleation dynamics by controllably mixing water and ethanol. It depicts the temporal evolution of the conventional exchange in a single field of view, detailing the conditions for surface nanobubble nucleation and the flow field that influences their spatial organization. This technique can also pattern surface nanobubbles with variable size distribution.

Volumes of critical bubbles from the nucleation theorem

NASA Astrophysics Data System (ADS)

Wilemski, Gerald

2006-09-01

A corollary of the nucleation theorem due to Kashchiev [Nucleation: Basic Theory with Applications (Butterworth-Heinemann, Oxford, 2000)] allows the volume V* of a critical bubble to be determined from nucleation rate measurements. The original derivation was limited to one-component, ideal gas bubbles with a vapor density much smaller than that of the ambient liquid. Here, an exact result is found for multicomponent, nonideal gas bubbles. Provided a weak density inequality holds, this result reduces to Kashchiev's simple form which thus has a much broader range of applicability than originally expected. Limited applications to droplets are also mentioned, and the utility of the pT,x form of the nucleation theorem as a sum rule is noted.

Microstructure development in Kolmogorov, Johnson-Mehl, and Avrami nucleation and growth kinetics

NASA Astrophysics Data System (ADS)

Pineda, Eloi; Crespo, Daniel

1999-08-01

A statistical model with the ability to evaluate the microstructure developed in nucleation and growth kinetics is built in the framework of the Kolmogorov, Johnson-Mehl, and Avrami theory. A populational approach is used to compute the observed grain-size distribution. The impingement process which delays grain growth is analyzed, and the effective growth rate of each population is estimated considering the previous grain history. The proposed model is integrated for a wide range of nucleation and growth protocols, including constant nucleation, pre-existing nuclei, and intermittent nucleation with interface or diffusion-controlled grain growth. The results are compared with Monte Carlo simulations, giving quantitative agreement even in cases where previous models fail.

Crystal nucleation in metallic alloys using x-ray radiography and machine learning

PubMed Central

Arteta, Carlos; Lempitsky, Victor

2018-01-01

The crystallization of solidifying Al-Cu alloys over a wide range of conditions was studied in situ by synchrotron x-ray radiography, and the data were analyzed using a computer vision algorithm trained using machine learning. The effect of cooling rate and solute concentration on nucleation undercooling, crystal formation rate, and crystal growth rate was measured automatically for thousands of separate crystals, which was impossible to achieve manually. Nucleation undercooling distributions confirmed the efficiency of extrinsic grain refiners and gave support to the widely assumed free growth model of heterogeneous nucleation. We show that crystallization occurred in temporal and spatial bursts associated with a solute-suppressed nucleation zone. PMID:29662954

Towards establishing a combined rate law of nucleation and crystal growth - The case study of gypsum precipitation

NASA Astrophysics Data System (ADS)

Rendel, Pedro M.; Gavrieli, Ittai; Wolff-Boenisch, Domenik; Ganor, Jiwchar

2018-03-01

The main obstacle in the formulation of a quantitative rate-model for mineral precipitation is the absence of a rigorous method for coupling nucleation and growth processes. In order to link both processes, we conducted a series of batch experiments in which gypsum nucleation was followed by crystal growth. Experiments were carried out using various stirring methods in several batch vessels made of different materials. In the experiments, the initial degree of supersaturation of the solution with respect to gypsum (Ωgyp) was set between 1.58 and 1.82. Under these conditions, heterogeneous nucleation is the dominant nucleation mode. Based on changes in SO42- concentration with time, the induction time of gypsum nucleation and the following rate of crystal growth were calculated for each experiment. The induction time (6-104 h) was found to be a function of the vessel material, while the rates of crystal growth, which varied over three orders of magnitude, were strongly affected by the stirring speed and its mode (i.e. rocking, shaking, magnetic stirrer, and magnetic impeller). The SO42- concentration data were then used to formulate a forward model that couples the simple rate laws for nucleation and crystal growth of gypsum into a single kinetic model. Accordingly, the obtained rate law is based on classical nucleation theory and heterogeneous crystal growth.

Taking directions: the role of microtubule-bound nucleation in the self-organization of the plant cortical array

NASA Astrophysics Data System (ADS)

Deinum, Eva E.; Tindemans, Simon H.; Mulder, Bela M.

2011-10-01

The highly aligned cortical microtubule array of interphase plant cells is a key regulator of anisotropic cell expansion. Recent computational and analytical work has shown that the non-equilibrium self-organization of this structure can be understood on the basis of experimentally observed collisional interactions between dynamic microtubules attached to the plasma membrane. Most of these approaches assumed that new microtubules are homogeneously and isotropically nucleated on the cortical surface. Experimental evidence, however, shows that nucleation mostly occurs from other microtubules and under specific relative angles. Here, we investigate the impact of directed microtubule-bound nucleations on the alignment process using computer simulations. The results show that microtubule-bound nucleations can increase the degree of alignment achieved, decrease the timescale of the ordering process and widen the regime of dynamic parameters for which the system can self-organize. We establish that the major determinant of this effect is the degree of co-alignment of the nucleations with the parent microtubule. The specific role of sideways branching nucleations appears to allow stronger alignment while maintaining a measure of overall spatial homogeneity. Finally, we investigate the suggestion that observed persistent rotation of microtubule domains can be explained through a handedness bias in microtubule-bound nucleations, showing that this is possible only for an extreme bias and over a limited range of parameters.

Controlled ice nucleation in cryopreservation--a review.

PubMed

Morris, G John; Acton, Elizabeth

2013-04-01

We review here for the first time, the literature on control of ice nucleation in cryopreservation. Water and aqueous solutions have a tendency to undercool before ice nucleation occurs. Control of ice nucleation has been recognised as a critical step in the cryopreservation of embryos and oocytes but is largely ignored for other cell types. We review the processes of ice nucleation and crystal growth in the solution around cells and tissues during cryopreservation with an emphasis on non IVF applications. The extent of undercooling that is encountered during the cooling of various cryocontainers is defined and the methods that have been employed to control the nucleation of ice are examined. The effects of controlled ice nucleation on the structure of the sample and the outcome of cryopreservation of a range of cell types and tissues are presented and the physical events which define the cellular response are discussed. Nucleation of ice is the most significant uncontrolled variable in conventional cryopreservation leading to sample to sample variation in cell recovery, viability and function and should be controlled to allow standardisation of cryopreservation protocols for cells for biobanking, cell based assays or clinical application. This intervention allows a way of increasing viability of cells and reducing variability between samples and should be included as standard operating procedures are developed. Copyright © 2012 Elsevier Inc. All rights reserved.

SUCCESS Evidence for Cirrus Cloud Ice Nucleation Mechanisms

NASA Technical Reports Server (NTRS)

Jensen, Eric; Gore, Warren J. Y. (Technical Monitor)

1997-01-01

During the SUCCESS mission, several measurements were made which should improve our understanding of ice nucleation processes in cirrus clouds. Temperature and water vapor concentration were made with a variety of instruments on the NASA DC-8. These observations should provide accurate upper tropospheric humidities. In particular, we will evaluate what humidities are required for ice nucleation. Preliminary results suggest that substantial supersaturations frequently exist in the upper troposphere. The leading-edge region of wave-clouds (where ice nucleation occurs) was sampled extensively at temperatures near -40 and -60C. These observations should give precise information about conditions required for ice nucleation. In addition, we will relate the observed aerosol composition and size distributions to the ice formation observed to evaluate the role of soot or mineral particles on ice nucleation. As an alternative technique for determining what particles act as ice nuclei, numerous samples of aerosols inside ice crystals were taken. In some cases, large numbers of aerosols were detected in each crystal, indicating that efficient scavenging occurred. Analysis of aerosols in ice crystals when only one particle per crystal was detected should help with the ice nucleation issue. Direct measurements of the ice nucleating activity of ambient aerosols drawn into airborne cloud chambers were also made. Finally, measurements of aerosols and ice crystals in contrails should indicate whether aircraft exhaust soot particles are effective ice nuclei.

Investigation of nucleation processes during dynamic recrystallization of ice using cryo-EBSD.

PubMed

Chauve, T; Montagnat, M; Barou, F; Hidas, K; Tommasi, A; Mainprice, D

2017-02-13

Nucleation mechanisms occurring during dynamic recrystallization play a crucial role in the evolution of microstructures and textures during high temperature deformation. In polycrystalline ice, the strong viscoplastic anisotropy induces high strain heterogeneities between grains which control the recrystallization mechanisms. Here, we study the nucleation mechanisms occurring during creep tests performed on polycrystalline columnar ice at high temperature and stress (T=-5°C;σ=0.5 MPa) by post-mortem analyses of deformation microstructures using cryogenic electron backscatter diffraction. The columnar geometry of the samples enables discrimination of the nuclei from the initial grains. Various nucleation mechanisms are deduced from the analysis of the nuclei relations with the dislocation sub-structures within grains and at grain boundaries. Tilt sub-grain boundaries and kink bands are the main structures responsible for development of polygonization and mosaic sub-structures. Nucleation by bulging at serrated grain boundaries is also an efficient nucleation mechanism near the grain boundaries where strain incompatibilities are high. Observation of nuclei with orientations not related to the 'parent' ones suggests the possibility of 'spontaneous' nucleation driven by the relaxation of the dislocation-related internal stress field. The complexity of the nucleation mechanisms observed here emphasizes the impact of stress and strain heterogeneities on dynamic recrystallization mechanisms.This article is part of the themed issue 'Microdynamics of ice'. © 2016 The Author(s).

Copper Benzenetricarboxylate Metal-Organic Framework Nucleation Mechanisms on Metal Oxide Powders and Thin Films formed by Atomic Layer Deposition.

PubMed

Lemaire, Paul C; Zhao, Junjie; Williams, Philip S; Walls, Howard J; Shepherd, Sarah D; Losego, Mark D; Peterson, Gregory W; Parsons, Gregory N

2016-04-13

Chemically functional microporous metal-organic framework (MOF) crystals are attractive for filtration and gas storage applications, and recent results show that they can be immobilized on high surface area substrates, such as fiber mats. However, fundamental knowledge is still lacking regarding initial key reaction steps in thin film MOF nucleation and growth. We find that thin inorganic nucleation layers formed by atomic layer deposition (ALD) can promote solvothermal growth of copper benzenetricarboxylate MOF (Cu-BTC) on various substrate surfaces. The nature of the ALD material affects the MOF nucleation time, crystal size and morphology, and the resulting MOF surface area per unit mass. To understand MOF nucleation mechanisms, we investigate detailed Cu-BTC MOF nucleation behavior on metal oxide powders and Al2O3, ZnO, and TiO2 layers formed by ALD on polypropylene substrates. Studying both combined and sequential MOF reactant exposure conditions, we find that during solvothermal synthesis ALD metal oxides can react with the MOF metal precursor to form double hydroxy salts that can further convert to Cu-BTC MOF. The acidic organic linker can also etch or react with the surface to form MOF from an oxide metal source, which can also function as a nucleation agent for Cu-BTC in the mixed solvothermal solution. We discuss the implications of these results for better controlled thin film MOF nucleation and growth.

Deviation from equilibrium conditions in molecular dynamic simulations of homogeneous nucleation.

PubMed

Halonen, Roope; Zapadinsky, Evgeni; Vehkamäki, Hanna

2018-04-28

We present a comparison between Monte Carlo (MC) results for homogeneous vapour-liquid nucleation of Lennard-Jones clusters and previously published values from molecular dynamics (MD) simulations. Both the MC and MD methods sample real cluster configuration distributions. In the MD simulations, the extent of the temperature fluctuation is usually controlled with an artificial thermostat rather than with more realistic carrier gas. In this study, not only a primarily velocity scaling thermostat is considered, but also Nosé-Hoover, Berendsen, and stochastic Langevin thermostat methods are covered. The nucleation rates based on a kinetic scheme and the canonical MC calculation serve as a point of reference since they by definition describe an equilibrated system. The studied temperature range is from T = 0.3 to 0.65 ϵ/k. The kinetic scheme reproduces well the isothermal nucleation rates obtained by Wedekind et al. [J. Chem. Phys. 127, 064501 (2007)] using MD simulations with carrier gas. The nucleation rates obtained by artificially thermostatted MD simulations are consistently lower than the reference nucleation rates based on MC calculations. The discrepancy increases up to several orders of magnitude when the density of the nucleating vapour decreases. At low temperatures, the difference to the MC-based reference nucleation rates in some cases exceeds the maximal nonisothermal effect predicted by classical theory of Feder et al. [Adv. Phys. 15, 111 (1966)].

Investigation of nucleation processes during dynamic recrystallization of ice using cryo-EBSD

PubMed Central

Barou, F.; Hidas, K.; Tommasi, A.; Mainprice, D.

2017-01-01

Nucleation mechanisms occurring during dynamic recrystallization play a crucial role in the evolution of microstructures and textures during high temperature deformation. In polycrystalline ice, the strong viscoplastic anisotropy induces high strain heterogeneities between grains which control the recrystallization mechanisms. Here, we study the nucleation mechanisms occurring during creep tests performed on polycrystalline columnar ice at high temperature and stress (T=−5°C;σ=0.5 MPa) by post-mortem analyses of deformation microstructures using cryogenic electron backscatter diffraction. The columnar geometry of the samples enables discrimination of the nuclei from the initial grains. Various nucleation mechanisms are deduced from the analysis of the nuclei relations with the dislocation sub-structures within grains and at grain boundaries. Tilt sub-grain boundaries and kink bands are the main structures responsible for development of polygonization and mosaic sub-structures. Nucleation by bulging at serrated grain boundaries is also an efficient nucleation mechanism near the grain boundaries where strain incompatibilities are high. Observation of nuclei with orientations not related to the ‘parent’ ones suggests the possibility of ‘spontaneous’ nucleation driven by the relaxation of the dislocation-related internal stress field. The complexity of the nucleation mechanisms observed here emphasizes the impact of stress and strain heterogeneities on dynamic recrystallization mechanisms. This article is part of the themed issue ‘Microdynamics of ice’. PMID:28025294

Deviation from equilibrium conditions in molecular dynamic simulations of homogeneous nucleation

NASA Astrophysics Data System (ADS)

Halonen, Roope; Zapadinsky, Evgeni; Vehkamäki, Hanna

2018-04-01

We present a comparison between Monte Carlo (MC) results for homogeneous vapour-liquid nucleation of Lennard-Jones clusters and previously published values from molecular dynamics (MD) simulations. Both the MC and MD methods sample real cluster configuration distributions. In the MD simulations, the extent of the temperature fluctuation is usually controlled with an artificial thermostat rather than with more realistic carrier gas. In this study, not only a primarily velocity scaling thermostat is considered, but also Nosé-Hoover, Berendsen, and stochastic Langevin thermostat methods are covered. The nucleation rates based on a kinetic scheme and the canonical MC calculation serve as a point of reference since they by definition describe an equilibrated system. The studied temperature range is from T = 0.3 to 0.65 ɛ/k. The kinetic scheme reproduces well the isothermal nucleation rates obtained by Wedekind et al. [J. Chem. Phys. 127, 064501 (2007)] using MD simulations with carrier gas. The nucleation rates obtained by artificially thermostatted MD simulations are consistently lower than the reference nucleation rates based on MC calculations. The discrepancy increases up to several orders of magnitude when the density of the nucleating vapour decreases. At low temperatures, the difference to the MC-based reference nucleation rates in some cases exceeds the maximal nonisothermal effect predicted by classical theory of Feder et al. [Adv. Phys. 15, 111 (1966)].

Controls of ionic strength and macromolecule chemistry on calcite nucleation: Salinity and ion hydration as levers for regulating biomineralization

NASA Astrophysics Data System (ADS)

Dove, P. M.; Giuffre, A. J.; Mergelsberg, S. T.; Han, N.; De Yoreo, J. J.

2016-12-01

Organisms form shells and skeletons with remarkable fidelity by controlling the timing and placement of the minerals that nucleate and subsequently grow. An extensive effort has identified features of the organic matrix that regulate this process. Recent measurements from our group show the energy barrier to nucleation onto polysaccharide (PS) substrates is dependent upon hydrophilicity through functional group chemistry and suggest that free energy of the macromolecule-liquid interface influences where and when mineral nucleation occurs (Giuffre et al., 2013, PNAS). The importance of interfacial free energy in regulating nucleation raises the question of whether local changes in salinity or electrolyte composition can be tuned to further modulate the onset of calcite nucleation. Using alginate (negatively charged by carboxyl groups) and chitosan (small positive charge by amine groups), the rate of calcite nucleation was measured at controlled supersaturations and pH as a function of NaCl concentration (65-600 mM). Analyses of the data show the thermodynamic barrier to calcite nucleation onto both types of PS increases with ionic strength. The evidence suggests this effect arises from an increasing concentration of solvated ions at the PS-water interface while also increasing the hydrophilic character of that interface; thus decreasing the substrate-liquid interfacial free energy. To test this explanation, a second group of nucleation experiments used a suite of electrolytes (alkali chlorides for alginate and sodium halides for chitosan) while holding ionic strength constant. Indeed, the nucleation barriers for calcite formation are electrolyte-specific and correlated with the hydration free energy of the ion. This suggests solvated electrolyte ions indirectly regulate calcite nucleation onto substrates through their competition with the substrate for water thereby influencing net interfacial free energy. These effects are consistent with the long-established salting-in phenomena of the Hofmeister series. The new insights from this experimental study reiterate the importance of electrolytes in reactions involving mineral-water interfaces and suggest a role for seemingly inert `spectator' ions in regulating the local timing and placement of minerals during biomineralization.

Nanoparticle Nucleation Is Termolecular in Metal and Involves Hydrogen: Evidence for a Kinetically Effective Nucleus of Three {Ir3H2x·P2W15Nb3O62}6- in Ir(0)n Nanoparticle Formation From [(1,5-COD)IrI·P2W15Nb3O62]8- Plus Dihydrogen.

PubMed

Özkar, Saim; Finke, Richard G

2017-04-19

The nucleation process yielding Ir(0) ∼300 nanoparticles from (Bu 4 N) 5 Na 3 [(1,5-COD)Ir·P 2 W 15 Nb 3 O 62 ] (abbreviated hereafter as (COD)Ir·POM 8- , where POM 9- = the polyoxometalate, P 2 W 15 Nb 3 O 62 9- ) under H 2 is investigated to learn the true molecularity, and hence the associated kinetically effective nucleus (KEN), for nanoparticle formation for the first time. Recent work with this prototype transition-metal nanoparticle formation system ( J. Am. Chem. Soc. 2014 , 136 , 17601 - 17615 ) revealed that nucleation in this system is an apparent second-order in the precatalyst, A = (COD)Ir·POM 8- , not the higher order implied by classic nucleation theory and its nA ⇌ A n , "critical nucleus", A n concept. Herein, the three most reasonable more intimate mechanisms of nucleation are tested: bimolecular nucleation, termolecular nucleation, and a mechanism termed "alternative termolecular nucleation" in which 2(COD)Ir + and 1(COD)Ir·POM 8- yield the transition state of the rate-determining step of nucleation. The results obtained definitively rule out a simple bimolecular nucleation mechanism and provide evidence for the alternative termolecular mechanism with a KEN of 3, Ir 3 . All higher molecularity nucleation mechanisms were also ruled out. Further insights into the KEN and its more detailed composition involving hydrogen, {Ir 3 H 2x POM} 6- , are also obtained from the established role of H 2 in the Ir(0) ∼300 formation balanced reaction stoichiometry, from the p(H 2 ) dependence of the kinetics, and from a D 2 /H 2 kinetic isotope effect of 1.2(±0.3). Eight insights and conclusions are presented. A section covering caveats in the current work, and thus needed future studies, is also included.

A~comprehensive parameterization of heterogeneous ice nucleation of dust surrogate: laboratory study with hematite particles and its application to atmospheric models

NASA Astrophysics Data System (ADS)

Hiranuma, N.; Paukert, M.; Steinke, I.; Zhang, K.; Kulkarni, G.; Hoose, C.; Schnaiter, M.; Saathoff, H.; Möhler, O.

2014-06-01

A new heterogeneous ice nucleation parameterization that covers a~wide temperature range (-36 to -78 °C) is presented. Developing and testing such an ice nucleation parameterization, which is constrained through identical experimental conditions, is critical in order to accurately simulate the ice nucleation processes in cirrus clouds. The surface-scaled ice nucleation efficiencies of hematite particles, inferred by ns, were derived from AIDA (Aerosol Interaction and Dynamics in the Atmosphere) cloud chamber measurements under water subsaturated conditions that were realized by continuously changing temperature (T) and relative humidity with respect to ice (RHice) in the chamber. Our measurements showed several different pathways to nucleate ice depending on T and RHice conditions. For instance, almost T-independent freezing was observed at -60 °C < T < -50 °C, where RHice explicitly controlled ice nucleation efficiency, while both T and RHice played roles in other two T regimes: -78 °C < T < -60 °C and -50 °C < T < -36 °C. More specifically, observations at T colder than -60 °C revealed that higher RHice was necessary to maintain constant ns, whereas T may have played a significant role in ice nucleation at T warmer than -50 °C. We implemented new ns parameterizations into two cloud models to investigate its sensitivity and compare with the existing ice nucleation schemes towards simulating cirrus cloud properties. Our results show that the new AIDA-based parameterizations lead to an order of magnitude higher ice crystal concentrations and inhibition of homogeneous nucleation in colder temperature regions. Our cloud simulation results suggest that atmospheric dust particles that form ice nuclei at lower temperatures, below -36 °C, can potentially have stronger influence on cloud properties such as cloud longevity and initiation when compared to previous parameterizations.

A comprehensive parameterization of heterogeneous ice nucleation of dust surrogate: laboratory study with hematite particles and its application to atmospheric models

NASA Astrophysics Data System (ADS)

Hiranuma, N.; Paukert, M.; Steinke, I.; Zhang, K.; Kulkarni, G.; Hoose, C.; Schnaiter, M.; Saathoff, H.; Möhler, O.

2014-12-01

A new heterogeneous ice nucleation parameterization that covers a wide temperature range (-36 to -78 °C) is presented. Developing and testing such an ice nucleation parameterization, which is constrained through identical experimental conditions, is important to accurately simulate the ice nucleation processes in cirrus clouds. The ice nucleation active surface-site density (ns) of hematite particles, used as a proxy for atmospheric dust particles, were derived from AIDA (Aerosol Interaction and Dynamics in the Atmosphere) cloud chamber measurements under water subsaturated conditions. These conditions were achieved by continuously changing the temperature (T) and relative humidity with respect to ice (RHice) in the chamber. Our measurements showed several different pathways to nucleate ice depending on T and RHice conditions. For instance, almost T-independent freezing was observed at -60 °C < T < -50 °C, where RHice explicitly controlled ice nucleation efficiency, while both T and RHice played roles in other two T regimes: -78 °C < T < -60 °C and -50 °C < T < -36 °C. More specifically, observations at T lower than -60 °C revealed that higher RHice was necessary to maintain a constant ns, whereas T may have played a significant role in ice nucleation at T higher than -50 °C. We implemented the new hematite-derived ns parameterization, which agrees well with previous AIDA measurements of desert dust, into two conceptual cloud models to investigate their sensitivity to the new parameterization in comparison to existing ice nucleation schemes for simulating cirrus cloud properties. Our results show that the new AIDA-based parameterization leads to an order of magnitude higher ice crystal concentrations and to an inhibition of homogeneous nucleation in lower-temperature regions. Our cloud simulation results suggest that atmospheric dust particles that form ice nuclei at lower temperatures, below -36 °C, can potentially have a stronger influence on cloud properties, such as cloud longevity and initiation, compared to previous parameterizations.

Overview of TANGENT (Tandem Aerosol Nucleation and Growth ENvironment Tube) 2017 IOP Study

NASA Astrophysics Data System (ADS)

Tiszenkel, L.

2017-12-01

New particle formation consists of two steps: nucleation and growth of nucleated particles. However, most laboratory studies have been conducted under conditions where these two processes are convoluted together, thereby hampering the detailed understanding of the effect of chemical species and atmospheric conditions on two processes. The objective of the Tandem Aerosol Nucleation and Growth ENvironment Tube (TANGENT) laboratory study is to investigate aerosol nucleation and growth properties independently by separating these two processes in two different flow tubes. This research is a collaboration between the University of Alabama in Huntsville and the University of Delaware. In this poster we will present the experimental setup of TANGENT and summarize the key results from the first IOP (intense observation period) experiments undertaken during Summer 2017. Nucleation takes place in a temperature- and RH-controlled fast flow reactor (FT-1) where sulfuric acid forms from OH radicals and sulfur dioxide. Sulfuric acid and impurity base compounds are detected with chemical ionization mass spectrometers (CIMS). Particle sizes and number concentrations of newly nucleated particles are measured with a scanning mobility particle sizer (SMPS) and particle size magnifier (PSM), providing concentrations of particles between 1-100 nm. The nucleation particles are transferred directly to the growth tube (FT-2) where oxidants and biogenic organic precursors are added to grow nucleated nanoparticles. Sizes of particles after growth are analyzed with an additional SMPS and elemental chemical composition of 50 nm and above particles detected with a nano-aerosol mass spectrometer (NAMS). TANGENT provides the unique ability to conduct experiments that can monitor and control reactant concentrations, aerosol size and aerosol chemical composition during nucleation and growth. Experiments during this first IOP study have elucidated the effects of sulfur dioxide, particle size, relative humidity, temperature, oxidants and biogenic organics on nanoparticle formation and growth. In another 3 companion posters, we will discuss findings of these results in detail.

A Comprehensive Parameterization of Heterogeneous Ice Nucleation of Dust Surrogate: Laboratory Study with Hematite Particles and Its Application to Atmospheric Models

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hiranuma, Naruki; Paukert, Marco; Steinke, Isabelle

2014-12-10

A new heterogeneous ice nucleation parameterization that covers a wide temperature range (-36 °C to -78 °C) is presented. Developing and testing such an ice nucleation parameterization, which is constrained through identical experimental conditions, is critical in order to accurately simulate the ice nucleation processes in cirrus clouds. The surface-scaled ice nucleation efficiencies of hematite particles, inferred by n s, were derived from AIDA (Aerosol Interaction and Dynamics in the Atmosphere) cloud chamber measurements under water subsaturated conditions that were realized by continuously changing temperature (T) and relative humidity with respect to ice (RH ice) in the chamber. Our measurementsmore » showed several different pathways to nucleate ice depending on T and RH ice conditions. For instance, almost independent freezing was observed at -60 °C < T < -50 °C, where RH ice explicitly controlled ice nucleation efficiency, while both T and RH ice played roles in other two T regimes: -78 °C < T < -60 °C and -50 °C < T < -36 °C. More specifically, observations at T colder than -60 °C revealed that higher RHice was necessary to maintain constant n s, whereas T may have played a significant role in ice nucleation at T warmer than -50 °C. We implemented new n s parameterizations into two cloud models to investigate its sensitivity and compare with the existing ice nucleation schemes towards simulating cirrus cloud properties. Our results show that the new AIDA-based parameterizations lead to an order of magnitude higher ice crystal concentrations and inhibition of homogeneous nucleation in colder temperature regions. Our cloud simulation results suggest that atmospheric dust particles that form ice nuclei at lower temperatures, below -36 °C, can potentially have stronger influence on cloud properties such as cloud longevity and initiation when compared to previous parameterizations.« less

Experimental investigation on the phenomena around the onset nucleate boiling during the impacting of a droplet on the hot surface

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mitrakusuma, Windy H., E-mail: windyhm@polban.ac.id; Refrigeration and Airconditioning Department, Politeknik Negeri Bandung, Jl. Gegerkalong Hilir, Ds. Ciwaruga Kotak Pos 1234 Bandung; Deendarlianto,

2016-06-03

Onset of nucleate boiling of a droplet when impacted onto hot surface was investigated. Three kinds of surfaces, normal stainless steel (NSS), stainless steel with TiO{sub 2} coating (UVN), and stainless steel with TiO{sub 2} coating and radiated by ultraviolet ray were employed to examine the effect of wettability. The droplet size was 2.4 mm diameter, and dropped under different We number. The image is generated by high speed camera with the frame speed of 1000 fps. The boiling conditions are identified as natural convection, nucleate boiling, critical heat flux, transition, and film boiling. In the present report, the discussionmore » will be focused on the beginning of nucleate boiling on the droplet. Nucleate boiling occurs when bubbles are generated. These bubbles are probably caused by nucleation on the impurities within the liquid rather than at nucleation sites on the heated surface because the bubbles appear to be in the bulk of the liquid instead of at the liquid-solid interface. In addition, the smaller the contact angle, the fastest the boiling.« less

Theoretical analysis to interpret projected image data from in-situ 3-dimensional equiaxed nucleation and growth

NASA Astrophysics Data System (ADS)

Mooney, Robin P.; McFadden, Shaun

2017-12-01

In-situ observation of crystal growth in transparent media allows us to observe solidification phase change in real-time. These systems are analogous to opaque systems such as metals. The interpretation of transient 2-dimensional area projections from 3-dimensional phase change phenomena occurring in a bulky sample is problematic due to uncertainty of impingement and hidden nucleation events; in stereology this problem is known as over-projection. This manuscript describes and demonstrates a continuous model for nucleation and growth using the well-established Johnson-Mehl-Avrami-Kolmogorov model, and provides a method to relate 3-dimensional volumetric data (nucleation events, volume fraction) to observed data in a 2-dimensional projection (nucleation count, area fraction). A parametric analysis is performed; the projection phenomenon is shown to be significant in cases where nucleation is occurring continuously with a relatively large variance. In general, area fraction on a projection plane will overestimate the volume fraction within the sample and the nuclei count recorded on the projection plane will underestimate the number of real nucleation events. The statistical framework given in this manuscript provides a methodology to deal with the differences between the observed (projected) data and the real (volumetric) measures.

Heterogeneous nucleation on rough surfaces: Generalized Gibbs' approach.

PubMed

Abyzov, Alexander S; Schmelzer, Jürn W P; Davydov, Leonid N

2017-12-07

Heterogeneous nucleation (condensation) of liquid droplets from vapor (gas) on a defective solid surface is considered. The vapor is described by the van der Waals equation of state. The dependence of nucleating droplet parameters on droplet size is accounted for within the generalized Gibbs approach. As a surface defect, a conic void is taken. This choice allows us to simplify the analysis and at the same time to follow the main aspects of the influence of the surface roughness on the nucleation process. Similar to condensation on ideal planar surfaces, the contact angle and catalytic factor for heterogeneous nucleation on a rough surface depend on the degree of vapor overcooling. In the case of droplet formation on a hydrophilic surface of a conic void, the nucleation rate considerably increases in comparison with the condensation on a planar interface. In fact, the presence of a defect on the hydrophilic surface leads to a considerable shift of the spinodal towards lower supersaturation in comparison with heterogeneous nucleation on a planar interface. With the decrease in the void cone angle, the heterogeneous spinodal approaches the binodal, and the region of metastability is diminished at the expense of the instability region.

Dislocation creation and void nucleation in FCC ductile metals under tensile loading: a general microscopic picture.

PubMed

Pang, Wei-Wei; Zhang, Ping; Zhang, Guang-Cai; Xu, Ai-Guo; Zhao, Xian-Geng

2014-11-10

Numerous theoretical and experimental efforts have been paid to describe and understand the dislocation and void nucleation processes that are fundamental for dynamic fracture modeling of strained metals. To date an essential physical picture on the self-organized atomic collective motions during dislocation creation, as well as the essential mechanisms for the void nucleation obscured by the extreme diversity in structural configurations around the void nucleation core, is still severely lacking in literature. Here, we depict the origin of dislocation creation and void nucleation during uniaxial high strain rate tensile processes in face-centered-cubic (FCC) ductile metals. We find that the dislocations are created through three distinguished stages: (i) Flattened octahedral structures (FOSs) are randomly activated by thermal fluctuations; (ii) The double-layer defect clusters are formed by self-organized stacking of FOSs on the close-packed plane; (iii) The stacking faults are formed and the Shockley partial dislocations are created from the double-layer defect clusters. Whereas, the void nucleation is shown to follow a two-stage description. We demonstrate that our findings on the origin of dislocation creation and void nucleation are universal for a variety of FCC ductile metals with low stacking fault energies.

Probing the Biomimetic Ice Nucleation Inhibition Activity of Poly(vinyl alcohol) and Comparison to Synthetic and Biological Polymers.

PubMed

Congdon, Thomas; Dean, Bethany T; Kasperczak-Wright, James; Biggs, Caroline I; Notman, Rebecca; Gibson, Matthew I

2015-09-14

Nature has evolved many elegant solutions to enable life to flourish at low temperatures by either allowing (tolerance) or preventing (avoidance) ice formation. These processes are typically controlled by ice nucleating proteins or antifreeze proteins, which act to either promote nucleation, prevent nucleation or inhibit ice growth depending on the specific need, respectively. These proteins can be expensive and their mechanisms of action are not understood, limiting their translation, especially into biomedical cryopreservation applications. Here well-defined poly(vinyl alcohol), synthesized by RAFT/MADIX polymerization, is investigated for its ice nucleation inhibition (INI) activity, in contrast to its established ice growth inhibitory properties and compared to other synthetic polymers. It is shown that ice nucleation inhibition activity of PVA has a strong molecular weight dependence; polymers with a degree of polymerization below 200 being an effective inhibitor at just 1 mg.mL(-1). Other synthetic and natural polymers, both with and without hydroxyl-functional side chains, showed negligible activity, highlighting the unique ice/water interacting properties of PVA. These findings both aid our understanding of ice nucleation but demonstrate the potential of engineering synthetic polymers as new biomimetics to control ice formation/growth processes.

Probing the Biomimetic Ice Nucleation Inhibition Activity of Poly(vinyl alcohol) and Comparison to Synthetic and Biological Polymers

PubMed Central

2015-01-01

Nature has evolved many elegant solutions to enable life to flourish at low temperatures by either allowing (tolerance) or preventing (avoidance) ice formation. These processes are typically controlled by ice nucleating proteins or antifreeze proteins, which act to either promote nucleation, prevent nucleation or inhibit ice growth depending on the specific need, respectively. These proteins can be expensive and their mechanisms of action are not understood, limiting their translation, especially into biomedical cryopreservation applications. Here well-defined poly(vinyl alcohol), synthesized by RAFT/MADIX polymerization, is investigated for its ice nucleation inhibition (INI) activity, in contrast to its established ice growth inhibitory properties and compared to other synthetic polymers. It is shown that ice nucleation inhibition activity of PVA has a strong molecular weight dependence; polymers with a degree of polymerization below 200 being an effective inhibitor at just 1 mg.mL–1. Other synthetic and natural polymers, both with and without hydroxyl-functional side chains, showed negligible activity, highlighting the unique ice/water interacting properties of PVA. These findings both aid our understanding of ice nucleation but demonstrate the potential of engineering synthetic polymers as new biomimetics to control ice formation/growth processes PMID:26258729

Phase field simulations of autocatalytic formation of alpha lamellar colonies in Ti-6Al-4V

DOE PAGES

Radhakrishnan, Bala; Gorti, Sarma; Babu, Suresh Sudharsanam

2016-09-13

Here, we present phase field simulations incorporating energy contributions due to thermodynamics, and anisotropic interfacial and strain energies, to demonstrate the nucleation and growth of multiple variants of alpha from beta in Ti-6Al-4V under isothermal conditions. The simulations focused on the effect of thermodynamic driving force and nucleation rate on the morphology of the transformed alpha assuming that the partitioning of V between beta and alpha is negligible for short isothermal holds. The results indicate that a high nucleation rate favors the formation of the basket-weave structure. However, at a lower nucleation rate the simulations show the intragranular nucleation ofmore » a colony structure by an autocatalytic nucleation mechanism adjacent to a pre-existing alpha variant. New side-plates of the same variant appear to nucleate progressively and grow to form the colony. The isothermal simulation results are used to offer a possible explanation for the transition from a largely basket weave structure to a colony structure inside narrow layer bands occurring during continuous heating and cooling conditions encountered during laser additive manufacturing of Ti-6Al-4V.« less

Supersaturation-nucleation behavior of poorly soluble drugs and its impact on the oral absorption of drugs in thermodynamically high-energy forms.

PubMed

Ozaki, Shunsuke; Minamisono, Takuma; Yamashita, Taro; Kato, Takashi; Kushida, Ikuo

2012-01-01

In order to better understand the oral absorption behavior of poorly water-soluble drugs, their supersaturation-nucleation behavior was characterized in fasted state simulated intestinal fluid. The induction time (t(ind)) for nucleation was measured for four model drugs: itraconazole, erlotinib, troglitazone, and PLX4032. Supersaturated solutions were prepared by solvent shift method, and nucleation initiation was monitored by ultraviolet detection. The relationship between t(ind) and degree of supersaturation was analyzed in terms of classical nucleation theory. The defined supersaturation stability proved to be compound specific. Clinical data on oral absorption were investigated for drugs in thermodynamically high-energy forms such as amorphous forms and salts and was compared with in vitro supersaturation-nucleation characteristics. Solubility-limited maximum absorbable dose was proportionate to intestinal effective drug concentrations, which are related to supersaturation stability and thermodynamic solubility. Supersaturation stability was shown to be an important factor in determining the effect of high-energy forms. The characterization of supersaturation-nucleation behavior by the presented method is, therefore, valuable for assessing the potential absorbability of poorly water-soluble drugs. Copyright © 2011 Wiley-Liss, Inc.

