Mapping carrier diffusion in single silicon core-shell nanowires with ultrafast optical microscopy.

PubMed

Seo, M A; Yoo, J; Dayeh, S A; Picraux, S T; Taylor, A J; Prasankumar, R P

2012-12-12

Recent success in the fabrication of axial and radial core-shell heterostructures, composed of one or more layers with different properties, on semiconductor nanowires (NWs) has enabled greater control of NW-based device operation for various applications. (1-3) However, further progress toward significant performance enhancements in a given application is hindered by the limited knowledge of carrier dynamics in these structures. In particular, the strong influence of interfaces between different layers in NWs on transport makes it especially important to understand carrier dynamics in these quasi-one-dimensional systems. Here, we use ultrafast optical microscopy (4) to directly examine carrier relaxation and diffusion in single silicon core-only and Si/SiO(2) core-shell NWs with high temporal and spatial resolution in a noncontact manner. This enables us to reveal strong coherent phonon oscillations and experimentally map electron and hole diffusion currents in individual semiconductor NWs for the first time.

Catalytic conversion in nanoporous materials: Concentration oscillations and spatial correlations due to inhibited transport and intermolecular interactions

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

Garcia, Andres; Evans, James W.

2016-11-03

We show that steady-state catalytic conversion in nanoporous materials can occur in a quasi-counter-diffusion mode with the reactant (product) concentration strongly decaying (growing) into the pore, but also with oscillations in the total concentration. These oscillations reflect the response of the fluid to the transition from an extended to a confined environment near the pore opening. We focus on the regime of strongly inhibited transport in narrow pores corresponding to single-file diffusion. Here, limited penetration of the reactant into the pores and the associated low reaction yield is impacted by strong spatial correlations induced by both reaction (non-equilibrium correlations) andmore » also by intermolecular interactions (thermodynamic correlations). We develop a generalized hydrodynamic formulation to effectively describe inhibited transport accounting for the effect of these correlations, and incorporate this description of transport into appropriate reaction-diffusion equations. These equations accurately describe both shorter-range concentration oscillations near the pore opening and the longer-range mesoscale variation of concentration profiles in the pore (and thus also describe reaction yield). Success of the analytic theory is validated by comparison with a precise kinetic Monte Carlo simulation of an appropriate molecular-level stochastic reaction-diffusion model. As a result, this work elucidates unconventional chemical kinetics in interacting confined systems.« less

Enhanced Atom Mobility on the Surface of a Metastable Film

NASA Astrophysics Data System (ADS)

Picone, A.; Riva, M.; Fratesi, G.; Brambilla, A.; Bussetti, G.; Finazzi, M.; Duò, L.; Ciccacci, F.

2014-07-01

A remarkable enhancement of atomic diffusion is highlighted by scanning tunneling microscopy performed on ultrathin metastable body-centered tetragonal Co films grown on Fe(001). The films follow a nearly perfect layer-by-layer growth mode with a saturation island density strongly dependent on the layer on which the nucleation occurs, indicating a lowering of the diffusion barrier. Density functional theory calculations reveal that this phenomenon is driven by the increasing capability of the film to accommodate large deformations as the thickness approaches the limit at which a structural transition occurs. These results disclose the possibility of tuning surface diffusion dynamics and controlling cluster nucleation and self-organization.

Enhanced atom mobility on the surface of a metastable film.

PubMed

Picone, A; Riva, M; Fratesi, G; Brambilla, A; Bussetti, G; Finazzi, M; Duò, L; Ciccacci, F

2014-07-25

A remarkable enhancement of atomic diffusion is highlighted by scanning tunneling microscopy performed on ultrathin metastable body-centered tetragonal Co films grown on Fe(001). The films follow a nearly perfect layer-by-layer growth mode with a saturation island density strongly dependent on the layer on which the nucleation occurs, indicating a lowering of the diffusion barrier. Density functional theory calculations reveal that this phenomenon is driven by the increasing capability of the film to accommodate large deformations as the thickness approaches the limit at which a structural transition occurs. These results disclose the possibility of tuning surface diffusion dynamics and controlling cluster nucleation and self-organization.

Diffusion Monte Carlo study of strongly interacting two-dimensional Fermi gases

DOE PAGES

Galea, Alexander; Dawkins, Hillary; Gandolfi, Stefano; ...

2016-02-01

Ultracold atomic Fermi gases have been a popular topic of research, with attention being paid recently to two-dimensional (2D) gases. In this work, we perform T=0 ab initio diffusion Monte Carlo calculations for a strongly interacting two-component Fermi gas confined to two dimensions. We first go over finite-size systems and the connection to the thermodynamic limit. After that, we illustrate pertinent 2D scattering physics and properties of the wave function. We then show energy results for the strong-coupling crossover, in between the Bose-Einstein condensation (BEC) and Bardeen-Cooper-Schrieffer (BCS) regimes. Our energy results for the BEC-BCS crossover are parametrized to producemore » an equation of state, which is used to determine Tan's contact. We carry out a detailed comparison with other microscopic results. Lastly, we calculate the pairing gap for a range of interaction strengths in the strong coupling regime, following from variationally optimized many-body wave functions.« less

ULTRA-SHARP solution of the Smith-Hutton problem

NASA Technical Reports Server (NTRS)

Leonard, B. P.; Mokhtari, Simin

1992-01-01

Highly convective scalar transport involving near-discontinuities and strong streamline curvature was addressed in a paper by Smith and Hutton in 1982, comparing several different convection schemes applied to a specially devised test problem. First order methods showed significant artificial diffusion, whereas higher order methods gave less smearing but had a tendency to overshoot and oscillate. Perhaps because unphysical oscillations are more obvious than unphysical smearing, the intervening period has seen a rise in popularity of low order artificially diffusive schemes, especially in the numerical heat transfer industry. The present paper describes an alternate strategy of using non-artificially diffusive high order methods, while maintaining strictly monotonic transitions through the use of simple flux limited constraints. Limited third order upwinding is usually found to be the most cost effective basic convection scheme. Tighter resolution of discontinuities can be obtained at little additional cost by using automatic adaptive stencil expansion to higher order in local regions, as needed.

Universal linear-temperature resistivity: possible quantum diffusion transport in strongly correlated superconductors.

PubMed

Hu, Tao; Liu, Yinshang; Xiao, Hong; Mu, Gang; Yang, Yi-Feng

2017-08-25

The strongly correlated electron fluids in high temperature cuprate superconductors demonstrate an anomalous linear temperature (T) dependent resistivity behavior, which persists to a wide temperature range without exhibiting saturation. As cooling down, those electron fluids lose the resistivity and condense into the superfluid. However, the origin of the linear-T resistivity behavior and its relationship to the strongly correlated superconductivity remain a mystery. Here we report a universal relation [Formula: see text], which bridges the slope of the linear-T-dependent resistivity (dρ/dT) to the London penetration depth λ L at zero temperature among cuprate superconductor Bi 2 Sr 2 CaCu 2 O 8+δ and heavy fermion superconductors CeCoIn 5 , where μ 0 is vacuum permeability, k B is the Boltzmann constant and ħ is the reduced Planck constant. We extend this scaling relation to different systems and found that it holds for other cuprate, pnictide and heavy fermion superconductors as well, regardless of the significant differences in the strength of electronic correlations, transport directions, and doping levels. Our analysis suggests that the scaling relation in strongly correlated superconductors could be described as a hydrodynamic diffusive transport, with the diffusion coefficient (D) approaching the quantum limit D ~ ħ/m*, where m* is the quasi-particle effective mass.

Diffusion of Sites versus Polymers in Polyelectrolyte Complexes and Multilayers.

PubMed

Fares, Hadi M; Schlenoff, Joseph B

2017-10-18

It has long been assumed that the spontaneous formation of materials such as complexes and multilayers from charged polymers depends on (inter)diffusion of these polyelectrolytes. Here, we separately examine the mass transport of polymer molecules and extrinsic sites-charged polyelectrolyte repeat units balanced by counterions-within thin films of polyelectrolyte complex, PEC, using sensitive isotopic labeling techniques. The apparent diffusion coefficients of these sites within PEC films of poly(diallyldimethylammonium), PDADMA, and poly(styrenesulfonate), PSS, are at least 2 orders of magnitude faster than the diffusion of polyelectrolytes themselves. This is because site diffusion requires only local rearrangements of polyelectrolyte repeat units, placing far fewer kinetic limitations on the assembly of polyelectrolyte complexes in all of their forms. Site diffusion strongly depends on the salt concentration (ionic strength) of the environment, and diffusion of PDADMA sites is faster than that of PSS sites, accounting for the asymmetric nature of multilayer growth. Site diffusion is responsible for multilayer growth in the linear and into the exponential regimes, which explains how PDADMA can mysteriously "pass through" layers of PSS. Using quantitative relationships between site diffusion coefficient and salt concentration, conditions were identified that allowed the diffusion length to always exceed the film thickness, leading to full exponential growth over 3 orders of magnitude thickness. Both site and polymer diffusion were independent of molecular weight, suggesting that ion pairing density is a limiting factor. Polyelectrolyte complexes are examples of a broader class of dynamic bulk polymeric materials that (self-) assemble via the transport of cross-links or defects rather than actual molecules.

Confinement and diffusion time-scales of CR hadrons in AGN-inflated bubbles

NASA Astrophysics Data System (ADS)

Prokhorov, D. A.; Churazov, E. M.

2017-09-01

While rich clusters are powerful sources of X-rays, γ-ray emission from these large cosmic structures has not been detected yet. X-ray radiative energy losses in the central regions of relaxed galaxy clusters are so strong that one needs to consider special sources of energy, likely active galactic nucleus (AGN) feedback, to suppress catastrophic cooling of the gas. We consider a model of AGN feedback that postulates that the AGN supplies the energy to the gas by inflating bubbles of relativistic plasma, whose energy content is dominated by cosmic-ray (CR) hadrons. If most of these hadrons can quickly escape the bubbles, then collisions of CRs with thermal protons in the intracluster medium (ICM) should lead to strong γ-ray emission, unless fast diffusion of CRs removes them from the cluster. Therefore, the lack of detections with modern γ-ray telescopes sets limits on the confinement time of CR hadrons in bubbles and CR diffusive propagation in the ICM.

Strong Asymmetric Limit of the Quasi-Potential of the Boundary Driven Weakly Asymmetric Exclusion Process

NASA Astrophysics Data System (ADS)

Bertini, Lorenzo; Gabrielli, Davide; Landim, Claudio

2009-07-01

We consider the weakly asymmetric exclusion process on a bounded interval with particles reservoirs at the endpoints. The hydrodynamic limit for the empirical density, obtained in the diffusive scaling, is given by the viscous Burgers equation with Dirichlet boundary conditions. In the case in which the bulk asymmetry is in the same direction as the drift due to the boundary reservoirs, we prove that the quasi-potential can be expressed in terms of the solution to a one-dimensional boundary value problem which has been introduced by Enaud and Derrida [16]. We consider the strong asymmetric limit of the quasi-potential and recover the functional derived by Derrida, Lebowitz, and Speer [15] for the asymmetric exclusion process.

Impact of Molecular Organization on Exciton Diffusion in Photosensitive Single-Crystal Halogenated Perylenediimides Charge Transfer Interfaces.

PubMed

Pinto, Rui M; Gouveia, Wilson; Maçôas, Ermelinda M S; Santos, Isabel C; Raja, Sebastian; Baleizão, Carlos; Alves, Helena

2015-12-23

The efficiency of organic photodetectors and optoelectronic devices is strongly limited by exciton diffusion, in particular for acceptor materials. Although mechanisms for exciton diffusion are well established, their correlation to molecular organization in real systems has received far less attention. In this report, organic single-crystals interfaces were probed with wavelength-dependent photocurrent spectroscopy and their crystal structure resolved using X-ray diffraction. All systems present a dynamic photoresponse, faster than 500 ms, up to 650 nm. A relationship between molecular organization and favorable exciton diffusion in substituted butyl-perylenediimides (PDIB) is established. This is demonstrated by a set of PDIBs with different intra- and interstack distances and short contacts and their impact on photoresponse. Given the short packing distances between PDIs cores along the same stacking direction (3.4-3.7 Å), and across parallel stacks (2.5 Å), singlet exciton in these PDIBs can follow both Förster and Dexter exciton diffusion, with the Dexter-type mechanism assuming special relevance for interstack exciton diffusion. Yet, the response is maximized in substituted PDIBs, where a 2D percolation network is formed through strong interstack contacts, allowing for PDIBs primary excitons to reach with great efficiency the splitting interface with crystalline rubrene. The importance of short contacts and molecular distances, which is often overlooked as a parameter to consider and optimize when choosing materials for excitonic devices, is emphasized.

Factors Affecting Educational Innovation with in Class Electronic Response Systems

ERIC Educational Resources Information Center

Freeman, Mark; Bell, Amani; Comerton-Forde, Carole; Pickering, Joanne; Blayney, Paul

2007-01-01

This paper reports the use of Rogers' diffusion of innovation perspective to understand the factors affecting educational innovation decisions, specifically in regard to in class electronic response systems. Despite decreasing costs and four decades of research showing strong student support, academic adoption is limited. Using data collected from…

Strange kinetics of bulk-mediated diffusion on lipid bilayers

PubMed Central

Campagnola, Grace; Nepal, Kanti; Peersen, Olve B.

2016-01-01

Diffusion at solid-liquid interfaces is crucial in many technological and biophysical processes. Although its behavior seems deceivingly simple, recent studies showing passive superdiffusive transport suggest diffusion on surfaces may hide rich complexities. In particular, bulk-mediated diffusion occurs when molecules are transiently released from the surface to perform three-dimensional excursions into the liquid bulk. This phenomenon bears the dichotomy where a molecule always return to the surface but the mean jump length is infinite. Such behavior is associated with a breakdown of the central limit theorem and weak ergodicity breaking. Here, we use single-particle tracking to study the statistics of bulk-mediated diffusion on a supported lipid bilayer. We find that the time-averaged mean square displacement (MSD) of individual trajectories, the archetypal measure in diffusion processes, does not converge to the ensemble MSD but it remains a random variable, even in the long observation-time limit. The distribution of time averages is shown to agree with a Lévy flight model. Our results also unravel intriguing anomalies in the statistics of displacements. The time averaged MSD is shown to depend on experimental time and investigations of fractional moments show a scaling 〈|r(t)|q〉 ∼ tqv(q) with non-linear exponents, i.e. v(q) ≠ const. This type of behavior is termed strong anomalous diffusion and is rare among experimental observations. PMID:27095275

Investigating fuel-cell transport limitations using hydrogen limiting current

DOE PAGES

Spingler, Franz B.; Phillips, Adam; Schuler, Tobias; ...