Effects of ensembles on methane hydrate nucleation kinetics.

PubMed

Zhang, Zhengcai; Liu, Chan-Juan; Walsh, Matthew R; Guo, Guang-Jun

2016-06-21

By performing molecular dynamics simulations to form a hydrate with a methane nano-bubble in liquid water at 250 K and 50 MPa, we report how different ensembles, such as the NPT, NVT, and NVE ensembles, affect the nucleation kinetics of the methane hydrate. The nucleation trajectories are monitored using the face-saturated incomplete cage analysis (FSICA) and the mutually coordinated guest (MCG) order parameter (OP). The nucleation rate and the critical nucleus are obtained using the mean first-passage time (MFPT) method based on the FS cages and the MCG-1 OPs, respectively. The fitting results of MFPT show that hydrate nucleation and growth are coupled together, consistent with the cage adsorption hypothesis which emphasizes that the cage adsorption of methane is a mechanism for both hydrate nucleation and growth. For the three different ensembles, the hydrate nucleation rate is quantitatively ordered as follows: NPT > NVT > NVE, while the sequence of hydrate crystallinity is exactly reversed. However, the largest size of the critical nucleus appears in the NVT ensemble, rather than in the NVE ensemble. These results are helpful for choosing a suitable ensemble when to study hydrate formation via computer simulations, and emphasize the importance of the order degree of the critical nucleus.

Chlorine-containing salts as water ice nucleating particles on Mars

NASA Astrophysics Data System (ADS)

Santiago-Materese, D. L.; Iraci, L. T.; Clapham, M. E.; Chuang, P. Y.

2018-03-01

Water ice cloud formation on Mars largely is expected to occur on the most efficient ice nucleating particle available. Salts have been observed on the Martian surface and have been known to facilitate water cloud formation on Earth. We examined heterogeneous ice nucleation onto sodium chloride and sodium perchlorate substrates under Martian atmospheric conditions, in the range of 150 to 180 K and 10-7 to 10-5 Torr water partial pressure. Sub-155 K data for the critical saturation ratio (Scrit) suggests an exponential model best describes the temperature-dependence of nucleation onset of water ice for all substrates tested. While sodium chloride does not facilitate water ice nucleation more easily than bare silicon, sodium perchlorate does support depositional nucleation at lower saturation levels than other substrates shown and is comparable to smectite-rich clay in its ability to support cloud initiation. Perchlorates could nucleate water ice at partial pressures up to 40% lower than other substrates examined to date under Martian atmospheric conditions. These findings suggest air masses on Mars containing uplifted salts such as perchlorates could form water ice clouds at lower saturation ratios than in air masses absent similar particles.

The Stress-Dependent Activation Parameters for Dislocation Nucleation in Molybdenum Nanoparticles.

PubMed

Chachamovitz, Doron; Mordehai, Dan

2018-03-02

Many specimens at the nanoscale are pristine of dislocations, line defects which are the main carriers of plasticity. As a result, they exhibit extremely high strengths which are dislocation-nucleation controlled. Since nucleation is a thermally activated process, it is essential to quantify the stress-dependent activation parameters for dislocation nucleation in order to study the strength of specimens at the nanoscale and its distribution. In this work, we calculate the strength of Mo nanoparticles in molecular dynamics simulations and we propose a method to extract the activation free-energy barrier for dislocation nucleation from the distribution of the results. We show that by deforming the nanoparticles at a constant strain rate, their strength distribution can be approximated by a normal distribution, from which the activation volumes at different stresses and temperatures are calculated directly. We found that the activation energy dependency on the stress near spontaneous nucleation conditions obeys a power-law with a critical exponent of approximately 3/2, which is in accordance with critical exponents found in other thermally activated processes but never for dislocation nucleation. Additionally, significant activation entropies were calculated. Finally, we generalize the approach to calculate the activation parameters for other driving-force dependent thermally activated processes.

Stimulation of microtubule-based transport by nucleation of microtubules on pigment granules

PubMed Central

Semenova, Irina; Gupta, Dipika; Usui, Takeo; Hayakawa, Ichiro; Cowan, Ann; Rodionov, Vladimir

2017-01-01

Microtubule (MT)-based transport can be regulated through changes in organization of MT transport tracks, but the mechanisms that regulate these changes are poorly understood. In Xenopus melanophores, aggregation of pigment granules in the cell center involves their capture by the tips of MTs growing toward the cell periphery, and granule aggregation signals facilitate capture by increasing the number of growing MT tips. This increase could be explained by stimulation of MT nucleation either on the centrosome or on the aggregate of pigment granules that gradually forms in the cell center. We blocked movement of pigment granules to the cell center and compared the MT-nucleation activity of the centrosome in the same cells in two signaling states. We found that granule aggregation signals did not stimulate MT nucleation on the centrosome but did increase MT nucleation activity of pigment granules. Elevation of MT-nucleation activity correlated with the recruitment to pigment granules of a major component of MT-nucleation templates, γ-tubulin, and was suppressed by γ-tubulin inhibitors. We conclude that generation of new MT transport tracks by concentration of the leading pigment granules provides a positive feedback loop that enhances delivery of trailing granules to the cell center. PMID:28381426

A marine biogenic source of atmospheric ice-nucleating particles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wilson, T. W.; Ladino, L. A.; Alpert, Peter A.

2015-09-09

The formation of ice in clouds is facilitated by the presence of airborne ice nucleating particles1,2. Sea spray is one of the major global sources of atmospheric particles, but it is unclear to what extent these particles are capable of nucleating ice3–11. Here we show that material in the sea surface microlayer, which is enriched in surface active organic material representative of that found in sub-micron sea- spray aerosol12–21, nucleates ice under conditions that occur in mixed-phase clouds and high-altitude ice clouds. The ice active material is likely biogenic and is less than ~0.2 ?m in size. We also showmore » that organic material (exudate) released by a common marine diatom nucleates ice when separated from cells and propose that organic material associated with phytoplankton cell exudates are a candidate for the observed ice nucleating ability of the microlayer samples. By combining our measurements with global model simulations of marine organic aerosol, we show that ice nucleating particles of marine origin are dominant in remote marine environments, such as the Southern Ocean, the North Pacific and the North Atlantic.« less

Implications of the Homogeneous Nucleation Barrier for Top-Down Crystallization in Mercury's Core

NASA Astrophysics Data System (ADS)

Huguet, L.; Hauck, S. A.; Van Orman, J. A.; Jing, Z.

2018-05-01

Crystallization of solids in planetary cores depends both on ambient temperatures falling below the liquidus and on the ability to nucleate crystal growth. We discuss the implications of the nucleation barrier for thermal evolution of Mercury's core.

A Comparative Study of Nucleation Parameterizations: 2. Three-Dimensional Model Application and Evaluation

EPA Science Inventory

Following the examination and evaluation of 12 nucleation parameterizations presented in part 1, 11 of them representing binary, ternary, kinetic, and cluster‐activated nucleation theories are evaluated in the U.S. Environmental Protection Agency Community Multiscale Air Quality ...

A simple apparatus for controlling nucleation and size in protein crystal growth

NASA Technical Reports Server (NTRS)

Gernert, Kim M.; Smith, Robert; Carter, Daniel C.

1988-01-01

A simple device is described for controlling vapor equilibrium in macromolecular crystallization as applied to the protein crystal growth technique commonly referred to as the 'hanging drop' method. Crystal growth experiments with hen egg white lysozyme have demonstrated control of the nucleation rate. Nucleation rate and final crystal size have been found to be highly dependent upon the rate at which critical supersaturation is approached. Slower approaches show a marked decrease in the nucleation rate and an increase in crystal size.

Molecular beam epitaxial growth and structural characterization of ZnS on (001) GaAs

NASA Technical Reports Server (NTRS)

Benz, R. G., II; Huang, P. C.; Stock, S. R.; Summers, C. J.

1988-01-01

The effect of surface nucleation processes on the quality of ZnS layers grown on (001) GaAs substrates by molecular beam epitaxy is reported. Reflection high energy electron diffraction indicated that nucleation at high temperatures produced more planar surfaces than nucleation at low temperatures, but the crystalline quality as assessed by X-ray double crystal diffractometry is relatively independent of nucleation temperature. A critical factor in layer quality was the initial roughness of the GaAs surfaces.

Nucleation of insulin crystals in a wide continuous supersaturation gradient.

PubMed

Penkova, Anita; Dimitrov, Ivaylo; Nanev, Christo

2004-11-01

Modifying the classical double pulse technique, by using a supersaturation gradient along an insulin solution contained in a glass capillary tube, we found conditions appropriate for the direct measurement of nucleation parameters. The nucleation time lag has been measured. Data for the number of crystal nuclei versus the nucleation time were obtained for this hormone. Insulin was chosen as a model protein because of the availability of solubility data in the literature. A comparison with the results for hen-egg-white lysozyme, HEWL was performed.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rebowski, Grzegorz; Namgoong, Suk; Boczkowska, Malgorzata

Actin filament nucleators initiate polymerization in cells in a regulated manner. A common architecture among these molecules consists of tandem WASP homology 2 domains (W domains) that recruit three to four actin subunits to form a polymerization nucleus. We describe a low-resolution crystal structure of an actin dimer assembled by tandem W domains, where the first W domain is cross-linked to Cys374 of the actin subunit bound to it, whereas the last W domain is followed by the C-terminal pointed end-capping helix of thymosin {beta}4. While the arrangement of actin subunits in the dimer resembles that of a long-pitch helixmore » of the actin filament, important differences are observed. These differences result from steric hindrance of the W domain with intersubunit contacts in the actin filament. We also determined the structure of the first W domain of Vibrio parahaemolyticus VopL cross-linked to actin Cys374 and show it to be nearly identical with non-cross-linked W-Actin structures. This result validates the use of cross-linking as a tool for the study of actin nucleation complexes, whose natural tendency to polymerize interferes with most structural methods. Combined with a biochemical analysis of nucleation, the structures may explain why nucleators based on tandem W domains with short inter-W linkers have relatively weak activity, cannot stay bound to filaments after nucleation, and are unlikely to influence filament elongation. The findings may also explain why nucleation-promoting factors of the Arp2/3 complex, which are related to tandem-W-domain nucleators, are ejected from branch junctions after nucleation. We finally show that the simple addition of the C-terminal pointed end-capping helix of thymosin {beta}4 to tandem W domains can change their activity from actin filament nucleation to monomer sequestration.« less

Ice formation on kaolinite: Insights from molecular dynamics simulations

NASA Astrophysics Data System (ADS)

Sosso, Gabriele C.; Tribello, Gareth A.; Zen, Andrea; Pedevilla, Philipp; Michaelides, Angelos

2016-12-01

The formation of ice affects many aspects of our everyday life as well as important technologies such as cryotherapy and cryopreservation. Foreign substances almost always aid water freezing through heterogeneous ice nucleation, but the molecular details of this process remain largely unknown. In fact, insight into the microscopic mechanism of ice formation on different substrates is difficult to obtain even if state-of-the-art experimental techniques are used. At the same time, atomistic simulations of heterogeneous ice nucleation frequently face extraordinary challenges due to the complexity of the water-substrate interaction and the long time scales that characterize nucleation events. Here, we have investigated several aspects of molecular dynamics simulations of heterogeneous ice nucleation considering as a prototypical ice nucleating material the clay mineral kaolinite, which is of relevance in atmospheric science. We show via seeded molecular dynamics simulations that ice nucleation on the hydroxylated (001) face of kaolinite proceeds exclusively via the formation of the hexagonal ice polytype. The critical nucleus size is two times smaller than that obtained for homogeneous nucleation at the same supercooling. Previous findings suggested that the flexibility of the kaolinite surface can alter the time scale for ice nucleation within molecular dynamics simulations. However, we here demonstrate that equally flexible (or non flexible) kaolinite surfaces can lead to very different outcomes in terms of ice formation, according to whether or not the surface relaxation of the clay is taken into account. We show that very small structural changes upon relaxation dramatically alter the ability of kaolinite to provide a template for the formation of a hexagonal overlayer of water molecules at the water-kaolinite interface, and that this relaxation therefore determines the nucleation ability of this mineral.

Heterogeneous ice nucleation on phase-separated organic-sulfate particles: effect of liquid vs. glassy coatings

NASA Astrophysics Data System (ADS)

Schill, G. P.; Tolbert, M. A.

2013-05-01

Atmospheric ice nucleation on aerosol particles relevant to cirrus clouds remains one of the least understood processes in the atmosphere. Upper tropospheric aerosols as well as sub-visible cirrus residues are known to be enhanced in both sulfates and organics. The hygroscopic phase transitions of organic-sulfate particles can have an impact on both the cirrus cloud formation mechanism and resulting cloud microphysical properties. In addition to deliquescence and efflorescence, organic-sulfate particles are known to undergo another phase transition known as liquid-liquid phase separation. The ice nucleation properties of particles that have undergone liquid-liquid phase separation are unknown. Here, Raman microscopy coupled with an environmental cell was used to study the low temperature deliquescence, efflorescence, and liquid-liquid phase separation behavior of 2 : 1 mixtures of organic polyols (1,2,6-hexanetriol and 1 : 1 1,2,6-hexanetriol + 2,2,6,6-tetrakis(hydroxymethyl)cyclohexanol) and ammonium sulfate from 240-265 K. Further, the ice nucleation efficiency of these organic-sulfate systems after liquid-liquid phase separation and efflorescence was investigated from 210-235 K. Raman mapping and volume-geometry analysis indicate that these particles contain solid ammonium sulfate cores fully engulfed in organic shells. For the ice nucleation experiments, we find that if the organic coatings are liquid, water vapor diffuses through the shell and ice nucleates on the ammonium sulfate core. In this case, the coatings minimally affect the ice nucleation efficiency of ammonium sulfate. In contrast, if the coatings become semi-solid or glassy, ice instead nucleates on the organic shell. Consistent with recent findings that glasses can be efficient ice nuclei, the phase-separated particles are nearly as efficient at ice nucleation as pure crystalline ammonium sulfate.

Heterogeneous ice nucleation on phase-separated organic-sulfate particles: effect of liquid vs. glassy coatings

NASA Astrophysics Data System (ADS)

Schill, G. P.; Tolbert, M. A.

2012-12-01

Atmospheric ice nucleation on aerosol particles relevant to cirrus clouds remains one of the least understood processes in the atmosphere. Upper tropospheric aerosols as well as sub-visible cirrus residues are known to be enhanced in both sulfates and organics. The hygroscopic phase transitions of organic-sulfate particles can have an impact on both the cirrus cloud formation mechanism and resulting cloud microphysical properties. In addition to deliquescence and efflorescence, organic-sulfate particles are known to undergo another phase transition known as liquid-liquid phase separation. The ice nucleation properties of particles that have undergone liquid-liquid phase separation are unknown. Here, Raman microscopy coupled with an environmental cell was used to study the low temperature deliquescence, efflorescence, and liquid-liquid phase separation behavior of 2:1 mixtures of organic polyols (1,2,6-hexanetriol, and 1:1 1,2,6-hexanetriol +2,2,6,6-tetrakis(hydroxymethyl)cycohexanol) and ammonium sulfate from 240-265 K. Further, the ice nucleation efficiency of these organic-sulfate systems after liquid-liquid phase separation and efflorescence was investigated from 210-235 K. Raman mapping and volume-geometry analysis indicates that these particles contain solid ammonium sulfate cores fully engulfed in organic shells. For the ice nucleation experiments, we find that if the organic coatings are liquid, water vapor diffuses through the shell and ice nucleates on the ammonium sulfate core. In this case, the coatings minimally affect the ice nucleation efficiency of ammonium sulfate. In contrast, if the coatings become semi-solid or glassy, ice instead nucleates on the organic shell. Consistent with recent findings that glasses can be efficient ice nuclei, the phase separated particles are nearly as efficient at ice nucleation as pure crystalline ammonium sulfate.

Oxygen induced strain field homogenization in AlN nucleation layers and its impact on GaN grown by metal organic vapor phase epitaxy on sapphire: An x-ray diffraction study

NASA Astrophysics Data System (ADS)

Bläsing, J.; Krost, A.; Hertkorn, J.; Scholz, F.; Kirste, L.; Chuvilin, A.; Kaiser, U.

2009-02-01

This paper presents an x-ray study of GaN, which is grown on nominally undoped and oxygen-doped AlN nucleation layers on sapphire substrates by metal organic vapor phase epitaxy. Without additional oxygen doping a trimodal nucleation distribution of AlN is observed leading to inhomogeneous in-plane strain fields, whereas in oxygen-doped layers a homogeneous distribution of nucleation centers is observed. In both types of nucleation layers extremely sharp correlation peaks occur in transverse ω-scans which are attributed to a high density of edge-type dislocations having an in-plane Burgers vector. The correlation peaks are still visible in the (0002) ω-scans of 500 nm GaN which might mislead an observer to conclude incorrectly that there exists an extremely high structural quality. For the undoped nucleation layers depth-sensitive measurements in grazing incidence geometry reveal a strong thickness dependence of the lattice parameter a, whereas no such dependence is observed for doped samples. For oxygen-doped nucleation layers, in cross-sectional transmission electron microscopy images a high density of stacking faults parallel to the substrate surface is found in contrast to undoped nucleation layers where a high density of threading dislocations is visible. GaN of 2.5 μm grown on top of 25 nm AlN nucleation layers with an additional in situ SiN mask show full widths at half maximum of 160″ and 190″ in (0002) and (10-10) high-resolution x-ray diffraction ω-scans, respectively.

Subtle charge balance controls surface-nucleated self-assembly of designed biopolymers.

PubMed

Charbonneau, Céline; Kleijn, J Mieke; Cohen Stuart, Martien A

2014-03-25

We report the surface-nucleated self-assembly into fibrils of a biosynthetic amino acid polymer synthesized by the yeast Pichia pastoris. This polymer has a block-like architecture, with a central silk-like block labeled SH, responsible for the self-assembly into fibrils, and two collagen-like random coil end blocks (C) that colloidally stabilize the fibers in aqueous solution. The silk-like block contains histidine residues (pKa≈6) that are positively charged in the low pH region, which hinders self-assembly. In aqueous solution, CSHC self-assembles into fibers above a pH-dependent critical nucleation concentration Ccb. Below Ccb, where no self-assembly occurs in solution, fibril formation can be induced by a negatively charged surface (silica) in the pH range of 3.5-7. The density of the fibers at the surface and their length are controlled by a subtle balance in charge between the protein polymer and the silica surface, which is evidenced from the dependence on pH. With increasing number density of the fibers at the surface, their average length decreases. The results can be explained on the basis of a nucleation-and-growth mechanism: the surface density of fibers depends on the rate of nucleation, while their growth rate is limited by transport of proteins from solution. Screening of the charges on the surface and histidine units by adding NaCl influences the nucleation-and-growth process in a complicated fashion: at low pH, the growth is improved, while at high pH, the nucleation is limited. Under conditions where nucleation in the bulk solution is not possible, growth of the surface-nucleated fibers into the solution--away from the surface--can still occur.

Comparison of the SAWNUC model with CLOUD measurements of sulphuric acid-water nucleation.

PubMed

Ehrhart, Sebastian; Ickes, Luisa; Almeida, Joao; Amorim, Antonio; Barmet, Peter; Bianchi, Federico; Dommen, Josef; Dunne, Eimear M; Duplissy, Jonathan; Franchin, Alessandro; Kangasluoma, Juha; Kirkby, Jasper; Kürten, Andreas; Kupc, Agnieszka; Lehtipalo, Katrianne; Nieminen, Tuomo; Riccobono, Francesco; Rondo, Linda; Schobesberger, Siegfried; Steiner, Gerhard; Tomé, António; Wimmer, Daniela; Baltensperger, Urs; Wagner, Paul E; Curtius, Joachim

2016-10-27

Binary nucleation of sulphuric acid-water particles is expected to be an important process in the free troposphere at low temperatures. SAWNUC (Sulphuric Acid Water Nucleation) is a model of binary nucleation that is based on laboratory measurements of the binding energies of sulphuric acid and water in charged and neutral clusters. Predictions of SAWNUC are compared for the first time comprehensively with experimental binary nucleation data from the CLOUD chamber at European Organization for Nuclear Research. The experimental measurements span a temperature range of 208-292 K, sulphuric acid concentrations from 1·10 6 to 1·10 9  cm -3 , and distinguish between ion-induced and neutral nucleation. Good agreement, within a factor of 5, is found between the experimental and modeled formation rates for ion-induced nucleation at 278 K and below and for neutral nucleation at 208 and 223 K. Differences at warm temperatures are attributed to ammonia contamination which was indicated by the presence of ammonia-sulphuric acid clusters, detected by an Atmospheric Pressure Interface Time of Flight (APi-TOF) mass spectrometer. APi-TOF measurements of the sulphuric acid ion cluster distributions ( (H2SO4)i·HSO4- with i = 0, 1, ..., 10) show qualitative agreement with the SAWNUC ion cluster distributions. Remaining differences between the measured and modeled distributions are most likely due to fragmentation in the APi-TOF. The CLOUD results are in good agreement with previously measured cluster binding energies and show the SAWNUC model to be a good representation of ion-induced and neutral binary nucleation of sulphuric acid-water clusters in the middle and upper troposphere.

New particle formation in rural areas - what is behind the story?

NASA Astrophysics Data System (ADS)

Boy, M.; Sihto, S. L.-; Lauros, J.; Bonn, B.; Guenther, A.

2009-04-01

New particle formation in rural areas - what's behind it? M. Boy(1), S.-L. Sihto(1), J. Lauros(1), B. Bonn(2) and A. Guenther(3) (1) Department of Physical Sciences, P.O. Box 64, 00014 University of Helsinki, Finland. (2) Institute of Atmospherical and Environmental Sciences, J. W. Goethe University, Altenhöferallee 1, D-60438 Frankfurt/Main, Germany (3) ACD, NCAR, P.O. Box 3000, 80305 Boulder, Colorado, USA. New particle formation has been observed at almost all sites, where both particle number concentrations and size distributions have been measured. Although many field campaigns, laboratory experiments and new modelling approaches have led to increased understanding, detailed mechanisms responsible for the formation of new particles in the troposphere have still not been completely elucidated. In MALTE (Model to predict new Aerosol formation in the Lower Troposphere) individually developed codes from different institutes around the globe merged into a one-dimensional model including aerosol dynamics, boundary layer meteorology, biology and chemistry in order to investigate the formation and growth processes of Secondary Organic Aerosols (SOA) under realistic atmospheric conditions. Our knowledge concerning the formation of very small particles or clusters is still limited. The question of which molecules are involved in the atmospheric nucleation processes remains controversial within the aerosol community. MALTE takes several hypothetical nucleation theories into account for the formation of secondary aerosols. To test the different hypothesis concerning atmospheric nucleation we used data from several field stations in Europe, USA, Africa and Australia. Furthermore, we investigated the role of certain organic vapours in the particle formation processes during intensive laboratory chamber experiments at NCAR - National Centre for Atmospheric Research in Boulder, Colorado, USA. Initial model simulations performed with the activation or kinetic nucleation mechanism predict nucleation rates that are strongly correlated with observed nucleation rates at background areas like boreal forest in Finland or somewhat more polluted areas east of Leipzig, Germany. In addition, many chamber experiments raise the possibility that organic vapours may form new particles by organic nucleation. Regarding the possibility that organic vapours can nucleate, as apparently observed in several chamber experiments, we estimated with MALTE that even with low SO2 concentrations, sufficient sulphuric acid is generated to explain observed particle number concentrations via the activation or kinetic nucleation mechanism alone. To investigate the possibility for nucleation of organic vapours we included in MALTE a parameterization developed by Bonn et al. (2008). Calculated nucleation rates with the organic nucleation code for data achieved during a field campaign in USA and the chamber experiments at NCAR showed high agreement with the measurements. It is currently not possible to draw a final conclusion if nucleation of organic vapours in the real atmosphere is present and if yes how important it is compared to other nucleation mechanisms.

Why are some Interfaces in Materials Stronger than others?

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fensin, Saryu J.; Cerreta, Ellen K.; Gray, George T.

2014-06-26

Grain boundaries (GBs) are often the preferred sites for void nucleation in ductile metals. However, it has been observed that all boundaries do not contribute equally to this process. We present a mechanistic rationale for the role of GBs in damage nucleation in copper, along with a quantitative map for predicting preferred void nucleation at GBs based on molecular dynamics simulations in copper. Simulations show a direct correlation between the void nucleation stress and the ability of a grain boundary to plastically deform by emitting dislocations, during shock compression. Plastic response of a GB, affects the development of stress concentrationsmore » believed to be responsible for void nucleation by acting as a dissipation mechanism for the applied stress.« less

First Measurements of Time-Dependent Nucleation as a Function of Composition in Na2O.2CaO.3SiO2 Glasses

NASA Technical Reports Server (NTRS)

Kelton, K. F.; Narayan, K. Lakshmi

1996-01-01

The first measurements in any system of the composition dependence of the time-dependent nucleation rate are presented Nucleation rates of the stoichiometric crystalline phase, Na2O.2CaO.3SiO2, from quenched glasses made with different SiO2 concentrations were determined as a function of temperature and glass composition. A strong compositional dependence of the nucleation rates and a weak dependence for the induction times are observed. Using measured values of the liquidus temperatures and growth velocities as a function of glass composition, these data are shown to be consistent with predictions from the classical theory of nucleation, assuming a composition-dependent interfacial energy.

Homogeneous nucleation and microstructure evolution in million-atom molecular dynamics simulation

PubMed Central

Shibuta, Yasushi; Oguchi, Kanae; Takaki, Tomohiro; Ohno, Munekazu

2015-01-01

Homogeneous nucleation from an undercooled iron melt is investigated by the statistical sampling of million-atom molecular dynamics (MD) simulations performed on a graphics processing unit (GPU). Fifty independent instances of isothermal MD calculations with one million atoms in a quasi-two-dimensional cell over a nanosecond reveal that the nucleation rate and the incubation time of nucleation as functions of temperature have characteristic shapes with a nose at the critical temperature. This indicates that thermally activated homogeneous nucleation occurs spontaneously in MD simulations without any inducing factor, whereas most previous studies have employed factors such as pressure, surface effect, and continuous cooling to induce nucleation. Moreover, further calculations over ten nanoseconds capture the microstructure evolution on the order of tens of nanometers from the atomistic viewpoint and the grain growth exponent is directly estimated. Our novel approach based on the concept of “melting pots in a supercomputer” is opening a new phase in computational metallurgy with the aid of rapid advances in computational environments. PMID:26311304

Influence of Mineral Dust On New Particle Formation and Growth: A Model Study of Heterogeneous Nucleation

NASA Astrophysics Data System (ADS)

Korhonen, H.; Kulmala, M.; Lauri, A.

Analyses of nucleation events have emphasized the importance of pre-existing par- ticles in new particle formation. When coarse mode aerosol is present, it typically dominates the condensation sink for trace gases and thus inhibits the onset of nucle- ation. A monodisperse aerosol dynamical box model is applied to investigate the effect of soluble coated mineral dust particles on new particle formation. The model in- cludes ternary H2SO4­H2O­NH3 nucleation, multicomponent condensation, coagu- lation and dry deposition. As the soluble coating can significantly change the ability of dust particles to serve as a condensation sink for condensable vapours, different mech- anisms of coating, including heterogeneous nucleation of sulphuric acid, are consid- ered. Preliminary results show that the presence of micron sized soluble coated dust par- ticles can even at relatively low concentrations prevent homogeneous nucleation or growth of nucleated particles to detectable size of 3 nm. Furthermore, critical conden- sation sinks obtained by model simulations correspond to measured sinks.

Ice-nucleating bacteria control the order and dynamics of interfacial water

DOE PAGES

Pandey, Ravindra; Usui, Kota; Livingstone, Ruth A.; ...

2016-04-22

Ice-nucleating organisms play important roles in the environment. With their ability to induce ice formation at temperatures just below the ice melting point, bacteria such as Pseudomonas syringae attack plants through frost damage using specialized ice-nucleating proteins. Besides the impact on agriculture and microbial ecology, airborne P. syringae can affect atmospheric glaciation processes, with consequences for cloud evolution, precipitation, and climate. Biogenic ice nucleation is also relevant for artificial snow production and for biomimetic materials for controlled interfacial freezing. We use interface-specific sum frequency generation (SFG) spectroscopy to show that hydrogen bonding at the water-bacteria contact imposes structural ordering onmore » the adjacent water network. Experimental SFG data and molecular dynamics simulations demonstrate that ice active sites within P. syringae feature unique hydrophilic-hydrophobic patterns to enhance ice nucleation. Finally, the freezing transition is further facilitated by the highly effective removal of latent heat from the nucleation site, as apparent from time-resolved SFG spectroscopy.« less

Overcoming the Time Limitation in Molecular Dynamics Simulation of Crystal Nucleation: A Persistent-Embryo Approach

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sun, Yang; Song, Huajing; Zhang, Feng

The crystal nucleation from liquid in most cases is too rare to be accessed within the limited time scales of the conventional molecular dynamics (MD) simulation. Here, we developed a “persistent embryo” method to facilitate crystal nucleation in MD simulations by preventing small crystal embryos from melting using external spring forces. We applied this method to the pure Ni case for a moderate undercooling where no nucleation can be observed in the conventional MD simulation, and obtained nucleation rate in good agreement with the experimental data. Moreover, the method is applied to simulate an even more sluggish event: the nucleationmore » of the B2 phase in a strong glass-forming Cu-Zr alloy. The nucleation rate was found to be 8 orders of magnitude smaller than Ni at the same undercooling, which well explains the good glass formability of the alloy. In conclusion, our work opens a new avenue to study solidification under realistic experimental conditions via atomistic computer simulation.« less

Analysis of the Effect of Water Activity on Ice Formation Using a New Theory of Nucleation

NASA Technical Reports Server (NTRS)

Barahona, Donifan

2013-01-01

In this work a new theory of nucleation is developed and used to investigate the effect of water activity on the formation of ice within super-cooled droplets. The new theory is based on a novel concept where the interface is assumed to be made of liquid molecules trapped by the solid matrix. Using this concept new expressions are developed for the critical ice germ size and the nucleation work, with explicit dependencies on temperature and water activity. However unlike previous approaches, the new theory does not depend on the interfacial tension between liquid and ice. Comparison against experimental results shows that the new theory is able to reproduce the observed effect of water activity on nucleation rate and freezing temperature. It allows for the first time a theoretical derivation of the constant shift in water activity between melting and nucleation. The new theory offers a consistent thermodynamic view of ice nucleation, simple enough to be applied in atmospheric models of cloud formation.

Overcoming the Time Limitation in Molecular Dynamics Simulation of Crystal Nucleation: A Persistent-Embryo Approach

DOE PAGES

Sun, Yang; Song, Huajing; Zhang, Feng; ...

2018-02-23

The crystal nucleation from liquid in most cases is too rare to be accessed within the limited time scales of the conventional molecular dynamics (MD) simulation. Here, we developed a “persistent embryo” method to facilitate crystal nucleation in MD simulations by preventing small crystal embryos from melting using external spring forces. We applied this method to the pure Ni case for a moderate undercooling where no nucleation can be observed in the conventional MD simulation, and obtained nucleation rate in good agreement with the experimental data. Moreover, the method is applied to simulate an even more sluggish event: the nucleationmore » of the B2 phase in a strong glass-forming Cu-Zr alloy. The nucleation rate was found to be 8 orders of magnitude smaller than Ni at the same undercooling, which well explains the good glass formability of the alloy. In conclusion, our work opens a new avenue to study solidification under realistic experimental conditions via atomistic computer simulation.« less

The role of the cubic structure in freezing of a supercooled water droplet on an ice substrate

NASA Astrophysics Data System (ADS)

Takahashi, T.; Kobayashi, T.

1983-12-01

The possibility of the formation of a metastable cubic (diamond) structure and its role in freezing of a supercooled water droplet on an ice substrate are discussed in terms of two-dimensional nucleation. The mode of stacking sequence of new layers formed by two-dimensional nucleation is divided into single and multi-nucleation according to the degree of supercooling and to the size of the supercooled droplet. In the case of single nucleation a frozen droplet develops into a complete hexagonal single crystal or an optically single crystal (containing discontinuous stacking faults). In the case of multi-nucleation attention is paid to the size effect and the stacking direction of the nucleus to calculate the waiting time in the nucleation. Then the frozen droplets are crystallographically divided into three categories: completely single crystals, optically single crystals (containing a small cubic structure, i.e. stacking faults) and polycrystals with a misorientation of 70.53° between the c-axes.

Drosophila Spire is an actin nucleation factor.

PubMed

Quinlan, Margot E; Heuser, John E; Kerkhoff, Eugen; Mullins, R Dyche

2005-01-27

The actin cytoskeleton is essential for many cellular functions including shape determination, intracellular transport and locomotion. Previous work has identified two factors--the Arp2/3 complex and the formin family of proteins--that nucleate new actin filaments via different mechanisms. Here we show that the Drosophila protein Spire represents a third class of actin nucleation factor. In vitro, Spire nucleates new filaments at a rate that is similar to that of the formin family of proteins but slower than in the activated Arp2/3 complex, and it remains associated with the slow-growing pointed end of the new filament. Spire contains a cluster of four WASP homology 2 (WH2) domains, each of which binds an actin monomer. Maximal nucleation activity requires all four WH2 domains along with an additional actin-binding motif, conserved among Spire proteins. Spire itself is conserved among metazoans and, together with the formin Cappuccino, is required for axis specification in oocytes and embryos, suggesting that multiple actin nucleation factors collaborate to construct essential cytoskeletal structures.

Martensite Embryology

NASA Astrophysics Data System (ADS)

Reid, Andrew C. E.; Olson, Gregory B.

2000-03-01

Heterogeneous nucleation of martensite is modeled by examining the strain field of a dislocation array in a nonlinear, nonlocal continuum elastic matrix. The dislocations are modeled by including effects from atomic length scales, which control the dislocation Burger's vector, into a mesoscopic continuum model. The dislocation array models the heterogeneous nucleation source of the Olson/Cohen defect dissociation model, and depending on the potency can give rise to embryos of different character. High potency dislocations give rise to fully developed, classical pre-existing embryos, whereas low-potency dislocations result in the formation of highly nonclassical strain embryos. Heterogeneous nucleation theory is related to nucleation kinetics through the critical driving force for nucleation at a defect of a given potency. Recent stereological and calorimetric kinetic studies in thermoelastic TiNi alloys confirm that these materials exhibit the same form of defect potency distribution and resulting sample-size dependent Martensite start temperature, M_s, as nonthermoelastic FeNi systems. These results together point towards a broad theory of heterogeneous nucleation for both thermoelastic and nonthermoelastic martensites.

Does hydrophilicity of carbon particles improve their ice nucleation ability?

PubMed

Lupi, Laura; Molinero, Valeria

2014-09-04

Carbonaceous particles account for 10% of the particulate matter in the atmosphere. Atmospheric oxidation and aging of soot modulates its ice nucleation ability. It has been suggested that an increase in the ice nucleation ability of aged soot results from an increase in the hydrophilicity of the surfaces upon oxidation. Oxidation, however, also impacts the nanostructure of soot, making it difficult to assess the separate effects of soot nanostructure and hydrophilicity in experiments. Here we use molecular dynamics simulations to investigate the effect of changes in hydrophilicity of model graphitic surfaces on the freezing temperature of ice. Our results indicate that the hydrophilicity of the surface is not in general a good predictor of ice nucleation ability. We find a correlation between the ability of a surface to promote nucleation of ice and the layering of liquid water at the surface. The results of this work suggest that ordering of liquid water in contact with the surface plays an important role in the heterogeneous ice nucleation mechanism.

Ice nucleation rates near ˜225 K

NASA Astrophysics Data System (ADS)

Amaya, Andrew J.; Wyslouzil, Barbara E.

2018-02-01

We have measured the ice nucleation rates, Jice, in supercooled nano-droplets with radii ranging from 6.6 nm to 10 nm and droplet temperatures, Td, ranging from 225 K to 204 K. The initial temperature of the 10 nm water droplets is ˜250 K, i.e., well above the homogeneous nucleation temperature for micron sized water droplets, TH ˜235 K. The nucleation rates increase systematically from ˜1021 cm-3 s-1 to ˜1022 cm-3 s-1 in this temperature range, overlap with the nucleation rates of Manka et al. [Phys. Chem. Chem. Phys. 14, 4505 (2012)], and suggest that experiments with larger droplets would extrapolate smoothly the rates of Hagen et al. [J. Atmos. Sci. 38, 1236 (1981)]. The sharp corner in the rate data as temperature drops is, however, difficult to match with available theory even if we correct classical nucleation theory and the physical properties of water for the high internal pressure of the nanodroplets.