2017-03-09

Reducing mass-transport losses in polymer-electrolyte fuel cells (PEFCs) is essential to increase their power density and reduce overall stack cost. At the same time, cost also motivates the reduction in expensive precious-metal catalysts, which results in higher local transport losses in the catalyst layers. Here, we use a hydrogen-pump limiting-current setup to explore the gas-phase transport losses through PEFC catalyst layers and various gas-diffusion and microporous layers. It is shown that the effective diffusivity in the gas-diffusion layers is a strong function of liquid saturation. Additionally, it is shown how the catalyst layer unexpectedly contributes significantly to the overall measuredmore » transport resistance. This is especially true for low catalyst loadings. It is also shown how the various losses can be separated into different mechanisms including diffusional processes and mass-dependent and independent ones, where the data suggests that a large part of the transport resistance in catalyst layers cannot be attributed to a gas-phase diffusional process. The technique is promising for deconvoluting transport losses in PEFCs.« less

CO2 diffusion into pore spaces limits weathering rate of an experimental basalt landscape

USGS Publications Warehouse

van Haren, Joost; Dontsova, Katerina; Barron-Gafford, Greg A.; Troch, Peter A.; Chorover, Jon; DeLong, Stephen B.; Breshears, David D.; Huxman, Travis E.; Pelletier, Jon D.; Saleska, Scott; Zeng, Xubin; Ruiz, Joaquin

2017-01-01

Basalt weathering is a key control over the global carbon cycle, though in situ measurements of carbon cycling are lacking. In an experimental, vegetation-free hillslope containing 330 m3 of ground basalt scoria, we measured real-time inorganic carbon dynamics within the porous media and seepage flow. The hillslope carbon flux (0.6–5.1 mg C m–2 h–1) matched weathering rates of natural basalt landscapes (0.4–8.8 mg C m–2 h–1) despite lacking the expected field-based impediments to weathering. After rainfall, a decrease in CO2 concentration ([CO2]) in pore spaces into solution suggested rapid carbon sequestration but slow reactant supply. Persistent low soil [CO2] implied that diffusion limited CO2 supply, while when sufficiently dry, reaction product concentrations limited further weathering. Strong influence of diffusion could cause spatial heterogeneity of weathering even in natural settings, implying that modeling studies need to include variable soil [CO2] to improve carbon cycling estimates associated with potential carbon sequestration methods.

Aging underdamped scaled Brownian motion: Ensemble- and time-averaged particle displacements, nonergodicity, and the failure of the overdamping approximation.

PubMed

Safdari, Hadiseh; Cherstvy, Andrey G; Chechkin, Aleksei V; Bodrova, Anna; Metzler, Ralf

2017-01-01

We investigate both analytically and by computer simulations the ensemble- and time-averaged, nonergodic, and aging properties of massive particles diffusing in a medium with a time dependent diffusivity. We call this stochastic diffusion process the (aging) underdamped scaled Brownian motion (UDSBM). We demonstrate how the mean squared displacement (MSD) and the time-averaged MSD of UDSBM are affected by the inertial term in the Langevin equation, both at short, intermediate, and even long diffusion times. In particular, we quantify the ballistic regime for the MSD and the time-averaged MSD as well as the spread of individual time-averaged MSD trajectories. One of the main effects we observe is that, both for the MSD and the time-averaged MSD, for superdiffusive UDSBM the ballistic regime is much shorter than for ordinary Brownian motion. In contrast, for subdiffusive UDSBM, the ballistic region extends to much longer diffusion times. Therefore, particular care needs to be taken under what conditions the overdamped limit indeed provides a correct description, even in the long time limit. We also analyze to what extent ergodicity in the Boltzmann-Khinchin sense in this nonstationary system is broken, both for subdiffusive and superdiffusive UDSBM. Finally, the limiting case of ultraslow UDSBM is considered, with a mixed logarithmic and power-law dependence of the ensemble- and time-averaged MSDs of the particles. In the limit of strong aging, remarkably, the ordinary UDSBM and the ultraslow UDSBM behave similarly in the short time ballistic limit. The approaches developed here open ways for considering other stochastic processes under physically important conditions when a finite particle mass and aging in the system cannot be neglected.

Aging underdamped scaled Brownian motion: Ensemble- and time-averaged particle displacements, nonergodicity, and the failure of the overdamping approximation

NASA Astrophysics Data System (ADS)

Safdari, Hadiseh; Cherstvy, Andrey G.; Chechkin, Aleksei V.; Bodrova, Anna; Metzler, Ralf

2017-01-01

We investigate both analytically and by computer simulations the ensemble- and time-averaged, nonergodic, and aging properties of massive particles diffusing in a medium with a time dependent diffusivity. We call this stochastic diffusion process the (aging) underdamped scaled Brownian motion (UDSBM). We demonstrate how the mean squared displacement (MSD) and the time-averaged MSD of UDSBM are affected by the inertial term in the Langevin equation, both at short, intermediate, and even long diffusion times. In particular, we quantify the ballistic regime for the MSD and the time-averaged MSD as well as the spread of individual time-averaged MSD trajectories. One of the main effects we observe is that, both for the MSD and the time-averaged MSD, for superdiffusive UDSBM the ballistic regime is much shorter than for ordinary Brownian motion. In contrast, for subdiffusive UDSBM, the ballistic region extends to much longer diffusion times. Therefore, particular care needs to be taken under what conditions the overdamped limit indeed provides a correct description, even in the long time limit. We also analyze to what extent ergodicity in the Boltzmann-Khinchin sense in this nonstationary system is broken, both for subdiffusive and superdiffusive UDSBM. Finally, the limiting case of ultraslow UDSBM is considered, with a mixed logarithmic and power-law dependence of the ensemble- and time-averaged MSDs of the particles. In the limit of strong aging, remarkably, the ordinary UDSBM and the ultraslow UDSBM behave similarly in the short time ballistic limit. The approaches developed here open ways for considering other stochastic processes under physically important conditions when a finite particle mass and aging in the system cannot be neglected.

Surface effects on exciton diffusion in non polar ZnO/ZnMgO heterostructures

NASA Astrophysics Data System (ADS)

Sakr, G.; Sartel, C.; Sallet, V.; Lusson, A.; Patriarche, G.; Galtier, P.; Barjon, J.

2017-12-01

The diffusion of excitons injected in ZnO/Zn0.92Mg0.08O quantum well heterostructures grown by metal-organic-vapor-phase-epitaxy on non-polar ZnO substrates is investigated at room temperature. Cathodoluminescence linescans in a field-emission-gun scanning-electron-microscope are performed across cleaved cross-sections. A 55 nm diffusion length is assessed for excitons in bulk ZnMgO. When prepared as small angle bevels using focused ion beam (FIB), the effective diffusion length of excitons is shown to decrease down to 8 nm in the thinner part of the slab. This effect is attributed to non-radiative surface recombinations, with a 7  ×  104 cm s-1 recombination velocity estimated at the FIB-machined ZnMgO surface. The strong reduction of the diffusion extent in such thin lamellae usually used for transmission electron microscopy could be use improve the spatial resolution of cathodoluminescence images, often limited by diffusion processes.

Ion Pairing and Diffusion in Magnesium Electrolytes Based on Magnesium Borohydride.

PubMed

Samuel, Devon; Steinhauser, Carl; Smith, Jeffrey G; Kaufman, Aaron; Radin, Maxwell D; Naruse, Junichi; Hiramatsu, Hidehiko; Siegel, Donald J

2017-12-20

One obstacle to realizing a practical, rechargeable magnesium-ion battery is the development of efficient Mg electrolytes. Electrolytes based on simple Mg(BH 4 ) 2 salts suffer from poor salt solubility and/or low conductivity, presumably due to strong ion pairing. Understanding the molecular-scale processes occurring in these electrolytes would aid in overcoming these performance limitations. Toward this goal, the present study examines the solvation, agglomeration, and transport properties of a family of Mg electrolytes based on the Mg(BH 4 ) 2 salt using classical molecular dynamics. These properties were examined across five different solvents (tetrahydrofuran and the glymes G1-G4) and at four salt concentrations ranging from the dilute limit up to 0.4 M. Significant and irreversible salt agglomeration was observed in all solvents at all nondilute Mg(BH 4 ) 2 concentrations. The degree of clustering observed in these divalent Mg systems is much larger than that reported for electrolytes containing monovalent cations, such as Li. The salt agglomeration rate and diffusivity of Mg 2+ were both observed to correlate with solvent self-diffusivity: electrolytes using longer- (shorter-) chain solvents had the lowest (highest) Mg 2+ diffusivity and agglomeration rates. Incorporation of Mg 2+ into Mg 2+ -BH 4 - clusters significantly reduces the diffusivity of Mg 2+ by restricting displacements to localized motion within largely immobile agglomerates. Consequently, diffusion is increasingly impeded with increasing Mg(BH 4 ) 2 concentration. These data are consistent with the solubility limitations observed experimentally for Mg(BH 4 ) 2 -based electrolytes and highlight the need for strategies that minimize salt agglomeration in electrolytes containing divalent cations.

Effect of probe diffusion on the SOFI imaging accuracy.

PubMed

Vandenberg, Wim; Dedecker, Peter

2017-03-23

Live-cell super-resolution fluorescence imaging is becoming commonplace for exploring biological systems, though sample dynamics can affect the imaging quality. In this work we evaluate the effect of probe diffusion on super-resolution optical fluctuation imaging (SOFI), using a theoretical model and numerical simulations based on the imaging of live cells labelled with photochromic fluorescent proteins. We find that, over a range of physiological conditions, fluorophore diffusion results in a change in the amplitude of the SOFI signal. The magnitude of this change is approximately proportional to the on-time ratio of the fluorophores. However, for photochromic fluorescent proteins this effect is unlikely to present a significant distortion in practical experiments in biological systems. Due to this lack of distortions, probe diffusion strongly enhances the SOFI imaging by avoiding spatial undersampling caused by the limited labeling density.

Dynamics of a magnetic active Brownian particle under a uniform magnetic field.

PubMed

Vidal-Urquiza, Glenn C; Córdova-Figueroa, Ubaldo M

2017-11-01

The dynamics of a magnetic active Brownian particle undergoing three-dimensional Brownian motion, both translation and rotation, under the influence of a uniform magnetic field is investigated. The particle self-propels at a constant speed along its magnetic dipole moment, which reorients due to the interplay between Brownian and magnetic torques, quantified by the Langevin parameter α. In this work, the time-dependent active diffusivity and the crossover time (τ^{cross})-from ballistic to diffusive regimes-are calculated through the time-dependent correlation function of the fluctuations of the propulsion direction. The results reveal that, for any value of α, the particle undergoes a directional (or ballistic) propulsive motion at very short times (t≪τ^{cross}). In this regime, the correlation function decreases linearly with time, and the active diffusivity increases with it. It the opposite time limit (t≫τ^{cross}), the particle moves in a purely diffusive regime with a correlation function that decays asymptotically to zero and an active diffusivity that reaches a constant value equal to the long-time active diffusivity of the particle. As expected in the absence of a magnetic field (α=0), the crossover time is equal to the characteristic time scale for rotational diffusion, τ_{rot}. In the presence of a magnetic field (α>0), the correlation function, the active diffusivity, and the crossover time decrease with increasing α. The magnetic field regulates the regimes of propulsion of the particle. Here, the field reduces the period of time at which the active particle undergoes a directional motion. Consequently, the active particle rapidly reaches a diffusive regime at τ^{cross}≪τ_{rot}. In the limit of weak fields (α≪1), the crossover time decreases quadratically with α, while in the limit of strong fields (α≫1) it decays asymptotically as α^{-1}. The results are in excellent agreement with those obtained by Brownian dynamics simulations.

Dynamics of a magnetic active Brownian particle under a uniform magnetic field

NASA Astrophysics Data System (ADS)

Vidal-Urquiza, Glenn C.; Córdova-Figueroa, Ubaldo M.

2017-11-01

The dynamics of a magnetic active Brownian particle undergoing three-dimensional Brownian motion, both translation and rotation, under the influence of a uniform magnetic field is investigated. The particle self-propels at a constant speed along its magnetic dipole moment, which reorients due to the interplay between Brownian and magnetic torques, quantified by the Langevin parameter α . In this work, the time-dependent active diffusivity and the crossover time (τcross)—from ballistic to diffusive regimes—are calculated through the time-dependent correlation function of the fluctuations of the propulsion direction. The results reveal that, for any value of α , the particle undergoes a directional (or ballistic) propulsive motion at very short times (t ≪τcross ). In this regime, the correlation function decreases linearly with time, and the active diffusivity increases with it. It the opposite time limit (t ≫τcross ), the particle moves in a purely diffusive regime with a correlation function that decays asymptotically to zero and an active diffusivity that reaches a constant value equal to the long-time active diffusivity of the particle. As expected in the absence of a magnetic field (α =0 ), the crossover time is equal to the characteristic time scale for rotational diffusion, τrot. In the presence of a magnetic field (α >0 ), the correlation function, the active diffusivity, and the crossover time decrease with increasing α . The magnetic field regulates the regimes of propulsion of the particle. Here, the field reduces the period of time at which the active particle undergoes a directional motion. Consequently, the active particle rapidly reaches a diffusive regime at τcross≪τrot . In the limit of weak fields (α ≪1 ), the crossover time decreases quadratically with α , while in the limit of strong fields (α ≫1 ) it decays asymptotically as α-1. The results are in excellent agreement with those obtained by Brownian dynamics simulations.

Transport and mixing in strongly coupled dusty plasma medium

NASA Astrophysics Data System (ADS)

Dharodi, Vikram; Das, Amita; Patel, Bhavesh

2016-10-01

The generalized hydrodynamic (GHD) fluid model has been employed to study the transport and mixing properties of Dusty plasma medium in strong coupling limit. The response of lighter electron and ion species to the dust motion is taken to be instantaneous i.e. inertia-less. Thus the electron and ion density are presumed to follow the Boltzman relation. In the incompressible limit (i-GHD) the model supports Transverse Shear wave in contrast to the Hydrodynamic fluids. It has been shown that the presence of these waves leads to a better mixing of fluid in this case. Several cases of flow configuration have been considered for the study. The transport and mixing attributes have been quantified by studying the dynamical evolution of tracer particles in the system. The diffusion and clustering of these test particles are directly linked to the mixing characteristic of a medium. The displacement of these particles provides for a quantitative estimate of the diffusion coefficient of the medium. It is shown that these test particles often organize themselves in spatially inhomogeneous pattern leading to the phenomena of clustering.

Strongly nonlinear dynamics of electrolytes in large ac voltages.

PubMed

Højgaard Olesen, Laurits; Bazant, Martin Z; Bruus, Henrik

2010-07-01

We study the response of a model microelectrochemical cell to a large ac voltage of frequency comparable to the inverse cell relaxation time. To bring out the basic physics, we consider the simplest possible model of a symmetric binary electrolyte confined between parallel-plate blocking electrodes, ignoring any transverse instability or fluid flow. We analyze the resulting one-dimensional problem by matched asymptotic expansions in the limit of thin double layers and extend previous work into the strongly nonlinear regime, which is characterized by two features--significant salt depletion in the electrolyte near the electrodes and, at very large voltage, the breakdown of the quasiequilibrium structure of the double layers. The former leads to the prediction of "ac capacitive desalination" since there is a time-averaged transfer of salt from the bulk to the double layers, via oscillating diffusion layers. The latter is associated with transient diffusion limitation, which drives the formation and collapse of space-charge layers, even in the absence of any net Faradaic current through the cell. We also predict that steric effects of finite ion sizes (going beyond dilute-solution theory) act to suppress the strongly nonlinear regime in the limit of concentrated electrolytes, ionic liquids, and molten salts. Beyond the model problem, our reduced equations for thin double layers, based on uniformly valid matched asymptotic expansions, provide a useful mathematical framework to describe additional nonlinear responses to large ac voltages, such as Faradaic reactions, electro-osmotic instabilities, and induced-charge electrokinetic phenomena.

Superdiffusion revisited in view of collisionless reconnection

NASA Astrophysics Data System (ADS)

Treumann, R. A.; Baumjohann, W.

2014-06-01

The concept of diffusion in collisionless space plasmas like those near the magnetopause and in the geomagnetic tail during reconnection is reexamined making use of the division of particle orbits into waiting orbits and break-outs into ballistic motion lying at the bottom, for instance, of Lévy flights. The rms average displacement in this case increases with time, describing superdiffusion, though faster than classical, is still a weak process, being however strong enough to support fast reconnection. Referring to two kinds of numerical particle-in-cell simulations we determine the anomalous diffusion coefficient, the anomalous collision frequency on which the diffusion process is based, and construct a relation between the diffusion coefficients and the resistive scale. The anomalous collision frequency from electron pseudo-viscosity in reconnection turns out to be of the order of the lower-hybrid frequency with the latter providing a lower limit, thus making similar assumptions physically meaningful. Tentative though not completely justified use of the κ distribution yields κ ≈ 6 in the reconnection diffusion region and, for the anomalous diffusion coefficient, the order of several times Bohm diffusivity.

A spectrophotometer-based diffusivity assay reveals that diffusion hindrance of small molecules in extracellular matrix gels used in 3D cultures is dominated by viscous effects.

PubMed

Galgoczy, Roland; Pastor, Isabel; Colom, Adai; Giménez, Alicia; Mas, Francesc; Alcaraz, Jordi

2014-08-01

The design of 3D culture studies remains challenging due to the limited understanding of extracellular matrix (ECM)-dependent hindered diffusion and the lack of simple diffusivity assays. To address these limitations, we set up a cost-effective diffusivity assay based on a Transwell plate and the spectrophotometer of a Microplate Reader, which are readily accessible to cell biology groups. The spectrophotometer-based assay was used to assess the apparent diffusivity D of FITC-dextrans with molecular weight (4-70kDa) spanning the physiological range of signaling factors in a panel of acellular ECM gels including Matrigel, fibrin and type I collagen. Despite their technical differences, D data exhibited ∼15% relative difference with respect to FRAP measurements. Our results revealed that diffusion hindrance of small particles is controlled by the enhanced viscosity of the ECM gel in conformance with the Stokes-Einstein equation rather than by geometrical factors. Moreover, we provided a strong rationale that the enhanced ECM viscosity is largely contributed to by unassembled ECM macromolecules. We also reported that gels with the lowest D exhibited diffusion hindrance closest to the large physiologic hindrance of brain tissue, which has a typical pore size much smaller than ECM gels. Conversely, sparse gels (≤1mg/ml), which are extensively used in 3D cultures, failed to reproduce the hindered diffusion of tissues, thereby supporting that dense (but not sparse) ECM gels are suitable tissue surrogates in terms of macromolecular transport. Finally, the consequences of reduced diffusivity in terms of optimizing the design of 3D culture experiments were addressed in detail. Copyright © 2014 Elsevier B.V. All rights reserved.

Quasi-linear diffusion coefficients for highly oblique whistler mode waves

NASA Astrophysics Data System (ADS)

Albert, J. M.

2017-05-01

Quasi-linear diffusion coefficients are considered for highly oblique whistler mode waves, which exhibit a singular "resonance cone" in cold plasma theory. The refractive index becomes both very large and rapidly varying as a function of wave parameters, making the diffusion coefficients difficult to calculate and to characterize. Since such waves have been repeatedly observed both outside and inside the plasmasphere, this problem has received renewed attention. Here the diffusion equations are analytically treated in the limit of large refractive index μ. It is shown that a common approximation to the refractive index allows the associated "normalization integral" to be evaluated in closed form and that this can be exploited in the numerical evaluation of the exact expression. The overall diffusion coefficient formulas for large μ are then reduced to a very simple form, and the remaining integral and sum over resonances are approximated analytically. These formulas are typically written for a modeled distribution of wave magnetic field intensity, but this may not be appropriate for highly oblique whistlers, which become quasi-electrostatic. Thus, the analysis is also presented in terms of wave electric field intensity. The final results depend strongly on the maximum μ (or μ∥) used to model the wave distribution, so realistic determination of these limiting values becomes paramount.

Monte-Carlo simulation of a stochastic differential equation

NASA Astrophysics Data System (ADS)

Arif, ULLAH; Majid, KHAN; M, KAMRAN; R, KHAN; Zhengmao, SHENG

2017-12-01

For solving higher dimensional diffusion equations with an inhomogeneous diffusion coefficient, Monte Carlo (MC) techniques are considered to be more effective than other algorithms, such as finite element method or finite difference method. The inhomogeneity of diffusion coefficient strongly limits the use of different numerical techniques. For better convergence, methods with higher orders have been kept forward to allow MC codes with large step size. The main focus of this work is to look for operators that can produce converging results for large step sizes. As a first step, our comparative analysis has been applied to a general stochastic problem. Subsequently, our formulization is applied to the problem of pitch angle scattering resulting from Coulomb collisions of charge particles in the toroidal devices.

Surface charge- and space-dependent transport of proteins in crowded environments of nanotailored posts.

PubMed

Choi, Chang Kyoung; Fowlkes, Jason D; Retterer, Scott T; Siuti, Piro; Iyer, Sukanya; Doktycz, Mitchel J

2010-06-22

The reaction and diffusion of molecules across barriers and through crowded environments is integral to biological system function and to separation technologies. Ordered, microfabricated post arrays are a promising route to creating synthetic barriers with controlled chemical and physical characteristics. They can be used to create crowded environments, to mimic aspects of cellular membranes, and to serve as engineered replacements of polymer-based separation media. Here, the translational diffusion of fluorescein isothiocyante and various forms of green fluorescent protein (GFP), including "supercharged" variants, are examined in a silicon-based post array environment. The technique of fluorescence recovery after photobleaching (FRAP) is combined with analytical approximations and numerical simulations to assess the relative effects of reaction and diffusion on molecular transport, respectively. FRAP experiments were conducted for 64 different cases where the molecular species, the density of the posts, and the chemical surface charge of the posts were varied. In all cases, the dense packing of the posts hindered the diffusive transport of the fluorescent species. The supercharged GFPs strongly interacted with oppositely charged surfaces. With similar molecular and surface charges, transport is primarily limited by hindered diffusion. For conventional, enhanced GFP in a positively charged surface environment, transport was limited by the coupled action of hindered diffusion and surface interaction with the posts. Quantification of the size-, space-, time-, and charge-dependent translational diffusion in the post array environments can provide insight into natural processes and guide the design and development of selective membrane systems.

Survey of upper band chorus and ECH waves: Implications for the diffuse aurora

NASA Astrophysics Data System (ADS)

Meredith, Nigel; Horne, Richard; Thorne, Richard; Anderson, Roger

2010-05-01

The origin of the diffuse aurora has been a source of controversy for many years. More recently the question has taken a new significance in view of the associated changes in atmospheric chemistry which may affect the middle atmosphere. Here we use CRRES data to assess the importance of upper band chorus and electron cyclotron harmonic (ECH) waves in the production of the diffuse aurora. Both wave modes increase with increasing geomagnetic activity, suggesting they are related to periods of enhanced convection and/or substorm activity. They are confined to the near-equatorial region which excludes the pre-noon sector from the wave survey. During active conditions intense ECH waves and upper band chorus, with amplitudes exceeding 1 mVm-1, are observed in the region 4 < L < 7 from 2100 to 0600 MLT approximately 20% and 6% of the time respectively. This suggests that both wave modes can put electrons on strong diffusion, but only during active conditions and not at all local times. Scattering rates fall below the strong diffusion limit at other times when the wave amplitudes are weaker. Fluxes of low energy electrons (100 eV < E < 30 keV) also increase with increasing geomagnetic activity in approximately the same region of geospace as the waves, suggesting that these electrons are responsible for the generation of the waves. The patterns of the upper band chorus, ECH waves and low energy electrons are similar to the global morphology of the diffuse aurora, suggesting that both wave modes play significant roles in the production of the diffuse aurora.

Time-dependent diffusion MRI in cancer: tissue modeling and applications

NASA Astrophysics Data System (ADS)

Reynaud, Olivier

2017-11-01

In diffusion weighted imaging (DWI), the apparent diffusion coefficient has been recognized as a useful and sensitive surrogate for cell density, paving the way for non-invasive tumor staging, and characterization of treatment efficacy in cancer. However, microstructural parameters, such as cell size, density and/or compartmental diffusivities affect diffusion in various fashions, making of conventional DWI a sensitive but non-specific probe into changes happening at cellular level. Alternatively, tissue complexity can be probed and quantified using the time dependence of diffusion metrics, sometimes also referred to as temporal diffusion spectroscopy when only using oscillating diffusion gradients. Time-dependent diffusion (TDD) is emerging as a strong candidate for specific and non-invasive tumor characterization. Despite the lack of a general analytical solution for all diffusion times / frequencies, TDD can be probed in various regimes where systems simplify in order to extract relevant information about tissue microstructure. The fundamentals of TDD are first reviewed (a) in the short time regime, disentangling structural and diffusive tissue properties, and (b) near the tortuosity limit, assuming weakly heterogeneous media near infinitely long diffusion times. Focusing on cell bodies (as opposed to neuronal tracts), a simple but realistic model for intracellular diffusion can offer precious insight on diffusion inside biological systems, at all times. Based on this approach, the main three geometrical models implemented so far (IMPULSED, POMACE, VERDICT) are reviewed. Their suitability to quantify cell size, intra- and extracellular spaces (ICS and ECS) and diffusivities are assessed. The proper modeling of tissue membrane permeability – hardly a newcomer in the field, but lacking applications - and its impact on microstructural estimates are also considered. After discussing general issues with tissue modeling and microstructural parameter estimation (i.e. fitting), potential solutions are detailed. The in vivo applications of this new, non-invasive, specific approach in cancer are reviewed, ranging from the characterization of gliomas in rodent brains and observation of time-dependence in breast tissue lesions and prostate cancer, to the recent preclinical evaluation of new treatments efficacy. It is expected that clinical applications of TDD will strongly benefit the community in terms of non-invasive cancer screening.

Cubic PdNP-based air-breathing cathodes integrated in glucose hybrid biofuel cells

NASA Astrophysics Data System (ADS)

Faggion Junior, D.; Haddad, R.; Giroud, F.; Holzinger, M.; Maduro de Campos, C. E.; Acuña, J. J. S.; Domingos, J. B.; Cosnier, S.

2016-05-01

Cubic Pd nanoparticles (PdNPs) were synthesized using ascorbic acid as a reducing agent and were evaluated for the catalytic oxygen reduction reaction. PdNPs were confined with multiwalled carbon nanotube (MWCNT) dispersions to form black suspensions and these inks were dropcast onto glassy carbon electrodes. Different nanoparticle sizes were synthesized and investigated upon oxygen reduction capacities (onset potential and electrocatalytic current densities) under O2 saturated conditions at varying pH values. Strong evidence of O2 diffusion limitation was demonstrated. In order to overcome oxygen concentration and diffusion limitations in solution, we used a gas diffusion layer to create a PdNP-based air-breathing cathode, which delivered -1.5 mA cm-2 at 0.0 V with an onset potential of 0.4 V. This air-breathing cathode was combined with a specially designed phenanthrolinequinone/glucose dehydrogenase-based anode to form a complete glucose/O2 hybrid bio-fuel cell providing an open circuit voltage of 0.554 V and delivering a maximal power output of 184 +/- 21 μW cm-2 at 0.19 V and pH 7.0.Cubic Pd nanoparticles (PdNPs) were synthesized using ascorbic acid as a reducing agent and were evaluated for the catalytic oxygen reduction reaction. PdNPs were confined with multiwalled carbon nanotube (MWCNT) dispersions to form black suspensions and these inks were dropcast onto glassy carbon electrodes. Different nanoparticle sizes were synthesized and investigated upon oxygen reduction capacities (onset potential and electrocatalytic current densities) under O2 saturated conditions at varying pH values. Strong evidence of O2 diffusion limitation was demonstrated. In order to overcome oxygen concentration and diffusion limitations in solution, we used a gas diffusion layer to create a PdNP-based air-breathing cathode, which delivered -1.5 mA cm-2 at 0.0 V with an onset potential of 0.4 V. This air-breathing cathode was combined with a specially designed phenanthrolinequinone/glucose dehydrogenase-based anode to form a complete glucose/O2 hybrid bio-fuel cell providing an open circuit voltage of 0.554 V and delivering a maximal power output of 184 +/- 21 μW cm-2 at 0.19 V and pH 7.0. Electronic supplementary information (ESI) available: Physical characterization, Fig. S1-S4 electrochemical experiments Fig. S5-S11. See DOI: 10.1039/c6nr01245k

Brownian motion of electrons in time-dependent magnetic fields.

NASA Technical Reports Server (NTRS)

Iverson, G. J.; Williams, R. M.

1973-01-01

The behavior of a weakly ionized plasma in slowly varying time-dependent magnetic fields is studied through an extension of Williamson's stochastic theory. In particular, attention is focused on the properties of electron diffusion in the plane perpendicular to the direction of the magnetic field, when the field strength is large. It is shown that, in the strong field limit, the classical 1/B-squared dependence of the perpendicular diffusion coefficient is obtained for two models in which the field B(t) is monotonic in t and for two models in which B(t) possesses at least one turning point.

Quite a lot of smoke but very limited fire--the use of methamphetamine in Europe.

PubMed

Griffiths, Paul; Mravcik, Viktor; Lopez, Dominique; Klempova, Danica

2008-05-01

This paper provides an overview of the historical development, current situation and potential future diffusion of methamphetamine (MA) use in Europe. The analysis is based on a review of published and grey literature, as well as data collected as part of the ongoing monitoring of the drug situation in Europe. Some qualitative surveys among high-risk populations do exist, but overall the general low prevalence of methamphetamine use in most of Europe means that the data available to explore patterns of use are limited. In many parts of Europe, amphetamine use is well established and the injecting of amphetamines has historically constituted an important component of the drug problem in many Nordic countries. Methamphetamine problems are long documented in the Czech and Slovak republics, but there is no current evidence of widespread use of MA elsewhere in Europe. Concern that MA use is spreading in Europe is prompted by some reports of use among high-risk groups. However, the evidence available suggests that even in high-risk populations, the use of MA currently remains uncommon. Europe accounted for less than 1% of worldwide MA seizures in 2005, and over the period 2004-05 European ephedrine seizures amounted for 6% of the global figure. The spread of MA use is limited and no strong evidence exists that significant diffusion is occurring. It appears likely that methamphetamine diffusion in Europe is impeded by a strong market for other stimulant drugs [cocaine, amphetamine and methylenedioxymethamphetamine (MDMA)]. The future potential for the diffusion of MA may be influenced by factors such as: the relative availability and popularity of other drugs; possible 'leakage' from areas of historical high prevalence; travel by young Europeans to areas of high prevalence; and how users perceive MA as a desirable, suitable and cost-effective alternative to other stimulants available on the European illicit drug market.