Control over phase separation and nucleation using a laser-tweezing potential

NASA Astrophysics Data System (ADS)

Walton, Finlay; Wynne, Klaas

2018-05-01

Control over the nucleation of new phases is highly desirable but elusive. Even though there is a long history of crystallization engineering by varying physicochemical parameters, controlling which polymorph crystallizes or whether a molecule crystallizes or forms an amorphous precipitate is still a poorly understood practice. Although there are now numerous examples of control using laser-induced nucleation, the absence of physical understanding is preventing progress. Here we show that the proximity of a liquid-liquid critical point or the corresponding binodal line can be used by a laser-tweezing potential to induce concentration gradients. A simple theoretical model shows that the stored electromagnetic energy of the laser beam produces a free-energy potential that forces phase separation or triggers the nucleation of a new phase. Experiments in a liquid mixture using a low-power laser diode confirm the effect. Phase separation and nucleation using a laser-tweezing potential explains the physics behind non-photochemical laser-induced nucleation and suggests new ways of manipulating matter.

Ice-nucleating bacteria control the order and dynamics of interfacial water

PubMed Central

Pandey, Ravindra; Usui, Kota; Livingstone, Ruth A.; Fischer, Sean A.; Pfaendtner, Jim; Backus, Ellen H. G.; Nagata, Yuki; Fröhlich-Nowoisky, Janine; Schmüser, Lars; Mauri, Sergio; Scheel, Jan F.; Knopf, Daniel A.; Pöschl, Ulrich; Bonn, Mischa; Weidner, Tobias

2016-01-01

Ice-nucleating organisms play important roles in the environment. With their ability to induce ice formation at temperatures just below the ice melting point, bacteria such as Pseudomonas syringae attack plants through frost damage using specialized ice-nucleating proteins. Besides the impact on agriculture and microbial ecology, airborne P. syringae can affect atmospheric glaciation processes, with consequences for cloud evolution, precipitation, and climate. Biogenic ice nucleation is also relevant for artificial snow production and for biomimetic materials for controlled interfacial freezing. We use interface-specific sum frequency generation (SFG) spectroscopy to show that hydrogen bonding at the water-bacteria contact imposes structural ordering on the adjacent water network. Experimental SFG data and molecular dynamics simulations demonstrate that ice-active sites within P. syringae feature unique hydrophilic-hydrophobic patterns to enhance ice nucleation. The freezing transition is further facilitated by the highly effective removal of latent heat from the nucleation site, as apparent from time-resolved SFG spectroscopy. PMID:27152346

Preferential nucleation during polymorphic transformations

DOE PAGES

Sharma, H.; Sietsma, J.; Offerman, S. E.

2016-08-03

Polymorphism is the ability of a solid material to exist in more than one phase or crystal structure. Polymorphism may occur in metals, alloys, ceramics, minerals, polymers, and pharmaceutical substances. Unresolved are the conditions for preferential nucleation during polymorphic transformations in which structural relationships or special crystallographic orientation relationships (OR’s) form between the nucleus and surrounding matrix grains. We measured in-situ and simultaneously the nucleation rates of grains that have zero, one, two, three and four special OR’s with the surrounding parent grains. These experiments show a trend in which the activation energy for nucleation becomes smaller – and thereforemore » nucleation more probable - with increasing number of special OR’s. As a result, these insights contribute to steering the processing of polymorphic materials with tailored properties, since preferential nucleation affects which crystal structure forms, the average grain size and texture of the material, and thereby - to a large extent - the final properties of the material.« less

Time-Resolved In Situ Liquid-Phase Atomic Force Microscopy and Infrared Nanospectroscopy during the Formation of Metal–Organic Framework Thin Films

PubMed Central

2018-01-01

Metal–organic framework (MOF) thin films show unmatched promise as smart membranes and photocatalytic coatings. However, their nucleation and growth resulting from intricate molecular assembly processes are not well understood yet are crucial to control the thin film properties. Here, we directly observe the nucleation and growth behavior of HKUST-1 thin films by real-time in situ AFM at different temperatures in a Cu-BTC solution. In combination with ex situ infrared (nano)spectroscopy, synthesis at 25 °C reveals initial nucleation of rapidly growing HKUST-1 islands surrounded by a continuously nucleating but slowly growing HKUST-1 carpet. Monitoring at 13 and 50 °C shows the strong impact of temperature on thin film formation, resulting in (partial) nucleation and growth inhibition. The nucleation and growth mechanisms as well as their kinetics provide insights to aid in future rational design of MOF thin films. PMID:29595980

Controlled nucleation in freeze-drying: effects on pore size in the dried product layer, mass transfer resistance, and primary drying rate.

PubMed

Konstantinidis, Alex K; Kuu, Wei; Otten, Lori; Nail, Steven L; Sever, Robert R

2011-08-01

A novel and scalable method has been developed to enable control of the ice nucleation step for the freezing process during lyophilization. This method manipulates the chamber pressure of the freeze dryer to simultaneously induce nucleation in all product vials at a desired temperature. The effects of controlled nucleation on the drying rate of various formulations including 5% (w/w) mannitol, 5% (w/w) sucrose, and a mixture of 3% (w/w) mannitol and 2% (w/w) sucrose were studied. For a 5% (w/w) mannitol, uncontrolled ice nucleation occurred randomly at product temperatures between -8.0°C and -15.9°C as the vials were cooled to -40°C. Controlled ice nucleation was achieved at product temperatures between -2.3°C and -3.7°C. The effect of nucleation control on the effective pore radius (r(e) ) of the cake was determined from the product temperature profiles using a pore diffusion model in combination with a nonlinear parameter estimation approach reported earlier. Results show that the value of r(e) for 5% (w/w) mannitol was enlarged from 13 to 27 μm by uniformly inducing nucleation at higher temperatures. Applying the resistance parameters obtained from the pore diffusion model for 5% (w/w) mannitol, optimized cycles were theoretically generated and experimentally tested, resulting in a 41% reduction in primary drying time. Copyright © 2011 Wiley-Liss, Inc.

BINARY: an optical freezing array for assessing temperature and time dependence of heterogeneous ice nucleation

NASA Astrophysics Data System (ADS)

Budke, C.; Koop, T.

2015-02-01

A new optical freezing array for the study of heterogeneous ice nucleation in microliter-sized droplets is introduced, tested and applied to the study of immersion freezing in aqueous Snomax® suspensions. In the Bielefeld Ice Nucleation ARraY (BINARY) ice nucleation can be studied simultaneously in 36 droplets at temperatures down to -40 °C (233 K) and at cooling rates between 0.1 and 10 K min-1. The droplets are separated from each other in individual compartments, thus preventing a Wegener-Bergeron-Findeisen type water vapor transfer between droplets as well as avoiding the seeding of neighboring droplets by formation and surface growth of frost halos. Analysis of freezing and melting occurs via an automated real-time image analysis of the optical brightness of each individual droplet. As an application ice nucleation in water droplets containing Snomax® at concentrations from 1 ng mL-1 to 1 mg mL-1 was investigated. Using different cooling rates, a small time dependence of ice nucleation induced by two different classes of ice nucleators (INs) contained in Snomax® was detected and the corresponding heterogeneous ice nucleation rate coefficient was quantified. The observed time dependence is smaller than those of other types of INs reported in the literature, suggesting that the BINARY setup is suitable for quantifying time dependence for most other INs of atmospheric interest, making it a useful tool for future investigations.

Homogeneous ice nucleation and supercooled liquid water in orographic wave clouds

NASA Technical Reports Server (NTRS)

Heymsfield, Andrew J.; Miloshevich, Larry M.

1993-01-01

This study investigates ice nucleation mechanisms in cold lenticular wave clouds, a cloud type characterized by quasi-steady-state air motions and microphysical properties. It is concluded that homogeneous ice nucleation is responsible for the ice production in these clouds at temperatures below about -33 C. The lack of ice nucleation observed above -33 C indicates a dearth of ice-forming nuclei, and hence heterogeneous ice nucleation, in these clouds. Aircraft measurements in the temperature range -31 to -41 C show the following complement of simultaneous and abrupt changes in cloud properties that indicate a transition from the liquid phase to ice: disappearance of liquid water; decrease in relative humidity from near water saturation to ice saturation; increase in mean particle size; change in particle concentration; and change in temperature due to the release of latent heat. A numerical model of cloud particle growth and homogeneous ice nucleation is used to aid in interpretation of our in situ measurements. The abrupt changes in observed cloud properties compare favorably, both qualitatively and quantitatively, with results from the homogeneous ice nucleation model. It is shown that the homogeneous ice nucleation rates from the measurements are consistent with the temperature-dependent rates employed by the model (within a factor of 100, corresponding to about 1 C in temperature) in the temperature range -35 deg to -38 C. Given the theoretical basis of the modeled rates, it may be reasonable to apply them throughout the -30 to -50 C temperature range considered by the theory.

A simulation study of homogeneous ice nucleation in supercooled salty water

NASA Astrophysics Data System (ADS)

Soria, Guiomar D.; Espinosa, Jorge R.; Ramirez, Jorge; Valeriani, Chantal; Vega, Carlos; Sanz, Eduardo

2018-06-01

We use computer simulations to investigate the effect of salt on homogeneous ice nucleation. The melting point of the employed solution model was obtained both by direct coexistence simulations and by thermodynamic integration from previous calculations of the water chemical potential. Using a seeding approach, in which we simulate ice seeds embedded in a supercooled aqueous solution, we compute the nucleation rate as a function of temperature for a 1.85 NaCl mol per water kilogram solution at 1 bar. To improve the accuracy and reliability of our calculations, we combine seeding with the direct computation of the ice-solution interfacial free energy at coexistence using the Mold Integration method. We compare the results with previous simulation work on pure water to understand the effect caused by the solute. The model captures the experimental trend that the nucleation rate at a given supercooling decreases when adding salt. Despite the fact that the thermodynamic driving force for ice nucleation is higher for salty water for a given supercooling, the nucleation rate slows down with salt due to a significant increase of the ice-fluid interfacial free energy. The salty water model predicts an ice nucleation rate that is in good agreement with experimental measurements, bringing confidence in the predictive ability of the model. We expect that the combination of state-of-the-art simulation methods here employed to study ice nucleation from solution will be of much use in forthcoming numerical investigations of crystallization in mixtures.

Persistent Daily Aerosol Nucleation Events at Mountain-Top Location

NASA Astrophysics Data System (ADS)

Hallar, A. G.; Wiedinmyer, C.; Lowenthal, D. H.

2009-12-01

Atmospheric aerosols are of great consequence since they can impact climate through direct and indirect forcing, degrade air quality and visibility, and have detrimental effects on human health. Thus, an important phenomenon is atmospheric aerosol formation, the production of nanometer-size particles by nucleation and their growth to detectable sizes. Storm Peak Laboratory (3210 m AMSL), owned and operated by the Desert Research Institute (DRI), is located on the west summit of Mt. Werner in the Park Range near Steamboat Springs in northwestern Colorado. This site has been used in aerosol studies for more than 20 years. Daily nucleation events have been observed Storm Peak Laboratory between 2002 and 2009 with a TSI Scanning Mobility Particle Sizer (SMPS) (model 3936) coupled with a TSI model 3022 condensation particle counter (CPC). This instrument was set to measure particles with diameters between 8 and 335 nm. These events were observed during all measurement periods in the spring, summer and winter months. Nucleation was consistently seen in the mid-afternoon each day. This study includes 422 days of data; in 320 of these days nucleation events were observed. Thus, the nucleation events occurred during 76% of the measurement days, including during cloud events, and appear to be associated with elevated levels of ultraviolet radiation. This work will compare and contrast days with and without nucleation events, by investigating the radiation and meteorological conditions present. The results presented will provide further insight to the insitu production of aerosols via nucleation.

Nonclassical nucleation pathways in protein crystallization

NASA Astrophysics Data System (ADS)

Zhang, Fajun

2017-11-01

Classical nucleation theory (CNT), which was established about 90 years ago, has been very successful in many research fields, and continues to be the most commonly used theory in describing the nucleation process. For a fluid-to-solid phase transition, CNT states that the solute molecules in a supersaturated solution reversibly form small clusters. Once the cluster size reaches a critical value, it becomes thermodynamically stable and favored for further growth. One of the most important assumptions of CNT is that the nucleation process is described by one reaction coordinate and all order parameters proceed simultaneously. Recent studies in experiments, computer simulations and theory have revealed nonclassical features in the early stage of nucleation. In particular, the decoupling of order parameters involved during a fluid-to-solid transition leads to the so-called two-step nucleation mechanism, in which a metastable intermediate phase (MIP) exists between the initial supersaturated solution and the final crystals. Depending on the exact free energy landscapes, the MIPs can be a high density liquid phase, mesoscopic clusters, or a pre-ordered state. In this review, we focus on the studies of nonclassical pathways in protein crystallization and discuss the applications of the various scenarios of two-step nucleation theory. In particular, we focus on protein solutions in the presence of multivalent salts, which serve as a model protein system to study the nucleation pathways. We wish to point out the unique features of proteins as model systems for further studies.

Impact of surface nanostructure on ice nucleation.

PubMed

Zhang, Xiang-Xiong; Chen, Min; Fu, Ming

2014-09-28

Nucleation of water on solid surface can be promoted noticeably when the lattice parameter of a surface matches well with the ice structure. However, the characteristic length of the surface lattice reported is generally less than 0.5 nm and is hardly tunable. In this paper, we show that a surface with nanoscale roughness can also remarkably promote ice nucleation if the characteristic length of the surface structure matches well with the ice crystal. A series of surfaces composed of periodic grooves with same depth but different widths are constructed in molecular dynamics simulations. Water cylinders are placed on the constructed surfaces and frozen at constant undercooling. The nucleation rates of the water cylinders are calculated in the simulation using the mean first-passage time method and then used to measure the nucleation promotion ability of the surfaces. Results suggest that the nucleation behavior of the supercooled water is significantly sensitive to the width of the groove. When the width of the groove matches well with the specific lengths of the ice crystal structure, the nucleation can be promoted remarkably. If the width does not match with the ice crystal, this kind of promotion disappears and the nucleation rate is even smaller than that on the smooth surface. Simulations also indicate that even when water molecules are adsorbed onto the surface structure in high-humidity environment, the solid surface can provide promising anti-icing ability as long as the characteristic length of the surface structure is carefully designed to avoid geometric match.

Internally mixed sulfate and organic particles as potential ice nuclei in the tropical tropopause region

PubMed Central

Tolbert, Margaret A.

2010-01-01

Cirrus clouds are ubiquitous in the tropical tropopause region and play a major role in the Earth’s climate. Any changes to cirrus abundance due to natural or anthropogenic influences must be considered to evaluate future climate change. The detailed impact of cirrus clouds on climate depends on ice particle number, size, morphology, and composition. These properties depend in turn on the nucleation mechanism of the ice particles. Although it is often assumed that ice nucleates via a homogeneous mechanism, recent work points to the possibility that heterogeneous ice nucleation is important in the tropical tropopause region. However, there are very few studies of depositional ice nucleation on the complex types of particles likely to be found in this region of the atmosphere. Here, we use a unique method to probe depositional ice nucleation on internally mixed ammonium sulfate/palmitic acid particles, namely optical microscopy coupled with Raman microscopy. The deliquescence and efflorescence phase transitions of the mixed particles were first studied to gain insight into whether the particles are likely to be liquid or solid in the tropical tropopause region. The ice nucleating ability of the particles was then measured under typical upper tropospheric conditions. It was found that coating the particles with insoluble palmitic acid had little effect on the deliquescence, efflorescence, or ice nucleating ability of ammonium sulfate. Additional experiments involving Raman mapping provide new insights into how the composition and morphology of mixed particles impact their ability to nucleate ice. PMID:20388912

Nonclassical nucleation pathways in protein crystallization.

PubMed

Zhang, Fajun

2017-11-08

Classical nucleation theory (CNT), which was established about 90 years ago, has been very successful in many research fields, and continues to be the most commonly used theory in describing the nucleation process. For a fluid-to-solid phase transition, CNT states that the solute molecules in a supersaturated solution reversibly form small clusters. Once the cluster size reaches a critical value, it becomes thermodynamically stable and favored for further growth. One of the most important assumptions of CNT is that the nucleation process is described by one reaction coordinate and all order parameters proceed simultaneously. Recent studies in experiments, computer simulations and theory have revealed nonclassical features in the early stage of nucleation. In particular, the decoupling of order parameters involved during a fluid-to-solid transition leads to the so-called two-step nucleation mechanism, in which a metastable intermediate phase (MIP) exists between the initial supersaturated solution and the final crystals. Depending on the exact free energy landscapes, the MIPs can be a high density liquid phase, mesoscopic clusters, or a pre-ordered state. In this review, we focus on the studies of nonclassical pathways in protein crystallization and discuss the applications of the various scenarios of two-step nucleation theory. In particular, we focus on protein solutions in the presence of multivalent salts, which serve as a model protein system to study the nucleation pathways. We wish to point out the unique features of proteins as model systems for further studies.

A simulation study of homogeneous ice nucleation in supercooled salty water.

PubMed

Soria, Guiomar D; Espinosa, Jorge R; Ramirez, Jorge; Valeriani, Chantal; Vega, Carlos; Sanz, Eduardo

2018-06-14

We use computer simulations to investigate the effect of salt on homogeneous ice nucleation. The melting point of the employed solution model was obtained both by direct coexistence simulations and by thermodynamic integration from previous calculations of the water chemical potential. Using a seeding approach, in which we simulate ice seeds embedded in a supercooled aqueous solution, we compute the nucleation rate as a function of temperature for a 1.85 NaCl mol per water kilogram solution at 1 bar. To improve the accuracy and reliability of our calculations, we combine seeding with the direct computation of the ice-solution interfacial free energy at coexistence using the Mold Integration method. We compare the results with previous simulation work on pure water to understand the effect caused by the solute. The model captures the experimental trend that the nucleation rate at a given supercooling decreases when adding salt. Despite the fact that the thermodynamic driving force for ice nucleation is higher for salty water for a given supercooling, the nucleation rate slows down with salt due to a significant increase of the ice-fluid interfacial free energy. The salty water model predicts an ice nucleation rate that is in good agreement with experimental measurements, bringing confidence in the predictive ability of the model. We expect that the combination of state-of-the-art simulation methods here employed to study ice nucleation from solution will be of much use in forthcoming numerical investigations of crystallization in mixtures.

Dynamic Nucleation of Ice Induced by a Single Stable Cavitation Bubble

NASA Technical Reports Server (NTRS)

Ohsaka, Kenichi; Trinh, Eugene H.

1997-01-01

Dynamic nucleation of ice induced by caviation bubble in undercooled water is observed using an acoustic levitation technique. The observation indicates that a high pressure pulse associated with a collapsing bubble is indeed responsible for the nucleation of a high pressure phase of ice.

Global atmospheric particle formation from CERN CLOUD measurements

NASA Astrophysics Data System (ADS)

Dunne, Eimear M.; Gordon, Hamish; Carslaw, Kenneth S.

2017-04-01

New particle formation (or nucleation) is acknowledged as a significant source of climate-relevant aerosol throughout the atmosphere. However, performing atmospherically relevant nucleation experiments in a laboratory setting is extremely challenging. As a result, until now, the parameterisations used to represent new particle formation in global aerosol models were largely based on in-situ observations or theoretical nucleation models, and usually only represented the binary H2SO4-H2O system. Several different chemicals can affect particle formation rates, even at extremely low trace concentrations, which are technically challenging to measure directly. Nucleation rates also respond to environmental changes in e.g. temperature in a highly non-linear fashion. The CERN CLOUD experiment was designed to provide the most controlled and accurate nucleation rate measurements to date, over the full range of free tropospheric temperatures and down to sulphuric acid concentrations of the order of 105 cm-3. We will present a parameterisation of inorganic nucleation rates for use in global models, based on these measurements, which includes four separate nucleation pathways: binary neutral, binary ion-induced, ternary neutral, and ternary ion-induced. Both inorganic and organic nucleation parameterisations derived from CLOUD measurements have been implemented in the GLOMAP global aerosol model. The parameterisations depend on temperature and on concentrations of sulphuric acid, ammonia, organic vapours, and ions. One of CLOUD's main original goals was to determine the sensitivity of atmospheric aerosol to changes in the nucleation rate over a solar cycle. We will show that, in a present-day atmosphere, the changes in climate-relevant aerosol (in the form of cloud-level cloud condensation nuclei) over a solar cycle are on average about 0.1%, with local changes of less than 1%. In contrast, anthropogenic changes in ammonia since pre-industrial times were estimated to have a much greater influence, resulting in a radiative forcing of between -0.62 and -0.66 W m-2. Including ternary inorganic pathways in GLOMAP improved the model's agreement with free tropospheric observations, especially aircraft measurements. The further inclusion of an organic parameterisation, which increased nucleation in the summertime boundary layer, brought our results more in line with observations made at surface stations. We therefore believe that, while the addition of other nucleation pathways (such as amine-induced nucleation) will doubtless improve agreement with local in-situ measurements, this model set-up provides a good representation of the global atmosphere as a whole. By presenting this novel parameterisation at EGU, we hope to encourage its uptake among the aerosol modelling community.

ACID-CATALYZED REACTIONS IN SULFURIC ACID AEROSOLS: CHARACTERIZATION AND IMPACT ON ICE NUCLEATION

EPA Science Inventory

Several different experimental results are possible. It may be that as long as the water content of the aerosol is known, ice nucleation conditions can be predicted using an accepted model for homogeneous ice nucleation. However, in aerosol systems where larger organics form...

On Capillary Rise and Nucleation

ERIC Educational Resources Information Center

Prasad, R.

2008-01-01

A comparison of capillary rise and nucleation is presented. It is shown that both phenomena result from a balance between two competing energy factors: a volume energy and a surface energy. Such a comparison may help to introduce nucleation with a topic familiar to the students, capillary rise. (Contains 1 table and 3 figures.)

Temperature dependence of nucleation rate in a binary solid solution

NASA Astrophysics Data System (ADS)

Wang, H. Y.; Philippe, T.; Duguay, S.; Blavette, D.

2012-12-01

The influence of regression (partial dissolution) effects on the temperature dependence of nucleation rate in a binary solid solution has been studied theoretically. The results of the analysis are compared with the predictions of the simplest Volmer-Weber theory. Regression effects are shown to have a strong influence on the shape of the curve of nucleation rate versus temperature. The temperature TM at which the maximum rate of nucleation occurs is found to be lowered, particularly for low interfacial energy (coherent precipitation) and high-mobility species (e.g. interstitial atoms).

Nucleation and chiral symmetry breaking under controlled hydrodynamic flows

NASA Technical Reports Server (NTRS)

Wu, Xiao-Lun; Martin, Brian; Tharrington, Arnold

1994-01-01

The effects of hydrodynamic convection on nucleation and broken chiral symmetry have been investigated for a simple inorganic molecule, sodium chlorate (NaClO3). Our experiment suggests that the symmetry breaking is a result of hydrodynamic amplification of rare nucleation events. The effect is more pronounced when the primary nucleation occurs on the solute-vapor interface, where mixing in the surface sublayer becomes important. The transition from the achiral to the chiral states appears to be smooth as the hydrodynamic parameters, such as flow rate, are varied.

Sublimating icy planetesimals as the source of nucleation seeds for grain condensation in classical novae

NASA Technical Reports Server (NTRS)

Matese, John J.; Whitmire, D. P.; Reynolds, R. T.

1989-01-01

The problem of grain nucleation during novae outbursts is a major obstacle to our understanding of dust formation in these systems. How nucleation seeds can form in the hostile post-outburst environment remains an unresolved matter. It is suggested that the material for seeding the condensation of ejecta outflow is stored in a primordial disk of icy planetesimals surrounding the system. Evidence is presented that the requisite number of nucleation seeds can be released by sublimation of the planetesimals during outbursts.

Particle nucleation in the tropical boundary layer and its coupling to marine sulfur sources

PubMed

Clarke; Davis; Kapustin; Eisele; Chen; Paluch; Lenschow; Bandy; Thornton; Moore; Mauldin; Tanner; Litchy; Carroll; Collins; Albercook

1998-10-02

New particle formation in a tropical marine boundary layer setting was characterized during NASA's Pacific Exploratory Mission-Tropics A program. It represents the clearest demonstration to date of aerosol nucleation and growth being linked to the natural marine sulfur cycle. This conclusion was based on real-time observations of dimethylsulfide, sulfur dioxide, sulfuric acid (gas), hydroxide, ozone, temperature, relative humidity, aerosol size and number distribution, and total aerosol surface area. Classic binary nucleation theory predicts no nucleation under the observed marine boundary layer conditions.

Nanostructures nucleation in carbon-metal gaseous phase: A molecular dynamics study

NASA Astrophysics Data System (ADS)

Galiullina, G. M.; Orekhov, N. D.; Stegailov, V. V.

2018-01-01

We perform nonequilibrium molecular dynamics simulation of carbon nanoclusters nucleation and early stages of growth from the gaseous phase. We analyze the catalytic effect of iron atoms on the nucleation kinetics and structure of the resultant nanoparticles. Reactive Force Field (ReaxFF) is used in the simulations for the description of bond formation and dissociation during the nucleation process at the nanoscale. The catalytic effect of iron reveals itself even on nanosecond simulation times: iron atoms accelerate the process of clustering but result in less graphitized carbon structures.

Observation of end-vortex nucleation in individual ferromagnetic nanotubes

NASA Astrophysics Data System (ADS)

Mehlin, A.; Gross, B.; Wyss, M.; Schefer, T.; Tütüncüoglu, G.; Heimbach, F.; Fontcuberta i Morral, A.; Grundler, D.; Poggio, M.

2018-04-01

The reversal of uniform axial magnetization in a ferromagnetic nanotube (FNT) has been predicted to occur through the nucleation and propagation of vortex domains forming at the ends. We provide experimental evidence for this behavior through dynamic cantilever magnetometry measurements of individual FNTs. In particular, we identify the nucleation of the vortex end domains as a function of applied magnetic field and show that they mark the onset of magnetization reversal. We find that the nucleation field depends sensitively on the angle between the end surface of the FNT and the applied field. Micromagnetic simulations substantiate the experimental results and highlight the importance of the ends in determining the reversal process. The control over end-vortex nucleation enabled by our findings is promising for the production of FNTs with tailored reversal properties.

Nucleation and growth mechanism of self-catalyzed InAs nanowires on silicon

NASA Astrophysics Data System (ADS)

Gomes, U. P.; Ercolani, D.; Zannier, V.; David, J.; Gemmi, M.; Beltram, F.; Sorba, L.

2016-06-01

We report on the nucleation and growth mechanism of self-catalyzed InAs nanowires (NWs) grown on Si (111) substrates by chemical beam epitaxy. Careful choices of the growth parameters lead to In-rich conditions such that the InAs NWs nucleate from an In droplet and grow by the vapor-liquid-solid mechanism while sustaining an In droplet at the tip. As the growth progresses, new NWs continue to nucleate on the Si (111) surface causing a spread in the NW size distribution. The observed behavior in NW nucleation and growth is described within a suitable existing theoretical model allowing us to extract relevant growth parameters. We argue that these results provide useful guidelines to rationally control the growth of self-catalyzed InAs NWs for various applications.

Combinatorial Screening Of Inorganic And Organometallic Materials

DOEpatents

Li, Yi , Li, Jing , Britton, Ted W.

2002-06-25

A method for differentiating and enumerating nucleated red blood cells in a blood sample is described. The method includes the steps of lysing red blood cells of a blood sample with a lytic reagent, measuring nucleated blood cells by DC impedance measurement in a non-focused flow aperture, differentiating nucleated red blood cells from other cell types, and reporting nucleated red blood cells in the blood sample. The method further includes subtracting nucleated red blood cells and other interference materials from the count of remaining blood cells, and reporting a corrected white blood cell count of the blood sample. Additionally, the method further includes measuring spectrophotometric absorbance of the sample mixture at a predetermined wavelength of a hemoglobin chromogen formed upon lysing the blood sample, and reporting hemoglobin concentration of the blood sample.

Preface: Special Topic on Nucleation: New Concepts and Discoveries.

PubMed

Kelton, K F; Frenkel, Daan

2016-12-07

Many phenomena in the world around us depend on infrequent, yet short-lived, events that completely alter how a system subsequently develops in time. In the physical sciences, there are many examples of such crucial "rare events." Among the most important of these are nucleation processes, in which, due to a rare fluctuation, a new phase forms spontaneously within a meta-stable parent phase. Because nucleation processes are both rare and rapid and happen on a microscopic spatial scale, their experimental study is challenging. In recent years, there have been major developments both in the experimental study of nucleation phenomena and in the numerical simulation of such processes. As the articles in this special issue demonstrate, these recent advances in the ability to probe nucleation phenomena have transformed our understanding of the field.

Nucleation and growth mechanisms of nano magnesium hydride from the hydrogen sorption kinetics.

PubMed

Mooij, Lennard; Dam, Bernard

2013-07-21

We use a combination of hydrogenography and Johnson-Mehl-Avrami-Kolmogorov (JMAK) analyses to identify (1) the driving force dependence of the nucleation and growth mechanism of MgH2 in thin film multilayers of Mg (10 nm) and (2) the nucleation and growth mechanism of Mg in the earlier formed MgH2, i.e. the hydrogen desorption process. We conclude that JMAK may be successfully applied to obtain the nucleation and growth mechanism of hydrogen absorption. The desorption mechanism, however, is not simply the reverse of the absorption mechanism. We find evidence that the barrier for nucleation of Mg is small. The dehydrogenation probably involves the formation of voids, which is energetically more favorable than elastic and plastic deformation of the multilayer.

Magnetization processes in core/shell exchange-spring structures.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jiang, J. S.

2015-03-27

The magnetization reversal processes in cylindrical and spherical soft core/hard shell exchange-spring structures are investigated via the analytical nucleation theory, and are verified with numerical micromagnetic simulations. At small core sizes, the nucleation of magnetic reversal proceeds via the modified bulging mode, where the transverse component of the magnetization is only semi-coherent in direction and the nucleation field contains a contribution from self-demagnetization. For large core sizes, the modified curling mode, where the magnetization configuration is vortex-like, is favored at nucleation. The preference for the modified curling mode is beneficial in that the fluxclosure allows cylindrical and spherical core/shell exchange-springmore » elements to be densely packed into bulk permanent magnets without affecting the nucleation field, thereby offering the potential for high energy product.« less

Zinc Nucleation and Growth in Microgravity

NASA Technical Reports Server (NTRS)

Michael, B. Patrick; Nuth, J. A., III; Lilleleht, L. U.; Vondrak, Richard R. (Technical Monitor)

2000-01-01

We report our experiences with zinc nucleation in a microgravity environment aboard NASA's Reduced Gravity Research Facility. Zinc vapor is produced by a heater in a vacuum chamber containing argon gas. Nucleation is induced by cooling and its onset is easily detected visually by the appearance of a cloud of solid, at least partially crystalline zinc particles. Size distribution of these particles is monitored in situ by photon correlation spectroscopy. Samples of particles are also extracted for later analysis by SEM. The initially rapid increase in particle size is followed by a slower period of growth. We apply Scaled Nucleation Theory to our data and find that the derived critical temperature of zinc, the critical cluster size at nucleation, and the surface tension values are all in reasonably good agreement with their accepted literature values.

Site-Specific Colloidal Crystal Nucleation by Template-enhanced Particle Transport

NASA Astrophysics Data System (ADS)

Mishra, Chandan K.; Sood, A. K.; Ganapathy, Rajesh

The deliberate positioning of nano- and microstructures on surfaces is often a prerequisite for fabricating functional devices. While template-assisted nucleation is a promising route to self-assemble these structures, its success hinges on particles reaching target sites prior to nucleation and for nano/microscale particles, this is hampered by their small surface mobilities. We tailored surface features, which in the presence of attractive depletion interactions not only directed micrometer-sized colloids to specific sites but also subsequently guided their growth into ordered crystalline arrays of well-defined size and symmetry. By following the nucleation kinetics with single-particle resolution, we demonstrate control over nucleation density in a growth regime that has hitherto remained inaccessible. Our findings pave the way towards realizing non-trivial surface architectures composed of complex colloids/nanoparticles as well.

Landscapes with megabasins: Polyamorphism in liquids and biopolymers and the role of nucleation in folding and folding diseases

NASA Astrophysics Data System (ADS)

Angell, C. A.

1997-02-01

We show how energy landscape concepts can rationalize the observations on glassforming liquids over the whole range of behavior, strong to fragile. In particular, we show how the existence of landscapes with both strong and fragile megabasins can provide a basis for understanding the nature of quasi-first-order transitions between amorphous states such as those observed to occur in the glassy states of “strong” glassformers. We show how this propensity originates in the liquid state and then emphasize the analogy provided, at the mesoscopic level, by the folding transition in proteins. Recognition that the folding transition is an equilibrium first-order transition between polyamorphic forms of a complex system implies recognition of the need for a nucleation step in the process. When nucleated phase transitions are kinetically retarded, their probability can be influenced by time-temperature history and by the presence of nucleating agents. Nucleation events are statistically rare in mesoscopic systems, hence the ability to fold rapidly should require special features in the folding molecular structure or the presence of nucleating agents. We propose that the unwanted folding events leading to pathogenic forms of certain proteins (prions) can be stimulated by nucleating agents, which thus may be the unidentified infectious agents in “mad cow” disease and related maladies.

Mechanisms for Ductile Rupture - FY16 ESC Progress Report

DOE Office of Scientific and Technical Information (OSTI.GOV)

Boyce, Brad L.; Carroll, Jay D.; Noell, Phillip

2017-01-01

Ductile rupture in metals is generally a multi-step process of void nucleation, growth, and coalescence. Particle decohesion and particle fracture are generally invoked as the primary microstructural mechanisms for room-temperature void nucleation. However, because high-purity materials also fail by void nucleation and coalescence, other microstructural features must also act as sites for void nucleation. Early studies of void initiation in high-purity materials, which included post-mortem fracture surface characterization using scanning electron microscopy (SEM) and high-voltage electron microscopy (HVEM) and in-situ HVEM observations of fracture, established the presence of dislocation cell walls as void initiation sites in high-purity materials. Direct experimentalmore » evidence for this contention was obtained during in-situ HVEM tensile tests of Be single crystals. Voids between 0.2 and 1 μm long appeared suddenly along dislocation cell walls during tensile straining. However, subsequent attempts to replicate these results in other materials, particularly α -Fe single crystals, were unsuccessful because of the small size of the dislocation cells, and these remain the only published in-situ HVEM observations of void nucleation at dislocation cell walls in the absence of a growing macrocrack. Despite this challenge, other approaches to studying void nucleation in high-purity metals also indicate that dislocation cell walls are nucleation sites for voids.« less

Surface Crystallization of Cloud Droplets: Implications for Climate Change and Ozone Depletion

NASA Technical Reports Server (NTRS)

Tabazadeh, A.; Djikaev, Y. S.; Reiss, H.; Gore, Warren J. (Technical Monitor)

2002-01-01

The process of supercooled liquid water crystallization into ice is still not well understood. Current experimental data on homogeneous freezing rates of ice nucleation in supercooled water droplets show considerable scatter. For example, at -33 C, the reported freezing nucleation rates vary by as much as 5 orders of magnitude, which is well outside the range of measurement uncertainties. Until now, experimental data on the freezing of supercooled water has been analyzed under the assumption that nucleation of ice took place in the interior volume of a water droplet. Here, the same data is reanalyzed assuming that the nucleation occurred "pseudoheterogeneously" at the air (or oil)-liquid water interface of the droplet. Our analysis suggest that the scatter in the nucleation data can be explained by two main factors. First, the current assumption that nucleation occurs solely inside the volume of a water droplet is incorrect. Second, because the nucleation process most likely occurs on the surface, the rates of nuclei formation could differ vastly when oil or air interfaces are involved. Our results suggest that ice freezing in clouds may initiate on droplet surfaces and such a process can allow for low amounts of liquid water (approx. 0.002 g per cubic meters) to remain supercooled down to -40 C as observed in the atmosphere.

Focus Article: Theoretical aspects of vapor/gas nucleation at structured surfaces

NASA Astrophysics Data System (ADS)

Meloni, Simone; Giacomello, Alberto; Casciola, Carlo Massimo

2016-12-01

Heterogeneous nucleation is the preferential means of formation of a new phase. Gas and vapor nucleation in fluids under confinement or at textured surfaces is central for many phenomena of technological relevance, such as bubble release, cavitation, and biological growth. Understanding and developing quantitative models for nucleation is the key to control how bubbles are formed and to exploit them in technological applications. An example is the in silico design of textured surfaces or particles with tailored nucleation properties. However, despite the fact that gas/vapor nucleation has been investigated for more than one century, many aspects still remain unclear and a quantitative theory is still lacking; this is especially true for heterogeneous systems with nanoscale corrugations, for which experiments are difficult. The objective of this focus article is analyzing the main results of the last 10-20 years in the field, selecting few representative works out of this impressive body of the literature, and highlighting the open theoretical questions. We start by introducing classical theories of nucleation in homogeneous and in simple heterogeneous systems and then discuss their extension to complex heterogeneous cases. Then we describe results from recent theories and computer simulations aimed at overcoming the limitations of the simpler theories by considering explicitly the diffuse nature of the interfaces, atomistic, kinetic, and inertial effects.