Mass transfer resistance in ASFF reactors for waste water treatment.

PubMed

Ettouney, H M; Al-Haddad, A A; Abu-Irhayem, T M

1996-01-01

Analysis of mass transfer resistances was performed for an aerated submerged fixed-film reactor (ASFF) for the treatment of waste water containing a mixture of sucrose and ammonia. Both external and internal mass transfer resistances were considered in the analysis, and characterized as a function of feed flow-rate and concentration. Results show that, over a certain operating regime, external mass transfer resistance in the system was greater for sucrose removal than ammonia. This is because the reaction rates for carbon removal were much larger than those of nitrogen. As a result, existence of any form of mass transfer resistance caused by inadequate mixing or diffusion limitations, strongly affects the overall removal rates of carbon more than nitrogen. Effects of the internal måss transfer resistance were virtually non-existent for ammonia removal. This behaviour was found over two orders of magnitude range for the effective diffusivity for ammonia, and one order of magnitude for the film specific surface area. However, over the same parameters' range, it is found that sucrose removal was strongly affected upon lowering its effective diffusivity and increasing the film specific surface area.

Lean limit phenomena

NASA Technical Reports Server (NTRS)

Law, C. K.

1984-01-01

The concept of flammability limits in the presence of flame interaction, and the existence of negative flame speeds are discussed. Downstream interaction between two counterflow premixed flames of different stoichiometries are experimentally studied. Various flame configurations are observed and quantified; these include the binary system of two lean or rich flames, the triplet system of a lean and a rich flame separated by a diffusion flame, and single diffusion flames with some degree of premixedness. Extinction limits are determined for methane/air and butane/air mixtures over the entire range of mixture concentrations. The results show that the extent of flame interaction depends on the separation distance between the flames which are functions of the mixtures' concentrations, the stretch rate, and the effective Lewis numbers (Le). In particular, in a positively-stretched flow field Le 1 ( 1) mixtures tend to interact strongly (weakly), while the converse holds for flames in a negatively-stretched flow. Also established was the existence of negative flames whose propagation velocity is in the same general direction as that of the bulk convective flow, being supported by diffusion alone. Their existence demonstrates the tendency of flames to resist extinction, and further emphasizes the possibility of very lean or rich mixtures to undergo combustion.

Monte Carlo simulations of particle acceleration at oblique shocks: Including cross-field diffusion

NASA Technical Reports Server (NTRS)

Baring, M. G.; Ellison, D. C.; Jones, F. C.

1995-01-01

The Monte Carlo technique of simulating diffusive particle acceleration at shocks has made spectral predictions that compare extremely well with particle distributions observed at the quasi-parallel region of the earth's bow shock. The current extension of this work to compare simulation predictions with particle spectra at oblique interplanetary shocks has required the inclusion of significant cross-field diffusion (strong scattering) in the simulation technique, since oblique shocks are intrinsically inefficient in the limit of weak scattering. In this paper, we present results from the method we have developed for the inclusion of cross-field diffusion in our simulations, namely model predictions of particle spectra downstream of oblique subluminal shocks. While the high-energy spectral index is independent of the shock obliquity and the strength of the scattering, the latter is observed to profoundly influence the efficiency of injection of cosmic rays into the acceleration process.

Simulations of Plasmasheet Electrons in a Model Magnetosphere with AMIE Potentials: Implications for Diffuse Aurora

NASA Astrophysics Data System (ADS)

Chen, M. W.; Schulz, M.; Lu, G.

2001-12-01

We obtain distributions of precipitating electrons by tracing drift shells of plasmasheet electrons in the limit of strong pitch angle diffusion in Dungey's model magnetosphere, which consists of a dipolar magnetic field plus a uniform southward field. Under strong pitch-angle diffusion particles drift so as to conserve an adiabatic invariant Λ equal to the enclosed phase-space volume (i.e., the cube of the particle momentum p times the occupied flux-tube volume per unit magnetic flux). In the past we applied a quiescent Stern-Volland electric-field model with a cross-tail potential drop of 25 kV and added to it a storm-associated Brice-Nishida cross-magnetospheric electric field with impulses to represent substorm effects. For the present study we use the more realistic Assimilative Model of Ionospheric Electrodynamics (AMIE). We use an analytical expansion to express the AMIE ionospheric potential as a function of latitude and magnetic local time. We map this AMIE potential to latitudes >= 50^o to magnetospheric field lines with (L \\ge 2.5) in Dungey's magnetic field model. We trace the bounce-averaged drift motion of representative plasmasheet electrons for values of \\Lambda corresponding to energies of 0.25-64 keV on field lines of equatorial radial distance r = 6 R_E (L = 5.7), which maps to \\approx 65^o$ latitude in the ionosphere. We use the simulation results to map stormtime phase space distributions taking into account loss due to precipitation. We consider 2 models of electron scattering: (1) the limit of strong scattering everywhere, and (2) an MLT-dependent scattering that is less than everywhere strong in the plasma sheet. From the phase space distributions we calculate the total precipitating electron energy flux into the ionosphere. For this study we focus on the October 19, 1998, storm. We compare qualitatively the simulated energy flux with X-ray intensity from Polar/PIXIE images during this storm.

Infinite densities for Lévy walks.

PubMed

Rebenshtok, A; Denisov, S; Hänggi, P; Barkai, E

2014-12-01

Motion of particles in many systems exhibits a mixture between periods of random diffusive-like events and ballistic-like motion. In many cases, such systems exhibit strong anomalous diffusion, where low-order moments 〈|x(t)|(q)〉 with q below a critical value q(c) exhibit diffusive scaling while for q>q(c) a ballistic scaling emerges. The mixed dynamics constitutes a theoretical challenge since it does not fall into a unique category of motion, e.g., the known diffusion equations and central limit theorems fail to describe both aspects. In this paper we resolve this problem by resorting to the concept of infinite density. Using the widely applicable Lévy walk model, we find a general expression for the corresponding non-normalized density which is fully determined by the particles velocity distribution, the anomalous diffusion exponent α, and the diffusion coefficient K(α). We explain how infinite densities play a central role in the description of dynamics of a large class of physical processes and discuss how they can be evaluated from experimental or numerical data.

Simulation of whistler waves excited in the presence of a cold plasma cloud - Implications for the CRRES mission. [Combined Release and Radiation Effects Satellite

NASA Technical Reports Server (NTRS)

Pritchett, P. L.; Schriver, D.; Ashour-Abdalla, M.

1991-01-01

A one-dimensional electromagnetic particle simulation model is constructed to study the excitation of whistler waves in the presence of a cold plasma cloud for conditions representative of those after the release of lithium in the inner plasma sheet during the Combined Release and Radiation Effect Satellite mission. The results indicate that a standing-wave pattern with discrete wave frequencies is formed within the cloud. The magnetic wave amplitude inside the cloud, which is limited by quasi-linear diffusion, is of the order of several nanoteslas. Assuming a magnetospheric loss cone of 5 deg, the observed pitch angle diffusion produced by the whistler waves is sufficient to put the electrons on strong diffusion.

Influences of Exciton Diffusion and Exciton-Exciton Annihilation on Photon Emission Statistics of Carbon Nanotubes.

PubMed

Ma, Xuedan; Roslyak, Oleskiy; Duque, Juan G; Pang, Xiaoying; Doorn, Stephen K; Piryatinski, Andrei; Dunlap, David H; Htoon, Han

2015-07-03

Pump-dependent photoluminescence imaging and second-order photon correlation studies have been performed on individual single-walled carbon nanotubes (SWCNTs) at room temperature. These studies enable the extraction of both the exciton diffusion constant and the Auger recombination coefficient. A linear correlation between these parameters is attributed to the effect of environmental disorder in setting the exciton mean free path and capture-limited Auger recombination at this length scale. A suppression of photon antibunching is attributed to the creation of multiple spatially nonoverlapping excitons in SWCNTs, whose diffusion length is shorter than the laser spot size. We conclude that complete antibunching at room temperature requires an enhancement of the exciton-exciton annihilation rate that may become realizable in SWCNTs allowing for strong exciton localization.

Density Functional Theory Investigation of Proton Diffusion in Tungsten Oxide And Its Hydrates

NASA Astrophysics Data System (ADS)

Lin, Hao

Fast proton conduction mechanism is of key importance for achieving high performance in fuel cell membranes, batteries, supercapacitors, and electrochromic materials. Enhanced proton diffusion is often observed in hydrated materials where it is thought to occur via the famous Grotthuss mechanism through pathways formed by structural water. Using first-principles calculations, we demonstrate that proton diffusion in tungsten oxide dihydrate (WO3·2H 2O), a known good proton conductor, takes place within the layers of corner-sharing WO6 octahedra without direct involvement of structural water. The calculated proton migration barrier in WO3·2H 2O is in good agreement with the experimental value inferred from the temperature dependence of conductivity. The preferred proton diffusion path in WO3·2H2O is essentially the same as in gamma-WO 3. In contrast to the small intercalation voltages calculated for WO 3 and WO3·2H2O, we find that proton absorption in the monohydrate WO3·H2O is energetically highly favorable. However, strong proton-proton repulsion limits the equilibrium H content at zero voltage. We find a fast one-dimensional diffusion channel in WO3·H2O at dilute proton concentrations, but much higher barriers are expected at near-equilibrium concentrations due to strong repulsive interactions with other protons. Our results illustrate that low proton diffusion barriers and low insertion voltages both contribute to fast proton transport in bulk WO3·2H2O and gamma-WO 3.

Radiation-enhanced self- and boron diffusion in germanium

NASA Astrophysics Data System (ADS)

Schneider, S.; Bracht, H.; Klug, J. N.; Hansen, J. Lundsgaard; Larsen, A. Nylandsted; Bougeard, D.; Haller, E. E.

2013-03-01

We report experiments on proton radiation-enhanced self- and boron (B) diffusion in germanium (Ge) for temperatures between 515 ∘C and 720 ∘C. Modeling of the experimental diffusion profiles measured by means of secondary ion mass spectrometry is achieved on the basis of the Frenkel pair reaction and the interstitialcy and dissociative diffusion mechanisms. The numerical simulations ascertain concentrations of Ge interstitials and B-interstitial pairs that deviate by several orders of magnitude from their thermal equilibrium values. The dominance of self-interstitial related defects under irradiation leads to an enhanced self- and B diffusion in Ge. Analysis of the experimental profiles yields data for the diffusion of self-interstitials (I) and the thermal equilibrium concentration of BI pairs in Ge. The temperature dependence of these quantities provides the migration enthalpy of I and formation enthalpy of BI that are compared with recent results of atomistic calculations. The behavior of self- and B diffusion in Ge under concurrent annealing and irradiation is strongly affected by the property of the Ge surface to hinder the annihilation of self-interstitials. The limited annihilation efficiency of the Ge surface can be caused by donor-type surface states favored under vacuum annealing, but the physical origin remains unsolved.

Electro-convective versus electroosmotic instability in concentration polarization.

PubMed

Rubinstein, Isaak; Zaltzman, Boris

2007-10-31

Electro-convection is reviewed as a mechanism of mixing in the diffusion layer of a strong electrolyte adjacent to a charge-selective solid, such as an ion exchange (electrodialysis) membrane or an electrode. Two types of electro-convection in strong electrolytes may be distinguished: bulk electro-convection, due to the action of the electric field upon the residual space charge of a quasi-electro-neutral bulk solution, and convection induced by electroosmotic slip, due to electric forces acting in the thin electric double layer of either quasi-equilibrium or non-equilibrium type near the solid/liquid interface. According to recent studies, the latter appears to be the likely source of mixing in the diffusion layer, leading to 'over-limiting' conductance in electrodialysis. Electro-convection near a planar uniform charge selective solid/liquid interface sets on as a result of hydrodynamic instability of one-dimensional steady state electric conduction through such an interface. We compare the results of linear stability analysis obtained for instabilities of this kind appearing in the full electro-convective and limiting non-equilibrium electroosmotic formulations. The short- and long-wave aspects of these instabilities are discussed along with the wave number selection principles.

Inhomogeneous diffusion and ergodicity breaking induced by global memory effects

NASA Astrophysics Data System (ADS)

Budini, Adrián A.

2016-11-01

We introduce a class of discrete random-walk model driven by global memory effects. At any time, the right-left transitions depend on the whole previous history of the walker, being defined by an urnlike memory mechanism. The characteristic function is calculated in an exact way, which allows us to demonstrate that the ensemble of realizations is ballistic. Asymptotically, each realization is equivalent to that of a biased Markovian diffusion process with transition rates that strongly differs from one trajectory to another. Using this "inhomogeneous diffusion" feature, the ergodic properties of the dynamics are analytically studied through the time-averaged moments. Even in the long-time regime, they remain random objects. While their average over realizations recovers the corresponding ensemble averages, departure between time and ensemble averages is explicitly shown through their probability densities. For the density of the second time-averaged moment, an ergodic limit and the limit of infinite lag times do not commutate. All these effects are induced by the memory effects. A generalized Einstein fluctuation-dissipation relation is also obtained for the time-averaged moments.

The limitation of the proposed collection efficiency for fiber probes on the visible and near-infrared diffuse spectroscopy

NASA Astrophysics Data System (ADS)

Zhang, Linna; Ding, Hongyan; Lin, Ling; Wang, Yimin; Guo, Xin

2017-12-01

A fiber is usually used as a probe in visible and near-infrared diffuse spectra measurement. However, the use of different fiber probes in the same measurement may cause data mismatch problems. Our group has researched the influence of the parameters of fiber probe, including the aperture angle, on the diffuse spectrum by a modified Monte Carlo model. To eliminate the influence of the aperture angle, we proposed a fitted equation of correction coefficient to correct its difference in practical range. However, we did not discuss the limitation of this method. In this work, we explored the collection efficiency in different optical environment with Monte Carlo simulation method, and find the suitable conditions-weak absorbing and strong scattering media, for the proposed collection efficiency. Furthermore, we tried to explain the stability of the collection efficiency in this condition. This work gives suitable conditions for the collection efficiency. The use of collection efficiency can help reduce the influence of different measurement systems and is also helpful to the model translation.