Dislocation creation and void nucleation in FCC ductile metals under tensile loading: A general microscopic picture

PubMed Central

Pang, Wei-Wei; Zhang, Ping; Zhang, Guang-Cai; Xu, Ai-Guo; Zhao, Xian-Geng

2014-01-01

Numerous theoretical and experimental efforts have been paid to describe and understand the dislocation and void nucleation processes that are fundamental for dynamic fracture modeling of strained metals. To date an essential physical picture on the self-organized atomic collective motions during dislocation creation, as well as the essential mechanisms for the void nucleation obscured by the extreme diversity in structural configurations around the void nucleation core, is still severely lacking in literature. Here, we depict the origin of dislocation creation and void nucleation during uniaxial high strain rate tensile processes in face-centered-cubic (FCC) ductile metals. We find that the dislocations are created through three distinguished stages: (i) Flattened octahedral structures (FOSs) are randomly activated by thermal fluctuations; (ii) The double-layer defect clusters are formed by self-organized stacking of FOSs on the close-packed plane; (iii) The stacking faults are formed and the Shockley partial dislocations are created from the double-layer defect clusters. Whereas, the void nucleation is shown to follow a two-stage description. We demonstrate that our findings on the origin of dislocation creation and void nucleation are universal for a variety of FCC ductile metals with low stacking fault energies. PMID:25382029

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mishchenko, L; Khan, M; Aizenberg, J

Certain natural organisms use micro-patterned surface chemistry, or ice-nucleating species, to control water condensation and ice nucleation for survival under extreme conditions. As an analogy to these biological approaches, it is shown that functionalized, hydrophilic polymers and particles deposited on the tips of superhydrophobic posts induce precise topographical control over water condensation and freezing at the micrometer scale. A bottom-up deposition process is used to take advantage of the limited contact area of a non-wetting aqueous solution on a superhydrophobic surface. Hydrophilic polymer deposition on the tips of these geometrical structures allows spatial control over the nucleation, growth, and coalescencemore » of micrometer-scale water droplets. The hydrophilic tips nucleate water droplets with extremely uniform nucleation and growth rates, uniform sizes, an increased stability against coalescence, and asymmetric droplet morphologies. Control of freezing behavior is also demonstrated via deposition of ice-nucleating AgI nanoparticles on the tips of these structures. This combination of the hydrophilic polymer and AgI particles on the tips was used to achieve templating of ice nucleation at the micrometer scale. Preliminary results indicate that control over ice crystal size, spatial symmetry, and position might be possible with this method. This type of approach can serve as a platform for systematically analyzing micrometer-scale condensation and freezing phenomena, and as a model for natural systems.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mishchenko, Lidiya; Khan, M.; Aizenberg, Joanna

Certain natural organisms use micro-patterned surface chemistry, or ice-nucleating species, to control water condensation and ice nucleation for survival under extreme conditions. As an analogy to these biological approaches, it is shown that functionalized, hydrophilic polymers and particles deposited on the tips of superhydrophobic posts induce precise topographical control over water condensation and freezing at the micrometer scale. A bottom-up deposition process is used to take advantage of the limited contact area of a non-wetting aqueous solution on a superhydrophobic surface. Hydrophilic polymer deposition on the tips of these geometrical structures allows spatial control over the nucleation, growth, and coalescencemore » of micrometer-scale water droplets. The hydrophilic tips nucleate water droplets with extremely uniform nucleation and growth rates, uniform sizes, an increased stability against coalescence, and asymmetric droplet morphologies. Furthermore, control of freezing behavior is also demonstrated via deposition of ice-nucleating AgI nanoparticles on the tips of these structures. The combination of the hydrophilic polymer and AgI particles on the tips was used to achieve templating of ice nucleation at the micrometer scale. Preliminary results indicate that control over ice crystal size, spatial symmetry, and position might be possible with this method. This type of approach can serve as a platform for systematically analyzing micrometer-scale condensation and freezing phenomena, and as a model for natural systems.« less

Computational modeling of soot nucleation

NASA Astrophysics Data System (ADS)

Chung, Seung-Hyun

Recent studies indicate that soot is the second most significant driver of climate change---behind CO2, but ahead of methane---and increased levels of soot particles in the air are linked to health hazards such as heart disease and lung cancer. Within the soot formation process, soot nucleation is the least understood step, and current experimental findings are still limited. This thesis presents computational modeling studies of the major pathways of the soot nucleation process. In this study, two regimes of soot nucleation---chemical growth and physical agglomeration---were evaluated and the results demonstrated that combustion conditions determine the relative importance of these two routes. Also, the dimerization process of polycyclic aromatic hydrocarbons, which has been regarded as one of the most important physical agglomeration processes in soot formation, was carefully examined with a new method for obtaining the nucleation rate using molecular dynamics simulation. The results indicate that the role of pyrene dimerization, which is the commonly accepted model, is expected to be highly dependent on various flame temperature conditions and may not be a key step in the soot nucleation process. An additional pathway, coronene dimerization in this case, needed to be included to improve the match with experimental data. The results of this thesis provide insight on the soot nucleation process and can be utilized to improve current soot formation models.

Heterogeneous nucleation of nitric acid trihydrate on clay minerals: relevance to type ia polar stratospheric clouds.

PubMed

Hatch, Courtney D; Gough, Raina V; Toon, Owen B; Tolbert, Margaret A

2008-01-17

Although critical to atmospheric modeling of stratospheric ozone depletion, selective heterogeneous nuclei that promote the formation of Type Ia polar stratospheric clouds (PSCs) are largely unknown. While mineral particles are known to be good ice nuclei, it is currently not clear whether they are also good nuclei for PSCs. In the present study, a high-vacuum chamber equipped with transmission Fourier transform infrared spectroscopy and a quadrupole mass spectrometer was used to study heterogeneous nucleation of nitric acid trihydrate (NAT) on two clay minerals-Na-montmorillonite and kaolinite-as analogs of atmospheric terrestrial and extraterrestrial minerals. The minerals are first coated with a 3:1 supercooled H2O/HNO3 solution prior to the observed nucleation of crystalline NAT. At 220 K, NAT formation was observed at low SNAT values of 12 and 7 on kaolinite and montmorillonite clays, respectively. These are the lowest SNAT values reported in the literature on any substrate. However, NAT nucleation exhibited significant temperature dependence. At lower temperatures, representative of typical polar stratospheric conditions, much higher supersaturations were required before nucleation was observed. Our results suggest that NAT nucleation on mineral particles, not previously treated with sulfuric acid, may not be an important nucleation platform for Type Ia PSCs under normal polar stratospheric conditions.

The Nucleation of Protein Aggregates - From Crystals to Amyloid Fibrils.

PubMed

Buell, Alexander K

2017-01-01

The condensation and aggregation of individual protein molecules into dense insoluble phases is of relevance in such diverse fields as materials science, medicine, structural biology and pharmacology. A common feature of these condensation phenomena is that they usually are nucleated processes, i.e. the first piece of the condensed phase is energetically costly to create and hence forms slowly compared to its subsequent growth. Here we give a compact overview of the differences and similarities of various protein nucleation phenomena, their theoretical description in the framework of colloid and polymer science and their experimental study. Particular emphasis is put on the nucleation of a specific type of filamentous protein aggregates, amyloid fibrils. The current experimentally derived knowledge on amyloid fibril nucleation is critically assessed, and we argue that it is less advanced than is generally believed. This is due to (I) the lack of emphasis that has been put on the distinction between homogeneous and heterogeneous nucleation in experimental studies (II) the use of oversimplifying and/or inappropriate theoretical frameworks for the analysis of kinetic data of amyloid fibril nucleation. A strategy is outlined and advocated of how our understanding of this important class of processes can be improved in the future. © 2017 Elsevier Inc. All rights reserved.

In vivo bubble nucleation probability in sheep brain tissue.

PubMed

Gateau, J; Aubry, J-F; Chauvet, D; Boch, A-L; Fink, M; Tanter, M

2011-11-21

Gas nuclei exist naturally in living bodies. Their activation initiates cavitation activity, and is possible using short ultrasonic excitations of high amplitude. However, little is known about the nuclei population in vivo, and therefore about the rarefaction pressure required to form bubbles in tissue. A novel method dedicated to in vivo investigations was used here that combines passive and active cavitation detection with a multi-element linear ultrasound probe (4-7 MHz). Experiments were performed in vivo on the brain of trepanated sheep. Bubble nucleation was induced using a focused single-element transducer (central frequency 660 kHz, f-number = 1) driven by a high power (up to 5 kW) electric burst of two cycles. Successive passive recording and ultrafast active imaging were shown to allow detection of a single nucleation event in brain tissue in vivo. Experiments carried out on eight sheep allowed statistical studies of the bubble nucleation process. The nucleation probability was evaluated as a function of the peak negative pressure. No nucleation event could be detected with a peak negative pressure weaker than -12.7 MPa, i.e. one order of magnitude higher than the recommendations based on the mechanical index. Below this threshold, bubble nucleation in vivo in brain tissues is a random phenomenon.

Self-narrowing of size distributions of nanostructures by nucleation antibunching

NASA Astrophysics Data System (ADS)

Glas, Frank; Dubrovskii, Vladimir G.

2017-08-01

We study theoretically the size distributions of ensembles of nanostructures fed from a nanosize mother phase or a nanocatalyst that contains a limited number of the growth species that form each nanostructure. In such systems, the nucleation probability decreases exponentially after each nucleation event, leading to the so-called nucleation antibunching. Specifically, this effect has been observed in individual nanowires grown in the vapor-liquid-solid mode and greatly affects their properties. By performing numerical simulations over large ensembles of nanostructures as well as developing two different analytical schemes (a discrete and a continuum approach), we show that nucleation antibunching completely suppresses fluctuation-induced broadening of the size distribution. As a result, the variance of the distribution saturates to a time-independent value instead of growing infinitely with time. The size distribution widths and shapes primarily depend on the two parameters describing the degree of antibunching and the nucleation delay required to initiate the growth. The resulting sub-Poissonian distributions are highly desirable for improving size homogeneity of nanowires. On a more general level, this unique self-narrowing effect is expected whenever the growth rate is regulated by a nanophase which is able to nucleate an island much faster than it is refilled from a surrounding macroscopic phase.

Recent developments in the kinetic theory of nucleation.

PubMed

Ruckenstein, E; Djikaev, Y S

2005-12-30

A review of recent progress in the kinetics of nucleation is presented. In the conventional approach to the kinetic theory of nucleation, it is necessary to know the free energy of formation of a new-phase particle as a function of its independent variables at least for near-critical particles. Thus the conventional kinetic theory of nucleation is based on the thermodynamics of the process. The thermodynamics of nucleation can be examined by using various approaches, such as the capillarity approximation, density functional theory, and molecular simulation, each of which has its own advantages and drawbacks. Relatively recently a new approach to the kinetics of nucleation was proposed [Ruckenstein E, Nowakowski B. J Colloid Interface Sci 1990;137:583; Nowakowski B, Ruckenstein E. J Chem Phys 1991;94:8487], which is based on molecular interactions and does not employ the traditional thermodynamics, thus avoiding such a controversial notion as the surface tension of tiny clusters involved in nucleation. In the new kinetic theory the rate of emission of molecules by a new-phase particle is determined with the help of a mean first passage time analysis. This time is calculated by solving the single-molecule master equation for the probability distribution function of a surface layer molecule moving in a potential field created by the rest of the cluster. The new theory was developed for both liquid-to-solid and vapor-to-liquid phase transitions. In the former case the single-molecule master equation is the Fokker-Planck equation in the phase space which can be reduced to the Smoluchowski equation owing to the hierarchy of characteristic time scales. In the latter case, the starting master equation is a Fokker-Planck equation for the probability distribution function of a surface layer molecule with respect to both its energy and phase coordinates. Unlike the case of liquid-to-solid nucleation, this Fokker-Planck equation cannot be reduced to the Smoluchowski equation, but the hierarchy of time scales does allow one to reduce it to the Fokker-Plank equation in the energy space. The new theory provides an equation for the critical radius of a new-phase particle which in the limit of large clusters (low supersaturations) yields the Kelvin equation and hence an expression for the macroscopic surface tension. The theory was illustrated with numerical calculations for a molecular pair interaction potential combining the dispersive attraction with the hard-sphere repulsion. The results for the liquid-to-solid nucleation clearly show that at given supersaturation the nucleation rate depends on the cluster structure (for three cluster structures considered-amorphous, fcc, and icosahedral). For both the liquid-to-solid and vapor-to-liquid nucleation, the predictions of the theory are consistent with the results of classical nucleation theory (CNT) in the limit of large critical clusters (low supersaturations). For small critical clusters the new theory provides higher nucleation rates than CNT. This can be accounted for by the fact that CNT uses the macroscopic interfacial tension which presumably overpredicts the surface tension of small clusters, and hence underpredicts nucleation rates.

Neonatal nucleated red blood cell counts in small-for-gestational age fetuses with abnormal umbilical artery Doppler studies.

PubMed

Bernstein, P S; Minior, V K; Divon, M Y

1997-11-01

The presence of elevated nucleated red blood cell counts in neonatal blood has been associated with fetal hypoxia. We sought to determine whether small-for-gestational-age fetuses with abnormal umbilical artery Doppler velocity waveforms have elevated nucleated red blood cell counts. Hospital charts of neonates with the discharge diagnosis of small for gestational age (birth weight < 10th percentile) who were delivered between October 1988 and June 1995 were reviewed for antepartum testing, delivery conditions, and neonatal outcome. We studied fetuses who had an umbilical artery systolic/diastolic ratio within 3 days of delivery and a complete blood cell count on the first day of life. Multiple gestations, anomalous fetuses, and infants of diabetic mothers were excluded. Statistical analysis included the Student t test, chi 2 analysis, analysis of variance, and simple and stepwise regression. Fifty-two infants met the inclusion criteria. Those with absent or reversed end-diastolic velocity (n = 19) had significantly greater nucleated red blood cell counts than did those with end-diastolic velocity present (n = 33) (nucleated red blood cells/100 nucleated cells +/- SD: 135.5 +/- 138 vs 17.4 +/- 23.7, p < 0.0001). These infants exhibited significantly longer time intervals for clearance of nucleated red blood cells from their circulation (p < 0.0001). They also had lower birth weights (p < 0.05), lower initial platelet count (p = 0.0006), lower arterial cord blood pH (p < 0.05), higher cord blood base deficit (p < 0.05), and an increased likelihood of cesarean section for "fetal distress" (p < 0.05). Multivariate analysis demonstrated that absent or reversed end-diastolic velocity (p < 0.0001) and low birth weight (p < 0.0001) contributed to the elevation of the nucleated red blood cell count, whereas gestational age at delivery was not a significant contributor. We observed significantly greater nucleated red blood cell counts and lower platelet counts in small-for-gestational-age fetuses with abnormal umbilical artery Doppler studies. This may suggest that antenatal thrombotic events lead to an increased placental impedance. Fetal response to this chronic condition may result in an increased nucleated red blood cell count.

Traffic and nucleation events as main sources of ultrafine particles in high-insolation developed world cities

NASA Astrophysics Data System (ADS)

Brines, M.; Dall'Osto, M.; Beddows, D. C. S.; Harrison, R. M.; Gómez-Moreno, F.; Núñez, L.; Artíñano, B.; Costabile, F.; Gobbi, G. P.; Salimi, F.; Morawska, L.; Sioutas, C.; Querol, X.

2015-05-01

Road traffic emissions are often considered the main source of ultrafine particles (UFP, diameter smaller than 100 nm) in urban environments. However, recent studies worldwide have shown that - in high-insolation urban regions at least - new particle formation events can also contribute to UFP. In order to quantify such events we systematically studied three cities located in predominantly sunny environments: Barcelona (Spain), Madrid (Spain) and Brisbane (Australia). Three long-term data sets (1-2 years) of fine and ultrafine particle number size distributions (measured by SMPS, Scanning Mobility Particle Sizer) were analysed. Compared to total particle number concentrations, aerosol size distributions offer far more information on the type, origin and atmospheric evolution of the particles. By applying k-means clustering analysis, we categorized the collected aerosol size distributions into three main categories: "Traffic" (prevailing 44-63% of the time), "Nucleation" (14-19%) and "Background pollution and Specific cases" (7-22%). Measurements from Rome (Italy) and Los Angeles (USA) were also included to complement the study. The daily variation of the average UFP concentrations for a typical nucleation day at each site revealed a similar pattern for all cities, with three distinct particle bursts. A morning and an evening spike reflected traffic rush hours, whereas a third one at midday showed nucleation events. The photochemically nucleated particles' burst lasted 1-4 h, reaching sizes of 30-40 nm. On average, the occurrence of particle size spectra dominated by nucleation events was 16% of the time, showing the importance of this process as a source of UFP in urban environments exposed to high solar radiation. Nucleation events lasting for 2 h or more occurred on 55% of the days, this extending to > 4 h in 28% of the days, demonstrating that atmospheric conditions in urban environments are not favourable to the growth of photochemically nucleated particles. In summary, although traffic remains the main source of UFP in urban areas, in developed countries with high insolation urban nucleation events are also a main source of UFP. If traffic-related particle concentrations are reduced in the future, nucleation events will likely increase in urban areas, due to the reduced urban condensation sinks.

Soot Aerosol Particles as Cloud Condensation Nuclei: from Ice Nucleation Activity to Ice Crystal Morphology

NASA Astrophysics Data System (ADS)

Pirim, Claire; Ikhenazene, Raouf; Ortega, Isamel Kenneth; Carpentier, Yvain; Focsa, Cristian; Chazallon, Bertrand; Ouf, François-Xavier

2016-04-01

Emissions of solid-state particles (soot) from engine exhausts due to incomplete fuel combustion is considered to influence ice and liquid water cloud droplet activation [1]. The activity of these aerosols would originate from their ability to be important centers of ice-particle nucleation, as they would promote ice formation above water homogeneous freezing point. Soot particles are reported to be generally worse ice nuclei than mineral dust because they activate nucleation at higher ice-supersaturations for deposition nucleation and at lower temperatures for immersion freezing than ratios usually expected for homogeneous nucleation [2]. In fact, there are still numerous opened questions as to whether and how soot's physico-chemical properties (structure, morphology and chemical composition) can influence their nucleation ability. Therefore, systematic investigations of soot aerosol nucleation activity via one specific nucleation mode, here deposition nucleation, combined with thorough structural and compositional analyzes are needed in order to establish any association between the particles' activity and their physico-chemical properties. In addition, since the morphology of the ice crystals can influence their radiative properties [3], we investigated their morphology as they grow over both soot and pristine substrates at different temperatures and humidity ratios. In the present work, Combustion Aerosol STandart soot samples were produced from propane using various experimental conditions. Their nucleation activity was studied in deposition mode (from water vapor), and monitored using a temperature-controlled reactor in which the sample's relative humidity is precisely measured with a cryo-hygrometer. Formation of water/ice onto the particles is followed both optically and spectroscopically, using a microscope coupled to a Raman spectrometer. Vibrational signatures of hydroxyls (O-H) emerge when the particle becomes hydrated and are used to characterize ice crystals. [1] Koop, T. Atmospheric water, Water: Fundamentals as the Basis for Understanding the Environment and Promoting Technology, 187, 45-75 (2015). [2] Hoose & Möhler. Atmospheric Chemistry and Physics. 12, 9817-9854. (2012) [3] Schumann et al. Journal of Applied Meteorology and Climatology. 51, 1391-1406 (2012)

How does bone sialoprotein promote the nucleation of hydroxyapatite? A molecular dynamics study using model peptides of different conformations.

PubMed

Yang, Yang; Cui, Qiang; Sahai, Nita

2010-06-15

Bone sialoprotein (BSP) is a highly phosphorylated, acidic, noncollagenous protein in bone matrix. Although BSP has been proposed to be a nucleator of hydroxyapatite (Ca(5)(PO(4))(3)OH), the major mineral component of bone, no detailed mechanism for the nucleation process has been elucidated at the atomic level to date. In the present work, using a peptide model, we apply molecular dynamics (MD) simulations to study the conformational effect of a proposed nucleating motif of BSP (a phosphorylated, acidic, 10 amino-acid residue sequence) on controlling the distributions of Ca(2+) and inorganic phosphate (Pi) ions in solution, and specifically, we explore whether a nucleating template for orientated hydroxyapatite could be formed in different peptide conformations. Both the alpha-helical conformation and the random coil structure have been studied, and inorganic solutions without the peptide are simulated as reference. Ca(2+) distributions around the peptide surface and interactions between Ca(2+) and Pi in the presence of the peptide are examined in detail. From the MD simulations, although in some cases for the alpha-helical conformation, we observe that a Ca(2+) equilateral triangle forms around the surface of peptide, which matches the distribution of Ca(2+) ions on the (001) face of the hydroxyapatite crystal, we do not consistently find a stable nucleating template formation in general for either the helical conformation or the random coil structure. Therefore, independent of conformations, the BSP nucleating motif is more likely to help nucleate an amorphous calcium phosphate cluster, which ultimately converts to crystalline hydroxyapatite.

Dynamic observations of vesiculation reveal the role of silicate crystals in bubble nucleation and growth in andesitic magmas

NASA Astrophysics Data System (ADS)

Pleše, P.; Higgins, M. D.; Mancini, L.; Lanzafame, G.; Brun, F.; Fife, J. L.; Casselman, J.; Baker, D. R.

2018-01-01

Bubble nucleation and growth control the explosivity of volcanic eruptions, and the kinetics of these processes are generally determined from examinations of natural samples and quenched experimental run products. These samples, however, only provide a view of the final state, from which the initial conditions of a time-evolving magmatic system are then inferred. The interpretations that follow are inexact due to the inability of determining the exact conditions of nucleation and the potential detachment of bubbles from their nucleation sites, an uncertainty that can obscure their nucleation location - either homogeneously within the melt or heterogeneously at the interface between crystals and melts. We present results of a series of dynamic, real-time 4D X-ray tomographic microscopy experiments where we observed the development of bubbles in crystal bearing silicate magmas. Experimentally synthesized andesitic glasses with 0.25-0.5 wt% H2O and seed silicate crystals were heated at 1 atm to induce bubble nucleation and track bubble growth and movement. In contrast to previous studies on natural and experimentally produced samples, we found that bubbles readily nucleated on plagioclase and clinopyroxene crystals, that their contact angle changes during growth and that they can grow to sizes many times that of the silicate on whose surface they originated. The rapid heterogeneous nucleation of bubbles at low degrees of supersaturation in the presence of silicate crystals demonstrates that silicates can affect when vesiculation ensues, influencing subsequent permeability development and effusive vs. explosive transition in volcanic eruptions.

Kinetic Monte Carlo simulations of electrodeposition: Crossover from continuous to instantaneous homogeneous nucleation within Avrami’s law

NASA Astrophysics Data System (ADS)

Frank, Stefan; Rikvold, Per Arne

2006-06-01

The influence of lateral adsorbate diffusion on the dynamics of the first-order phase transition in a two-dimensional Ising lattice gas with attractive nearest-neighbor interactions is investigated by means of kinetic Monte Carlo simulations. For example, electrochemical underpotential deposition proceeds by this mechanism. One major difference from adsorption in vacuum surface science is that under control of the electrode potential and in the absence of mass-transport limitations, local adsorption equilibrium is approximately established. We analyze our results using the theory of Kolmogorov, Johnson and Mehl, and Avrami (KJMA), which we extend to an exponentially decaying nucleation rate. Such a decay may occur due to a suppression of nucleation around existing clusters in the presence of lateral adsorbate diffusion. Correlation functions prove the existence of such exclusion zones. By comparison with microscopic results for the nucleation rate I and the interface velocity of the growing clusters v, we can show that the KJMA theory yields the correct order of magnitude for Iv2. This is true even though the spatial correlations mediated by diffusion are neglected. The decaying nucleation rate causes a gradual crossover from continuous to instantaneous nucleation, which is complete when the decay of the nucleation rate is very fast on the time scale of the phase transformation. Hence, instantaneous nucleation can be homogeneous, producing negative minima in the two-point correlation functions. We also present in this paper an n-fold way Monte Carlo algorithm for a square lattice gas with adsorption/desorption and lateral diffusion.

Effect and interactions of commercial additives and chloride ion in copper electrowinning

NASA Astrophysics Data System (ADS)

Cui, Wenyuan

This thesis is to understand and compare the effects and interactions of modified polysaccharide (HydroStar), polyacrylamide (Cyquest N-900) and chloride ion on copper electrowinning. A study of the nucleation and growth was conducted in a synthetic electrolyte (40 g/L Cu, 160 g/L H2SO 4, 20 mg/L Cl-) with the addition of HydroStar or Cyquest N-900 using potential step measurements. The current responses generated were compared to theoretical models of nucleation and growth mechanisms. The nucleation and growth mechanism changed as function of potential and the presence of organic additives. The nucleation and growth mechanisms were confirmed using scanning electron microscopy (SEM). At low overpotentials, electrodeposition from the electrolyte without additives proceeded by progressive nucleation with three-dimensional (3-D) growth. The addition of HydroStar produced smaller nuclei and changed the mechanism to progressive nucleation and 2-D growth. Cyquest N-900 used there appeared to be progressive nucleation with 2-D growth and polarize the cathodes. In addition, instantaneous nucleation under diffusion control occurred at high overpotentials. Chloride ion and its interaction with HydroStar and Cyquest N-900 were further characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The trends observed from Nyquist plots and equivalent circuit models were consistent with the CV results. Chloride, on its own, depolarized copper electrodeposition, while chloride ion associated with Cyquest N-900 inhibited the reaction. It is proposed that Cl- acted as a bridging ligand between copper and Cyquest N-900. The addition of HydroStar depolarized copper deposition, but it did not interact with.

Structural match of heterogeneously nucleated Mn(OH) 2(s) nanoparticles on quartz under various pH conditions

DOE PAGES

Jung, Haesung; Lee, Byeongdu; Jun, Young -Shin

2016-09-14

The early nucleation stage of Mn (hydr)oxide on mineral surfaces is crucial to understand its occurrence and the cycling of nutrients in environmental systems. However, there are only limited studies on the heterogeneous nucleation of Mn(OH) 2(s) as the initial stage of Mn (hydr)oxide precipitation. Here, we investigated the effect of pH on the initial nucleation of Mn(OH) 2(s) on quartz. Under various pH conditions of 9.8, 9.9, and 10.1, we analyzed the structural matches between quartz and heterogeneously nucleated Mn(OH) 2(s). The structural matches were calculated by measuring lateral and vertical dimensions using grazing incidence small angle X-ray scatteringmore » (GISAXS) and atomic force microscopy (AFM), respectively. We found that a poorer structural match occurred at a higher pH than at a lower pH. The faster nucleation at a higher pH condition accounted for the observed poorer structural match. By fitting the structural match using classical nucleation theory, we also calculated the interfacial energy between Mn(OH) 2(s) and water (γ nf = 71 ± 7 mJ/m 2). The calculated m values and γ nf provided the variance of interfacial energy between quartz and Mn(OH) 2(s): γ sn = 262–272 mJ/m 2. As a result, this study provides new qualitative and quantitative information about heterogeneous nucleation on environmentally an abundant mineral surface, quartz, and it offers important underpinnings for understanding the fate and transport of trace ions in environmental systems.« less

Structural match of heterogeneously nucleated Mn(OH) 2(s) nanoparticles on quartz under various pH conditions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jung, Haesung; Lee, Byeongdu; Jun, Young -Shin

The early nucleation stage of Mn (hydr)oxide on mineral surfaces is crucial to understand its occurrence and the cycling of nutrients in environmental systems. However, there are only limited studies on the heterogeneous nucleation of Mn(OH) 2(s) as the initial stage of Mn (hydr)oxide precipitation. Here, we investigated the effect of pH on the initial nucleation of Mn(OH) 2(s) on quartz. Under various pH conditions of 9.8, 9.9, and 10.1, we analyzed the structural matches between quartz and heterogeneously nucleated Mn(OH) 2(s). The structural matches were calculated by measuring lateral and vertical dimensions using grazing incidence small angle X-ray scatteringmore » (GISAXS) and atomic force microscopy (AFM), respectively. We found that a poorer structural match occurred at a higher pH than at a lower pH. The faster nucleation at a higher pH condition accounted for the observed poorer structural match. By fitting the structural match using classical nucleation theory, we also calculated the interfacial energy between Mn(OH) 2(s) and water (γ nf = 71 ± 7 mJ/m 2). The calculated m values and γ nf provided the variance of interfacial energy between quartz and Mn(OH) 2(s): γ sn = 262–272 mJ/m 2. As a result, this study provides new qualitative and quantitative information about heterogeneous nucleation on environmentally an abundant mineral surface, quartz, and it offers important underpinnings for understanding the fate and transport of trace ions in environmental systems.« less

Elucidating Key Motifs Required for Arp2/3-Dependent and Independent Actin Nucleation by Las17/WASP

PubMed Central

Urbanek, Agnieszka N.; Smaczynska-de Rooij, Iwona I.

2016-01-01

Actin nucleation is the key rate limiting step in the process of actin polymerization, and tight regulation of this process is critical to ensure actin filaments form only at specific times and at defined regions of the cell. Arp2/3 is a well-characterised protein complex that can promote nucleation of new filaments, though its activity requires additional nucleation promotion factors (NPFs). The best recognized of these factors are the WASP family of proteins that contain binding motifs for both monomeric actin and for Arp2/3. Previously we demonstrated that the yeast WASP homologue, Las17, in addition to activating Arp2/3 can also nucleate actin filaments de novo, independently of Arp2/3. This activity is dependent on its polyproline rich region. Through biochemical and in vivo analysis we have now identified key motifs within the polyproline region that are required for nucleation and elongation of actin filaments, and have addressed the role of the WH2 domain in the context of actin nucleation without Arp2/3. We have also demonstrated that full length Las17 is able to bind liposomes giving rise to the possibility of direct linkage of nascent actin filaments to specific membrane sites to which Las17 has been recruited. Overall, we propose that Las17 functions as the key initiator of de novo actin filament formation at endocytic sites by nucleating, elongating and tethering nascent filaments which then serve as a platform for Arp2/3 recruitment and function. PMID:27637067

Heterogeneous nucleation on convex spherical substrate surfaces: A rigorous thermodynamic formulation of Fletcher's classical model and the new perspectives derived.

PubMed

Qian, Ma; Ma, Jie

2009-06-07

Fletcher's spherical substrate model [J. Chem. Phys. 29, 572 (1958)] is a basic model for understanding the heterogeneous nucleation phenomena in nature. However, a rigorous thermodynamic formulation of the model has been missing due to the significant complexities involved. This has not only left the classical model deficient but also likely obscured its other important features, which would otherwise have helped to better understand and control heterogeneous nucleation on spherical substrates. This work presents a rigorous thermodynamic formulation of Fletcher's model using a novel analytical approach and discusses the new perspectives derived. In particular, it is shown that the use of an intermediate variable, a selected geometrical angle or pseudocontact angle between the embryo and spherical substrate, revealed extraordinary similarities between the first derivatives of the free energy change with respect to embryo radius for nucleation on spherical and flat substrates. Enlightened by the discovery, it was found that there exists a local maximum in the difference between the equivalent contact angles for nucleation on spherical and flat substrates due to the existence of a local maximum in the difference between the shape factors for nucleation on spherical and flat substrate surfaces. This helps to understand the complexity of the heterogeneous nucleation phenomena in a practical system. Also, it was found that the unfavorable size effect occurs primarily when R<5r( *) (R: radius of substrate and r( *): critical embryo radius) and diminishes rapidly with increasing value of R/r( *) beyond R/r( *)=5. This finding provides a baseline for controlling the size effects in heterogeneous nucleation.

Crack Nucleation in β Titanium Alloys under High Cycle Fatigue Conditions - A Review

NASA Astrophysics Data System (ADS)

Benjamin, Rohit; Nageswara Rao, M.

2017-05-01

Beta titanium (β-Ti) alloys have emerged over the last 3 to 4 decades as an important class of titanium alloys. Many of the applications that they found, particularly in aerospace sector, are such that their high cycle fatigue (HCF) behavior becomes critical. In HCF regime, crack nucleation accounts for major part of the life. Consequently it becomes important to understand the mechanisms underlying the nucleation of cracks under HCF type loading conditions. The purpose of this review is to document the best understanding we have on date on crack nucleation in β-Ti alloys under HCF conditions. Role of various microstructural features encountered in β-Ti alloys in influencing the crack nucleation under HCF conditions has been reviewed. It has been brought out that changes in processing can result in changes in microstructure which in turn influence the time for crack nucleation/fatigue life and fatigue limit. While majority of fatigue failures originate at the surface, subsurface cracking is not uncommon with β-Ti alloys and the factors leading to subsurface cracking have been discussed in this review.

A spatially resolved network spike in model neuronal cultures reveals nucleation centers, circular traveling waves and drifting spiral waves.

PubMed

Paraskevov, A V; Zendrikov, D K

2017-03-23

We show that in model neuronal cultures, where the probability of interneuronal connection formation decreases exponentially with increasing distance between the neurons, there exists a small number of spatial nucleation centers of a network spike, from where the synchronous spiking activity starts propagating in the network typically in the form of circular traveling waves. The number of nucleation centers and their spatial locations are unique and unchanged for a given realization of neuronal network but are different for different networks. In contrast, if the probability of interneuronal connection formation is independent of the distance between neurons, then the nucleation centers do not arise and the synchronization of spiking activity during a network spike occurs spatially uniform throughout the network. Therefore one can conclude that spatial proximity of connections between neurons is important for the formation of nucleation centers. It is also shown that fluctuations of the spatial density of neurons at their random homogeneous distribution typical for the experiments in vitro do not determine the locations of the nucleation centers. The simulation results are qualitatively consistent with the experimental observations.

The void nucleation mechanism within lead phase during spallation of leaded brass

NASA Astrophysics Data System (ADS)

Yang, Yang; Wang, Can; Chen, Xingzhi; Chen, Kaiguo; Hu, Haibo; Fu, Yanan

2018-07-01

The incipient spall behaviours of Cu-34%Zn-3%Pb leaded brass samples with annealed and cryogenic-treated conditions were loaded using one-stage light gas gun experiments. The effect of Pb-phase on dynamic damage nucleation in leaded brass specimens was investigated by means of optical microscopy, scanning electron microscopy and x-ray computer tomography. It was found that the voids of incipient spall were mainly nucleated in the interior of the lead (no tensile stress would be produced within lead according to the impact theory) instead of nucleated at the phase interface as expected by quasi-static damage fracture theory. A nucleation model is proposed in the present work that is the asymmetry high compression zones in the centre of the lead-phase were formed by the rarefaction wave convergence effects of matrix/quasi-spherical lead interface, which caused adiabatic temperature rise that exceeded melting point of lead due to severe plastic deformation, finally led to local melting and void nucleation. In addition, the spall strength and damage rate increased with the increase in the Pb-phase number.

Zinc-Nucleated D 2 and H 2 Crystal Formation from Their Liquids

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bernat, T. P.; Petta, N.; Kozioziemski, B.

Calorimetric measurements at University of Rochester Laboratory for Laser Energetics of D 2 crystallization from the melt indicate that zinc can act as a heterogeneous nucleation seed with suppressed supercooling. We further studied in this paper this effect for a variety of zinc substrates using the optical-access cryogenic sample cell at Lawrence Livermore National Laboratory. Small supercoolings are observed, some as low as 5 mK, but results depend on the zinc history and sample preparation. In general, thin samples prepared by physical vapor deposition were not effective in nucleating crystal formation. Larger (several-millimeter) granules showed greater supercooling suppression, depending onmore » surface modification and granule size. Surfaces of these granules are morphologically varied and not uniform. Scanning electron microscope images were not able to correlate any particular surface feature with enhanced nucleation. Finally, application of classical nucleation theory to the observed variation of supercooling level with granule size is consistent with nucleation features with sizes <100 nm and with wetting angles of a few degrees.« less

An improved ice cloud formation parameterization in the EMAC model

NASA Astrophysics Data System (ADS)

Bacer, Sara; Pozzer, Andrea; Karydis, Vlassis; Tsimpidi, Alexandra; Tost, Holger; Sullivan, Sylvia; Nenes, Athanasios; Barahona, Donifan; Lelieveld, Jos

2017-04-01

Cirrus clouds cover about 30% of the Earth's surface and are an important modulator of the radiative energy budget of the atmosphere. Despite their importance in the global climate system, there are still large uncertainties in understanding the microphysical properties and interactions with aerosols. Ice crystal formation is quite complex and a variety of mechanisms exists for ice nucleation, depending on aerosol characteristics and environmental conditions. Ice crystals can be formed via homogeneous nucleation or heterogeneous nucleation of ice-nucleating particles in different ways (contact, immersion, condensation, deposition). We have implemented the computationally efficient cirrus cloud formation parameterization by Barahona and Nenes (2009) into the EMAC (ECHAM5/MESSy Atmospheric Chemistry) model in order to improve the representation of ice clouds and aerosol-cloud interactions. The parameterization computes the ice crystal number concentration from precursor aerosols and ice-nucleating particles accounting for the competition between homogeneous and heterogeneous nucleation and among different freezing modes. Our work shows the differences and the improvements obtained after the implementation with respect to the previous version of EMAC.