Enhanced reaction kinetics and reactive mixing scale dynamics in mixing fronts under shear flow for arbitrary Damköhler numbers

NASA Astrophysics Data System (ADS)

Bandopadhyay, Aditya; Le Borgne, Tanguy; Méheust, Yves; Dentz, Marco

2017-02-01

Mixing fronts, where fluids of different chemical compositions mix with each other, are known to represent hotspots of chemical reaction in hydrological systems. These fronts are typically subjected to velocity gradients, ranging from the pore scale due to no slip boundary conditions at fluid solid interfaces, to the catchment scale due to permeability variations and complex geometry of the Darcy velocity streamlines. A common trait of these processes is that the mixing interface is strained by shear. Depending on the Péclet number Pe , which represents the ratio of the characteristic diffusion time to the characteristic shear time, and the Damköhler number Da , which represents the ratio of the characteristic diffusion time to the characteristic reaction time, the local reaction rates can be strongly impacted by the dynamics of the mixing interface. So far, this impact has been characterized mostly either in kinetics-limited or in mixing-limited conditions, that is, for either low or high Da. Here the coupling of shear flow and chemical reactivity is investigated for arbitrary Damköhler numbers, for a bimolecular reaction and an initial interface with separated reactants. Approximate analytical expressions for the global production rate and reactive mixing scale are derived based on a reactive lamella approach that allows for a general coupling between stretching enhanced mixing and chemical reactions. While for Pe < Da , reaction kinetics and stretching effects are decoupled, a scenario which we name "weak stretching", for Pe > Da , we uncover a "strong stretching" scenario where new scaling laws emerge from the interplay between reaction kinetics, diffusion, and stretching. The analytical results are validated against numerical simulations. These findings shed light on the effect of flow heterogeneity on the enhancement of chemical reaction and the creation of spatially localized hotspots of reactivity for a broad range of systems ranging from kinetic limited to mixing limited situations.

Local renormalization group functions from quantum renormalization group and holographic bulk locality

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

Nakayama, Yu

Here, the bulk locality in the constructive holographic renormalization group requires miraculous cancellations among various local renormalization group functions. The cancellation is not only from the properties of the spectrum but from more detailed aspects of operator product expansions in relation to conformal anomaly. It is remarkable that one-loop computation of the universal local renormalization group functions in the weakly coupled limit of the N = 4 super Yang-Mills theory fulfils the necessary condition for the cancellation in the strongly coupled limit in its SL(2, Z) duality invariant form. From the consistency between the quantum renormalization group and the holographicmore » renormalization group, we determine some unexplored local renormalization group functions (e.g. diffusive term in the beta function for the gauge coupling constant) in the strongly coupled limit of the planar N = 4 super Yang-Mills theory.« less

Strong Surface Diffusion Mediated Glancing-Angle Deposition: Growth, Recrystallization and Reorientation of Tin Nanorods

NASA Astrophysics Data System (ADS)

Wang, Huan-Hua; Shi, Yi-Jian; William, Chu; Yigal, Blum

2008-01-01

Different from usual glancing-angle deposition where low surface diffusion is necessary to form nanorods, strong surface diffusion mediated glancing-angle deposition is exemplified by growing tin nanorod films on both silicon and glass substrates simultaneously via thermal evaporation. During growth, the nanorods were simultaneously baked by the high-temperature evaporator, and therefore re-crystallized into single crystals in consequence of strong surface diffusion. The monocrystalline tin nanorods have a preferred orientation perpendicular to the substrate surface, which is quite different from the usual uniformly oblique nanorods without recrystallization.

The improvement of the method of equivalent cross section in HTR

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

Guo, J.; Li, F.

The Method of Equivalence Cross-Sections (MECS) is a combined transport-diffusion method. By appropriately adjusting the diffusion coefficient of homogenized absorber region, the diffusion theory could yield satisfactory results for the full core model with strong neutron absorber material, for example the control rod in High temperature gas cooled reactor (HTR). Original implementation of MECS based on 1-D cell transport model has some limitation on accuracy and applicability, a new implementation of MECS based on 2-D transport model are proposed and tested in this paper. This improvement can extend the MECS to the calculation of twin small absorber ball system whichmore » have a non-circular boring in graphite reflector and different radial position. A least-square algorithm for the calculation of equivalent diffusion coefficient is adopted, and special treatment for diffusion coefficient for higher energy group is proposed in the case that absorber is absent. Numerical results to adopt MECS into control rod calculation in HTR are encouraging. However, there are some problems left. (authors)« less

Acoustic instability driven by cosmic-ray streaming

NASA Technical Reports Server (NTRS)

Begelman, Mitchell C.; Zweibel, Ellen G.

1994-01-01

We study the linear stability of compressional waves in a medium through which cosmic rays stream at the Alfven speed due to strong coupling with Alfven waves. Acoustic waves can be driven unstable by the cosmic-ray drift, provided that the streaming speed is sufficiently large compared to the thermal sound speed. Two effects can cause instability: (1) the heating of the thermal gas due to the damping of Alfven waves driven unstable by cosmic-ray streaming; and (2) phase shifts in the cosmic-ray pressure perturbation caused by the combination of cosmic-ray streaming and diffusion. The instability does not depend on the magnitude of the background cosmic-ray pressure gradient, and occurs whether or not cosmic-ray diffusion is important relative to streaming. When the cosmic-ray pressure is small compared to the gas pressure, or cosmic-ray diffusion is strong, the instability manifests itself as a weak overstability of slow magnetosonic waves. Larger cosmic-ray pressure gives rise to new hybrid modes, which can be strongly unstable in the limits of both weak and strong cosmic-ray diffusion and in the presence of thermal conduction. Parts of our analysis parallel earlier work by McKenzie & Webb (which were brought to our attention after this paper was accepted for publication), but our treatment of diffusive effects, thermal conduction, and nonlinearities represent significant extensions. Although the linear growth rate of instability is independent of the background cosmic-ray pressure gradient, the onset of nonlinear eff ects does depend on absolute value of DEL (vector differential operator) P(sub c). At the onset of nonlinearity the fractional amplitude of cosmic-ray pressure perturbations is delta P(sub C)/P(sub C) approximately (kL) (exp -1) much less than 1, where k is the wavenumber and L is the pressure scale height of the unperturbed cosmic rays. We speculate that the instability may lead to a mode of cosmic-ray transport in which plateaus of uniform cosmic-ray pressure are separated by either laminar or turbulent jumps in which the thermal gas is subject to intense heating.

Imaging fluorescence-correlation spectroscopy for measuring fast surface diffusion at liquid/solid interfaces.

PubMed

Cooper, Justin T; Harris, Joel M

2014-08-05

The development of techniques to probe interfacial molecular transport is important for understanding and optimizing surface-based analytical methods including surface-enhanced spectroscopies, biological assays, and chemical separations. Single-molecule-fluorescence imaging and tracking has been used to measure lateral diffusion rates of fluorescent molecules at surfaces, but the technique is limited to the study of slower diffusion, where molecules must remain relatively stationary during acquisition of an image in order to build up sufficient intensity in a spot to detect and localize the molecule. Although faster time resolution can be achieved by fluorescence-correlation spectroscopy (FCS), where intensity fluctuations in a small spot are related to the motions of molecules on the surface, long-lived adsorption events arising from surface inhomogeneity can overwhelm the correlation measurement and mask the surface diffusion of the moving population. Here, we exploit a combination of these two techniques, imaging-FCS, for measurement of fast interfacial transport at a model chromatographic surface. This is accomplished by rapid imaging of the surface using an electron-multiplied-charged-coupled-device (CCD) camera, while limiting the acquisition to a small area on the camera to allow fast framing rates. The total intensity from the sampled region is autocorrelated to determine surface diffusion rates of molecules with millisecond time resolution. The technique allows electronic control over the acquisition region, which can be used to avoid strong adsorption sites and thus minimize their contribution to the measured autocorrelation decay and to vary the acquisition area to resolve surface diffusion from adsorption and desorption kinetics. As proof of concept, imaging-FCS was used to measure surface diffusion rates, interfacial populations, and adsorption-desorption rates of 1,1'-dioctadecyl-3,3,3'3'-tetramethylindocarbocyanine (DiI) on planar C18- and C1-modified surfaces.

Optical second-harmonic diffraction study of anisotropic surface diffusion: CO on Ni(110)

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

Xiao, X.; Zhu, X.D.; Daum, W.

We describe in detail a technique using optical second-harmonic (SH) diffraction from a one-dimensional laser-induced monolayer grating to probe surface diffusion of adsorbates and its anisotropy on a solid surface. The case of CO on Ni(110) is used as a demonstration. The two orthogonal and independent diffusion tensor components along (1{bar 1}0) and (001) are measured, exhibiting a strong anisotropy in both the activation energy {ital E}{sub diff} and the preexponential factor {ital D}{sub 0} in the diffusion coefficients. A compensation effect between {ital E}{sub diff} and {ital D}{sub 0} is observed. In comparison with CO/Ni(111) and CO/Ni(100), our resultmore » suggests that the Ni(110) surface seen by CO is much smoother than Ni(111) and Ni(100). Both advantages and limitations of the present technique are mentioned and possible complications in the data analysis are discussed.« less

CdS quantum dots in a novel glass with a very low activation energy and its variation of diffusivity with temperature

NASA Astrophysics Data System (ADS)

Nagpal, Swati

2011-07-01

CdS quantum dots of different average sizes in the range 2 to 3.8 nm were grown by diffusion-limited growth process in indigenously made silicate glass. The absorption spectra showed a strong quantum confinement effect with a blue shift of the order of 500 meV depending on the average size. Critical radius of quantum dots was found to be 1.8 nm. The size dispersion decreased from 15.2 to 12.5% with a 20% increase in the particle size. The activation energy for diffusion was found to be very low i.e. 193 kJ mol-1 and the diffusion coefficient increased by 60% for 10 K rise in temperature. The PL emission spectra showed the presence of only deep traps around 600 nm with a red shift of 200 nm. No shallow traps or band edge emission was observed. The PL peak position changed from 560 to 640 nm with a 35 K increase in annealing temperature.

The effects of surface chemistry of mesoporous silica materials and solution pH on kinetics of molsidomine adsorption

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

Dolinina, E.S.; Parfenyuk, E.V., E-mail: terrakott37@mail.ru

2014-01-15

Adsorption kinetics of molsidomine on mesoporous silica material (UMS), the phenyl- (PhMS) and mercaptopropyl-functionalized (MMS) derivatives from solution with different pH and 298 K was studied. The adsorption kinetics was found to follow the pseudo-second-order kinetic model for all studied silica materials and pH. Effects of surface functional groups and pH on adsorption efficiency and kinetic adsorption parameters were investigated. At all studied pH, the highest molsidomine amount is adsorbed on PhMS due to π–π interactions and hydrogen bonding between surface groups of PhMS and molsidomine molecules. An increase of pH results in a decrease of the amounts of adsorbedmore » molsidomine onto the silica materials. Furthermore, the highest adsorption rate kinetically evaluated using a pseudo-second-order model, is observed onto UMS and it strongly depends on pH. The mechanism of the adsorption process was determined from the intraparticle diffusion and Boyd kinetic film–diffusion models. The results showed that the molsidomine adsorption on the silica materials is controlled by film diffusion. Effect of pH on the diffusion parameters is discussed. - Graphical abstract: The kinetic study showed that the k{sub 2} value, the rate constant of pseudo-second order kinetic model, is the highest for molsidomine adsorption on UMS and strongly depends on pH because it is determined by availability and accessibility of the reaction sites of the adsorbents molsidomine binding. Display Omitted - Highlights: • The adsorption capacities of UMS, PhMS and MMS were dependent on the pH. • At all studied pH, the highest molsidomine amount is adsorbed on PhMS. • The highest adsorption rate, k{sub 2}, is observed onto UMS and strongly depends on pH. • Film diffusion was the likely rate-limiting step in the adsorption process.« less

Confinement regulates complex biochemical networks: initiation of blood clotting by "diffusion acting".

PubMed

Shen, Feng; Pompano, Rebecca R; Kastrup, Christian J; Ismagilov, Rustem F

2009-10-21

This study shows that environmental confinement strongly affects the activation of nonlinear reaction networks, such as blood coagulation (clotting), by small quantities of activators. Blood coagulation is sensitive to the local concentration of soluble activators, initiating only when the activators surpass a threshold concentration, and therefore is regulated by mass transport phenomena such as flow and diffusion. Here, diffusion was limited by decreasing the size of microfluidic chambers, and it was found that microparticles carrying either the classical stimulus, tissue factor, or a bacterial stimulus, Bacillus cereus, initiated coagulation of human platelet-poor plasma only when confined. A simple analytical argument and numerical model were used to describe the mechanism for this phenomenon: confinement causes diffusible activators to accumulate locally and surpass the threshold concentration. To interpret the results, a dimensionless confinement number, Cn, was used to describe whether a stimulus was confined, and a Damköhler number, Da(2), was used to describe whether a subthreshold stimulus could initiate coagulation. In the context of initiation of coagulation by bacteria, this mechanism can be thought of as "diffusion acting", which is distinct from "diffusion sensing". The ability of confinement and diffusion acting to change the outcome of coagulation suggests that confinement should also regulate other biological "on" and "off" processes that are controlled by thresholds.

Mechanisms limiting the performance of large grain polycrystalline silicon solar cells

NASA Technical Reports Server (NTRS)

Culik, J. S.; Alexander, P.; Dumas, K. A.; Wohlgemuth, J. W.

1984-01-01

The open-circuit voltage and short-circuit current of large-grain (1 to 10 mm grain diameter) polycrystalline silicon solar cells is determined by the minority-carrier diffusion length within the bulk of the grains. This was demonstrated by irradiating polycrystalline and single-crystal (Czochralski) silicon solar cells with 1 MeV electrons to reduce their bulk lifetime. The variation of short-circuit current with minority-carrier diffusion length for the polycrystalline solar cells is identical to that of the single-crystal solar cells. The open-circuit voltage versus short-circuit current characteristic of the polycrystalline solar cells for reduced diffusion lengths is also identical to that of the single-crystal solar cells. The open-circuit voltage of the polycrystalline solar cells is a strong function of quasi-neutral (bulk) recombination, and is reduced only slightly, if at all, by grain-boundary recombination.

Kinetic theory of nonlinear diffusion in a weakly disordered nonlinear Schrödinger chain in the regime of homogeneous chaos.

PubMed

Basko, D M

2014-02-01

We study the discrete nonlinear Schröinger equation with weak disorder, focusing on the regime when the nonlinearity is, on the one hand, weak enough for the normal modes of the linear problem to remain well resolved but, on the other, strong enough for the dynamics of the normal mode amplitudes to be chaotic for almost all modes. We show that in this regime and in the limit of high temperature, the macroscopic density ρ satisfies the nonlinear diffusion equation with a density-dependent diffusion coefficient, D(ρ) = D(0)ρ(2). An explicit expression for D(0) is obtained in terms of the eigenfunctions and eigenvalues of the linear problem, which is then evaluated numerically. The role of the second conserved quantity (energy) in the transport is also quantitatively discussed.

Accurate reaction-diffusion operator splitting on tetrahedral meshes for parallel stochastic molecular simulations

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

Hepburn, I.; De Schutter, E., E-mail: erik@oist.jp; Theoretical Neurobiology & Neuroengineering, University of Antwerp, Antwerp 2610

Spatial stochastic molecular simulations in biology are limited by the intense computation required to track molecules in space either in a discrete time or discrete space framework, which has led to the development of parallel methods that can take advantage of the power of modern supercomputers in recent years. We systematically test suggested components of stochastic reaction-diffusion operator splitting in the literature and discuss their effects on accuracy. We introduce an operator splitting implementation for irregular meshes that enhances accuracy with minimal performance cost. We test a range of models in small-scale MPI simulations from simple diffusion models to realisticmore » biological models and find that multi-dimensional geometry partitioning is an important consideration for optimum performance. We demonstrate performance gains of 1-3 orders of magnitude in the parallel implementation, with peak performance strongly dependent on model specification.« less

Joint estimation of phase and phase diffusion for quantum metrology.

PubMed

Vidrighin, Mihai D; Donati, Gaia; Genoni, Marco G; Jin, Xian-Min; Kolthammer, W Steven; Kim, M S; Datta, Animesh; Barbieri, Marco; Walmsley, Ian A

2014-04-14

Phase estimation, at the heart of many quantum metrology and communication schemes, can be strongly affected by noise, whose amplitude may not be known, or might be subject to drift. Here we investigate the joint estimation of a phase shift and the amplitude of phase diffusion at the quantum limit. For several relevant instances, this multiparameter estimation problem can be effectively reshaped as a two-dimensional Hilbert space model, encompassing the description of an interferometer phase probed with relevant quantum states--split single-photons, coherent states or N00N states. For these cases, we obtain a trade-off bound on the statistical variances for the joint estimation of phase and phase diffusion, as well as optimum measurement schemes. We use this bound to quantify the effectiveness of an actual experimental set-up for joint parameter estimation for polarimetry. We conclude by discussing the form of the trade-off relations for more general states and measurements.

On the Grain-modified Magnetic Diffusivities in Protoplanetary Disks

NASA Astrophysics Data System (ADS)

Xu, Rui; Bai, Xue-Ning

2016-03-01

Weakly ionized protoplanetary disks (PPDs) are subject to nonideal magnetohydrodynamic (MHD) effects, including ohmic resistivity, the Hall effect, and ambipolar diffusion (AD), and the resulting magnetic diffusivities ({η }{{O}},{η }{{H}}, and {η }{{A}}) largely control the disk gas dynamics. The presence of grains not only strongly reduces the disk ionization fraction, but also modifies the scalings of {η }{{H}} and {η }{{A}} with magnetic field strength. We analytically derive asymptotic expressions of {η }{{H}} and {η }{{A}} in both the strong and weak field limits and show that toward a strong field, {η }{{H}} can change sign (at a threshold field strength {B}{{th}}), mimicking a flip of field polarity, and AD is substantially reduced. Applied to PPDs, we find that when small ˜0.1 (0.01)μm grains are sufficiently abundant (mass ratio ˜0.01 (10-4)), {η }{{H}} can change sign up to ˜2-3 scale heights above the midplane at a modest field strength (plasma β ˜ 100) over a wide range of disk radii. The reduction of AD is also substantial toward the AD-dominated outer disk and may activate the magnetorotational instability. We further perform local nonideal MHD simulations of the inner disk (within 10 au) and show that, with sufficiently abundant small grains, the magnetic field amplification due to the Hall-shear instability saturates at a very low level near the threshold field strength {B}{{th}}. Together with previous studies, we conclude by discussing the grain-abundance-dependent phenomenology of PPD gas dynamics.

Asymmetric photon transport in organic semiconductor nanowires through electrically controlled exciton diffusion

PubMed Central

Cui, Qiu Hong; Peng, Qian; Luo, Yi; Jiang, Yuqian; Yan, Yongli; Wei, Cong; Shuai, Zhigang; Sun, Cheng; Yao, Jiannian; Zhao, Yong Sheng

2018-01-01

The ability to steer the flow of light toward desired propagation directions is critically important for the realization of key functionalities in optical communication and information processing. Although various schemes have been proposed for this purpose, the lack of capability to incorporate an external electric field to effectively tune the light propagation has severely limited the on-chip integration of photonics and electronics. Because of the noninteractive nature of photons, it is only possible to electrically control the flow of light by modifying the refractive index of materials through the electro-optic effect. However, the weak optical effects need to be strongly amplified for practical applications in high-density photonic integrations. We show a new strategy that takes advantage of the strong exciton-photon coupling in active waveguides to effectively manipulate photon transport by controlling the interaction between excitons and the external electric field. Single-crystal organic semiconductor nanowires were used to generate highly stable Frenkel exciton polaritons with strong binding and diffusion abilities. By making use of directional exciton diffusion in an external electric field, we have realized an electrically driven asymmetric photon transport and thus directional light propagation in a single nanowire. With this new concept, we constructed a dual-output single wire–based device to build an electrically controlled single-pole double-throw optical switch with fast temporal response and high switching frequency. Our findings may lead to the innovation of concepts and device architectures for optical information processing. PMID:29556529

Stochastic simulation of reaction-diffusion systems: A fluctuating-hydrodynamics approach

NASA Astrophysics Data System (ADS)

Kim, Changho; Nonaka, Andy; Bell, John B.; Garcia, Alejandro L.; Donev, Aleksandar

2017-03-01

We develop numerical methods for stochastic reaction-diffusion systems based on approaches used for fluctuating hydrodynamics (FHD). For hydrodynamic systems, the FHD formulation is formally described by stochastic partial differential equations (SPDEs). In the reaction-diffusion systems we consider, our model becomes similar to the reaction-diffusion master equation (RDME) description when our SPDEs are spatially discretized and reactions are modeled as a source term having Poisson fluctuations. However, unlike the RDME, which becomes prohibitively expensive for an increasing number of molecules, our FHD-based description naturally extends from the regime where fluctuations are strong, i.e., each mesoscopic cell has few (reactive) molecules, to regimes with moderate or weak fluctuations, and ultimately to the deterministic limit. By treating diffusion implicitly, we avoid the severe restriction on time step size that limits all methods based on explicit treatments of diffusion and construct numerical methods that are more efficient than RDME methods, without compromising accuracy. Guided by an analysis of the accuracy of the distribution of steady-state fluctuations for the linearized reaction-diffusion model, we construct several two-stage (predictor-corrector) schemes, where diffusion is treated using a stochastic Crank-Nicolson method, and reactions are handled by the stochastic simulation algorithm of Gillespie or a weakly second-order tau leaping method. We find that an implicit midpoint tau leaping scheme attains second-order weak accuracy in the linearized setting and gives an accurate and stable structure factor for a time step size of an order of magnitude larger than the hopping time scale of diffusing molecules. We study the numerical accuracy of our methods for the Schlögl reaction-diffusion model both in and out of thermodynamic equilibrium. We demonstrate and quantify the importance of thermodynamic fluctuations to the formation of a two-dimensional Turing-like pattern and examine the effect of fluctuations on three-dimensional chemical front propagation. By comparing stochastic simulations to deterministic reaction-diffusion simulations, we show that fluctuations accelerate pattern formation in spatially homogeneous systems and lead to a qualitatively different disordered pattern behind a traveling wave.

Stochastic simulation of reaction-diffusion systems: A fluctuating-hydrodynamics approach

DOE PAGES

Kim, Changho; Nonaka, Andy; Bell, John B.; ...

2017-03-24

Here, we develop numerical methods for stochastic reaction-diffusion systems based on approaches used for fluctuating hydrodynamics (FHD). For hydrodynamic systems, the FHD formulation is formally described by stochastic partial differential equations (SPDEs). In the reaction-diffusion systems we consider, our model becomes similar to the reaction-diffusion master equation (RDME) description when our SPDEs are spatially discretized and reactions are modeled as a source term having Poisson fluctuations. However, unlike the RDME, which becomes prohibitively expensive for an increasing number of molecules, our FHD-based description naturally extends from the regime where fluctuations are strong, i.e., each mesoscopic cell has few (reactive) molecules,more » to regimes with moderate or weak fluctuations, and ultimately to the deterministic limit. By treating diffusion implicitly, we avoid the severe restriction on time step size that limits all methods based on explicit treatments of diffusion and construct numerical methods that are more efficient than RDME methods, without compromising accuracy. Guided by an analysis of the accuracy of the distribution of steady-state fluctuations for the linearized reaction-diffusion model, we construct several two-stage (predictor-corrector) schemes, where diffusion is treated using a stochastic Crank-Nicolson method, and reactions are handled by the stochastic simulation algorithm of Gillespie or a weakly second-order tau leaping method. We find that an implicit midpoint tau leaping scheme attains second-order weak accuracy in the linearized setting and gives an accurate and stable structure factor for a time step size of an order of magnitude larger than the hopping time scale of diffusing molecules. We study the numerical accuracy of our methods for the Schlögl reaction-diffusion model both in and out of thermodynamic equilibrium. We demonstrate and quantify the importance of thermodynamic fluctuations to the formation of a two-dimensional Turing-like pattern and examine the effect of fluctuations on three-dimensional chemical front propagation. Furthermore, by comparing stochastic simulations to deterministic reaction-diffusion simulations, we show that fluctuations accelerate pattern formation in spatially homogeneous systems and lead to a qualitatively different disordered pattern behind a traveling wave.« less

Enzyme localization, crowding, and buffers collectively modulate diffusion-influenced signal transduction: Insights from continuum diffusion modeling

PubMed Central

Kekenes-Huskey, Peter M.; Eun, Changsun; McCammon, J. A.

2015-01-01

Biochemical reaction networks consisting of coupled enzymes connect substrate signaling events with biological function. Substrates involved in these reactions can be strongly influenced by diffusion “barriers” arising from impenetrable cellular structures and macromolecules, as well as interactions with biomolecules, especially within crowded environments. For diffusion-influenced reactions, the spatial organization of diffusion barriers arising from intracellular structures, non-specific crowders, and specific-binders (buffers) strongly controls the temporal and spatial reaction kinetics. In this study, we use two prototypical biochemical reactions, a Goodwin oscillator, and a reaction with a periodic source/sink term to examine how a diffusion barrier that partitions substrates controls reaction behavior. Namely, we examine how conditions representative of a densely packed cytosol, including reduced accessible volume fraction, non-specific interactions, and buffers, impede diffusion over nanometer length-scales. We find that diffusion barriers can modulate the frequencies and amplitudes of coupled diffusion-influenced reaction networks, as well as give rise to “compartments” of decoupled reactant populations. These effects appear to be intensified in the presence of buffers localized to the diffusion barrier. These findings have strong implications for the role of the cellular environment in tuning the dynamics of signaling pathways. PMID:26342355

Microtearing turbulence: Magnetic braiding and disruption limit

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

Firpo, Marie-Christine

2015-12-15

A realistic reduced model involving a large poloidal spectrum of microtearing modes is used to probe the existence of some stochasticity of magnetic field lines. Stochasticity is shown to occur even for the low values of the magnetic perturbation δB/B devoted to magnetic turbulence that have been experimentally measured. Because the diffusion coefficient may strongly depend on the radial (or magnetic-flux) coordinate, being very low near some resonant surfaces, and because its evaluation implicitly makes a normal diffusion hypothesis, one turns to another indicator appropriate to diagnose the confinement: the mean residence time of magnetic field lines. Their computation inmore » the microturbulence frame points to the existence of a disruption limit, namely of a critical order of magnitude of δB/B above which stochasticity is no longer benign yet, leads to a macroscopic loss of confinement in some tens to hundred of electron toroidal excursions. Since the level of magnetic turbulence δB/B has been measured to grow with the plasma electron density, this would also be a density limit.« less

Core-based intrinsic fiber-optic absorption sensor for the detection of volatile organic compounds

NASA Astrophysics Data System (ADS)

Klunder, Gregory L.; Russo, Richard E.

1995-03-01

A core-based intrinsic fiber-optic absorption sensor has been developed and tested for the detection of volatile organic compounds. The distal ends of transmitting and receiving fibers are connected by a small cylindrical section of an optically clear silicone rubber. The silicone rubber acts both as a light pipe and as a selective membrane into which the analyte molecules can diffuse. The sensor has been used to detect volatile organics (trichloroethylene, 1,1-dichloroethylene, and benzene) in both aqueous solutions and in the vapor phase or headspace. Absorption spectra obtained in the near-infrared (near-IR) provide qualitative and quantitative information about the analyte. Water, which has strong broad-band absorption in the near-IR, is excluded from the spectra because of the hydrophobic properties of the silicone rubber. The rate-limiting step is shown to be the diffusion through the Nernstian boundary layer surrounding the sensor and not the diffusion through the silicone polymer. The rate of analyte diffusion into the sensor, as measured by the t(sub 90) values (the time required for the sensor to reach 90% of the equilibrium value), is 30 min for measurements in aqueous solutions and approximately 3 min for measurements made in the headspace. The limit of detection obtained with this sensor is approximately 1.1 ppm for trichloroethylene in an aqueous solution.

Insights into Mechanistic Models for Evaporation of Organic Liquids in the Environment Obtained by Position-Specific Carbon Isotope Analysis.

PubMed

Julien, Maxime; Nun, Pierrick; Robins, Richard J; Remaud, Gérald S; Parinet, Julien; Höhener, Patrick

2015-11-03

Position-specific isotope effects (PSIEs) have been measured by isotope ratio monitoring (13)C nuclear magnetic resonance spectrometry during the evaporation of 10 liquids of different polarities under 4 evaporation modes (passive evaporation, air-vented evaporation, low pressure evaporation, distillation). The observed effects are used to assess the validity of the Craig-Gordon isotope model for organic liquids. For seven liquids the overall isotope effect (IE) includes a vapor-liquid contribution that is strongly position-specific in polar compounds but less so in apolar compounds and a diffusive IE that is not position-specific, except in the alcohols, ethanol and propan-1-ol. The diffusive IE is diminished under forced evaporation. The position-specific isotope pattern created by liquid-vapor IEs is manifest in five liquids, which have an air-side limitation for volatilization. For the alcohols, undefined processes in the liquid phase create additional PSIEs. Three other liquids with limitations on the liquid side have a lower, highly position-specific, bulk diffusive IE. It is concluded that evaporation of organic pollutants creates unique position-specific isotope patterns that may be used to assess the progress of remediation or natural attenuation of pollution and that the Craig-Gordon isotope model is valid for the volatilization of nonpolar organic liquids with air-side limitation of the volatilization rate.

Strong Pitch-Angle Diffusion of Ring Current Ions in Geomagnetic Storm-Associated Conditions

NASA Technical Reports Server (NTRS)

Khazanov, G. V.; Gamayunov, K. V.; Gallagher, D. L.; Spann, J. F.