Strong control of Southern Ocean cloud reflectivity by ice-nucleating particles

NASA Astrophysics Data System (ADS)

Vergara-Temprado, Jesús; Miltenberger, Annette K.; Furtado, Kalli; Grosvenor, Daniel P.; Shipway, Ben J.; Hill, Adrian A.; Wilkinson, Jonathan M.; Field, Paul R.; Murray, Benjamin J.; Carslaw, Ken S.

2018-03-01

Large biases in climate model simulations of cloud radiative properties over the Southern Ocean cause large errors in modeled sea surface temperatures, atmospheric circulation, and climate sensitivity. Here, we combine cloud-resolving model simulations with estimates of the concentration of ice-nucleating particles in this region to show that our simulated Southern Ocean clouds reflect far more radiation than predicted by global models, in agreement with satellite observations. Specifically, we show that the clouds that are most sensitive to the concentration of ice-nucleating particles are low-level mixed-phase clouds in the cold sectors of extratropical cyclones, which have previously been identified as a main contributor to the Southern Ocean radiation bias. The very low ice-nucleating particle concentrations that prevail over the Southern Ocean strongly suppress cloud droplet freezing, reduce precipitation, and enhance cloud reflectivity. The results help explain why a strong radiation bias occurs mainly in this remote region away from major sources of ice-nucleating particles. The results present a substantial challenge to climate models to be able to simulate realistic ice-nucleating particle concentrations and their effects under specific meteorological conditions.

Effect of wetting on nucleation and growth of D2 in confinement

NASA Astrophysics Data System (ADS)

Zepeda-Ruiz, L. A.; Sadigh, B.; Shin, S. J.; Kozioziemski, B. J.; Chernov, A. A.

2018-04-01

We have performed a computational study to determine how the wetting of liquid deuterium to the walls of the material influences nucleation. We present the development of a pair-wise interatomic potential that includes zero-point motion of molecular deuterium. Deuterium is used in this study because of its importance to inertial confinement fusion and the potential to generate a superfluid state if the solidification can be suppressed. Our simulations show that wetting dominates undercooling compared to the pore geometries. We observe a transition from heterogeneous nucleation at the confining wall to homogeneous nucleation at the bulk of the liquid (and intermediate cases) as the interaction with the confining wall changes from perfect wetting to non-wetting. When nucleation is heterogeneous, the temperature needed for solidification changes by 4 K with decreasing deuterium-wall interaction, but it remains independent (and equal to the one from bulk samples) when homogeneous nucleation dominates. We find that growth and quality of the resulting microstructure also depends on the magnitude of liquid deuterium-wall interaction strength.

Nucleus-size pinning for determination of nucleation free-energy barriers and nucleus geometry

NASA Astrophysics Data System (ADS)

Sharma, Abhishek K.; Escobedo, Fernando A.

2018-05-01

Classical Nucleation Theory (CNT) has recently been used in conjunction with a seeding approach to simulate nucleation phenomena at small-to-moderate supersaturation conditions when large free-energy barriers ensue. In this study, the conventional seeding approach [J. R. Espinosa et al., J. Chem. Phys. 144, 034501 (2016)] is improved by a novel, more robust method to estimate nucleation barriers. Inspired by the interfacial pinning approach [U. R. Pedersen, J. Chem. Phys. 139, 104102 (2013)] used before to determine conditions where two phases coexist, the seed of the incipient phase is pinned to a preselected size to iteratively drive the system toward the conditions where the seed becomes a critical nucleus. The proposed technique is first validated by estimating the critical nucleation conditions for the disorder-to-order transition in hard spheres and then applied to simulate and characterize the highly non-trivial (prolate) morphology of the critical crystal nucleus in hard gyrobifastigia. A generalization of CNT is used to account for nucleus asphericity and predict nucleation free-energy barriers for gyrobifastigia. These predictions of nuclei shape and barriers are validated by independent umbrella sampling calculations.

The SPectrometer for Ice Nuclei (SPIN): An instrument to investigate ice nucleation

DOE PAGES

Garimella, Sarvesh; Kristensen, Thomas Bjerring; Ignatius, Karolina; ...

2016-07-06

The SPectrometer for Ice Nuclei (SPIN) is a commercially available ice nucleating particle (INP) counter manufactured by Droplet Measurement Technologies in Boulder, CO. The SPIN is a continuous flow diffusion chamber with parallel plate geometry based on the Zurich Ice Nucleation Chamber and the Portable Ice Nucleation Chamber. This study presents a standard description for using the SPIN instrument and also highlights methods to analyze measurements in more advanced ways. It characterizes and describes the behavior of the SPIN chamber, reports data from laboratory measurements, and quantifies uncertainties associated with the measurements. Experiments with ammonium sulfate are used to investigatemore » homogeneous freezing of deliquesced haze droplets and droplet breakthrough. Experiments with kaolinite, NX illite, and silver iodide are used to investigate heterogeneous ice nucleation. SPIN nucleation results are compared to those from the literature. A machine learning approach for analyzing depolarization data from the SPIN optical particle counter is also presented (as an advanced use). Altogether, we report that the SPIN is able to reproduce previous INP counter measurements.« less

An Improved Model for Nucleation-Limited Ice Formation in Living Cells during Freezing

PubMed Central

Zhao, Gang; He, Xiaoming

2014-01-01

Ice formation in living cells is a lethal event during freezing and its characterization is important to the development of optimal protocols for not only cryopreservation but also cryotherapy applications. Although the model for probability of ice formation (PIF) in cells developed by Toner et al. has been widely used to predict nucleation-limited intracellular ice formation (IIF), our data of freezing Hela cells suggest that this model could give misleading prediction of PIF when the maximum PIF in cells during freezing is less than 1 (PIF ranges from 0 to 1). We introduce a new model to overcome this problem by incorporating a critical cell volume to modify the Toner's original model. We further reveal that this critical cell volume is dependent on the mechanisms of ice nucleation in cells during freezing, i.e., surface-catalyzed nucleation (SCN) and volume-catalyzed nucleation (VCN). Taken together, the improved PIF model may be valuable for better understanding of the mechanisms of ice nucleation in cells during freezing and more accurate prediction of PIF for cryopreservation and cryotherapy applications. PMID:24852166

Effects of side chains in helix nucleation differ from helix propagation

PubMed Central

Miller, Stephen E.; Watkins, Andrew M.; Kallenbach, Neville R.; Arora, Paramjit S.

2014-01-01

Helix–coil transition theory connects observable properties of the α-helix to an ensemble of microstates and provides a foundation for analyzing secondary structure formation in proteins. Classical models account for cooperative helix formation in terms of an energetically demanding nucleation event (described by the σ constant) followed by a more facile propagation reaction, with corresponding s constants that are sequence dependent. Extensive studies of folding and unfolding in model peptides have led to the determination of the propagation constants for amino acids. However, the role of individual side chains in helix nucleation has not been separately accessible, so the σ constant is treated as independent of sequence. We describe here a synthetic model that allows the assessment of the role of individual amino acids in helix nucleation. Studies with this model lead to the surprising conclusion that widely accepted scales of helical propensity are not predictive of helix nucleation. Residues known to be helix stabilizers or breakers in propagation have only a tenuous relationship to residues that favor or disfavor helix nucleation. PMID:24753597

Can Hail and Rain Nucleate Cloud Droplets?

NASA Astrophysics Data System (ADS)

Weiss, S.; Prabhakaran, P.; Krekhov, A.; Pumir, A.; Bodenschatz, E.

2017-12-01

We present results from a laboratory scale moist convection experiment composed of a mixture of pressurized sulphur hexafluoride (SF6 - liquid and vapor phase) and helium (He - gas phase) to mimic the wet (saturated water vapor) and dry components (nitrogen, oxygen etc.) of the earth's atmosphere. We operate the experiments close to critical conditions to allow for homogeneous nucleation of sulphur hexafluoride droplets. The liquid SF6 pool is heated from below and the warm SF6 vapor from the liquid-vapor interface rise and condense underneath the cold top plate. We observe the nucleation of microdroplets in the wake of cold drops falling through the SF6-He atmosphere. Using classical nucleation theory, we show that the nucleation is caused by isobaric cooling of SF6 vapor in the wake of the cold drop. Furthermore, we argue that in an atmospheric cloud, falling hail and large cold raindrops may induce heterogeneous nucleation of microdroplets in their wake. We also observe that under appropriate conditions these microdroplets form a stable horizontal layer, thus separating regions of super and sub-critical saturation.

Can hail and rain nucleate cloud droplets?

NASA Astrophysics Data System (ADS)

Prabhakaran, Prasanth; Weiss, Stephan; Krekhov, Alexei; Pumir, Alain; Bodenschatz, Eberhard

2017-11-01

We present results from a laboratory scale moist convection experiment composed of a mixture of pressurized sulphur hexafluoride (SF6 - liquid and vapor phase) and helium (He - gas phase) to mimic the wet (saturated water vapor) and dry components (nitrogen, oxygen etc.) of the earth's atmosphere. We operate the experiments close to critical conditions to allow for homogeneous nucleation of sulphur hexafluoride droplets. The liquid SF6 pool is heated from below and the warm SF6 vapor from the liquid-vapor interface rise and condense underneath the cold top plate. We observe the nucleation of microdroplets in the wake of cold drops falling through the SF6-He atmosphere. Using classical nucleation theory, we show that the nucleation is caused by isobaric cooling of SF6 vapor in the wake of the cold drop. Furthermore, we argue that in an atmospheric cloud, falling hail and large cold raindrops may induce heterogeneous nucleation of microdroplets in their wake. We also observe that under appropriate conditions these microdroplets form a stable horizontal layer, thus separating regions of super and sub-critical saturation.

A spatially resolved network spike in model neuronal cultures reveals nucleation centers, circular traveling waves and drifting spiral waves

NASA Astrophysics Data System (ADS)

Paraskevov, A. V.; Zendrikov, D. K.

2017-04-01

We show that in model neuronal cultures, where the probability of interneuronal connection formation decreases exponentially with increasing distance between the neurons, there exists a small number of spatial nucleation centers of a network spike, from where the synchronous spiking activity starts propagating in the network typically in the form of circular traveling waves. The number of nucleation centers and their spatial locations are unique and unchanged for a given realization of neuronal network but are different for different networks. In contrast, if the probability of interneuronal connection formation is independent of the distance between neurons, then the nucleation centers do not arise and the synchronization of spiking activity during a network spike occurs spatially uniform throughout the network. Therefore one can conclude that spatial proximity of connections between neurons is important for the formation of nucleation centers. It is also shown that fluctuations of the spatial density of neurons at their random homogeneous distribution typical for the experiments in vitro do not determine the locations of the nucleation centers. The simulation results are qualitatively consistent with the experimental observations.

Strong control of Southern Ocean cloud reflectivity by ice-nucleating particles

PubMed Central

Miltenberger, Annette K.; Furtado, Kalli; Grosvenor, Daniel P.; Shipway, Ben J.; Hill, Adrian A.; Wilkinson, Jonathan M.; Field, Paul R.

2018-01-01

Large biases in climate model simulations of cloud radiative properties over the Southern Ocean cause large errors in modeled sea surface temperatures, atmospheric circulation, and climate sensitivity. Here, we combine cloud-resolving model simulations with estimates of the concentration of ice-nucleating particles in this region to show that our simulated Southern Ocean clouds reflect far more radiation than predicted by global models, in agreement with satellite observations. Specifically, we show that the clouds that are most sensitive to the concentration of ice-nucleating particles are low-level mixed-phase clouds in the cold sectors of extratropical cyclones, which have previously been identified as a main contributor to the Southern Ocean radiation bias. The very low ice-nucleating particle concentrations that prevail over the Southern Ocean strongly suppress cloud droplet freezing, reduce precipitation, and enhance cloud reflectivity. The results help explain why a strong radiation bias occurs mainly in this remote region away from major sources of ice-nucleating particles. The results present a substantial challenge to climate models to be able to simulate realistic ice-nucleating particle concentrations and their effects under specific meteorological conditions. PMID:29490918

3D Study of the Morphology and Dynamics of Zeolite Nucleation.

PubMed

Melinte, Georgian; Georgieva, Veselina; Springuel-Huet, Marie-Anne; Nossov, Andreï; Ersen, Ovidiu; Guenneau, Flavien; Gedeon, Antoine; Palčić, Ana; Bozhilov, Krassimir N; Pham-Huu, Cuong; Qiu, Shilun; Mintova, Svetlana; Valtchev, Valentin

2015-12-07

The principle aspects and constraints of the dynamics and kinetics of zeolite nucleation in hydrogel systems are analyzed on the basis of a model Na-rich aluminosilicate system. A detailed time-series EMT-type zeolite crystallization study in the model hydrogel system was performed to elucidate the topological and temporal aspects of zeolite nucleation. A comprehensive set of analytical tools and methods was employed to analyze the gel evolution and complement the primary methods of transmission electron microscopy (TEM) and nuclear magnetic resonance (NMR) spectroscopy. TEM tomography reveals that the initial gel particles exhibit a core-shell structure. Zeolite nucleation is topologically limited to this shell structure and the kinetics of nucleation is controlled by the shell integrity. The induction period extends to the moment when the shell is consumed and the bulk solution can react with the core of the gel particles. These new findings, in particular the importance of the gel particle shell in zeolite nucleation, can be used to control the growth process and properties of zeolites formed in hydrogels. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Heterogeneous Nucleation of Dicalcium Phosphate Dihydrate on Modified Silica Surfaces

PubMed Central

Miller, Carrie; Komunjer, Ljepša; Hlady, Vladimir

2012-01-01

Heterogeneous nucleation of dicalcium phosphate dihydrate, CaHPO4•2H2O (DCPD) was studied on untreated planar fused silica and on three modified silica surfaces: octadecylsilyl (OTS) modified silica, human serum albumin treated OTS silica, and UV-oxidized 3-mercaptopropyltriethoxysilyl (MTS) modified silica. The supersaturation ratio of calcium and phosphate solution with respect to DCPD was kept below ~10. The nucleated crystals were observed 24 hours and one week after initial contact between supersaturated solutions and substrate surfaces using bright field and reflectance interference contrast microscopy. No DCPD crystals nucleated on albumin-treated OTS-silica. Majority of the DCDP crystals formed on the other modified silica surfaces appeared to be morphologically similar irrespective of the nature of nucleating substrate. Reflectance interference contrast microscopy provided a proof that the majority of the crystals on these substrates do not develop an extended contact with the substrate surface. The images showed that the most extended contact planes were between the DCPD crystals and MTS modified silica surface. The crystals nucleated on OTS-treated and untreated silica surfaces showed only few or none well-developed contact planes. PMID:25264399

Multiple pathways of crystal nucleation in an extremely supersaturated aqueous potassium dihydrogen phosphate (KDP) solution droplet

PubMed Central

Lee, Sooheyong; Wi, Haeng Sub; Jo, Wonhyuk; Cho, Yong Chan; Lee, Hyun Hwi; Jeong, Se-Young; Kim, Yong-Il; Lee, Geun Woo

2016-01-01

Solution studies have proposed that crystal nucleation can take more complex pathways than previously expected in classical nucleation theory, such as formation of prenucleation clusters or densified amorphous/liquid phases. These findings show that it is possible to separate fluctuations in the different order parameters governing crystal nucleation, that is, density and structure. However, a direct observation of the multipathways from aqueous solutions remains a great challenge because heterogeneous nucleation sites, such as container walls, can prevent these paths. Here, we demonstrate the existence of multiple pathways of nucleation in highly supersaturated aqueous KH2PO4 (KDP) solution using the combination of a containerless device (electrostatic levitation), and in situ micro-Raman and synchrotron X-ray scattering. Specifically, we find that, at an unprecedentedly deep level of supersaturation, a high-concentration KDP solution first transforms into a metastable crystal before reaching stability at room temperature. However, a low-concentration solution, with different local structures, directly transforms into the stable crystal phase. These apparent multiple pathways of crystallization depend on the degree of supersaturation. PMID:27791068

Microtubule nucleation and organization in dendrites

PubMed Central

Delandre, Caroline; Amikura, Reiko; Moore, Adrian W.

2016-01-01

ABSTRACT Dendrite branching is an essential process for building complex nervous systems. It determines the number, distribution and integration of inputs into a neuron, and is regulated to create the diverse dendrite arbor branching patterns characteristic of different neuron types. The microtubule cytoskeleton is critical to provide structure and exert force during dendrite branching. It also supports the functional requirements of dendrites, reflected by differential microtubule architectural organization between neuron types, illustrated here for sensory neurons. Both anterograde and retrograde microtubule polymerization occur within growing dendrites, and recent studies indicate that branching is enhanced by anterograde microtubule polymerization events in nascent branches. The polarities of microtubule polymerization events are regulated by the position and orientation of microtubule nucleation events in the dendrite arbor. Golgi outposts are a primary microtubule nucleation center in dendrites and share common nucleation machinery with the centrosome. In addition, pre-existing dendrite microtubules may act as nucleation sites. We discuss how balancing the activities of distinct nucleation machineries within the growing dendrite can alter microtubule polymerization polarity and dendrite branching, and how regulating this balance can generate neuron type-specific morphologies. PMID:27097122

Multiple pathways of crystal nucleation in an extremely supersaturated aqueous potassium dihydrogen phosphate (KDP) solution droplet.

PubMed

Lee, Sooheyong; Wi, Haeng Sub; Jo, Wonhyuk; Cho, Yong Chan; Lee, Hyun Hwi; Jeong, Se-Young; Kim, Yong-Il; Lee, Geun Woo

2016-11-29

Solution studies have proposed that crystal nucleation can take more complex pathways than previously expected in classical nucleation theory, such as formation of prenucleation clusters or densified amorphous/liquid phases. These findings show that it is possible to separate fluctuations in the different order parameters governing crystal nucleation, that is, density and structure. However, a direct observation of the multipathways from aqueous solutions remains a great challenge because heterogeneous nucleation sites, such as container walls, can prevent these paths. Here, we demonstrate the existence of multiple pathways of nucleation in highly supersaturated aqueous KH 2 PO 4 (KDP) solution using the combination of a containerless device (electrostatic levitation), and in situ micro-Raman and synchrotron X-ray scattering. Specifically, we find that, at an unprecedentedly deep level of supersaturation, a high-concentration KDP solution first transforms into a metastable crystal before reaching stability at room temperature. However, a low-concentration solution, with different local structures, directly transforms into the stable crystal phase. These apparent multiple pathways of crystallization depend on the degree of supersaturation.

Vapor-liquid nucleation: the solid touch.

PubMed

Yarom, Michal; Marmur, Abraham

2015-08-01

Vapor-liquid nucleation is a ubiquitous process that has been widely researched in many disciplines. Yet, case studies are quite scattered in the literature, and the implications of some of its basic concepts are not always clearly stated. This is especially noticeable for heterogeneous nucleation, which involves a solid surface in touch with the liquid and vapor. The current review attempts to offer a comprehensive, though concise, thermodynamic discussion of homogeneous and heterogeneous nucleation in vapor-liquid systems. The fundamental concepts of nucleation are detailed, with emphasis on the role of the chemical potential, and on intuitive explanations whenever possible. We review various types of nucleating systems and discuss the effect of the solid geometry on the characteristics of the new phase formation. In addition, we consider the effect of mixing on the vapor-liquid equilibrium. An interesting sub-case is that of a non-volatile solute that modifies the chemical potential of the liquid, but not of the vapor. Finally, we point out topics that need either further research or more exact, accurate presentation. Copyright © 2014 Elsevier B.V. All rights reserved.

Isothermal Ice Crystallization Kinetics in the Gas-Diffusion Layer of a Proton-Exchange-Membrane Fuel Cell

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dursch, Thomas J.; Ciontea, Monica A.; Radke, Clayton J.

2011-12-01

Nucleation and growth of ice in the fibrous gas-diffusion layer (GDL) of a proton-exchange membrane fuel cell (PEMFC) are studied using isothermal differential scanning calorimetry (DSC). Isothermal crystallization rates and pseudo-steady-state nucleation rates are obtained as a function of subcooling from heat-flow and induction-time measurements. Kinetics of ice nucleation and growth are studied at two polytetrafluoroethylene (PTFE) loadings (0 and 10 wt %) in a commercial GDL for temperatures between 240 and 273 K. A nonlinear ice-crystallization rate expression is developed using Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory, in which the heat-transfer-limited growth rate is determined from the moving-boundary Stefan problem. Induction timesmore » follow a Poisson distribution and increase upon addition of PTFE, indicating that nucleation occurs more slowly on a hydrophobic fiber than on a hydrophilic fiber. The determined nucleation rates and induction times follow expected trends from classical nucleation theory. Finally, a validated rate expression is now available for predicting ice-crystallization kinetics in GDLs.« less

Mutagenic analysis of the nucleation propensity of oxidized Alzheimer's beta-amyloid peptide.

PubMed

Christopeit, Tony; Hortschansky, Peter; Schroeckh, Volker; Gührs, Karlheinz; Zandomeneghi, Giorgia; Fändrich, Marcus

2005-08-01

The formation of polypeptide aggregates represents a nucleated polymerization reaction in which an initial nucleation event (lag phase) is followed by the extension of newly formed nuclei into larger aggregates, including fibrils (growth phase). The efficiencies of these reactions relate to the lag time (lag phase) and to the rate of aggregation (growth phase), which can be determined from experimental aggregation curves. Here we present a mutagenic analysis in which we replace valine 18 of the Alzheimer's Abeta (1-40) peptide with 17 different amino acids and determine its effect on the lag time, and therefore, on the propensity of nucleation. Comparison with various physico-chemical properties shows that nucleation is affected in a predictable manner depending on the beta-sheet propensity and hydrophobicity of residue 18. In addition, we observe a direct proportionality between the lag time and the rate of aggregation. These data imply that the two reactions, nucleation and polymerization, are governed by very similar physicochemical principles and that they involve the formation of the same types of noncovalent interactions.

Zinc-Nucleated D 2 and H 2 Crystal Formation from Their Liquids

DOE PAGES

Bernat, T. P.; Petta, N.; Kozioziemski, B.; ...

2016-09-01

Calorimetric measurements at University of Rochester Laboratory for Laser Energetics of D 2 crystallization from the melt indicate that zinc can act as a heterogeneous nucleation seed with suppressed supercooling. We further studied in this paper this effect for a variety of zinc substrates using the optical-access cryogenic sample cell at Lawrence Livermore National Laboratory. Small supercoolings are observed, some as low as 5 mK, but results depend on the zinc history and sample preparation. In general, thin samples prepared by physical vapor deposition were not effective in nucleating crystal formation. Larger (several-millimeter) granules showed greater supercooling suppression, depending onmore » surface modification and granule size. Surfaces of these granules are morphologically varied and not uniform. Scanning electron microscope images were not able to correlate any particular surface feature with enhanced nucleation. Finally, application of classical nucleation theory to the observed variation of supercooling level with granule size is consistent with nucleation features with sizes <100 nm and with wetting angles of a few degrees.« less

Nucleation in the presence of long-range interactions. [performed on ferroelectric barium titanate

NASA Technical Reports Server (NTRS)

Chandra, P.

1989-01-01

Unlike droplet nucleation near a liquid-gas critical point, the decay of metastable phases in crystalline materials is strongly affected by the presence of long-range forces. Field quench experiments performed on the ferroelectric barium titanate indicate that nucleation in this material is markedly different from that observed in liquids. In this paper, a theory for nucleation at a first-order phase transition in which the mediating forces are long range is presented. It is found that the long-range force induces cooperative nucleation and growth processes, and that this feedback mechanism produces a well-defined delay time with a sharp onset in the transformation to the stable phase. Closed-form expressions for the characteristic onset time and width of the transition are developed, in good agreement with numerical and experimental results.

Quantification of dislocation nucleation stress in TiN through high-resolution in situ indentation experiments and first principles calculations

DOE PAGES

Li, N.; Yadav, S. K.; Liu, X. -Y.; ...

2015-11-05

Using the in situ indentation of TiN in a high-resolution transmission electron microscope, the nucleation of full as well as partial dislocations has been observed from {001} and {111} surfaces, respectively. The critical elastic strains associated with the nucleation of the dislocations were analyzed from the recorded atomic displacements, and the nucleation stresses corresponding to the measured critical strains were computed using density functional theory. The resolved shear stress was estimated to be 13.8 GPa for the partial dislocation 1/6 <110> {111} and 6.7 GPa for the full dislocation ½ <110> {110}. Moreover, such an approach of quantifying nucleation stressesmore » for defects via in situ high-resolution experiment coupled with density functional theory calculation may be applied to other unit processes.« less

Direct observation of hierarchical nucleation of martensite and size-dependent superelasticity in shape memory alloys.

PubMed

Liu, Lifeng; Ding, Xiangdong; Li, Ju; Lookman, Turab; Sun, Jun

2014-02-21

Martensitic transformation usually creates hierarchical internal structures beyond mere change of the atomic crystal structure. Multi-stage nucleation is thus required, where nucleation (level-1) of the underlying atomic crystal lattice does not have to be immediately followed by the nucleation of higher-order superstructures (level-2 and above), such as polysynthetic laths. Using in situ transmission electron microscopy (TEM), we directly observe the nucleation of the level-2 superstructure in a Cu-Al-Ni single crystal under compression, with critical super-nuclei size L2c around 500 nm. When the sample size D decreases below L2c, the superelasticity behavior changes from a flat stress plateau to a continuously rising stress-strain curve. Such size dependence definitely would impact the application of shape memory alloys in miniaturized MEMS/NEMS devices.

Monte-Carlo simulation of defect-cluster nucleation in metals during irradiation

NASA Astrophysics Data System (ADS)

Nakasuji, Toshiki; Morishita, Kazunori; Ruan, Xiaoyong

2017-02-01

A multiscale modeling approach was applied to investigate the nucleation process of CRPs (copper rich precipitates, i.e., copper-vacancy clusters) in α-Fe containing 1 at.% Cu during irradiation. Monte-Carlo simulations were performed to investigate the nucleation process, with the rate theory equation analysis to evaluate the concentration of displacement defects, along with the molecular dynamics technique to know CRP thermal stabilities in advance. Our MC simulations showed that there is long incubation period at first, followed by a rapid growth of CRPs. The incubation period depends on irradiation conditions such as the damage rate and temperature. CRP's composition during nucleation varies with time. The copper content of CRPs shows relatively rich at first, and then becomes poorer as the precipitate size increases. A widely-accepted model of CRP nucleation process is finally proposed.

Effect of controlled ice nucleation on primary drying stage and protein recovery in vials cooled in a modified freeze-dryer.

PubMed

Passot, Stéphanie; Tréléa, Ioan Cristian; Marin, Michèle; Galan, Miquel; Morris, G John; Fonseca, Fernanda

2009-07-01

The freezing step influences lyophilization efficiency and protein stability. The main objective of this work was to investigate the impact on the primary drying stage of an ultrasound controlled ice nucleation technology, compared with usual freezing protocols. Lyophilization cycles involving different freezing protocols (applying a constant shelf cooling rate of 1 degrees C/min or 0.2 degrees C/min, putting vials on a precooled shelf, and controlling nucleation by ultrasounds or by addition of a nucleating agent) were performed in a prototype freeze-dryer. Three protective media including sucrose or maltodextrin and differing by their thermal properties and their ability to preserve a model protein (catalase) were used. The visual aspect of the lyophilized cake, residual water content, and enzymatic activity recovery of catalase were assessed after each lyophilization cycle and after 1 month of storage of the lyophilized product at 4 degrees C and 25 degrees C. The freezing protocols allowing increasing nucleation temperature (precooled shelf and controlled nucleation by using ultrasounds or a nucleating agent) induced a faster sublimation step and higher sublimation rate homogeneity. Whatever the composition of the protective medium, applying the ultrasound technology made it possible to decrease the sublimation time by 14%, compared with the freezing method involving a constant shelf cooling rate of 1 degrees C/min. Concerning the enzyme activity recovery, the impact of the freezing protocol was observed only for the protective medium involving maltodextrin, a less effective protective agent than sucrose. Higher activity recovery results were obtained after storage when the ultrasound technology or the precooled shelf method was applied. Controlling ice nucleation during the freezing step of the lyophilization process improved the homogeneity of the sublimation rates, which will, in turn, reduce the intervial heterogeneity. The freeze-dryer prototype including the system of controlled nucleation by ultrasounds appears to be a promising tool in accelerating sublimation and improving intrabatch homogeneity.

Results from the FIN-2 formal comparison

NASA Astrophysics Data System (ADS)

Connolly, Paul; Hoose, Corinna; Liu, Xiaohong; Moehler, Ottmar; Cziczo, Daniel; DeMott, Paul

2017-04-01

During the Fifth International Ice Nucleation Workshop (FIN-2) at the AIDA Ice Nucleation facility in Karlsruhe, Germany in March 2015, a formal comparison of ice nucleation measurement methods was conducted. During the experiments the samples of ice nucleating particles were not revealed to the instrument scientists, hence this was referred to as a "blind comparison". The two samples used were later revealed to be Arizona Test Dust and an Argentina soil sample. For these two samples seven mobile ice nucleating particle counters sampled directly from the AIDA chamber or from the aerosol preparation chamber at specified temperatures, whereas filter samples were taken for two offline deposition nucleation instruments. Wet suspension methods for determining IN concentrations were also used with 10 different methods employed. For the wet suspension methods experiments were conducted using INPs collected from the air inside the chambers (impinger sampling) and INPs taken from the bulk samples (vial sampling). Direct comparisons of the ice nucleating particle concentrations are reported as well as derived ice nucleation active site densities. The study highlights the difficulties in performing such analyses, but generally indicates that there is reasonable agreement between the wet suspension techniques. It is noted that ice nucleation efficiency derived from the AIDA chamber (quantified using the ice active surface site density approach) is higher than that for the cold stage techniques. This is both true for the Argentina soil sample and, to a lesser extent, for the Arizona Test Dust sample too. Other interesting effects were noted: for the ATD the impinger sampling demonstrated higher INP efficiency at higher temperatures (>255 K) than the vial sampling, but agreed at the lower temperatures (<255K), whereas the opposite was true for the Argentina soil sample. The results are analysed to better understand the performance of the various techniques and to address any size-sorting effects and / or sampling line loses.

Slow Slip and Earthquake Nucleation in Meter-Scale Laboratory Experiments

NASA Astrophysics Data System (ADS)

Mclaskey, G.

2017-12-01

The initiation of dynamic rupture is thought to be preceded by a quasistatic nucleation phase. Observations of recent earthquakes sometimes support this by illuminating slow slip and foreshocks in the vicinity of the eventual hypocenter. I describe laboratory earthquake experiments conducted on two large-scale loading machines at Cornell University that provide insight into the way earthquake nucleation varies with normal stress, healing time, and loading rate. The larger of the two machines accommodates a 3 m long granite sample, and when loaded to 7 MPa stress levels, we observe dynamic rupture events that are preceded by a measureable nucleation zone with dimensions on the order of 1 m. The smaller machine accommodates a 0.76 m sample that is roughly the same size as the nucleation zone. On this machine, small variations in nucleation properties result in measurable differences in slip events, and we generate both dynamic rupture events (> 0.1 m/s slip rates) and slow slip events ( 0.001 to 30 mm/s slip rates). Slow events occur when instability cannot fully nucleate before reaching the sample ends. Dynamic events occur after long healing times or abrupt increases in loading rate which suggests that these factors shrink the spatial and temporal extents of the nucleation zone. Arrays of slip, strain, and ground motion sensors installed on the sample allow us to quantify seismic coupling and study details of premonitory slip and afterslip. The slow slip events we observe are primarily aseismic (less than 1% of the seismic coupling of faster events) and produce swarms of very small M -6 to M -8 events. These mechanical and seismic interactions suggest that faults with transitional behavior—where creep, small earthquakes, and tremor are often observed—could become seismically coupled if loaded rapidly, either by a slow slip front or dynamic rupture of an earthquake that nucleated elsewhere.

Slip-weakening zone sizes at nucleation of catastrophic subaerial and submarine landslides by gradually increasing pore pressure

NASA Astrophysics Data System (ADS)

Viesca, R. C.; Rice, J. R.

2011-12-01

We address the nucleation of dynamic landslide rupture in response to gradual pore pressure increases. Nucleation marks the onset of acceleration of the overlying slope mass due to the suddenly rapid enlargement of a sub-surface zone of shear failure, previously deforming quasi-statically. We model that zone as a planar surface undergoing initially linear slip-weakening frictional failure within a bordering linear-elastic medium. The results are also relevant to earthquake nucleation. The sub-surface rupture zone considered runs parallel to the free surface of a uniform slope, under a 2D plane-strain deformation state. We show results for ruptures with friction coefficients following linear slip weakening (i.e., the residual friction is not yet reached). For spatially broad increases in pore pressure, the nucleation length depends on a ratio of depth to a cohesive zone length scale. In the very broad-increase limit, a direct numerical solution for nucleation lengths compares well with solutions to a corresponding eigenvalue problem (similar to Uenishi and Rice [JGR '03]), in which spatial variations in normal stress are neglected. We estimate nucleation lengths for subaerial and submarine conditions using data [e.g., Bishop et al., Géotech. '71; Stark et al., JGGE '05] from ring-shear tests on sediments (peak friction fp = 0.5, frictional slip-weakening rate within the range w = -df/d(slip) = 0.1/cm-1/cm). We assume that only pre-stresses, and not material properties, vary with depth. With such fp and w, we find for a range of subsurface depths and shear moduli μ that nucleation lengths are typically several hundred meters long for shallow undersea slopes, and up to an order of magnitude less for steeper slopes on the Earth's surface. In the submarine case, this puts nucleation lengths in a size range comparable to observed pore-pressure-generated seafloor disturbances as pockmarks [e.g., Gay et al., MG '06].

Analysis of supercooling activity of tannin-related polyphenols.

PubMed

Kuwabara, Chikako; Wang, Donghui; Endoh, Keita; Fukushi, Yukiharu; Arakawa, Keita; Fujikawa, Seizo

2013-08-01

Based on the discovery of novel supercooling-promoting hydrolyzable gallotannins from deep supercooling xylem parenchyma cells (XPCs) in Katsura tree (see Wang et al. (2012) [38]), supercooling capability of a wide variety of tannin-related polyphenols (TRPs) was examined in order to find more effective supercooling-promoting substances for their applications. The TRPs examined were single compounds including six kinds of hydrolyzable tannins, 11 kinds of catechin derivatives, two kinds of structural analogs of catechin and six kinds of phenolcarboxylic acid derivatives, 11 kinds of polyphenol mixtures and five kinds of crude plant tannin extracts. The effects of these TRPs on freezing were examined by droplet freezing assays using various solutions containing different kinds of identified ice nucleators such as the ice nucleation bacterium (INB) Erwinia ananas, the INB Xanthomonas campestris, silver iodide and phloroglucinol as well as a solution containing only unintentionally included unidentified airborne ice nucleators. Among the 41 kinds of TRPs examined, all of the hydrolyzable tannins, catechin derivatives, polyphenol mixtures and crude plant tannin extracts as well as a few structural analogs of catechin and phenolcarboxylic acid derivatives exhibited supercooling-promoting activity (SCA) with significant differences (p>0.05) from at least one of the solutions containing different kinds of ice nucleators. It should be noted that there were no TRPs exhibiting ice nucleation-enhancing activity (INA) in all solutions containing identified ice nucleators, whereas there were many TRPs exhibiting INA with significant differences in solutions containing unidentified ice nucleators alone. An emulsion freezing assay confirmed that these TRPs did not essentially affect homogeneous ice nucleation temperatures. It is thought that not only SCA but also INA in the TRPs are produced by interactions with heterogeneous ice nucleators, not by direct interaction with water molecules. In the present study, several TRPs that might be useful for applications due to their high SCA in many solutions were identified. Copyright © 2013 Elsevier Inc. All rights reserved.

Heterogeneous nucleation from a supercooled ionic liquid on a carbon surface

NASA Astrophysics Data System (ADS)

He, Xiaoxia; Shen, Yan; Hung, Francisco R.; Santiso, Erik E.