2005-01-01

Do electromagnetic ion cyclotron (EMIC) waves cause strong pitch-angle diffusion of RC ions? This question is the primary motivation of this paper and has been affirmatively answered from the theoretical point of view. The materials that are presented in the Results section show clear evidence that strong pitch-angle diffusion takes place in the inner magnetosphere indicating an important role for the wave-particle interaction mechanism in the formation of RC ions and EMIC waves.

Humidity-insensitive water evaporation from molecular complex fluids.

PubMed

Salmon, Jean-Baptiste; Doumenc, Frédéric; Guerrier, Béatrice

2017-09-01

We investigated theoretically water evaporation from concentrated supramolecular mixtures, such as solutions of polymers or amphiphilic molecules, using numerical resolutions of a one-dimensional model based on mass transport equations. Solvent evaporation leads to the formation of a concentrated solute layer at the drying interface, which slows down evaporation in a long-time-scale regime. In this regime, often referred to as the falling rate period, evaporation is dominated by diffusive mass transport within the solution, as already known. However, we demonstrate that, in this regime, the rate of evaporation does not also depend on the ambient humidity for many molecular complex fluids. Using analytical solutions in some limiting cases, we first demonstrate that a sharp decrease of the water chemical activity at high solute concentration leads to evaporation rates which depend weakly on the humidity, as the solute concentration at the drying interface slightly depends on the humidity. However, we also show that a strong decrease of the mutual diffusion coefficient of the solution enhances considerably this effect, leading to nearly independent evaporation rates over a wide range of humidity. The decrease of the mutual diffusion coefficient indeed induces strong concentration gradients at the drying interface, which shield the concentration profiles from humidity variations, except in a very thin region close to the drying interface.

Step Permeability on the Pt(111) Surface

NASA Astrophysics Data System (ADS)

Altman, Michael

2005-03-01

Surface morphology will be affected, or even dictated, by kinetic limitations that may be present during growth. Asymmetric step attachment is recognized to be an important and possibly common cause of morphological growth instabilities. However, the impact of this kinetic limitation on growth morphology may be hindered by other factors such as the rate limiting step and step permeability. This strongly motivates experimental measurements of these quantities in real systems. Using low energy electron microscopy, we have measured step flow velocities in growth on the Pt(111) surface. The dependence of step velocity upon adjacent terrace width clearly shows evidence of asymmetric step attachment and step permeability. Step velocity is modeled by solving the diffusion equation simultaneously on several adjacent terraces subject to boundary conditions at intervening steps that include asymmetric step attachment and step permeability. This analysis allows a quantitative evaluation of step permeability and the kinetic length, which characterizes the rate limiting step continuously between diffusion and attachment-detachment limited regimes. This work provides information that is greatly needed to set physical bounds on the parameters that are used in theoretical treatments of growth. The observation that steps are permeable even on a simple metal surface should also stimulate more experimental measurements and theoretical treatments of this effect.

Sharp-front wave of strong magnetic field diffusion in solid metal

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

Xiao, Bo; Gu, Zhuo-wei; Kan, Ming-xian

When a strong magnetic field diffuses into a solid metal, if the metal's resistance possesses an abrupt rise at some critical temperature and the magnetic field strength is above some critical value, the magnetic field will diffuse into the metal in the form of a sharp-front wave. Formulas for the critical conditions under which a sharp-front magnetic diffusion wave emerges and a formula for the wave-front velocity are derived in this work.

A minimal model of an autonomous thermal motor

NASA Astrophysics Data System (ADS)

Fogedby, Hans C.; Imparato, Alberto

2017-09-01

We consider a model of a Brownian motor composed of two coupled overdamped degrees of freedom moving in periodic potentials and driven by two heat reservoirs. This model exhibits a spontaneous breaking of symmetry and gives rise to directed transport in the case of a non-vanishing interparticle interaction strength. For strong coupling between the particles we derive an expression for the propagation velocity valid for arbitrary periodic potentials. In the limit of strong coupling the model is equivalent to the Büttiker-Landauer model for a single particle diffusing in an environment with position-dependent temperature. By using numerical calculations of the Fokker-Planck equation and simulations of the Langevin equations we study the model for arbitrary coupling, retrieving many features of the strong-coupling limit. In particular, directed transport emerges even for symmetric potentials. For distinct heat reservoirs the heat currents are well-defined quantities allowing a study of the motor efficiency. We show that the optimal working regime occurs for moderate coupling. Finally, we introduce a model with discrete phase space which captures the essential features of the continuous model, can be solved in the limit of weak coupling, and exhibits a larger efficiency than the continuous counterpart.

Biodegradation of sorbed chemicals in soil

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

Scow, K.M.; Fan, S.; Johnson, C.

Rates of biodegradation of sorbed chemicals are usually lower in soil than in aqueous systems, in part because sorption reduces the availability of the chemical to microorganisms. Biodegradation, sorption, and diffusion occur simultaneously and are tightly coupled. In soil, the rate of biodegradation is a function of a chemical`s diffusion coefficient, sorption partition coefficient, the distance it must diffuse from the site of sorption to microbial populations that can degrade it, and its biodegradation rate constant. A model (DSB model) was developed that describes biodegradation of chemicals limited in the availability by sorption and diffusion. Different kinetics expressions describe biodegradationmore » depending on whether the reaction is controlled by mass transfer (diffusion and sorption) or the intrinsic biodegradation rate, and whether biodegradation begins during or after the majority of sorption has occurred. We tested the hypothesis that there is a direct relationship between how strongly a chemical is sorbed and the chemical`s biodegradation rate. In six soils with different organic carbon contents, there was no relationship between the extent or rate of biodegradation and the sorption partition coefficient for phenanthrene. Aging of phenanthrene residues in soil led to a substantial reduction in the rate of biodegradation compared to biodegradation rates of recently added phenanthrene. Considerable research has focused on identification and development of techniques for enhancing in situ biodegradation of sorbed chemicals. Development of such techniques, especially those involving inoculation with microbial strains, should consider physical mass transfer limitations and potential decreases in bioavailability over time. 4 refs., 3 figs., 1 tab.« less

Effects of water-contaminated air on blowoff limits of opposed jet hydrogen-air diffusion flames

NASA Technical Reports Server (NTRS)

Pellett, Gerald L.; Jentzen, Marilyn E.; Wilson, Lloyd G.; Northam, G. Burton

1988-01-01

The effects of water-contaminated air on the extinction and flame restoration of the central portion of N2-diluted H2 versus air counterflow diffusion flames are investigated using a coaxial tubular opposed jet burner. The results show that the replacement of N2 contaminant in air by water on a mole for mole basis decreases the maximum sustainable H2 mass flow, just prior to extinction, of the flame. This result contrasts strongly with the analogous substitution of water for N2 in a relatively hot premixed H2-O2-N2 flame, which was shown by Koroll and Mulpuru (1986) to lead to a significant, kinetically controlled increase in laminar burning velocity.

Confinement Regulates Complex Biochemical Networks: Initiation of Blood Clotting by “Diffusion Acting”

PubMed Central

Shen, Feng; Pompano, Rebecca R.; Kastrup, Christian J.; Ismagilov, Rustem F.

2009-01-01

Abstract This study shows that environmental confinement strongly affects the activation of nonlinear reaction networks, such as blood coagulation (clotting), by small quantities of activators. Blood coagulation is sensitive to the local concentration of soluble activators, initiating only when the activators surpass a threshold concentration, and therefore is regulated by mass transport phenomena such as flow and diffusion. Here, diffusion was limited by decreasing the size of microfluidic chambers, and it was found that microparticles carrying either the classical stimulus, tissue factor, or a bacterial stimulus, Bacillus cereus, initiated coagulation of human platelet-poor plasma only when confined. A simple analytical argument and numerical model were used to describe the mechanism for this phenomenon: confinement causes diffusible activators to accumulate locally and surpass the threshold concentration. To interpret the results, a dimensionless confinement number, Cn, was used to describe whether a stimulus was confined, and a Damköhler number, Da2, was used to describe whether a subthreshold stimulus could initiate coagulation. In the context of initiation of coagulation by bacteria, this mechanism can be thought of as “diffusion acting”, which is distinct from “diffusion sensing”. The ability of confinement and diffusion acting to change the outcome of coagulation suggests that confinement should also regulate other biological “on” and “off” processes that are controlled by thresholds. PMID:19843446

Diffusion of small molecules into medaka embryos improved by electroporation

PubMed Central

2013-01-01

Background Diffusion of small molecules into fish embryos is essential for many experimental procedures in developmental biology and toxicology. Since we observed a weak uptake of lithium into medaka eggs we started a detailed analysis of its diffusion properties using small fluorescent molecules. Results Contrary to our expectations, not the rigid outer chorion but instead membrane systems surrounding the embryo/yolk turned out to be the limiting factor for diffusion into medaka eggs. The consequence is a bi-phasic uptake of small molecules first reaching the pervitelline space with a diffusion half-time in the range of a few minutes. This is followed by a slow second phase (half-time in the range of several hours) during which accumulation in the embryo/yolk takes place. Treatment with detergents improved the uptake, but strongly affected the internal distribution of the molecules. Testing electroporation we could establish conditions to overcome the diffusion barrier. Applying this method to lithium chloride we observed anterior truncations in medaka embryos in agreement with its proposed activation of Wnt signalling. Conclusions The diffusion of small molecules into medaka embryos is slow, caused by membrane systems underneath the chorion. These results have important implications for pharmacologic/toxicologic techniques like the fish embryo test, which therefore require extended incubation times in order to reach sufficient concentrations in the embryos. PMID:23815821

The effect of pre-plasma formation under nonlocal transport conditions for ultra-relativistic laser-plasma interaction

NASA Astrophysics Data System (ADS)

Holec, M.; Nikl, J.; Vranic, M.; Weber, S.

2018-04-01

Interaction of high-power lasers with solid targets is in general strongly affected by the limited contrast available. The laser pre-pulse ionizes the target and produces a pre-plasma which can strongly modify the interaction of the main part of the laser pulse with the target. This is of particular importance for future experiments which will use laser intensities above 1021 W cm-2 and which are subject to the limited contrast. As a consequence the main part of the laser pulse will be modified while traversing the pre-plasma, interacting with it partially. A further complication arises from the fact that the interaction of a high-power pre-pulse with solid targets very often takes place under nonlocal transport conditions, i.e. the characteristic mean-free-path of the particles and photons is larger than the characteristic scale-lengths of density and temperature. The classical diffusion treatment of radiation and heat transport in the hydrodynamic model is then insufficient for the description of the pre-pulse physics. These phenomena also strongly modify the formation of the pre-plasma which in turn affects the propagation of the main laser pulse. In this paper nonlocal radiation-hydrodynamic simulations are carried out and serve as input for subsequent kinetic simulations of ultra-high intensity laser pulses interacting with the plasma in the ultra-relativistic regime. It is shown that the results of the kinetic simulations differ considerably whether a diffusive or nonlocal transport is used for the radiation-hydrodynamic simulations.

Mesoscopic-microscopic spatial stochastic simulation with automatic system partitioning.

PubMed

Hellander, Stefan; Hellander, Andreas; Petzold, Linda

2017-12-21

The reaction-diffusion master equation (RDME) is a model that allows for efficient on-lattice simulation of spatially resolved stochastic chemical kinetics. Compared to off-lattice hard-sphere simulations with Brownian dynamics or Green's function reaction dynamics, the RDME can be orders of magnitude faster if the lattice spacing can be chosen coarse enough. However, strongly diffusion-controlled reactions mandate a very fine mesh resolution for acceptable accuracy. It is common that reactions in the same model differ in their degree of diffusion control and therefore require different degrees of mesh resolution. This renders mesoscopic simulation inefficient for systems with multiscale properties. Mesoscopic-microscopic hybrid methods address this problem by resolving the most challenging reactions with a microscale, off-lattice simulation. However, all methods to date require manual partitioning of a system, effectively limiting their usefulness as "black-box" simulation codes. In this paper, we propose a hybrid simulation algorithm with automatic system partitioning based on indirect a priori error estimates. We demonstrate the accuracy and efficiency of the method on models of diffusion-controlled networks in 3D.

Role of boundary layer diffusion in vapor deposition growth of chalcogenide nanosheets: the case of GeS.

PubMed

Li, Chun; Huang, Liang; Snigdha, Gayatri Pongur; Yu, Yifei; Cao, Linyou

2012-10-23

We report a synthesis of single-crystalline two-dimensional GeS nanosheets using vapor deposition processes and show that the growth behavior of the nanosheet is substantially different from those of other nanomaterials and thin films grown by vapor depositions. The nanosheet growth is subject to strong influences of the diffusion of source materials through the boundary layer of gas flows. This boundary layer diffusion is found to be the rate-determining step of the growth under typical experimental conditions, evidenced by a substantial dependence of the nanosheet's size on diffusion fluxes. We also find that high-quality GeS nanosheets can grow only in the diffusion-limited regime, as the crystalline quality substantially deteriorates when the rate-determining step is changed away from the boundary layer diffusion. We establish a simple model to analyze the diffusion dynamics in experiments. Our analysis uncovers an intuitive correlation of diffusion flux with the partial pressure of source materials, the flow rate of carrier gas, and the total pressure in the synthetic setup. The observed significant role of boundary layer diffusions in the growth is unique for nanosheets. It may be correlated with the high growth rate of GeS nanosheets, ~3-5 μm/min, which is 1 order of magnitude higher than other nanomaterials (such as nanowires) and thin films. This fundamental understanding of the effect of boundary layer diffusions may generally apply to other chalcogenide nanosheets that can grow rapidly. It can provide useful guidance for the development of general paradigms to control the synthesis of nanosheets.

Long-time behavior and Turing instability induced by cross-diffusion in a three species food chain model with a Holling type-II functional response.

PubMed

Haile, Dawit; Xie, Zhifu

2015-09-01

In this paper, we study a strongly coupled reaction-diffusion system describing three interacting species in a food chain model, where the third species preys on the second one and simultaneously the second species preys on the first one. An intra-species competition b2 among the second predator is introduced to the food chain model. This parameter produces some very interesting result in linear stability and Turing instability. We first show that the unique positive equilibrium solution is locally asymptotically stable for the corresponding ODE system when the intra-species competition exists among the second predator. The positive equilibrium solution remains linearly stable for the reaction diffusion system without cross diffusion, hence it does not belong to the classical Turing instability scheme. But it becomes linearly unstable only when cross-diffusion also plays a role in the reaction-diffusion system, hence the instability is driven solely from the effect of cross diffusion. Our results also exhibit some interesting combining effects of cross-diffusion, intra-species competitions and inter-species interactions. Numerically, we conduct a one parameter analysis which illustrate how the interactions change the existence of stable equilibrium, limit cycle, and chaos. Some interesting dynamical phenomena occur when we perform analysis of interactions in terms of self-production of prey and intra-species competition of the middle predator. By numerical simulations, it illustrates the existence of nonuniform steady solutions and new patterns such as spot patterns, strip patterns and fluctuations due to the diffusion and cross diffusion in two-dimension. Published by Elsevier Inc.