2016-12-01

Classical molecular dynamics simulations were used to study the nucleation of the crystal phase of the ionic liquid [dmim+][Cl-] from its supercooled liquid phase, both in the bulk and in contact with a graphitic surface of D = 3 nm. By combining the string method in collective variables [Maragliano et al., J. Chem. Phys. 125, 024106 (2006)], with Markovian milestoning with Voronoi tessellations [Maragliano et al., J. Chem. Theory Comput. 5, 2589-2594 (2009)] and order parameters for molecular crystals [Santiso and Trout, J. Chem. Phys. 134, 064109 (2011)], we computed minimum free energy paths, the approximate size of the critical nucleus, the free energy barrier, and the rates involved in these nucleation processes. For homogeneous nucleation, the subcooled liquid phase has to overcome a free energy barrier of ˜85 kcal/mol to form a critical nucleus of size ˜3.6 nm, which then grows into the monoclinic crystal phase. This free energy barrier becomes about 42% smaller (˜49 kcal/mol) when the subcooled liquid phase is in contact with a graphitic disk, and the critical nucleus formed is about 17% smaller (˜3.0 nm) than the one observed for homogeneous nucleation. The crystal formed in the heterogeneous nucleation scenario has a structure that is similar to that of the bulk crystal, with the exception of the layers of ions next to the graphene surface, which have larger local density and the cations lie with their imidazolium rings parallel to the graphitic surface. The critical nucleus forms near the graphene surface separated only by these layers of ions. The heterogeneous nucleation rate (˜4.8 × 1011 cm-3 s-1) is about one order of magnitude faster than the homogeneous rate (˜6.6 × 1010 cm-3 s-1). The computed free energy barriers and nucleation rates are in reasonable agreement with experimental and simulation values obtained for the homogeneous and heterogeneous nucleation of other systems (ice, urea, Lennard-Jones spheres, and oxide glasses).

Microbial ice nucleators are scavenged from the atmosphere during artificial rain events

NASA Astrophysics Data System (ADS)

Hanlon, Regina; Powers, Craig; Failor, Kevin; Vinatzer, Boris; Schmale, David

2016-04-01

Some microorganisms associated with rain may catalyze the nucleation of ice crystals at significantly warmer temperatures than would normally be required for ice formation, suggesting that they may play an important role in the onset of precipitation. Rain samples collected near the surface of the earth contain an array of microbial ice nucleators, but the little is known about their source(s) and life history. We conducted a series of field experiments to test the hypothesis that microbial ice nucleators are scavenged from the atmosphere by rainfall. Thirty three artificial rain events were conducted over four months (Nov 2014, Dec 2014, April 2015, and June 2015) off the side of the Smart Road Bridge in Blacksburg, VA, USA. In each event, sterile water was dispensed over the side of the bridge and recovered in sterile containers following gravitational settling from the bridge to an open fallow agricultural field below (a distance of ~55m). Microbes scavenged from the artificial rain events were cultured on six different types of agar media (R2A, TSA, CA; +/-cycloheximide), and the ice nucleation activity was examined for colonies cultured from the different media types. Mean CFUs scavenged by artificial rain ranged from 2 to 267 CFUs/mL. Microbial ice nucleators were cultured from 94% (31/33) of the simulated rain events, and represented 1.4% (121/8871) of the total number of colonies assayed. This percentage is similar to the percentage of culturable microbial ice nucleators occurring in about half of the natural rain events studied in Blacksburg, VA. Sequence-assisted identification of the repeatable microbial ice nucleators that were scavenged from the atmosphere showed a number of unique prokaryotic and eukaryotic taxa. This work expands our knowledge of the scavenging properties of rainfall, and suggests that at least some ice nucleators in natural precipitation events may have been scrubbed from the atmosphere during rainfall, and thus are not likely to be involved in the onset of precipitation.

Heterogeneous nucleation from a supercooled ionic liquid on a carbon surface.

PubMed

He, Xiaoxia; Shen, Yan; Hung, Francisco R; Santiso, Erik E

2016-12-07

Classical molecular dynamics simulations were used to study the nucleation of the crystal phase of the ionic liquid [dmim + ][Cl - ] from its supercooled liquid phase, both in the bulk and in contact with a graphitic surface of D = 3 nm. By combining the string method in collective variables [Maragliano et al., J. Chem. Phys. 125, 024106 (2006)], with Markovian milestoning with Voronoi tessellations [Maragliano et al., J. Chem. Theory Comput. 5, 2589-2594 (2009)] and order parameters for molecular crystals [Santiso and Trout, J. Chem. Phys. 134, 064109 (2011)], we computed minimum free energy paths, the approximate size of the critical nucleus, the free energy barrier, and the rates involved in these nucleation processes. For homogeneous nucleation, the subcooled liquid phase has to overcome a free energy barrier of ∼85 kcal/mol to form a critical nucleus of size ∼3.6 nm, which then grows into the monoclinic crystal phase. This free energy barrier becomes about 42% smaller (∼49 kcal/mol) when the subcooled liquid phase is in contact with a graphitic disk, and the critical nucleus formed is about 17% smaller (∼3.0 nm) than the one observed for homogeneous nucleation. The crystal formed in the heterogeneous nucleation scenario has a structure that is similar to that of the bulk crystal, with the exception of the layers of ions next to the graphene surface, which have larger local density and the cations lie with their imidazolium rings parallel to the graphitic surface. The critical nucleus forms near the graphene surface separated only by these layers of ions. The heterogeneous nucleation rate (∼4.8 × 10 11 cm -3 s -1 ) is about one order of magnitude faster than the homogeneous rate (∼6.6 × 10 10 cm -3 s -1 ). The computed free energy barriers and nucleation rates are in reasonable agreement with experimental and simulation values obtained for the homogeneous and heterogeneous nucleation of other systems (ice, urea, Lennard-Jones spheres, and oxide glasses).

GIT1/βPIX signaling proteins and PAK1 kinase regulate microtubule nucleation.

PubMed

Černohorská, Markéta; Sulimenko, Vadym; Hájková, Zuzana; Sulimenko, Tetyana; Sládková, Vladimíra; Vinopal, Stanislav; Dráberová, Eduarda; Dráber, Pavel

2016-06-01

Microtubule nucleation from γ-tubulin complexes, located at the centrosome, is an essential step in the formation of the microtubule cytoskeleton. However, the signaling mechanisms that regulate microtubule nucleation in interphase cells are largely unknown. In this study, we report that γ-tubulin is in complexes containing G protein-coupled receptor kinase-interacting protein 1 (GIT1), p21-activated kinase interacting exchange factor (βPIX), and p21 protein (Cdc42/Rac)-activated kinase 1 (PAK1) in various cell lines. Immunofluorescence microscopy revealed association of GIT1, βPIX and activated PAK1 with centrosomes. Microtubule regrowth experiments showed that depletion of βPIX stimulated microtubule nucleation, while depletion of GIT1 or PAK1 resulted in decreased nucleation in the interphase cells. These data were confirmed for GIT1 and βPIX by phenotypic rescue experiments, and counting of new microtubules emanating from centrosomes during the microtubule regrowth. The importance of PAK1 for microtubule nucleation was corroborated by the inhibition of its kinase activity with IPA-3 inhibitor. GIT1 with PAK1 thus represent positive regulators, and βPIX is a negative regulator of microtubule nucleation from the interphase centrosomes. The regulatory roles of GIT1, βPIX and PAK1 in microtubule nucleation correlated with recruitment of γ-tubulin to the centrosome. Furthermore, in vitro kinase assays showed that GIT1 and βPIX, but not γ-tubulin, serve as substrates for PAK1. Finally, direct interaction of γ-tubulin with the C-terminal domain of βPIX and the N-terminal domain of GIT1, which targets this protein to the centrosome, was determined by pull-down experiments. We propose that GIT1/βPIX signaling proteins with PAK1 kinase represent a novel regulatory mechanism of microtubule nucleation in interphase cells. Copyright © 2016 Elsevier B.V. All rights reserved.

Urediospores of Puccinia spp. and other rusts are warm-temperature ice nucleators and harbor ice nucleation active bacteria

NASA Astrophysics Data System (ADS)

Morris, C. E.; Sands, D. C.; Glaux, C.; Samsatly, J.; Asaad, S.; Moukahel, A. R.; Gonçalves, F. L. T.; Bigg, E. K.

2012-10-01

In light of various features of the biology of the rust fungi and of the epidemiology of the plant diseases they cause that illustrate the important role of rainfall in their life history, we have characterized the ice nucleation activity (INA) of the aerially disseminated spores (urediospores) of this group of fungi. Urediospores of this obligate plant parasite were collected from natural infections from 7 species of weeds in France, from coffee in Brazil and from field and greenhouse-grown wheat in France, the USA, Turkey and Syria. Immersion freezing was used to determine freezing onset temperatures and the abundance of ice nuclei in suspensions of washed spores. Microbiological analyses of spores and subsequent tests of the ice nucleation activity of the bacteria associated with spores were deployed to quantify the contribution of bacteria to the ice nucleation activity of the spores. All samples of spores were ice nucleation active having freezing onset temperatures as warm as -4 °C. Spores in most of the samples carried cells of ice nucleation-active strains of the bacterium Pseudomonas syringae (at rates of less than 1 bacterial cell per 100 urediospores), but bacterial INA accounted for only a small fraction of the INA observed in spore suspensions. Changes in the INA of spore suspensions after treatment with lysozyme suggest that the INA of urediospores involves a polysaccharide. Based on data from the literature, we have estimated the concentrations of urediospores in air at cloud height and in rainfall. These quantities are very similar to those reported for other biological ice nucleators in these same substrates. We suggest that air sampling techniques have ignored the spatial and temporal variability of atmospheric concentrations that occur under conditions propitious for precipitation that could increase their local abundance intermittently. Nevertheless, we propose that the relative low abundance of warm-temperature biological ice nucleators in the atmosphere corresponds to optimal conditions for the processes of evolution to positively select for INA.

Understanding nanoparticle-mediated nucleation pathways of anisotropic nanoparticles

NASA Astrophysics Data System (ADS)

Laramy, Christine R.; Fong, Lam-Kiu; Jones, Matthew R.; O'Brien, Matthew N.; Schatz, George C.; Mirkin, Chad A.

2017-09-01

Several seed-mediated syntheses of low symmetry anisotropic nanoparticles yield broad product distributions with multiple defect structures. This observation challenges the role of the nanoparticle precursor as a seed for certain syntheses and suggests the possibility of alternate nucleation pathways. Herein, we report a method to probe the role of the nanoparticle precursor in anisotropic nanoparticle nucleation with compositional and structural 'labels' to track their fate. We use the synthesis of gold triangular nanoprisms (Au TPs) as a model system. We propose a mechanism in which, rather than acting as a template, the nanoparticle precursor catalyzes homogenous nucleation of Au TPs.

In-situ small angle x-ray scattering investigation on nucleation and growth of silica colloids

NASA Astrophysics Data System (ADS)

Bahadur, J.; Tripathi, B. M.; Prakash, J.; Das, Avik; Sen, D.; Mazumder, S.

2018-04-01

The nucleation and growth of silica colloids has been studied using real time small-angle X-ray scattering measurements. The ammonium fluorosilicate was used as precursor and both weak (NH3) and strong base (NaOH) has been used as reducing agent for the precursor. It is observed that nucleation, growth and aggregation phenomenon occur simultaneously. The kinetics of the nucleation and growth of silica colloids depends on the strength of the reducing agent as well on its concentration. The kinetics is slow for NH3 but is very fast for higher concentration of NaOH.

Delayed, disequilibrium degassing in rhyolite magma: Decompression experiments and implications for explosive volcanism

USGS Publications Warehouse

Mangan, M.; Sisson, T.

2000-01-01

Recent numerical models and analog shock tube experiments show that disequilibrium degassing during magma ascent may lead to violent vesiculation very near the surface. In this study a series of decompression experiments using crystal-free, rhyolite melt were conducted to examine the development of large supersaturations due to delayed, homogenous (spontaneous) bubble nucleation. Melts were saturated at 900??C and 200 MPa with either 5.2 wt% dissolved H2O, or with 4.2 wt% H2O and 640 ppm CO2, and isothermally decompressed at linear rates of either 0.003, 0.025, or 8.5 MPa/s to final pressures between 25 and 175 MPa. Additional isobaric saturation experiments (900??C, 200-25 MPa) using pure H2O or mixed H2O-CO2 fluids establish reference equilibrium solubility curves/values. Homogenous nucleation is triggered in both H2O-only and H2O-CO2 experiments once the supersaturation pressure (??Pss) reaches ?? 120-150 MPa and the melt contains ?? two times its equilibrium water contents. Bubble number density and nucleation rate depend on the supersaturation pressure, with values on the order of 102/cm3 and < 1/cm3/s for ??Pss~120 MPa; 106/cm3 and 103-105/cm3/s for ??Pss??~130-150 MPa; and 107/cm3 and 106/cm3/s for ??Pss??160-175 MPa. Nucleation rates are consistent with classical nucleation theory, and infer an activation energy for nucleation of 1.5 x 10-18 J/nucleus, a critical bubble radius of 2 x 10-9 m, and an effective surface tension for rhyolite at 5.2 wt% H2O and 900??C of 0.10-0.11 N/m. The long nucleation delay limits the time available for subsequent diffusion such that disequilibrium dissolved H2O and CO2 contents persist to the end of our runs. The disequilibrium degassing paths inferred from our experiments contrast markedly with the equilibrium or quasi-equilibrium paths found in other studies where bubble nucleation occurs heterogenously on crystals or other discontinuities in the melt at low ??Pss. Homogenous and heterogenous nucleation rates are comparable, however, as are bubble number densities, so that at a given decompression rate it appears that nucleation mechanism, rather than nucleation rate, determines degassing efficiency by fixing the pressure (depth) at which vesiculation commences and hence the time available for equilibration prior to eruption. Although real systems are probably never truly crystal-free, our results show that rhyolitic magmas containing up to 104 crystals/cm3, and perhaps as high as 106 crystals/cm3, are controlled by homogenous, rather than heterogenous, nucleation during ascent. ?? 2000 Elsevier Science B.V. All rights reserved.

The Nucleation Rate of Single O2 Nanobubbles at Pt Nanoelectrodes.

PubMed

Soto, Álvaro Moreno; German, Sean R; Ren, Hang; van der Meer, Devaraj; Lohse, Detlef; Edwards, Martin A; White, Henry S

2018-06-13

Nanobubble nucleation is a problem that affects efficiency in electrocatalytic reactions since those bubbles can block the surface of the catalytic sites. In this article, we focus on the nucleation rate of O 2 nanobubbles resulting from the electrooxidation of H 2 O 2 at Pt disk nanoelectrodes. Bubbles form almost instantaneously when a critical peak current, i nb p , is applied, but for lower currents, bubble nucleation is a stochastic process in which the nucleation (induction) time, t ind , dramatically decreases as the applied current approaches i nb p , a consequence of the local supersaturation level, ζ, increasing at high currents. Here, by applying different currents below i nb p , nanobubbles take some time to nucleate and block the surface of the Pt electrode at which the reaction occurs, providing a means to measure the stochastic t ind . We study in detail the different conditions in which nanobubbles appear, concluding that the electrode surface needs to be preconditioned to achieve reproducible results. We also measure the activation energy for bubble nucleation, E a , which varies in the range from (6 to 30) kT, and assuming a spherically cap-shaped nanobubble nucleus, we determine the footprint diameter L = 8-15 nm, the contact angle to the electrode surface θ = 135-155°, and the number of O 2 molecules contained in the nucleus (50 to 900 molecules).

Externally applied electric fields up to 1.6 × 10(5) V/m do not affect the homogeneous nucleation of ice in supercooled water.

PubMed

Stan, Claudiu A; Tang, Sindy K Y; Bishop, Kyle J M; Whitesides, George M

2011-02-10

The freezing of water can initiate at electrically conducting electrodes kept at a high electric potential or at charged electrically insulating surfaces. The microscopic mechanisms of these phenomena are unknown, but they must involve interactions between water molecules and electric fields. This paper investigates the effect of uniform electric fields on the homogeneous nucleation of ice in supercooled water. Electric fields were applied across drops of water immersed in a perfluorinated liquid using a parallel-plate capacitor; the drops traveled in a microchannel and were supercooled until they froze due to the homogeneous nucleation of ice. The distribution of freezing temperatures of drops depended on the rate of nucleation of ice, and the sensitivity of measurements allowed detection of changes by a factor of 1.5 in the rate of nucleation. Sinusoidal alternation of the electric field at frequencies from 3 to 100 kHz prevented free ions present in water from screening the electric field in the bulk of drops. Uniform electric fields in water with amplitudes up to (1.6 ± 0.4) × 10(5) V/m neither enhanced nor suppressed the homogeneous nucleation of ice. Estimations based on thermodynamic models suggest that fields in the range of 10(7)-10(8) V/m might cause an observable increase in the rate of nucleation.

Cirrus Parcel Model Comparison Project. Phase 1

NASA Technical Reports Server (NTRS)

Lin, Ruei-Fong; Starr, David O'C.; DeMott, Paul J.; Cotton, Richard; Jensen, Eric; Sassen, Kenneth

2000-01-01

The Cirrus Parcel Model Comparison (CPMC) is a project of the GEWEX Cloud System Study Working Group on Cirrus Cloud Systems (GCSS WG2). The primary goal of this project is to identify cirrus model sensitivities to the state of our knowledge of nucleation and microphysics. Furthermore, the common ground of the findings may provide guidelines for models with simpler cirrus microphysics modules. We focus on the nucleation regimes of the warm (parcel starting at -40 C and 340 hPa) and cold (-60 C and 170 hPa) cases studied in the GCSS WG2 Idealized Cirrus Model Comparison Project. Nucleation and ice crystal growth were forced through an externally imposed rate of lift and consequent adiabatic cooling. The background haze particles are assumed to be lognormally-distributed H2SO4 particles. Only the homogeneous nucleation mode is allowed to form ice crystals in the HN-ONLY runs; all nucleation modes are switched on in the ALL-MODE runs. Participants were asked to run the HN-lambda-fixed runs by setting lambda = 2 (lambda is further discussed in section 2) or tailoring the nucleation rate calculation in agreement with lambda = 2 (exp 1). The depth of parcel lift (800 m) was set to assure that parcels underwent complete transition through the nucleation regime to a stage of approximate equilibrium between ice mass growth and vapor supplied by the specified updrafts.

Microtubule-organizing centers and nucleating sites in land plants.

PubMed

Vaughn, K C; Harper, J D

1998-01-01

Microtubule-organizing centers (MTOCs) are morphologically diverse cellular sites involved in the nucleation and organization of microtubules (MTs). These structures are synonymous with the centrosome in mammalian cells. In most land plant cells, however, no such structures are observed and some have argued that plant cells may not have MTOCs. This review summarizes a number of experimental approaches toward the elucidation of those subcellular sites involved in microtubule nucleation and organization. In lower land plants, structurally well-defined MTOCs are present, such as the blepharoplast, multilayered structure, and polar organizer. In higher plants, much of the nucleation and organization of MTs occurs on the nuclear envelope or other endomembranes, such as the plasmalemma and smooth (tubular) endoplasmic reticulum. In some instances, one endomembrane may serve as a site of nucleation whereas others serve as the site of organization. Structural and motor microtubule-associated proteins also appear to be involved in MT nucleation and organization. Immunochemical evidence indicates that at least several of the proteins found in mammalian centrosomes, gamma-tubulin, centrin, pericentrin, and polypeptides recognized by the monoclonal antibodies MPM-2, 6C6, and C9 also recognize putative lower land plant MTOCs, indicating shared mechanisms of nucleation/organization in plants and animals. The most recent data from tubulin incorporation in vivo, mutants with altered MT organization, and molecular studies indicate the potential of these research tools in investigation of MTOCs in plants.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, Bingbing; Knopf, Daniel A.; China, Swarup

Heterogeneous ice nucleation is a physical chemistry process of critical relevance to a range of topics in the fundamental and the applied sciences and technologies. Heterogeneous ice nucleation remains insufficiently understood. This is in part due to the lack of experimental methods capable of in situ visualization of ice formation over nucleating substrates with microscopically characterized morphology and composition. We present development, validation and first applications of a novel electron microscopy platform allowing observation of individual ice nucleation events at temperature and relative humidity (RH) relevant for ice formation in a broad range of environmental and applied technology processes. Themore » approach utilizes a custom-built ice nucleation cell, interfaced with an Environmental Scanning Electron Microscope (IN-ESEM system). The IN-ESEM system allows dynamic observations of individual ice formation events over particles of atmospheric relevance and determination of the ice nucleation mechanisms. Additional IN-ESEM experiments allow examination of the location of ice formation on the surface of individual particles and micro-spectroscopy analysis of the ice nucleating particles (INPs). This includes elemental composition detected by the energy dispersed analysis of X-rays (EDX), speciation of the organic content in particles using scanning transmission X-ray microscopy with near edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS), and Helium ion microscopy (HeIM). The capabilities of the IN-ESEM experimental platform are demonstrated first on laboratory standards and then by chemical imaging of INPs using a complex sample of ambient particles.« less

Nucleation, aggregative growth and detachment of metal nanoparticles during electrodeposition at electrode surfaces.

PubMed

Lai, Stanley C S; Lazenby, Robert A; Kirkman, Paul M; Unwin, Patrick R

2015-02-01

The nucleation and growth of metal nanoparticles (NPs) on surfaces is of considerable interest with regard to creating functional interfaces with myriad applications. Yet, key features of these processes remain elusive and are undergoing revision. Here, the mechanism of the electrodeposition of silver on basal plane highly oriented pyrolytic graphite (HOPG) is investigated as a model system at a wide range of length scales, spanning electrochemical measurements from the macroscale to the nanoscale using scanning electrochemical cell microscopy (SECCM), a pipette-based approach. The macroscale measurements show that the nucleation process cannot be modelled as either truly instantaneous or progressive, and that step edge sites of HOPG do not play a dominant role in nucleation events compared to the HOPG basal plane, as has been widely proposed. Moreover, nucleation numbers extracted from electrochemical analysis do not match those determined by atomic force microscopy (AFM). The high time and spatial resolution of the nanoscale pipette set-up reveals individual nucleation and growth events at the graphite basal surface that are resolved and analysed in detail. Based on these results, corroborated with complementary microscopy measurements, we propose that a nucleation-aggregative growth-detachment mechanism is an important feature of the electrodeposition of silver NPs on HOPG. These findings have major implications for NP electrodeposition and for understanding electrochemical processes at graphitic materials generally.

Nucleation and growth of electrodeposited Mn oxide rods for supercapacitor electrodes

NASA Astrophysics Data System (ADS)

Clark, Michael; Ivey, Douglas G.

2015-09-01

The nucleation and growth of electrodeposited Mn oxide rods has been investigated by preparing deposits on Au coated Si at varying deposition times between 0.5 s and 10 min. The deposits were investigated using high resolution scanning and transmission electron microscopy. A model for the nucleation and growth of Mn oxide rods has been proposed. Nucleation begins as thin sheets along Au grain boundaries and triple points. As these nucleation sites are consumed, nucleation spreads across the grains. Nucleation of sheets in close proximity causes agglomeration and the formation of rounded particles. Some of these rounded particles then accelerate in growth, initially in all directions and then primarily in the direction normal to the sample surface. Accelerated growth normal to the sample surface leads to the formation of rods. As rods grow, the growth of other particles accelerates and they become rods themselves. Eventually the entire sample surface is covered with rods 15-20 μm long and about 2 μm wide. The sheet-like morphology of the deposits is retained at all stages of deposition. Electron diffraction analysis of 3 s and 6 s deposits shows that the sheets are initially amorphous and then begin to crystallize into a cubic spinel Mn3O4 crystal structure. High resolution imaging of the 6 s sample shows small crystalline regions (˜5 nm in size) within an amorphous matrix.

Analysis of experimental nucleation data for silver and SiO using scaled nucleation theory

NASA Astrophysics Data System (ADS)

Hale, Barbara N.; Kemper, Paul; Nuth, Joseph A.

1989-10-01

The experimental vapor phase nucleation data of Nuth et al., for silver [J. A. Nuth, K. A. Donnelly, B. Donn, and L. U. Lilleleht, J. Chem. Phys. 77, 2639 (1982)] and SiO [J. A. Nuth and B. Donn, J. Chem. Phys. 85, 1116 (1986)] are reanalyzed using a scaled model for homogeneous nucleation [B. N. Hale, Phys. Rev. A 33, 4156 (1986)]. The approximation is made that the vapor pressure at the nucleation site is not diminished significantly from that at the source (crucible). It is found that the data for ln S have a temperature dependence consistent with the scaled theory ln S≊ΓΩ3/2 [Tc/T-1]3/2, and predict critical temperatures 3800±200 K for silver and 3700±200 K for SiO. One can also extract an effective excess surface entropy per atom Ω=2.1±0.1 and an effective surface tension σ≊1500-0.45T ergs/cm2 for the small silver clusters (assuming a range of nucleation rates from 105 to 1011 cm-3 s-1). The corresponding values for SiO are Ω≊1.7±0.1 and σ≊820-0.22T ergs/cm2 (assuming a range of nucleation rates from 109 to 1012 cm-3 s-1).

Sensitivity of Cirrus Properties to Ice Nuclei Abundance

NASA Technical Reports Server (NTRS)

Jensen, Eric

2014-01-01

The relative importance of heterogeneous and homogeneous ice nucleation for cirrus formation remains an active area of debate in the cloud physics community. From a theoretical perspective, a number of modeling studies have investigated the sensitivity of ice number concentration to the nucleation mechanism and the abundance of ice nuclei. However, these studies typically only addressed ice concentration immediately after ice nucleation. Recent modeling work has shown that the high ice concentrations produced by homogeneous freezing may not persist very long, which is consistent with the low frequency of occurrence of high ice concentrations indicated by cirrus measurements. Here, I use idealized simulations to investigate the impact of ice nucleation mechanism and ice nuclei abundance on the full lifecycle of cirrus clouds. The primary modeling framework used includes different modes of ice nucleation, deposition growth/sublimation, aggregation, sedimentation, and radiation. A limited number of cloud-resolving simulations that treat radiation/dynamics interactions will also been presented. I will show that for typical synoptic situations with mesoscale waves present, the time-averaged cirrus ice crystal size distributions and bulk cloud properties are less sensitive to ice nucleation processes than might be expected from the earlier simple ice nucleation calculations. I will evaluate the magnitude of the ice nuclei impact on cirrus for a range of temperatures and mesoscale wave specifications, and I will discuss the implications for cirrus aerosol indirect effects in general.

Heterogeneous ice nucleation of viscous secondary organic aerosol produced from ozonolysis of α-pinene

NASA Astrophysics Data System (ADS)

Ignatius, Karoliina; Kristensen, Thomas B.; Järvinen, Emma; Nichman, Leonid; Fuchs, Claudia; Gordon, Hamish; Herenz, Paul; Hoyle, Christopher R.; Duplissy, Jonathan; Garimella, Sarvesh; Dias, Antonio; Frege, Carla; Höppel, Niko; Tröstl, Jasmin; Wagner, Robert; Yan, Chao; Amorim, Antonio; Baltensperger, Urs; Curtius, Joachim; Donahue, Neil M.; Gallagher, Martin W.; Kirkby, Jasper; Kulmala, Markku; Möhler, Ottmar; Saathoff, Harald; Schnaiter, Martin; Tomé, Antonio; Virtanen, Annele; Worsnop, Douglas; Stratmann, Frank

2016-05-01

There are strong indications that particles containing secondary organic aerosol (SOA) exhibit amorphous solid or semi-solid phase states in the atmosphere. This may facilitate heterogeneous ice nucleation and thus influence cloud properties. However, experimental ice nucleation studies of biogenic SOA are scarce. Here, we investigated the ice nucleation ability of viscous SOA particles. The SOA particles were produced from the ozone initiated oxidation of α-pinene in an aerosol chamber at temperatures in the range from -38 to -10 °C at 5-15 % relative humidity with respect to water to ensure their formation in a highly viscous phase state, i.e. semi-solid or glassy. The ice nucleation ability of SOA particles with different sizes was investigated with a new continuous flow diffusion chamber. For the first time, we observed heterogeneous ice nucleation of viscous α-pinene SOA for ice saturation ratios between 1.3 and 1.4 significantly below the homogeneous freezing limit. The maximum frozen fractions found at temperatures between -39.0 and -37.2 °C ranged from 6 to 20 % and did not depend on the particle surface area. Global modelling of monoterpene SOA particles suggests that viscous biogenic SOA particles are indeed present in regions where cirrus cloud formation takes place. Hence, they could make up an important contribution to the global ice nucleating particle budget.

Laboratory observations of temperature and humidity dependencies of nucleation and growth rates of sub-3 nm particles

NASA Astrophysics Data System (ADS)

Yu, Huan; Dai, Liang; Zhao, Yi; Kanawade, Vijay P.; Tripathi, Sachchida N.; Ge, Xinlei; Chen, Mindong; Lee, Shan-Hu

2017-02-01

Temperature and relative humidity (RH) are the most important thermodynamic parameters in aerosol formation, yet laboratory studies of nucleation and growth dependencies on temperature and RH are lacking. Here we report the experimentally observed temperature and RH dependences of sulfuric acid aerosol nucleation and growth. Experiments were performed in a flow tube in the temperature range from 248 to 313 K, RH from 0.8% to 79%, and relative acidity (RA) of sulfuric acid from 6 × 10-5 to 0.38 (2 × 107-109 cm-3). The impurity levels of base compounds were determined to be NH3 < 23 pptv (parts per thousand by volume), methylamine < 1.5 pptv, and dimethylamine < 0.52 pptv. Our results showed that low temperatures favor nucleation at fixed sulfuric acid concentration but impede nucleation when RA is fixed. It is also shown that binary nucleation of sulfuric acid and water is negligible in planetary boundary layer temperature and sulfuric acid ranges. An empirical algorithm was derived to correlate the nucleation rate with RA, RH, and temperature together. Collision-limited condensation of free-sulfuric acid molecules fails to predict the observed growth rate in the sub-3 nm size range, as well as its dependence on temperature and RH. This suggests that evaporation, sulfuric acid hydration, and possible involvement of other ternary molecules should be considered for the sub-3 nm particle growth.

A classical view on nonclassical nucleation.

PubMed

Smeets, Paul J M; Finney, Aaron R; Habraken, Wouter J E M; Nudelman, Fabio; Friedrich, Heiner; Laven, Jozua; De Yoreo, James J; Rodger, P Mark; Sommerdijk, Nico A J M

2017-09-19

Understanding and controlling nucleation is important for many crystallization applications. Calcium carbonate (CaCO 3 ) is often used as a model system to investigate nucleation mechanisms. Despite its great importance in geology, biology, and many industrial applications, CaCO 3 nucleation is still a topic of intense discussion, with new pathways for its growth from ions in solution proposed in recent years. These new pathways include the so-called nonclassical nucleation mechanism via the assembly of thermodynamically stable prenucleation clusters, as well as the formation of a dense liquid precursor phase via liquid-liquid phase separation. Here, we present results from a combined experimental and computational investigation on the precipitation of CaCO 3 in dilute aqueous solutions. We propose that a dense liquid phase (containing 4-7 H 2 O per CaCO 3 unit) forms in supersaturated solutions through the association of ions and ion pairs without significant participation of larger ion clusters. This liquid acts as the precursor for the formation of solid CaCO 3 in the form of vaterite, which grows via a net transfer of ions from solution according to z Ca 2+ + z CO 3 2- → z CaCO 3 The results show that all steps in this process can be explained according to classical concepts of crystal nucleation and growth, and that long-standing physical concepts of nucleation can describe multistep, multiphase growth mechanisms.

The dynamics of nucleation and growth of a particle in the ternary alloy melt with anisotropic surface tension.

PubMed

Chen, Ming-Wen; Li, Lin-Yan; Guo, Hui-Min

2017-08-28

The dynamics of nucleation and growth of a particle affected by anisotropic surface tension in the ternary alloy melt is studied. The uniformly valid asymptotic solution for temperature field, concentration field, and interface evolution of nucleation and particle growth is obtained by means of the multiple variable expansion method. The asymptotic solution reveals the critical radius of nucleation in the ternary alloy melt and an inward melting mechanism of the particle induced by the anisotropic effect of surface tension. The critical radius of nucleation is dependent on isotropic surface tension, temperature undercooling, and constitutional undercooling in the ternary alloy melt, and the solute diffusion melt decreases the critical radius of nucleation. Immediately after a nucleus forms in the initial stage of solidification, the anisotropic effect of surface tension makes some parts of its interface grow inward while some parts grow outward. Until the inward melting attains a certain distance (which is defined as "the melting depth"), these parts of interface start to grow outward with other parts. The interface of the particle evolves into an ear-like deformation, whose inner diameter may be less than two times the critical radius of nucleation within a short time in the initial stage of solidification. The solute diffusion in the ternary alloy melt decreases the effect of anisotropic surface tension on the interface deformation.

Ice Nucleation of Bare and Sulfuric Acid-coated Mineral Dust Particles and Implication for Cloud Properties

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kulkarni, Gourihar R.; Sanders, Cassandra N.; Zhang, Kai

2014-08-27

Ice nucleation properties of different dust species coated with soluble material are not well understood. We determined the ice nucleation ability of bare and sulfuric acid coated mineral dust particles as a function of temperature (-25 to -35 deg C) and relative humidity with respect to water (RHw). Five different mineral dust species: Arizona test dust (ATD), illite, montmorillonite, quartz and kaolinite were dry dispersed and size-selected at 150 nm and exposed to sulfuric acid vapors in the coating apparatus. The condensed sulfuric acid soluble mass fraction per particle was estimated from the cloud condensation nuclei activated fraction measurements. Themore » fraction of dust particles nucleating ice at various temperatures and RHw was determined using a compact ice chamber. In water-subsaturated conditions, compared to bare dust particles, we found that only coated ATD particles showed suppression of ice nucleation ability while other four dust species did not showed the effect of coating on the fraction of particles nucleating ice. The results suggest that interactions between the dust surface and sulfuric acid vapor are important, such that interactions may or may not modify the surface via chemical reactions with sulfuric acid. At water-supersaturated conditions we did not observed the effect of coating, i.e. the bare and coated dust particles had similar ice nucleation behavior.« less

Investigating Freezing Point Depression and Cirrus Cloud Nucleation Mechanisms Using a Differential Scanning Calorimeter

ERIC Educational Resources Information Center

Bodzewski, Kentaro Y.; Caylor, Ryan L.; Comstock, Ashley M.; Hadley, Austin T.; Imholt, Felisha M.; Kirwan, Kory D.; Oyama, Kira S.; Wise, Matthew E.

2016-01-01

A differential scanning calorimeter was used to study homogeneous nucleation of ice from micron-sized aqueous ammonium sulfate aerosol particles. It is important to understand the conditions at which these particles nucleate ice because of their connection to cirrus cloud formation. Additionally, the concept of freezing point depression, a topic…

Leipzig Ice Nucleation chamber Comparison (LINC): intercomparison of four online ice nucleation counters

NASA Astrophysics Data System (ADS)

Burkert-Kohn, Monika; Wex, Heike; Welti, André; Hartmann, Susan; Grawe, Sarah; Hellner, Lisa; Herenz, Paul; Atkinson, James D.; Stratmann, Frank; Kanji, Zamin A.

2017-09-01

Ice crystal formation in atmospheric clouds has a strong effect on precipitation, cloud lifetime, cloud radiative properties, and thus the global energy budget. Primary ice formation above 235 K is initiated by nucleation on seed aerosol particles called ice-nucleating particles (INPs). Instruments that measure the ice-nucleating potential of aerosol particles in the atmosphere need to be able to accurately quantify ambient INP concentrations. In the last decade several instruments have been developed to investigate the ice-nucleating properties of aerosol particles and to measure ambient INP concentrations. Therefore, there is a need for intercomparisons to ensure instrument differences are not interpreted as scientific findings.In this study, we intercompare the results from parallel measurements using four online ice nucleation chambers. Seven different aerosol types are tested including untreated and acid-treated mineral dusts (microcline, which is a K-feldspar, and kaolinite), as well as birch pollen washing waters. Experiments exploring heterogeneous ice nucleation above and below water saturation are performed to cover the whole range of atmospherically relevant thermodynamic conditions that can be investigated with the intercompared chambers. The Leipzig Aerosol Cloud Interaction Simulator (LACIS) and the Portable Immersion Mode Cooling chAmber coupled to the Portable Ice Nucleation Chamber (PIMCA-PINC) performed measurements in the immersion freezing mode. Additionally, two continuous-flow diffusion chambers (CFDCs) PINC and the Spectrometer for Ice Nuclei (SPIN) are used to perform measurements below and just above water saturation, nominally presenting deposition nucleation and condensation freezing.The results of LACIS and PIMCA-PINC agree well over the whole range of measured frozen fractions (FFs) and temperature. In general PINC and SPIN compare well and the observed differences are explained by the ice crystal growth and different residence times in the chamber. To study the mechanisms responsible for the ice nucleation in the four instruments, the FF (from LACIS and PIMCA-PINC) and the activated fraction, AF (from PINC and SPIN), are compared. Measured FFs are on the order of a factor of 3 higher than AFs, but are not consistent for all aerosol types and temperatures investigated. It is shown that measurements from CFDCs cannot be assumed to produce the same results as those instruments exclusively measuring immersion freezing. Instead, the need to apply a scaling factor to CFDCs operating above water saturation has to be considered to allow comparison with immersion freezing devices. Our results provide further awareness of factors such as the importance of dispersion methods and the quality of particle size selection for intercomparing online INP counters.

Sensitivity Studies of Dust Ice Nuclei Effect on Cirrus Clouds with the Community Atmosphere Model CAM5

NASA Technical Reports Server (NTRS)

Liu, Xiaohong; Zhang, Kai; Jensen, Eric J.; Gettelman, Andrew; Barahona, Donifan; Nenes, Athanasios; Lawson, Paul

2012-01-01

In this study the effect of dust aerosol on upper tropospheric cirrus clouds through heterogeneous ice nucleation is investigated in the Community Atmospheric Model version 5 (CAM5) with two ice nucleation parameterizations. Both parameterizations consider homogeneous and heterogeneous nucleation and the competition between the two mechanisms in cirrus clouds, but differ significantly in the number concentration of heterogeneous ice nuclei (IN) from dust. Heterogeneous nucleation on dust aerosol reduces the occurrence frequency of homogeneous nucleation and thus the ice crystal number concentration in the Northern Hemisphere (NH) cirrus clouds compared to simulations with pure homogeneous nucleation. Global and annual mean shortwave and longwave cloud forcing are reduced by up to 2.0+/-0.1Wm (sup-2) (1 uncertainty) and 2.4+/-0.1Wm (sup-2), respectively due to the presence of dust IN, with the net cloud forcing change of -0.40+/-0.20W m(sup-2). Comparison of model simulations with in situ aircraft data obtained in NH mid-latitudes suggests that homogeneous ice nucleation may play an important role in the ice nucleation at these regions with temperatures of 205-230 K. However, simulations overestimate observed ice crystal number concentrations in the tropical tropopause regions with temperatures of 190- 205 K, and overestimate the frequency of occurrence of high ice crystal number concentration (greater than 200 L(sup-1) and underestimate the frequency of low ice crystal number concentration (less than 30 L(sup-1) at NH mid-latitudes. These results highlight the importance of quantifying the number concentrations and properties of heterogeneous IN (including dust aerosol) in the upper troposphere from the global perspective.

BINARY: an optical freezing array for assessing temperature and time dependence of heterogeneous ice nucleation

NASA Astrophysics Data System (ADS)

Budke, C.; Koop, T.

2014-09-01

A new optical freezing array for the study of heterogeneous ice nucleation in microliter-sized droplets is introduced, tested and applied to the study of immersion freezing in aqueous Snomax® suspensions. In the Bielefeld Ice Nucleation ARraY (BINARY) ice nucleation can be studied simultaneously in 36 droplets at temperatures down to -40 °C (233 K) and at cooling rates between 0.1 K min-1 and 10 K min-1. The droplets are separated from each other in individual compartments, thus preventing a Wegener-Bergeron-Findeisen type water vapor transfer between droplets as well as avoiding the seeding of neighboring droplets by formation and surface growth of frost halos. Analysis of freezing and melting occurs via an automated real time image analysis of the optical brightness of each individual droplet. As an application ice nucleation in water droplets containing Snomax® at concentrations from 1 ng mL-1 to 1 mg mL-1 was investigated. Using different cooling rates a minute time dependence of ice nucleation induced by Class A and Class C ice nucleators contained in Snomax® was detected. For the Class A IN a very strong increase of the heterogeneous ice nucleation rate coefficient with decreasing temperature of λ ≡ -dln(jhet)/dT = 8.7 K-1 was observed emphasizing the capability of the BINARY device. This value is larger than those of other types of IN reported in the literature, suggesting that the BINARY setup is suitable for quantifying time dependence for most other IN of atmospheric interest, making it a useful tool for future investigations.

Cavitation in a metallic liquid: Homogeneous nucleation and growth of nanovoids

NASA Astrophysics Data System (ADS)

Cai, Y.; Wu, H. A.; Luo, S. N.

2014-06-01

Large-scale molecular dynamics (MD) simulations are performed to investigate homogeneous nucleation and growth of nanovoids during cavitation in liquid Cu. We characterize in detail the atomistic cavitation processes by following the temporal evolution of cavities or voids, analyze the nucleation behavior with the mean first-passage time (MFPT) and survival probability (SP) methods, and discuss the results against classical nucleation theory (CNT), the Tolman equation for surface energy, independent calculation of surface tension via integrating the stress profiles, the Johnson-Mehl-Avrami (JMA) growth law, and the power law for nucleus size distributions. Cavitation in this representative metallic liquid is a high energy barrier Poisson processes, and the steady-state nucleation rates obtained from statistical runs with the MFPT and SP methods are in agreement. The MFPT method also yields the critical nucleus size and the Zeldovich factor. Fitting with the Tolman's equation to the MD simulations yields the surface energy of a planar interface (˜0.9 J {m}^{-2}) and the Tolman length (0.4-0.5 Å), and those values are in accord with those from integrating the stress profiles of a planar interface. Independent CNT predictions of the nucleation rate (1033 - 34 s-1 m-3) and critical size (3-4 Å in radius) are in agreement with the MFPT and SP results. The JMA law can reasonably describe the nucleation and growth process. The size distribution of subcritical nuclei appears to follow a power law with an exponent decreasing with increasing tension owing to coupled nucleation and growth, and that of the supercritical nuclei becomes flattened during further stress relaxation due to void coalescence.

Cavitation in a metallic liquid: homogeneous nucleation and growth of nanovoids.

PubMed

Cai, Y; Wu, H A; Luo, S N

2014-06-07

Large-scale molecular dynamics (MD) simulations are performed to investigate homogeneous nucleation and growth of nanovoids during cavitation in liquid Cu. We characterize in detail the atomistic cavitation processes by following the temporal evolution of cavities or voids, analyze the nucleation behavior with the mean first-passage time (MFPT) and survival probability (SP) methods, and discuss the results against classical nucleation theory (CNT), the Tolman equation for surface energy, independent calculation of surface tension via integrating the stress profiles, the Johnson-Mehl-Avrami (JMA) growth law, and the power law for nucleus size distributions. Cavitation in this representative metallic liquid is a high energy barrier Poisson processes, and the steady-state nucleation rates obtained from statistical runs with the MFPT and SP methods are in agreement. The MFPT method also yields the critical nucleus size and the Zeldovich factor. Fitting with the Tolman's equation to the MD simulations yields the surface energy of a planar interface (~0.9 J m⁻²) and the Tolman length (0.4-0.5 Å), and those values are in accord with those from integrating the stress profiles of a planar interface. Independent CNT predictions of the nucleation rate (10(33 - 34) s(-1) m(-3)) and critical size (3-4 Å in radius) are in agreement with the MFPT and SP results. The JMA law can reasonably describe the nucleation and growth process. The size distribution of subcritical nuclei appears to follow a power law with an exponent decreasing with increasing tension owing to coupled nucleation and growth, and that of the supercritical nuclei becomes flattened during further stress relaxation due to void coalescence.

In Situ Characterization of Twin Nucleation in Pure Ti Using 3D-XRD

NASA Astrophysics Data System (ADS)

Bieler, Thomas R.; Wang, Leyun; Beaudoin, Armand J.; Kenesei, Peter; Lienert, Ulrich

2014-01-01

A small tensile specimen of grade 1 commercially pure titanium was deformed to a few percent strain with concurrent synchrotron X-ray diffraction measurements to identify subsurface {102} twin nucleation events. This sample was from the same piece of material in which a prior study showed that twin nucleation stimulated by slip transfer across a grain boundary accounted for many instances of twin nucleation. The sample had a strong c-axis texture of about eight times random aligned with the tensile axis. After 1.5 pct tensile strain, three twin nucleation events were observed in grains where the c-axis was nearly parallel to the tensile direction. Far-field 3-D X-ray diffraction data were analyzed to obtain the positional center of mass, the average lattice strain, and stress tensors in each grain and twin. In one case where the parent grain was mostly surrounded by hard grain orientations, the twin system with the highest resolved shear stress (RSS) among the six {102} twin variants was activated and the stress in the parent grain decreased after twin nucleation. In two other parent grains with a majority of softer neighboring grain orientations, the observed twins did not occur on the twin system with the highest RSS. Their nucleation could be geometrically attributed to slip transfer from neighboring grains with geometrically favorable basal slip systems, and the stress in the parent grain increased after twin nucleation. In all three twin events, the stress in the twin was 10 to 30 pct lower than the stress in the parent grain, indicating load partitioning between the hard-oriented parent grain and the soft-oriented twin.

A theory-based parameterization for heterogeneous ice nucleation and implications for the simulation of ice processes in atmospheric models

NASA Astrophysics Data System (ADS)

Savre, J.; Ekman, A. M. L.

2015-05-01

A new parameterization for heterogeneous ice nucleation constrained by laboratory data and based on classical nucleation theory is introduced. Key features of the parameterization include the following: a consistent and modular modeling framework for treating condensation/immersion and deposition freezing, the possibility to consider various potential ice nucleating particle types (e.g., dust, black carbon, and bacteria), and the possibility to account for an aerosol size distribution. The ice nucleating ability of each aerosol type is described using a contact angle (θ) probability density function (PDF). A new modeling strategy is described to allow the θ PDF to evolve in time so that the most efficient ice nuclei (associated with the lowest θ values) are progressively removed as they nucleate ice. A computationally efficient quasi Monte Carlo method is used to integrate the computed ice nucleation rates over both size and contact angle distributions. The parameterization is employed in a parcel model, forced by an ensemble of Lagrangian trajectories extracted from a three-dimensional simulation of a springtime low-level Arctic mixed-phase cloud, in order to evaluate the accuracy and convergence of the method using different settings. The same model setup is then employed to examine the importance of various parameters for the simulated ice production. Modeling the time evolution of the θ PDF is found to be particularly crucial; assuming a time-independent θ PDF significantly overestimates the ice nucleation rates. It is stressed that the capacity of black carbon (BC) to form ice in the condensation/immersion freezing mode is highly uncertain, in particular at temperatures warmer than -20°C. In its current version, the parameterization most likely overestimates ice initiation by BC.

Dynamics of protein aggregation and oligomer formation governed by secondary nucleation

NASA Astrophysics Data System (ADS)

Michaels, Thomas C. T.; Lazell, Hamish W.; Arosio, Paolo; Knowles, Tuomas P. J.

2015-08-01

The formation of aggregates in many protein systems can be significantly accelerated by secondary nucleation, a process where existing assemblies catalyse the nucleation of new species. In particular, secondary nucleation has emerged as a central process controlling the proliferation of many filamentous protein structures, including molecular species related to diseases such as sickle cell anemia and a range of neurodegenerative conditions. Increasing evidence suggests that the physical size of protein filaments plays a key role in determining their potential for deleterious interactions with living cells, with smaller aggregates of misfolded proteins, oligomers, being particularly toxic. It is thus crucial to progress towards an understanding of the factors that control the sizes of protein aggregates. However, the influence of secondary nucleation on the time evolution of aggregate size distributions has been challenging to quantify. This difficulty originates in large part from the fact that secondary nucleation couples the dynamics of species distant in size space. Here, we approach this problem by presenting an analytical treatment of the master equation describing the growth kinetics of linear protein structures proliferating through secondary nucleation and provide closed-form expressions for the temporal evolution of the resulting aggregate size distribution. We show how the availability of analytical solutions for the full filament distribution allows us to identify the key physical parameters that control the sizes of growing protein filaments. Furthermore, we use these results to probe the dynamics of the populations of small oligomeric species as they are formed through secondary nucleation and discuss the implications of our work for understanding the factors that promote or curtail the production of these species with a potentially high deleterious biological activity.

Dynamics of protein aggregation and oligomer formation governed by secondary nucleation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Michaels, Thomas C. T., E-mail: tctm3@cam.ac.uk; Lazell, Hamish W.; Arosio, Paolo

2015-08-07

The formation of aggregates in many protein systems can be significantly accelerated by secondary nucleation, a process where existing assemblies catalyse the nucleation of new species. In particular, secondary nucleation has emerged as a central process controlling the proliferation of many filamentous protein structures, including molecular species related to diseases such as sickle cell anemia and a range of neurodegenerative conditions. Increasing evidence suggests that the physical size of protein filaments plays a key role in determining their potential for deleterious interactions with living cells, with smaller aggregates of misfolded proteins, oligomers, being particularly toxic. It is thus crucial tomore » progress towards an understanding of the factors that control the sizes of protein aggregates. However, the influence of secondary nucleation on the time evolution of aggregate size distributions has been challenging to quantify. This difficulty originates in large part from the fact that secondary nucleation couples the dynamics of species distant in size space. Here, we approach this problem by presenting an analytical treatment of the master equation describing the growth kinetics of linear protein structures proliferating through secondary nucleation and provide closed-form expressions for the temporal evolution of the resulting aggregate size distribution. We show how the availability of analytical solutions for the full filament distribution allows us to identify the key physical parameters that control the sizes of growing protein filaments. Furthermore, we use these results to probe the dynamics of the populations of small oligomeric species as they are formed through secondary nucleation and discuss the implications of our work for understanding the factors that promote or curtail the production of these species with a potentially high deleterious biological activity.« less

Study of homogeneous bubble nucleation in liquid carbon dioxide by a hybrid approach combining molecular dynamics simulation and density gradient theory

NASA Astrophysics Data System (ADS)

Langenbach, K.; Heilig, M.; Horsch, M.; Hasse, H.

2018-03-01

A new method for predicting homogeneous bubble nucleation rates of pure compounds from vapor-liquid equilibrium (VLE) data is presented. It combines molecular dynamics simulation on the one side with density gradient theory using an equation of state (EOS) on the other. The new method is applied here to predict bubble nucleation rates in metastable liquid carbon dioxide (CO2). The molecular model of CO2 is taken from previous work of our group. PC-SAFT is used as an EOS. The consistency between the molecular model and the EOS is achieved by adjusting the PC-SAFT parameters to VLE data obtained from the molecular model. The influence parameter of density gradient theory is fitted to the surface tension of the molecular model. Massively parallel molecular dynamics simulations are performed close to the spinodal to compute bubble nucleation rates. From these simulations, the kinetic prefactor of the hybrid nucleation theory is estimated, whereas the nucleation barrier is calculated from density gradient theory. This enables the extrapolation of molecular simulation data to the whole metastable range including technically relevant densities. The results are tested against available experimental data and found to be in good agreement. The new method does not suffer from typical deficiencies of classical nucleation theory concerning the thermodynamic barrier at the spinodal and the bubble size dependence of surface tension, which is typically neglected in classical nucleation theory. In addition, the density in the center of critical bubbles and their surface tension is determined as a function of their radius. The usual linear Tolman correction to the capillarity approximation is found to be invalid.

Experimental particle formation rates spanning tropospheric sulfuric acid and ammonia abundances, ion production rates, and temperatures

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kürten, Andreas; Bianchi, Federico; Almeida, Joao

Binary nucleation of sulfuric acid and water as well as ternary nucleation involving ammonia are thought to be the dominant processes responsible for new particle formation (NPF) in the cold temperatures of the middle and upper troposphere. Ions are also thought to be important for particle nucleation in these regions. However, global models presently lack experimentally measured NPF rates under controlled laboratory conditions and so at present must rely on theoretical or empirical parameterizations. Here with data obtained in the European Organization for Nuclear Research CLOUD (Cosmics Leaving OUtdoor Droplets) chamber, we present the first experimental survey of NPF ratesmore » spanning free tropospheric conditions. The conditions during nucleation cover a temperature range from 208 to 298 K, sulfuric acid concentrations between 5 × 10 5 and 1 × 10 9cm -3, and ammonia mixing ratios from zero added ammonia, i.e., nominally pure binary, to a maximumof ~1400 parts per trillion by volume (pptv).We performed nucleation studies under pure neutral conditions with zero ions being present in the chamber and at ionization rates of up to 75 ion pairs cm -3 s -1 to study neutral and ion-induced nucleation. We found that the contribution from ion-induced nucleation is small at temperatures between 208 and 248 K when ammonia is present at several pptv or higher. However, the presence of charges significantly enhances the nucleation rates, especially at 248 K with zero added ammonia, and for higher temperatures independent of NH 3 levels. In conclusion, we compare these experimental data with calculated cluster formation rates from the Atmospheric Cluster Dynamics Code with cluster evaporation rates obtained from quantum chemistry.« less

Particle formation and growth at five rural and urban sites

NASA Astrophysics Data System (ADS)

Jeong, C.-H.; Evans, G. J.; McGuire, M. L.; Chang, R. Y.-W.; Abbatt, J. P. D.; Zeromskiene, K.; Mozurkewich, M.; Li, S.-M.; Leaitch, W. R.

2010-08-01

Ultrafine particle (UFP) number and size distributions were simultaneously measured at five urban and rural sites during the summer of 2007 in Ontario, Canada as part of the Border Air Quality and Meteorology Study (BAQS-Met 2007). Particle formation and growth events at these five sites were classified based on their strength and persistence as well as the variation in geometric mean diameter. Regional nucleation and growth events and local short-lived strong nucleation events were frequently observed at the near-border rural sites, upwind of industrial sources. Surprisingly, the particle number concentrations at one of these sites were higher than the concentrations at a downtown site in a major city, despite its high traffic density. Regional nucleation and growth events were favored during intense solar irradiance and in less polluted cooler drier air. The most distinctive regional particle nucleation and growth event during the campaign was observed simultaneously at all five sites, which were up to 350 km apart. Although the ultrafine particle concentrations and size distributions generally were spatially heterogeneous across the region, a more uniform spatial distribution of UFP across the five areas was observed during this regional nucleation event. Thus, nucleation events can cover large regions, contributing to the burden of UFP in cities and potentially to the associated health impacts on urban populations. Local short-lived nucleation events at the three near-border sites during this summer three-week campaign were associated with high SO2, which likely originated from US and Canadian industrial sources. Hence, particle formation in southwestern Ontario appears to often be related to anthropogenic gaseous emissions but biogenic emissions at times also contribute. Longer-term studies are needed to help resolve the relative contributions of anthropogenic and biogenic emissions to nucleation and growth in this region.

First Demonstration of a Scintillating Xenon Bubble Chamber for Detecting Dark Matter and Coherent Elastic Neutrino-Nucleus Scattering

NASA Astrophysics Data System (ADS)

Baxter, D.; Chen, C. J.; Crisler, M.; Cwiok, T.; Dahl, C. E.; Grimsted, A.; Gupta, J.; Jin, M.; Puig, R.; Temples, D.; Zhang, J.

2017-06-01

A 30-g xenon bubble chamber, operated at Northwestern University in June and November 2016, has for the first time observed simultaneous bubble nucleation and scintillation by nuclear recoils in a superheated liquid. This chamber is instrumented with a CCD camera for near-IR bubble imaging, a solar-blind photomultiplier tube to detect 175-nm xenon scintillation light, and a piezoelectric acoustic transducer to detect the ultrasonic emission from a growing bubble. The time of nucleation determined from the acoustic signal is used to correlate specific scintillation pulses with bubble-nucleating events. We report on data from this chamber for thermodynamic "Seitz" thresholds from 4.2 to 15.0 keV. The observed single- and multiple-bubble rates when exposed to a Cf 252 neutron source indicate that, for an 8.3-keV thermodynamic threshold, the minimum nuclear recoil energy required to nucleate a bubble is 19 ±6 keV (1 σ uncertainty). This is consistent with the observed scintillation spectrum for bubble-nucleating events. We see no evidence for bubble nucleation by gamma rays at any of the thresholds studied, setting a 90% C.L. upper limit of 6.3 ×10-7 bubbles per gamma interaction at a 4.2-keV thermodynamic threshold. This indicates stronger gamma discrimination than in CF3 I bubble chambers, supporting the hypothesis that scintillation production suppresses bubble nucleation by electron recoils, while nuclear recoils nucleate bubbles as usual. These measurements establish the noble-liquid bubble chamber as a promising new technology for the detection of weakly interacting massive particle dark matter and coherent elastic neutrino-nucleus scattering.

Experimental particle formation rates spanning tropospheric sulfuric acid and ammonia abundances, ion production rates, and temperatures

DOE PAGES

Kürten, Andreas; Bianchi, Federico; Almeida, Joao; ...

2016-10-27

Binary nucleation of sulfuric acid and water as well as ternary nucleation involving ammonia are thought to be the dominant processes responsible for new particle formation (NPF) in the cold temperatures of the middle and upper troposphere. Ions are also thought to be important for particle nucleation in these regions. However, global models presently lack experimentally measured NPF rates under controlled laboratory conditions and so at present must rely on theoretical or empirical parameterizations. Here with data obtained in the European Organization for Nuclear Research CLOUD (Cosmics Leaving OUtdoor Droplets) chamber, we present the first experimental survey of NPF ratesmore » spanning free tropospheric conditions. The conditions during nucleation cover a temperature range from 208 to 298 K, sulfuric acid concentrations between 5 × 10 5 and 1 × 10 9cm -3, and ammonia mixing ratios from zero added ammonia, i.e., nominally pure binary, to a maximumof ~1400 parts per trillion by volume (pptv).We performed nucleation studies under pure neutral conditions with zero ions being present in the chamber and at ionization rates of up to 75 ion pairs cm -3 s -1 to study neutral and ion-induced nucleation. We found that the contribution from ion-induced nucleation is small at temperatures between 208 and 248 K when ammonia is present at several pptv or higher. However, the presence of charges significantly enhances the nucleation rates, especially at 248 K with zero added ammonia, and for higher temperatures independent of NH 3 levels. In conclusion, we compare these experimental data with calculated cluster formation rates from the Atmospheric Cluster Dynamics Code with cluster evaporation rates obtained from quantum chemistry.« less

First demonstration of a scintillating xenon bubble chamber for detecting dark matter and coherent elastic neutrino-nucleus scattering

DOE PAGES

Baxter, D.; Chen, C. J.; Crisler, M.; ...

2017-06-08

A 30-g xenon bubble chamber, operated at Northwestern University in June and November 2016, has for the first time observed simultaneous bubble nucleation and scintillation by nuclear recoils in a superheated liquid. This chamber is instrumented with a CCD camera for near-IR bubble imaging, a solar-blind photomultiplier tube to detect 175-nm xenon scintillation light, and a piezoelectric acoustic transducer to detect the ultrasonic emission from a growing bubble. The time of nucleation determined from the acoustic signal is used to correlate specific scintillation pulses with bubble-nucleating events. We report on data from this chamber for thermodynamic "Seitz" thresholds from 4.2 to 15.0 keV. The observed single- and multiple-bubble rates when exposed to amore » $$^{252}$$Cf neutron source indicate that, for an 8.3-keV thermodynamic threshold, the minimum nuclear recoil energy required to nucleate a bubble is $$19\\pm6$$ keV (1$$\\sigma$$ uncertainty). This is consistent with the observed scintillation spectrum for bubble-nucleating events. We see no evidence for bubble nucleation by gamma rays at any of the thresholds studied, setting a 90% C.L. upper limit of $$6.3\\times10^{-7}$$ bubbles per gamma interaction at a 4.2-keV thermodynamic threshold. This indicates stronger gamma discrimination than in CF$$_3$$I bubble chambers, supporting the hypothesis that scintillation production suppresses bubble nucleation by electron recoils while nuclear recoils nucleate bubbles as usual. Finally, these measurements establish the noble-liquid bubble chamber as a promising new technology for the detection of weakly interacting massive particle dark matter and coherent elastic neutrino-nucleus scattering.« less

Effect of photochemical aging on the ice nucleation properties of diesel and wood burning particles

NASA Astrophysics Data System (ADS)

Chou, C.; Stetzer, O.; Tritscher, T.; Chirico, R.; Heringa, M. F.; Kanji, Z. A.; Weingartner, E.; Prévôt, A. S. H.; Baltensperger, U.; Lohmann, U.

2012-06-01

A measurement campaign (IMBALANCE) was conducted in 2009 and aimed at characterizing the physical and chemical properties of freshly emitted and photochemically aged combustion particles emitted from a log wood burner and diesel vehicles: a EURO3 Opel Astra with a diesel oxidation catalyst (DOC) but no particle filter and a EURO2 Volkswagen Transporter TDI Syncro with no emission after-treatment. Ice nucleation experiments in the deposition and condensation freezing modes were conducted with the Portable Ice Nucleation Chamber (PINC) at three nominal temperatures, -30 °C, -35 °C and -40 °C. Freshly emitted diesel particles showed ice formation only at -40 °C in the deposition mode at 137% relative humidity with respect to ice (RHi) and 92% relative humidity with respect to water (RHw), and photochemical aging did not play a role in modifying their ice nucleation behavior. Only one diesel experiment where α-pinene was added, showed an ice nucleation enhancement after the aging at -35 °C. Wood burning particles also act as ice nuclei (IN) at -40 °C in the deposition mode at the same conditions as for diesel particles and photochemical aging did also not alter the ice formation properties of the wood burning particles. Unlike diesel particles, wood burning particles form ice via condensation freezing at -35 °C with no ice nucleation observed at -30 °C for wood burning particles. Photochemical aging did not affect the ice nucleation ability of the diesel and wood burning particles at the three different temperatures investigated but a broader range of temperatures below -30 °C need to be investigated in order to draw an overall conclusion on the effect of photochemical aging on deposition/condensation ice nucleation across the entire temperature range relevant to cold clouds.

Effect of photochemical ageing on the ice nucleation properties of diesel and wood burning particles

NASA Astrophysics Data System (ADS)

Chou, C.; Kanji, Z. A.; Stetzer, O.; Tritscher, T.; Chirico, R.; Heringa, M. F.; Weingartner, E.; Prévôt, A. S. H.; Baltensperger, U.; Lohmann, U.

2013-01-01

A measurement campaign (IMBALANCE) conducted in 2009 was aimed at characterizing the physical and chemical properties of freshly emitted and photochemically aged combustion particles emitted from a log wood burner and diesel vehicles: a EURO3 Opel Astra with a diesel oxidation catalyst (DOC) but no particle filter and a EURO2 Volkswagen Transporter TDI Syncro without emission aftertreatment. Ice nucleation experiments in the deposition and condensation freezing modes were conducted with the Portable Ice Nucleation Chamber (PINC) at three nominal temperatures, -30 °C, -35 °C and -40 °C. Freshly emitted diesel particles showed ice formation only at -40 °C in the deposition mode at 137% relative humidity with respect to ice (RHi) and 92% relative humidity with respect to water (RHw), and photochemical ageing did not play a role in modifying their ice nucleation behaviour. Only one diesel experiment where α-pinene was added for the ageing process, showed an ice nucleation enhancement at -35 °C. Wood burning particles also act as ice nuclei (IN) at -40 °C in the deposition mode at the same conditions as for diesel particles and photochemical ageing also did not alter the ice formation properties of the wood burning particles. Unlike diesel particles, wood burning particles form ice via condensation freezing at -35 °C whereas no ice nucleation was observed at -30 °C. Photochemical ageing did not affect the ice nucleation ability of the diesel and wood burning particles at the three different temperatures investigated but a broader range of temperatures below -40 °C need to be investigated in order to draw an overall conclusion on the effect of photochemical ageing on deposition/condensation ice nucleation across the entire temperature range relevant to cold clouds.

Predictions of homogeneous nucleation rates for n-alkanes accounting for the diffuse phase interface and capillary waves.

PubMed

Planková, Barbora; Vinš, Václav; Hrubý, Jan

2017-10-28

Homogeneous droplet nucleation has been studied for almost a century but has not yet been fully understood. In this work, we used the density gradient theory (DGT) and considered the influence of capillary waves (CWs) on the predicted size-dependent surface tensions and nucleation rates for selected n-alkanes. The DGT model was completed by an equation of state (EoS) based on the perturbed-chain statistical associating fluid theory and compared to the classical nucleation theory and the Peng-Robinson EoS. It was found that the critical clusters are practically free of CWs because they are so small that even the smallest wavelengths of CWs do not fit into their finite dimensions. The CWs contribute to the entropy of the system and thus decrease the surface tension. A correction for the effect of CWs on the surface tension is presented. The effect of the different EoSs is relatively small because by a fortuitous coincidence their predictions are similar in the relevant range of critical cluster sizes. The difference of the DGT predictions to the classical nucleation theory computations is important but not decisive. Of the effects investigated, the most pronounced is the suppression of CWs which causes a sizable decrease of the predicted nucleation rates. The major difference between experimental nucleation rate data and theoretical predictions remains in the temperature dependence. For normal alkanes, this discrepancy is much stronger than observed, e.g., for water. Theoretical corrections developed here have a minor influence on the temperature dependency. We provide empirical equations correcting the predicted nucleation rates to values comparable with experiments.

Predictions of homogeneous nucleation rates for n-alkanes accounting for the diffuse phase interface and capillary waves

NASA Astrophysics Data System (ADS)

Planková, Barbora; Vinš, Václav; Hrubý, Jan

2017-10-01

Homogeneous droplet nucleation has been studied for almost a century but has not yet been fully understood. In this work, we used the density gradient theory (DGT) and considered the influence of capillary waves (CWs) on the predicted size-dependent surface tensions and nucleation rates for selected n-alkanes. The DGT model was completed by an equation of state (EoS) based on the perturbed-chain statistical associating fluid theory and compared to the classical nucleation theory and the Peng-Robinson EoS. It was found that the critical clusters are practically free of CWs because they are so small that even the smallest wavelengths of CWs do not fit into their finite dimensions. The CWs contribute to the entropy of the system and thus decrease the surface tension. A correction for the effect of CWs on the surface tension is presented. The effect of the different EoSs is relatively small because by a fortuitous coincidence their predictions are similar in the relevant range of critical cluster sizes. The difference of the DGT predictions to the classical nucleation theory computations is important but not decisive. Of the effects investigated, the most pronounced is the suppression of CWs which causes a sizable decrease of the predicted nucleation rates. The major difference between experimental nucleation rate data and theoretical predictions remains in the temperature dependence. For normal alkanes, this discrepancy is much stronger than observed, e.g., for water. Theoretical corrections developed here have a minor influence on the temperature dependency. We provide empirical equations correcting the predicted nucleation rates to values comparable with experiments.

Study of homogeneous bubble nucleation in liquid carbon dioxide by a hybrid approach combining molecular dynamics simulation and density gradient theory.

PubMed

Langenbach, K; Heilig, M; Horsch, M; Hasse, H

2018-03-28

A new method for predicting homogeneous bubble nucleation rates of pure compounds from vapor-liquid equilibrium (VLE) data is presented. It combines molecular dynamics simulation on the one side with density gradient theory using an equation of state (EOS) on the other. The new method is applied here to predict bubble nucleation rates in metastable liquid carbon dioxide (CO 2 ). The molecular model of CO 2 is taken from previous work of our group. PC-SAFT is used as an EOS. The consistency between the molecular model and the EOS is achieved by adjusting the PC-SAFT parameters to VLE data obtained from the molecular model. The influence parameter of density gradient theory is fitted to the surface tension of the molecular model. Massively parallel molecular dynamics simulations are performed close to the spinodal to compute bubble nucleation rates. From these simulations, the kinetic prefactor of the hybrid nucleation theory is estimated, whereas the nucleation barrier is calculated from density gradient theory. This enables the extrapolation of molecular simulation data to the whole metastable range including technically relevant densities. The results are tested against available experimental data and found to be in good agreement. The new method does not suffer from typical deficiencies of classical nucleation theory concerning the thermodynamic barrier at the spinodal and the bubble size dependence of surface tension, which is typically neglected in classical nucleation theory. In addition, the density in the center of critical bubbles and their surface tension is determined as a function of their radius. The usual linear Tolman correction to the capillarity approximation is found to be invalid.

Cavitation in a metallic liquid: Homogeneous nucleation and growth of nanovoids

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cai, Y.; The Peac Institute of Multiscale Sciences, Chengdu, Sichuan 610207; Wu, H. A., E-mail: wuha@ustc.edu.cn

2014-06-07

Large-scale molecular dynamics (MD) simulations are performed to investigate homogeneous nucleation and growth of nanovoids during cavitation in liquid Cu. We characterize in detail the atomistic cavitation processes by following the temporal evolution of cavities or voids, analyze the nucleation behavior with the mean first-passage time (MFPT) and survival probability (SP) methods, and discuss the results against classical nucleation theory (CNT), the Tolman equation for surface energy, independent calculation of surface tension via integrating the stress profiles, the Johnson-Mehl-Avrami (JMA) growth law, and the power law for nucleus size distributions. Cavitation in this representative metallic liquid is a high energymore » barrier Poisson processes, and the steady-state nucleation rates obtained from statistical runs with the MFPT and SP methods are in agreement. The MFPT method also yields the critical nucleus size and the Zeldovich factor. Fitting with the Tolman's equation to the MD simulations yields the surface energy of a planar interface (∼0.9 J m{sup −2}) and the Tolman length (0.4–0.5 Å), and those values are in accord with those from integrating the stress profiles of a planar interface. Independent CNT predictions of the nucleation rate (10{sup 33−34} s{sup −1} m{sup −3}) and critical size (3–4 Å in radius) are in agreement with the MFPT and SP results. The JMA law can reasonably describe the nucleation and growth process. The size distribution of subcritical nuclei appears to follow a power law with an exponent decreasing with increasing tension owing to coupled nucleation and growth, and that of the supercritical nuclei becomes flattened during further stress relaxation due to void coalescence.« less

The relevance of nanoscale biological fragments for ice nucleation in clouds

NASA Astrophysics Data System (ADS)

O‧Sullivan, D.; Murray, B. J.; Ross, J. F.; Whale, T. F.; Price, H. C.; Atkinson, J. D.; Umo, N. S.; Webb, M. E.

2015-01-01

Most studies of the role of biological entities as atmospheric ice-nucleating particles have focused on relatively rare supermicron particles such as bacterial cells, fungal spores and pollen grains. However, it is not clear that there are sufficient numbers of these particles in the atmosphere to strongly influence clouds. Here we show that the ice-nucleating activity of a fungus from the ubiquitous genus Fusarium is related to the presence of nanometre-scale particles which are far more numerous, and therefore potentially far more important for cloud glaciation than whole intact spores or hyphae. In addition, we quantify the ice-nucleating activity of nano-ice nucleating particles (nano-INPs) washed off pollen and also show that nano-INPs are present in a soil sample. Based on these results, we suggest that there is a reservoir of biological nano-INPs present in the environment which may, for example, become aerosolised in association with fertile soil dust particles.

Molecular simulations of heterogeneous ice nucleation. I. Controlling ice nucleation through surface hydrophilicity

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cox, Stephen J.; Kathmann, Shawn M.; Slater, B.

2015-05-14

Ice formation is one of the most common and important processes on earth and almost always occurs at the surface of a material. A basic understanding of how the physicochemical properties of a material’s surface affect its ability to form ice has remained elusive. Here, we use molecular dynamics simulations to directly probe heterogeneous ice nucleation at a hexagonal surface of a nanoparticle of varying hydrophilicity. Surprisingly, we find that structurally identical surfaces can both inhibit and promote ice formation and analogous to a chemical catalyst, it is found that an optimal interaction between the surface and the water existsmore » for promoting ice nucleation.We use our microscopic understanding of the mechanism to design a modified surface in silico with enhanced ice nucleating ability. C 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.« less

Corrosion and corrosion fatigue of airframe aluminum alloys

NASA Technical Reports Server (NTRS)

Chen, G. S.; Gao, M.; Harlow, D. G.; Wei, R. P.

1994-01-01

Localized corrosion and corrosion fatigue crack nucleation and growth are recognized as degradation mechanisms that effect the durability and integrity of commercial transport aircraft. Mechanically based understanding is needed to aid the development of effective methodologies for assessing durability and integrity of airframe components. As a part of the methodology development, experiments on pitting corrosion, and on corrosion fatigue crack nucleation and early growth from these pits were conducted. Pitting was found to be associated with constituent particles in the alloys and pit growth often involved coalescence of individual particle-nucleated pits, both laterally and in depth. Fatigue cracks typically nucleated from one of the larger pits that formed by a cluster of particles. The size of pit at which fatigue crack nucleates is a function of stress level and fatigue loading frequency. The experimental results are summarized, and their implications on service performance and life prediction are discussed.

Quantifying the Effect of Stress on Sn Whisker Nucleation Kinetics

NASA Astrophysics Data System (ADS)

Chason, Eric; Vasquez, Justin; Pei, Fei; Jain, Nupur; Hitt, Andrew

2018-01-01

Although Sn whiskers have been studied extensively, there is still a need to understand the driving forces behind whisker nucleation and growth. Many studies point to the role of stress, but confirming this requires a quantitative comparison between controlled stress and the resulting whisker evolution. Recent experimental studies applied stress to a Sn layer via thermal cycling and simultaneously monitored the evolution of the temperature, stress and number of nuclei. In this work, we analyze these nucleation kinetics in terms of classical nucleation theory to relate the observed behavior to underlying mechanisms including a stress dependent activation energy and a temperature and stress-dependent whisker growth rate. Non-linear least squares fitting of the data taken at different temperatures and strain rates to the model shows that the results can be understood in terms of stress decreasing the barrier for whisker nucleation.

Using small angle x-ray scattering to measure the homogeneous nucleation rates of n-propanol, n-butanol, and n-pentanol in supersonic nozzle expansions

NASA Astrophysics Data System (ADS)

Ghosh, David; Manka, Alexandra; Strey, Reinhard; Seifert, Soenke; Winans, Randall E.; Wyslouzil, Barbara E.

2008-09-01

In our earlier publication [M. Gharibeh et al., J. Chem. Phys. 122, 094512 (2005)] we determined the temperatures and partial pressures corresponding to the maximum nucleation rate for a series n-alcohols (CiH2i+lOH; i =3-5) during condensation in a supersonic nozzle. Although we were able to determine the characteristic time ΔtJmax corresponding to the peak nucleation rate, we were unable to measure the number density of the aerosol and, thus, unable to directly quantify the nucleation rate J. In this paper we report the results of our pioneering small angle x-ray scattering (SAXS) experiments of n-alcohol droplets formed in a supersonic nozzle together with a new series of complementary pressure trace measurements. By combining the SAXS and pressure trace measurement data we determine the nucleation rates as a function of temperature and supersaturation.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yu, Bingke; Cheng, Hui-Chun; Brautigam, Chad A.

Vibrio parahaemolyticus protein L (VopL) is an actin nucleation factor that induces stress fibers when injected into eukaryotic host cells. VopL contains three N-terminal Wiskott-Aldrich homology 2 (WH2) motifs and a unique VopL C-terminal domain (VCD). We describe crystallographic and biochemical analyses of filament nucleation by VopL. The WH2 element of VopL does not nucleate on its own and requires the VCD for activity. The VCD forms a U-shaped dimer in the crystal, stabilized by a terminal coiled coil. Dimerization of the WH2 motifs contributes strongly to nucleation activity, as do contacts of the VCD to actin. Our data leadmore » to a model in which VopL stabilizes primarily lateral (short-pitch) contacts between actin monomers to create the base of a two-stranded filament. Stabilization of lateral contacts may be a common feature of actin filament nucleation by WH2-based factors.« less

Heterogeneous Nucleation of Trichloroethylene Ozonation Products in the Formation of New Fine Particles

NASA Astrophysics Data System (ADS)

Wang, Ning; Sun, Xiaomin; Chen, Jianmin; Li, Xiang

2017-02-01

Free radicals in atmosphere have played an important role in the atmospheric chemistry. The chloro-Criegee free radicals are produced easily in the decomposition of primary ozonide (POZ) of the trichloroethylene, and can react with O2, NO, NO2, SO2 and H2O subsequently. Then the inorganic salts, polar organic nitrogen and organic sulfur compounds, oxygen-containing heterocyclic intermediates and polyhydroxy compounds can be obtained. The heterogeneous nucleation of oxidation intermediates in the formation of fine particles is investigated using molecular dynamics simulation. The detailed nucleation processes are reported. According to molecular dynamics simulation, the nucleation with a diameter of 2 nm is formed in the Organic Compounds-(NH4)2SO4-H2O system. The spontaneous nucleation is an important process in the formation of fine particles in atmosphere. The model study gives a good example from volatile organic compounds to new fine particles.

Energy dispersive-EXAFS of Pd nucleation at a liquid/liquid interface

NASA Astrophysics Data System (ADS)

Chang, S.-Y.; Booth, S. G.; Uehara, A.; Mosselmans, J. F. W.; Cibin, G.; Pham, V.-T.; Nataf, L.; Dryfe, R. A. W.; Schroeder, S. L. M.

2016-05-01

Energy dispersive extended X-ray absorption fine structure (EDE) has been applied to Pd nanoparticle nucleation at a liquid/liquid interface under control over the interfacial potential and thereby the driving force for nucleation. Preliminary analysis focusing on Pd K edge-step height determination shows that under supersaturated conditions the concentration of Pd near the interface fluctuate over a period of several hours, likely due to the continuous formation and dissolution of sub-critical nuclei. Open circuit potential measurements conducted ex-situ in a liquid/liquid electrochemical cell support this view, showing that the fluctuations in Pd concentration are also visible as variations in potential across the liquid/liquid interface. By decreasing the interfacial potential through inclusion of a common ion (tetraethylammonium, TEA+) the Pd nanoparticle growth rate could be slowed down, resulting in a smooth nucleation process. Eventually, when the TEA+ ions reached an equilibrium potential, Pd nucleation and particle growth were inhibited.

Bubble nucleation in stout beers

NASA Astrophysics Data System (ADS)

Lee, W. T.; McKechnie, J. S.; Devereux, M. G.

2011-05-01

Bubble nucleation in weakly supersaturated solutions of carbon dioxide—such as champagne, sparkling wines, and carbonated beers—is well understood. Bubbles grow and detach from nucleation sites: gas pockets trapped within hollow cellulose fibers. This mechanism appears not to be active in stout beers that are supersaturated solutions of nitrogen and carbon dioxide. In their canned forms these beers require additional technology (widgets) to release the bubbles which will form the head of the beer. We extend the mathematical model of bubble nucleation in carbonated liquids to the case of two gases and show that this nucleation mechanism is active in stout beers, though substantially slower than in carbonated beers and confirm this by observation. A rough calculation suggests that despite the slowness of the process, applying a coating of hollow porous fibers to the inside of a can or bottle could be a potential replacement for widgets.

Nucleation of nitric acid hydrates in polar stratospheric clouds by meteoric material

NASA Astrophysics Data System (ADS)

James, Alexander D.; Brooke, James S. A.; Mangan, Thomas P.; Whale, Thomas F.; Plane, John M. C.; Murray, Benjamin J.

2018-04-01

Heterogeneous nucleation of crystalline nitric acid hydrates in polar stratospheric clouds (PSCs) enhances ozone depletion. However, the identity and mode of action of the particles responsible for nucleation remains unknown. It has been suggested that meteoric material may trigger nucleation of nitric acid trihydrate (NAT, or other nitric acid phases), but this has never been quantitatively demonstrated in the laboratory. Meteoric material is present in two forms in the stratosphere: smoke that results from the ablation and re-condensation of vapours, and fragments that result from the break-up of meteoroids entering the atmosphere. Here we show that analogues of both materials have a capacity to nucleate nitric acid hydrates. In combination with estimates from a global model of the amount of meteoric smoke and fragments in the polar stratosphere we show that meteoric material probably accounts for NAT observations in early season polar stratospheric clouds in the absence of water ice.

Functional display of ice nucleation protein InaZ on the surface of bacterial ghosts.

PubMed

Kassmannhuber, Johannes; Rauscher, Mascha; Schöner, Lea; Witte, Angela; Lubitz, Werner

2017-09-03

In a concept study the ability to induce heterogeneous ice formation by Bacterial Ghosts (BGs) from Escherichia coli carrying ice nucleation protein InaZ from Pseudomonas syringae in their outer membrane was investigated by a droplet-freezing assay of ultra-pure water. As determined by the median freezing temperature and cumulative ice nucleation spectra it could be demonstrated that both the living recombinant E. coli and their corresponding BGs functionally display InaZ on their surface. Under the production conditions chosen both samples belong to type II ice-nucleation particles inducing ice formation at a temperature range of between -5.6 °C and -6.7 °C, respectively. One advantage for the application of such BGs over their living recombinant mother bacteria is that they are non-living native cell envelopes retaining the biophysical properties of ice nucleation and do no longer represent genetically modified organisms (GMOs).

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pandey, Ravindra; Usui, Kota; Livingstone, Ruth A.

Ice-nucleating organisms play important roles in the environment. With their ability to induce ice formation at temperatures just below the ice melting point, bacteria such as Pseudomonas syringae attack plants through frost damage using specialized ice-nucleating proteins. Besides the impact on agriculture and microbial ecology, airborne P. syringae can affect atmospheric glaciation processes, with consequences for cloud evolution, precipitation, and climate. Biogenic ice nucleation is also relevant for artificial snow production and for biomimetic materials for controlled interfacial freezing. We use interface-specific sum frequency generation (SFG) spectroscopy to show that hydrogen bonding at the water-bacteria contact imposes structural ordering onmore » the adjacent water network. Experimental SFG data and molecular dynamics simulations demonstrate that ice active sites within P. syringae feature unique hydrophilic-hydrophobic patterns to enhance ice nucleation. Finally, the freezing transition is further facilitated by the highly effective removal of latent heat from the nucleation site, as apparent from time-resolved SFG spectroscopy.« less

Hysteresis and the role of nucleation and growth in the hydrogenation of Mg nanolayers.

PubMed

Mooij, Lennard; Dam, Bernard

2013-02-28

We investigated the hydrogenation of 3 and 10 nm Mg layers sandwiched between Ti using an optical transmission technique (hydrogenography). We observe in situ the two dimensional nucleation and growth of single hydride domains of up to several millimeters in diameter. The low density of nuclei points to preferential nucleation at heterogeneous sites. From an analysis of the growth kinetics we deduce an extremely large edge boundary energy, which we relate to the plastic deformations inherent to the 30% volume expansion of the MgH(2). We find that the nucleation and growth process affects the hysteresis between absorption and desorption. Especially, the absorption branch can be lowered when nucleation barriers are removed. Our results show that when discussing the effect of nano-structuring on hydrogenation it may be quite complex to distinguish the thermodynamic and kinetic effects involved.

The undercooling of liquids

NASA Technical Reports Server (NTRS)

Turnbull, D.

1984-01-01

The formation by melt quenching of such metastable structures as glassy or microcrystalline solids and highly supersaturated solutions is made possible by the extreme resistance of most melts to homophase crystal nucleation at deep undercooling. This nucleation resistance contrasts sharply with the very low kinetic resistance to the movement of crystal-melt interfaces, once formed, in metals and other fluid systems at even minute undercooling. The methods of nucleation study which have proven especially effective in bypassing nucleation by heterophase impurities thereby exposing the high resistance of melts to homophase nucleation may be summarized as follows: observation of the crystallization behavior of dispersed small droplets; drop tube experiments in which liquid drops solidify, under containerless conditions, during their fall in the tube; and observation of the crystallization of bulk specimens immersed in fluxes chosen to dissolve or otherwise deactivate (e.g., by wetting) heterophase nucleants. This method has proven to be remarkably effective in deactivating such nucleants in certain pure metals.

How Properties of Solid Surfaces Modulate the Nucleation of Gas Hydrate

PubMed Central

Bai, Dongsheng; Chen, Guangjin; Zhang, Xianren; Sum, Amadeu K.; Wang, Wenchuan

2015-01-01

Molecular dynamics simulations were performed for CO2 dissolved in water near silica surfaces to investigate how the hydrophilicity and crystallinity of solid surfaces modulate the local structure of adjacent molecules and the nucleation of CO2 hydrates. Our simulations reveal that the hydrophilicity of solid surfaces can change the local structure of water molecules and gas distribution near liquid-solid interfaces, and thus alter the mechanism and dynamics of gas hydrate nucleation. Interestingly, we find that hydrate nucleation tends to occur more easily on relatively less hydrophilic surfaces. Different from surface hydrophilicity, surface crystallinity shows a weak effect on the local structure of adjacent water molecules and on gas hydrate nucleation. At the initial stage of gas hydrate growth, however, the structuring of molecules induced by crystalline surfaces are more ordered than that induced by amorphous solid surfaces. PMID:26227239

The relevance of nanoscale biological fragments for ice nucleation in clouds.

PubMed

O'Sullivan, D; Murray, B J; Ross, J F; Whale, T F; Price, H C; Atkinson, J D; Umo, N S; Webb, M E

2015-01-28

Most studies of the role of biological entities as atmospheric ice-nucleating particles have focused on relatively rare supermicron particles such as bacterial cells, fungal spores and pollen grains. However, it is not clear that there are sufficient numbers of these particles in the atmosphere to strongly influence clouds. Here we show that the ice-nucleating activity of a fungus from the ubiquitous genus Fusarium is related to the presence of nanometre-scale particles which are far more numerous, and therefore potentially far more important for cloud glaciation than whole intact spores or hyphae. In addition, we quantify the ice-nucleating activity of nano-ice nucleating particles (nano-INPs) washed off pollen and also show that nano-INPs are present in a soil sample. Based on these results, we suggest that there is a reservoir of biological nano-INPs present in the environment which may, for example, become aerosolised in association with fertile soil dust particles.

Charge Effects on the Efflorescence in Single Levitated Droplets.

PubMed

Hermann, Gunter; Zhang, Yan; Wassermann, Bernhard; Fischer, Henry; Quennet, Marcel; Rühl, Eckart

2017-09-14

The influence of electrical excess charges on the crystallization from supersaturated aqueous sodium chloride solutions is reported. This is accomplished by efflorescence studies on single levitated microdroplets using optical and electrodynamic levitation. Specifically, a strong increase in efflorescence humidity is observed as a function of the droplet's negative excess charge, ranging up to -2.1 pC, with a distinct threshold behavior, increasing the relative efflorescence humidity, at which spontaneous nucleation occurs, from 44% for the neutral microparticle to 60%. These findings are interpreted by using molecular dynamics simulations for determining plausible structural patterns located near the particle surface that could serve as suitable precursors for the formation of critical clusters overcoming the nucleation barrier. These results, facilitating heterogeneous nucleation in the case of negatively charged microparticles, are compared to recent work on charge-induced nucleation of neat supercooled water, where a distinctly different nucleation behavior as a function of droplet charge has been observed.

Polo-like kinase 1 regulates Nlp, a centrosome protein involved in microtubule nucleation.

PubMed

Casenghi, Martina; Meraldi, Patrick; Weinhart, Ulrike; Duncan, Peter I; Körner, Roman; Nigg, Erich A

2003-07-01

In animal cells, most microtubules are nucleated at centrosomes. At the onset of mitosis, centrosomes undergo a structural reorganization, termed maturation, which leads to increased microtubule nucleation activity. Centrosome maturation is regulated by several kinases, including Polo-like kinase 1 (Plk1). Here, we identify a centrosomal Plk1 substrate, termed Nlp (ninein-like protein), whose properties suggest an important role in microtubule organization. Nlp interacts with two components of the gamma-tubulin ring complex and stimulates microtubule nucleation. Plk1 phosphorylates Nlp and disrupts both its centrosome association and its gamma-tubulin interaction. Overexpression of an Nlp mutant lacking Plk1 phosphorylation sites severely disturbs mitotic spindle formation. We propose that Nlp plays an important role in microtubule organization during interphase, and that the activation of Plk1 at the onset of mitosis triggers the displacement of Nlp from the centrosome, allowing the establishment of a mitotic scaffold with enhanced microtubule nucleation activity.

A study of the applicability of nucleation theory to quasi-thermodynamic transformations of second and higher Ehrenfest-order

NASA Technical Reports Server (NTRS)

Barker, R. E., Jr.

1985-01-01

Transient and steady-state phenomena in temperature, stress, and electric, field intensity in ferroelectric polymers were investigated. The application and extension of the theory in the primary stage to the polarization domain nucleation and growth in ferroelectric polymers were developed. The kinetics of this growth were investigated. Expressions describing nucleation under the influence of an electric field were found through the expansion of the Gibbs' free energy in a Maclaurin series. The series was expanded in the electric field strength rather than the degree of undercooling. The resulting expressions were manipulated and applied to the case of nucleation of polarized domains in ferroelectric polymers. The kinetics of the nucleation and growth of polarized domains are also investigated. This was accomplished through the modification of the Johnson-Mehl-Avrami treatment of crystallization kinetics to be applicable to the growth of polarization domains in ferroelectric materials.

Functional display of ice nucleation protein InaZ on the surface of bacterial ghosts

PubMed Central

Kassmannhuber, Johannes; Rauscher, Mascha; Schöner, Lea; Witte, Angela; Lubitz, Werner

2017-01-01

ABSTRACT In a concept study the ability to induce heterogeneous ice formation by Bacterial Ghosts (BGs) from Escherichia coli carrying ice nucleation protein InaZ from Pseudomonas syringae in their outer membrane was investigated by a droplet-freezing assay of ultra-pure water. As determined by the median freezing temperature and cumulative ice nucleation spectra it could be demonstrated that both the living recombinant E. coli and their corresponding BGs functionally display InaZ on their surface. Under the production conditions chosen both samples belong to type II ice-nucleation particles inducing ice formation at a temperature range of between −5.6 °C and −6.7 °C, respectively. One advantage for the application of such BGs over their living recombinant mother bacteria is that they are non-living native cell envelopes retaining the biophysical properties of ice nucleation and do no longer represent genetically modified organisms (GMOs). PMID:28121482

Nucleation study for an undercooled melt of intermetallic NiZr

NASA Astrophysics Data System (ADS)

Kobold, R.; Kolbe, M.; Hornfeck, W.; Herlach, D. M.

2018-03-01

Electrostatic levitation is applied in order to undercool liquid glass forming NiZr significantly below its melting temperature. For NiZr large undercoolings are found to be highly reproducible with this experimental method. One single NiZr sample of high purity is undercooled 200 consecutive times which leads to a distribution function of undercooling temperatures. Within a statistical approach of classical nucleation theory, the undercooling distribution is analyzed yielding parameters, e.g., a pre-exponential factor of KV ≈ 1035 m-3 s-1, which indicates homogeneous nucleation. This result is consistent with the crystallization behavior of NiZr at high undercooling and with the corresponding microstructural analysis. Since NiZr is a representative of the very common CrB structure type, with 132 isostructural phases existing, understanding its nucleation behavior adds important knowledge to the nucleation of binary alloys in general.

Adherent nanoparticles-mediated micro- and nanobubble nucleation

NASA Astrophysics Data System (ADS)

Chan, Chon U.; Chen, Long Quan; Lippert, Alexander; Arora, Manish; Ohl, Claus-Dieter

2014-11-01

Surface nanobubbles are commonly nucleated through water-ethanol-water exchange. It is believed that the higher gas solubility in ethanol and exothermic mixing leads to a supersaturation of gas in water. However details of the nucleation dynamic are still unknown. Here we apply the exchange process onto a glass surface deposited with nanoparticles and monitor the dynamics optically at video frame rates. During exchange bubbles of a few micron in diameter nucleate at the sites of nanoparticles. These microbubbles eventually dissolve in ethanol but are stable in water. This agrees with the nucleation process observed for surface nanobubbles. Also we find a reduction of surface attached nanobubbles near the particles, which might be due to gas uptake from the microbubble growth. Finally, high speed recordings reveal stick-slip motion of the triple contact line during the growth process. We will discuss possibilities of utilizing the findings for contamination detection and ultrasonic cleaning.

The role of nanopore shape in surface-induced crystallization

NASA Astrophysics Data System (ADS)

Diao, Ying; Harada, Takuya; Myerson, Allan S.; Alan Hatton, T.; Trout, Bernhardt L.

2011-11-01

Crystallization of a molecular liquid from solution often initiates at solid-liquid interfaces, and nucleation rates are generally believed to be enhanced by surface roughness. Here we show that, on a rough surface, the shape of surface nanopores can also alter nucleation kinetics. Using lithographic methods, we patterned polymer films with nanopores of various shapes and found that spherical nanopores 15-120 nm in diameter hindered nucleation of aspirin crystals, whereas angular nanopores of the same size promoted it. We also show that favourable surface-solute interactions are required for angular nanopores to promote nucleation, and propose that pore shape affects nucleation kinetics through the alteration of the orientational order of the crystallizing molecule near the angles of the pores. Our findings have clear technological implications, for instance in the control of pharmaceutical polymorphism and in the design of ‘seed’ particles for the regulation of crystallization of fine chemicals.

Differences in ice nucleation behavior of arable and desert soil dust in deposition nucleation regime

NASA Astrophysics Data System (ADS)

Ullrich, Romy; Vogel, Franziska; Möhler, Ottmar; Höhler, Kristina; Schiebel, Thea

2017-04-01

Soil dust from arid and semi-arid regions is one of the most abundant aerosol types in the atmosphere with emission rates of about 1600 Tg per year (Andreae et al. (2009)). Therewith, soil dust plays an important role for the atmospheric radiative transfer and also for the formation of clouds. Soil dust refers to dust sampled from agricultural used areas, to dust from bare soil as well as to dust from desert regions. By mass-spectrometric measurements of the chemical composition of ice residuals, mineral dust as component of soil dust was found to be the major heterogeneous ice nucleating particle (INP) type (e.g. Cziczo et al. (2013)), in particular in the upper troposphere. Also in laboratory studies the ice nucleation efficiency of the different soil dusts was investigated. It was shown that desert dusts (Ullrich et al. (2017)) as well as soil dusts from arable regions (O'Sullivan et al. (2014), Tobo et al. (2014)) are efficient INP. However, there is still a lack of data for ice nucleation on soil dusts for temperatures below about 220 K. With the AIDA (Aerosol Interactions and Dynamics in the Atmosphere) cloud chamber, we are able to characterize the ice nucleation efficiency for different aerosol types to temperatures down to 180 K and high ice supersaturations. In order to extend the already existing AIDA data base for deposition nucleation on desert dusts and agricultural soil dusts, new experiments were done in the upper tropospheric temperature regime. This contribution will show the results of the new experiments with desert dust in comparison to existing data for higher temperatures. The first data analysis confirms the temperature dependent trend of the ice nucleation activity as discussed and parameterized in a recent paper by Ullrich et al. (2017). Furthermore, the update and extension of the recently published parameterization of deposition nucleation for desert dust to lower temperatures will be discussed. The experiments with agricultural soil dust will be compared to existing AIDA experiments at higher temperatures published by Steinke et al. (2016). Finally, the ice nucleation activity of both desert dust and agricultural soil dust will be compared for the upper tropospheric temperature regime. Andreae et al. (2009), Sources and Nature of Atmospheric Aerosols, in Aerosol Pollution Impact on Precipitation - A Scientific Review, Ch.3, Springer Netherlands, 45-89 Cziczo et al. (2013), Clarifying the Dominant Sources and Mechanisms of Cirrus Cloud Formation, Science, 340, 1320-1324 O'Sullivan et al. (2014), Ice nucleation by fertile soil dusts: relative importance of mineral and biogenic components, Atmos. Chem. Phys., 14, 1853-1867 Steinke et al. (2016), Ice nucleation activity of agricultural soil dust aerosols from Mongolia, Argentina and Germany, J. Geophys. Res., 121 Tobo et al. (2014), Organic matter matters for ice nuclei of agricultural soil origin, Atmos. Chem. Phys., 14, 8521-8531 Ullrich et al. (2017), A new ice nucleation active site parametrization for desert dust and soot, J. Atmos. Sci., in press

Cirrus Parcel Model Comparison Project. Phase 1: The Critical Components to Simulate Cirrus Initiation Explicitly.

NASA Astrophysics Data System (ADS)

Lin, Ruei-Fong; O'C. Starr, David; Demott, Paul J.; Cotton, Richard; Sassen, Kenneth; Jensen, Eric; Kärcher, Bernd; Liu, Xiaohong

2002-08-01

The Cirrus Parcel Model Comparison Project, a project of the GCSS [Global Energy and Water Cycle Experiment (GEWEX) Cloud System Studies] Working Group on Cirrus Cloud Systems, involves the systematic comparison of current models of ice crystal nucleation and growth for specified, typical, cirrus cloud environments. In Phase 1 of the project reported here, simulated cirrus cloud microphysical properties from seven models are compared for `warm' (40°C) and `cold' (60°C) cirrus, each subject to updrafts of 0.04, 0.2, and 1 m s1. The models employ explicit microphysical schemes wherein the size distribution of each class of particles (aerosols and ice crystals) is resolved into bins or the evolution of each individual particle is traced. Simulations are made including both homogeneous and heterogeneous ice nucleation mechanisms (all-mode simulations). A single initial aerosol population of sulfuric acid particles is prescribed for all simulations. Heterogeneous nucleation is disabled for a second parallel set of simulations in order to isolate the treatment of the homogeneous freezing (of haze droplets) nucleation process. Analysis of these latter simulations is the primary focus of this paper.Qualitative agreement is found for the homogeneous-nucleation-only simulations; for example, the number density of nucleated ice crystals increases with the strength of the prescribed updraft. However, significant quantitative differences are found. Detailed analysis reveals that the homogeneous nucleation rate, haze particle solution concentration, and water vapor uptake rate by ice crystal growth (particularly as controlled by the deposition coefficient) are critical components that lead to differences in the predicted microphysics.Systematic differences exist between results based on a modified classical theory approach and models using an effective freezing temperature approach to the treatment of nucleation. Each method is constrained by critical freezing data from laboratory studies, but each includes assumptions that can only be justified by further laboratory research. Consequently, it is not yet clear if the two approaches can be made consistent. Large haze particles may deviate considerably from equilibrium size in moderate to strong updrafts (0.2-1 m s1) at 60°C. The equilibrium assumption is commonly invoked in cirrus parcel models. The resulting difference in particle-size-dependent solution concentration of haze particles may significantly affect the ice particle formation rate during the initial nucleation interval. The uptake rate for water vapor excess by ice crystals is another key component regulating the total number of nucleated ice crystals. This rate, the product of particle number concentration and ice crystal diffusional growth rate, which is particularly sensitive to the deposition coefficient when ice particles are small, modulates the peak particle formation rate achieved in an air parcel and the duration of the active nucleation time period. The consequent differences in cloud microphysical properties, and thus cloud optical properties, between state-of-the-art models of ice crystal initiation are significant.Intermodel differences in the case of all-mode simulations are correspondingly greater than in the case of homogeneous nucleation acting alone. Definitive laboratory and atmospheric benchmark data are needed to improve the treatment of heterogeneous nucleation processes.

Deposition nucleation viewed as homogeneous or immersion freezing in pores and cavities

NASA Astrophysics Data System (ADS)

Marcolli, C.

2013-06-01

Heterogeneous ice nucleation is an important mechanism for the glaciation of mixed phase clouds and may also be relevant for cloud formation and dehydration at the cirrus cloud level. It is thought to proceed through different mechanisms, namely contact, condensation, immersion and deposition nucleation. Supposedly, deposition nucleation is the only pathway which does not involve liquid water but occurs by direct water vapor deposition on a surface. This study challenges this classical view by putting forward the hypothesis that what is called deposition nucleation is in fact homogeneous or immersion nucleation occurring in pores and cavities that may form between aggregated primary particles and fill with water at relative humidity RHw < 100% because of the inverse Kelvin effect. Evidence for this hypothesis of pore condensation and freezing (PCF) originates from a number of only loosely connected scientific areas. The prime example for PCF is ice nucleation in clay minerals and mineral dusts, for which the data base is best. Studies on freezing in confinement carried out on mesoporous silica materials such as SBA-15, SBA-16, MCM-41, zeolites and KIT have shown that homogeneous ice nucleation occurs abruptly at T=230-235 K in pores with diameters (D) of 3.5-4 nm or larger but only gradually at T=210-230 K in pores with D=2.5-3.5 nm. Melting temperatures in pores are depressed by an amount that can be described by the Gibbs-Thomson equation. Water adsorption isotherms of MCM-41 show that pores with D=3.5-4 nm fill with water at RHw = 56-60% in accordance with an inverse Kelvin effect. Water in such pores should freeze homogeneously for T < 235 K even before relative humidity with respect to ice (RHi) reaches ice saturation. Ice crystal growth by water vapor deposition from the gas phase is therefore expected to set in as soon as RHw > 100%. Pores with D > 7.5 nm fill with water at RHi > 100% for T < 235 K and are likely to freeze homogeneously as soon as they are filled with water. Water in pores can freeze in immersion mode at T > 235 K if the pore walls contain an active site. Pore analysis of clay minerals shows that kaolinites exhibit pore structures with pore diameters of 20-50 nm. The mesoporosity of illites and montmorillonites is characterized by pores with T = 2-5 nm. The number and size of pores is distinctly increased in acid treated montmorillonites like K10. Many clay minerals and mineral dusts show a strong increase in ice nucleation efficiency when temperature is decreased below 235 K. Such an increase is difficult to explain when ice nucleation is supposed to occur by a deposition mechanism, but evident when assuming freezing in pores, because for homogeneous ice nucleation only small pore volumes are needed, while heterogeneous ice nucleation requires larger pore structures to contain at least one active site for immersion nucleation. Together, these pieces of evidence strongly suggest that ice nucleation within pores should be the prevailing freezing mechanism of clay minerals for RHw below water saturation. Extending the analysis to other types of ice nuclei shows that freezing in pores and cracks is probably the prevailing ice nucleation mechanism for glassy and volcanic ash aerosols at RHw below water saturation. Freezing of water in carbon nanotubes might be of significance for ice nucleation by soot aerosols. No case could be identified that gives clear evidence of ice nucleation by water vapor deposition on a solid surface. Inspection of ice nuclei with a close lattice match to ice, such as silver iodide or SnomaxTM, show that for high ice nucleation efficiency below water saturation the presence of impurities or cracks on the surface may be essential. Soluble impurities promote the formation of a liquid phase below water saturation in patches on the surface or as a complete surface layer that offers an environment for immersion freezing. If porous aerosol particles come in contact with semivolatile vapors, these will condense preferentially in pores before a coating on the surface of the particles is formed. A pore partially filled with condensed species attracts water at lower RHw than an empty pore, but the aqueous solution that forms in the pore will freeze at a higher RHi than pure water. The ice nucleation ability of pores completely filled with condensed organic species might be totally impeded. Pores might also be important for preactivation, the capability of a particle to nucleate ice at lower RHi in subsequent experiments when compared to the first initial ice nucleation event. Preactivation has often been explained by persistence of ice embryos at specific sites like dislocations, steps, kinks or pores. However, it is not clear how such features can preserve an ice embryo at RHi < 100%. Rather, ice embryos could be preserved when embedded in water. To keep liquid water at RHw well below 100%, narrow pores are needed but to avoid a strong melting point depression large pores are favorable. A narrow pore opening and a large inner volume are combined in "ink bottle" pores. Such "ink bottle" pores would be suited to preserve ice at RHi < 100% and can arise e.g. in spaces between aggregated particles.

Effect of Pt Doping on Nucleation and Crystallization in Li2O.2SiO2 Glass: Experimental Measurements and Computer Modeling

NASA Technical Reports Server (NTRS)

Narayan, K. Lakshmi; Kelton, K. F.; Ray, C. S.

1996-01-01

Heterogeneous nucleation and its effects on the crystallization of lithium disilicate glass containing small amounts of Pt are investigated. Measurements of the nucleation frequencies and induction times with and without Pt are shown to be consistent with predictions based on the classical nucleation theory. A realistic computer model for the transformation is presented. Computed differential thermal analysis data (such as crystallization rates as a function of time and temperature) are shown to be in good agreement with experimental results. This modeling provides a new, more quantitative method for analyzing calorimetric data.

Champagne experiences various rhythmical bubbling regimes in a flute.

PubMed

Liger-Belair, Gérard; Tufaile, Alberto; Jeandet, Philippe; Sartorelli, José-Carlos

2006-09-20

Bubble trains are seen rising gracefully from a few points on the glass wall (called nucleation sites) whenever champagne is poured into a glass. As time passes during the gas-discharging process, the careful observation of some given bubble columns reveals that the interbubble distance may change suddenly, thus revealing different rhythmical bubbling regimes. Here, it is reported that the transitions between the different bubbling regimes of some nucleation sites during gas discharging is a process which may be ruled by a strong interaction between tiny gas pockets trapped inside the nucleation site and/or also by an interaction between the tiny bubbles just blown from the nucleation site.

Continuum analysis of the nucleus growth of reverse domains in large ferroelectric crystals

NASA Astrophysics Data System (ADS)

Neumeister, Peter; Balke, Herbert; Lupascu, Doru C.

2009-04-01

Polarization reversal in ferroelectrics arises due to domain nucleation and domain wall motion. The nucleation of reverse domains at crystal boundaries is the fundamental initiation process observed in single crystals. The classical continuum approach by Landauer determines an insurmountable energy barrier to extrinsic domain nucleation. We rediscuss the continuum approach. Predetermined surface states are found to be a misleading concept. Alternate energy contributions, for example, due to a dead layer or due to charge injection as well as reduced domain wall energy and anisotropy of domain wall energy, have to be included into a convincing picture of domain nucleation.

Metadynamics studies of crystal nucleation

PubMed Central

Giberti, Federico; Salvalaglio, Matteo; Parrinello, Michele

2015-01-01

Crystallization processes are characterized by activated events and long timescales. These characteristics prevent standard molecular dynamics techniques from being efficiently used for the direct investigation of processes such as nucleation. This short review provides an overview on the use of metadynamics, a state-of-the-art enhanced sampling technique, for the simulation of phase transitions involving the production of a crystalline solid. In particular the principles of metadynamics are outlined, several order parameters are described that have been or could be used in conjunction with metadynamics to sample nucleation events and then an overview is given of recent metadynamics results in the field of crystal nucleation. PMID:25866662

Nucleation and growth in one dimension

NASA Astrophysics Data System (ADS)

Ben-Naim, E.; Krapivsky, P. L.

1996-10-01

We study statistical properties of the Kolmogorov-Avrami-Johnson-Mehl nucleation-and-growth model in one dimension. We obtain exact results for the gap density as well as the island distribution. When all nucleation events occur simultaneously, we show that the island distribution has discontinuous derivatives on the rays xn(t)=nt, n=1,2,3... . We introduce an accelerated growth mechanism with growth rate increasing linearly with the island size. We solve for the interisland gap density and show that the system reaches complete coverage in a finite time and that the near-critical behavior of the system is robust; i.e., it is insensitive to details such as the nucleation mechanism.

Crosslinked Aspartic Acids as Helix-Nucleating Templates.

PubMed

Zhao, Hui; Liu, Qi-Song; Geng, Hao; Tian, Yuan; Cheng, Min; Jiang, Yan-Hong; Xie, Ming-Sheng; Niu, Xiao-Gang; Jiang, Fan; Zhang, Ya-Ou; Lao, Yuan-Zhi; Wu, Yun-Dong; Xu, Nai-Han; Li, Zi-Gang

2016-09-19

Described is a facile helix-nucleating template based on a tethered aspartic acid at the N-terminus [terminal aspartic acid (TD)]. The nucleating effect of the template is subtly influenced by the substituent at the end of the side-chain-end tether as indicated by circular dichroism, nuclear magnetic resonance, and molecular dynamics simulations. Unlike most nucleating strategies, the N-terminal amine is preserved, thus enabling further modification. Peptidomimetic estrogen receptor modulators (PERMs) constructed using this strategy show improved therapeutic properties. The current strategy can be regarded as a good complement to existing helix-stabilizing methods. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

A Theoretical Study of Vapour Phase Nucleation of the Rocket Propellant N2O4

NASA Astrophysics Data System (ADS)

Pal, P.

2003-05-01

The residual vapour of a rocket fuel at the venting stage develops a potential aerodynamic problem which is linked with the vapour phase nucleation phenomena of the propellant. This study, based entirely on molecular treatment, addresses the problem by focusing specifically on the N2O4 propellant which is used in the ARIANE flight. The phenomenon is examined by considering the thermodynamic free energies of N2O4 clusters, leading to the evaluation of nucleation flux rates of critical nuclei at incipient nucleation. Preliminary examinations of the kinetics of flux pulses provide basic explanation from a molecular perspective.

Determining whether metals nucleate homogeneously on graphite: A case study with copper

DOE PAGES

Appy, David; Lei, Huaping; Han, Yong; ...

2014-11-05

In this study, we observe that Cu clusters grow on surface terraces of graphite as a result of physical vapor deposition in ultrahigh vacuum. We show that the observation is incompatible with a variety of models incorporating homogeneous nucleation and calculations of atomic-scale energetics. An alternative explanation, ion-mediated heterogeneous nucleation, is proposed and validated, both with theory and experiment. This serves as a case study in identifying when and whether the simple, common observation of metal clusters on carbon-rich surfaces can be interpreted in terms of homogeneous nucleation. We describe a general approach for making system-specific and laboratory-specific predictions.

Navigating ligand protein binding free energy landscapes: universality and diversity of protein folding and molecular recognition mechanisms

NASA Astrophysics Data System (ADS)

Verkhivker, Gennady M.; Rejto, Paul A.; Bouzida, Djamal; Arthurs, Sandra; Colson, Anthony B.; Freer, Stephan T.; Gehlhaar, Daniel K.; Larson, Veda; Luty, Brock A.; Marrone, Tami; Rose, Peter W.

2001-03-01

Thermodynamic and kinetic aspects of ligand-protein binding are studied for the methotrexate-dihydrofolate reductase system from the binding free energy profile constructed as a function of the order parameter. Thermodynamic stability of the native complex and a cooperative transition to the unique native structure suggest the nucleation kinetic mechanism at the equilibrium transition temperature. Structural properties of the transition state ensemble and the ensemble of nucleation conformations are determined by kinetic simulations of the transmission coefficient and ligand-protein association pathways. Structural analysis of the transition states and the nucleation conformations reconciles different views on the nucleation mechanism in protein folding.