Materials corrosion and protection from first principles

NASA Astrophysics Data System (ADS)

Johnson, Donald F.

Materials erode under environmental stresses such as high temperature, high pressure, and mechanical shock/stress, but erosion is often exacerbated by chemical corrosion. In this dissertation, periodic density functional theory (DFT) is employed to simulate interfacial adhesion, absorption kinetics, bulk diffusion, and other material phenomena (e.g., hydrogen-enhanced decohesion and shock-induced phase changes) with the intention of understanding corrosion and subsequent failure processes and guiding the design of new protective coatings. This work examines corrosion and/or protection of materials ( i.e., Fe, Ni, W) with important applications: structural steel, gun tubes, high-pressure oil recovery vessels, jet engine turbine blades, and fusion reactor walls. We use DFT to model the pressure-induced, bcc-to-hcp phase transformation in Fe, in which a new low energy pathway is predicted exhibiting nonadiabatic behavior coupling magnetic and structural changes. Protection of steel is addressed in two aspects: interfacial adhesion of protective coatings and assessment of corrosion resistance provided by a surface alloy. First, the current chrome-coated steel system is examined where extremely strong adhesion is predicted at the Cr/Fe interface originating in strong spin correlations. A ceramic coating, SiC, is considered as a possible replacement for Cr. Strong adhesion is predicted, especially for C-Fe interfacial bonds. To assess corrosion resistance, we model ingress of two common corrosive elements, H and C, into two Fe alloys, FeAl and Fe3Si. Adsorption and absorption thermodynamics and kinetics, as well as bulk dissolution and diffusion are calculated in order to determine whether these two alloys can inhibit uptake of H and C. Relative to pure Fe, dissolved H and C are less stable in the alloys, as the dissolution enthalpy is predicted to be more endothermic. Overall, the energy barriers and rate constants for adsorbed H/C diffusing into Fe3Si subsurface layers suggests that alloying Fe with Si can be an effective means to limit uptake of these elements into steel. Spallation of protective layers on jet engine turbine blades is a problem that arises during thermal cycling. An alternative thermal barrier coating system involving MoSi2 is considered and calculations predict strong adhesion at the MoSi2/Ni interface. The interfacial bonding structure reveals a mixture of metallic and covalent cross-interface bonds. The adhesion energy is similar across all three MoSi2 facets studied. Upon exposure to oxygen, this MoSi2 alloy will form a strongly adhered oxide scale, which in turn may strongly adhere the heat shield material (yttria-stabilized zirconia), thereby potentially extending the lifetime of the barrier coating. Lastly, the interaction of hydrogen isotopes (fusion fuel) with tungsten (a proposed fusion reactor wall material) is examined. Exothermic dissociative adsorption is predicted, along with endothermic absorption and dissolution. Surface-to-subsurface diffusion energy barriers for H incorporation into bulk W are large and the corresponding outward diffusion barriers are very small. In bulk W, deep energetic traps (trapping multiple H atoms) are predicted at vacancy defects. Thus, under high neutron fluxes that will produce vacancies in W, H are predicted to collect at these vacancies. In turn, locally high concentrations of H at such vacancies will enhance decohesion of bulk W, consistent with observed blistering under deuterium implantation. Limiting vacancy formation may be key to the survival of W as a fusion reactor wall material.

Inhibitory effect on the uptake and diffusion of Cd(2+) onto soybean hull sorbent in Cd-Pb binary sorption systems.

PubMed

Módenes, Aparecido N; Espinoza-Quiñones, Fernando R; Colombo, Andréia; Geraldi, Claudinéia L; Trigueros, Daniela E G

2015-05-01

The uptake of Cd(2+) and Pb(2+) ions by a soybean hull (SH) biosorbent in single and binary systems has been investigated. Sorption tests regarding SH in natura and chemically treated were carried out testing a suitable value range of solution pH, sorption temperature and shaking velocity. Sorption capacity is improved at pH 4, 30 °C temperature and 100 rpm. When a strong base is applied, a related-to-untreated SH increasing of 20% in the sorption capacity of Pb(2+) ions was observed, but with poor results for Cd(2+) uptake. Additionally, a relatively strong decreasing in both sorption capacities of Pb(2+) and Cd(2+) ions was evidenced for all acidic treatments. Regarding untreated SH, kinetic sorption data of both metals were well-interpreted by a pseudo second-order model and a rate-limiting step on the basis of an intra-particle diffusion model was suggested to occur. An inhibitory effect of Pb(2+) diffusion over Cd(2+) one was observed, limiting to reach the obtained maximum sorption capacity in single system. Maximum adsorption capacities of 0.49 and 0.67mequivg(-1) for Cd(2+) and Pb(2+), respectively, were predicted by the Langmuir isotherm model that reproduced well the equilibrium sorption data for single systems. The inhibitory effect of one metal over the other one was verified in equilibrium sorption data for binary systems interpreted on the basis of a modified extended Langmuir isotherm model, predicting changes in metal affinity onto the SH surface. Finally, SH is an alternative biosorbent with a great potential for the wastewater treatment containing cadmium and lead ions. Copyright © 2015 Elsevier Ltd. All rights reserved.

A Huygens principle for diffusion and anomalous diffusion in spatially extended systems

PubMed Central

Gottwald, Georg A.; Melbourne, Ian

2013-01-01

We present a universal view on diffusive behavior in chaotic spatially extended systems for anisotropic and isotropic media. For anisotropic systems, strong chaos leads to diffusive behavior (Brownian motion with drift) and weak chaos leads to superdiffusive behavior (Lévy processes with drift). For isotropic systems, the drift term vanishes and strong chaos again leads to Brownian motion. We establish the existence of a nonlinear Huygens principle for weakly chaotic systems in isotropic media whereby the dynamics behaves diffusively in even space dimension and exhibits superdiffusive behavior in odd space dimensions. PMID:23653481

The Harrison Diffusion Kinetics Regimes in Solute Grain Boundary Diffusion

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

Belova, Irina; Fiedler, T; Kulkarni, Nagraj S

2012-01-01

Knowledge of the limits of the principal Harrison kinetics regimes (Type-A, B and C) for grain boundary diffusion is very important for the correct analysis of the depth profiles in a tracer diffusion experiment. These regimes for self-diffusion have been extensively studied in the past by making use of the phenomenological Lattice Monte Carlo (LMC) method with the result that the limits are now well established. The relationship of those self-diffusion limits to the corresponding ones for solute diffusion in the presence of solute segregation to the grain boundaries remains unclear. In the present study, the influence of solute segregationmore » on the limits is investigated with the LMC method for the well-known parallel grain boundary slab model by showing the equivalence of two diffusion models. It is shown which diffusion parameters are useful for identifying the limits of the Harrison kinetics regimes for solute grain boundary diffusion. It is also shown how the measured segregation factor from the diffusion experiment in the Harrison Type-B kinetics regime may differ from the global segregation factor.« less

Development of a sorption rate technique for single zeolite crystals using an electrodynamic balance

NASA Astrophysics Data System (ADS)

Welegala, Mark Joseph

Conventional means for evaluating intracrystalline diffusion in zeolites are complicated by extracrystalline mass transport resistances, crystallite size distribution, sorption heat effects, and finite instrument response times. A potentially direct means of overcoming these problems is to study sorption uptake on a single crystal suspended within a flowing gas stream in an electrodynamic balance (EDB). The objectives of this research were to design, build and investigate the viability of using such a device for obtaining diffusion coefficients from simple sorbate/zeolite systems, by computing the sorption uptake curve from the levitation voltage as a function of time. The initial electronic cell design was strongly influenced by flow mixing considerations. Accordingly, the conventional bihyperboloid electrode configuration was discarded in favor of novel four-ring (4R), and later two-ring/two-screen (2R/2S) designs with cylindrical interior geometries. A detailed numerical model based on the Method of Discrete Charges (MDC) was developed and used to aid in the design and operational understanding of these cells. Several 2R/2S designs were built and tested, including teflon/mica composite and ceramic cells capable of withstanding up to 750oF, for in situ activation of the zeolites. The diffusion of carbon dioxide in zeolite A was selected for testing due to the large differential weight change (10-20%) which occurs at ambient conditions and the availability of reliable experimental diffusion results (Yucel and Ruthven, 1980a). In addition to the carbon dioxide sorbate, water on zeolite 4A and a system relatively immune to atmospheric contamination, CO2 on activated carbon were also studied. Laboratory 4A crystals of up to 45 μm were grown using Charnell's method. These large solid particles were captured using a dry charging technique, and held during elevated temperature dehydration. Preliminary experimentation introduced externally dried crystals to the cell chamber in 0.5-3 minutes. Only minimal desorption results with carbon dioxide and later, adsorption for water vapor, were obtained. Further experiments revealed that crystal contamination from laboratory air can be considerable in less than one minute, thereby preadsorbing airborne water vapor. The experimental methodology was changed to include in situ heating. Subsequent attempts to circumvent laser heating of the particle had limited success. Particle loss, (due to excessive charge loss) and cell material degradation limited the process to null point temperatures of approximately 260oC, which is insufficient for complete zeolite dehydration. Early, it was demonstrated that gas compositions could be switched while flowing without losing the particle. However, the resulting concentration transient imposes an ultimate limitation on the technique for application to rapidly diffusing systems. Also, the fact that the technique is gravimetric requires that the diffusing species must be appreciably adsorbed at ambient conditions. Thus the single crystal sorption apparatus based on the electrodynamic containment device would appear to have use primarily for strongly adsorbed and slowly diffusing species. (Abstract shortened by UMI.)

Dynamic characterization of external and internal mass transport in heterotrophic biofilms from microsensors measurements.

PubMed

Guimerà, Xavier; Dorado, Antonio David; Bonsfills, Anna; Gabriel, Gemma; Gabriel, David; Gamisans, Xavier

2016-10-01

Knowledge of mass transport mechanisms in biofilm-based technologies such as biofilters is essential to improve bioreactors performance by preventing mass transport limitation. External and internal mass transport in biofilms was characterized in heterotrophic biofilms grown on a flat plate bioreactor. Mass transport resistance through the liquid-biofilm interphase and diffusion within biofilms were quantified by in situ measurements using microsensors with a high spatial resolution (<50 μm). Experimental conditions were selected using a mathematical procedure based on the Fisher Information Matrix to increase the reliability of experimental data and minimize confidence intervals of estimated mass transport coefficients. The sensitivity of external and internal mass transport resistances to flow conditions within the range of typical fluid velocities over biofilms (Reynolds numbers between 0.5 and 7) was assessed. Estimated external mass transfer coefficients at different liquid phase flow velocities showed discrepancies with studies considering laminar conditions in the diffusive boundary layer near the liquid-biofilm interphase. The correlation of effective diffusivity with flow velocities showed that the heterogeneous structure of biofilms defines the transport mechanisms inside biofilms. Internal mass transport was driven by diffusion through cell clusters and aggregates at Re below 2.8. Conversely, mass transport was driven by advection within pores, voids and water channels at Re above 5.6. Between both flow velocities, mass transport occurred by a combination of advection and diffusion. Effective diffusivities estimated at different biofilm densities showed a linear increase of mass transport resistance due to a porosity decrease up to biofilm densities of 50 g VSS·L(-1). Mass transport was strongly limited at higher biofilm densities. Internal mass transport results were used to propose an empirical correlation to assess the effective diffusivity within biofilms considering the influence of hydrodynamics and biofilm density. Copyright © 2016 Elsevier Ltd. All rights reserved.

Boundary-layer exchange by bubble: A novel method for generating transient nanofluidic layers

NASA Astrophysics Data System (ADS)

Jennissen, Herbert P.

2005-10-01

Unstirred layers (i.e., Nernst boundary layers) occur on every dynamic solid-liquid interface, constituting a diffusion barrier, since the velocity of a moving liquid approaches zero at the surface (no slip). If a macromolecule-surface reaction rate is higher than the diffusion rate, the Nernst layer is solute depleted and the reaction rate becomes mass-transport limited. The thickness of a Nernst boundary layer (δN) generally lies between 5 and 50μm. In an evanescent wave rheometer, measuring fibrinogen adsorption to fused silica, we made the fundamental observation that an air bubble preceding the sample through the flow cell abolishes the mass-transport limitation of the Nernst diffusion layer. Instead exponential kinetics are found. Experimental and simulation studies strongly indicate that these results are due to the elimination of the Nernst diffusion layer and its replacement by a dynamic nanofluidic layer (δν) maximally 200-300nm thick. It is suggested that the air bubble leads to a transient boundary-layer separation into a novel nanoboundary layer on the surface and the bulk fluid velocity profile separated by a vortex sheet with an estimated lifetime of 30-60s. A bubble-induced boundary-layer exchange from the Nernst to the nanoboundary layer and back is obtained, giving sufficient time for the measurement of unbiased exponential surface kinetics. Noteworthy is that the nanolayer can exist at all and displays properties such as (i) a long persistence and resistance to dissipation by the bulk liquid (boundary-layer-exchange-hysteresis) and (ii) a lack of solute depletion in spite of boundary-layer separation. The boundary-layer-exchange by bubble (BLEB) method therefore appears ideal for enhancing the rates of all types of diffusion-limited macromolecular reactions on surfaces with contact angles between 0° and 90° and only appears limited by slippage due to nanobubbles or an air gap beneath the nanofluidic layer on very hydrophobic surfaces. The possibility of producing nanoboundary layers without any nanostructuring or nanomachining should also be useful for fundamental physical studies in nanofluidics.

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

Ressler, Sean M.; Katsuda, Satoru; Reynolds, Stephen P.

Several young supernova remnants, including SN 1006, emit synchrotron X-rays in narrow filaments, hereafter thin rims, along their periphery. The widths of these rims imply 50-100 μG fields in the region immediately behind the shock, far larger than expected for the interstellar medium compressed by unmodified shocks, assuming electron radiative losses limit rim widths. However, magnetic field damping could also produce thin rims. Here we review the literature on rim width calculations, summarizing the case for magnetic field amplification. We extend these calculations to include an arbitrary power-law dependence of the diffusion coefficient on energy, D∝E {sup μ}. Loss-limited rimmore » widths should shrink with increasing photon energy, while magnetic-damping models predict widths almost independent of photon energy. We use these results to analyze Chandra observations of SN 1006, in particular the southwest limb. We parameterize the FWHM in terms of energy as FWHM ∝E{sub γ}{sup m{sub E}}. Filament widths in SN 1006 decrease with energy; m{sub E} ∼ –0.3 to –0.8, implying magnetic field amplification by factors of 10-50, above the factor of four expected in strong unmodified shocks. For SN 1006, the rapid shrinkage rules out magnetic damping models. It also favors short mean free paths (small diffusion coefficients) and strong dependence of D on energy (μ ≥ 1).« less

Strong Deformation of the Thick Electric Double Layer around a Charged Particle during Sedimentation or Electrophoresis.

PubMed

Khair, Aditya S

2018-01-23

The deformation of the electric double layer around a charged colloidal particle during sedimentation or electrophoresis in a binary, symmetric electrolyte is studied. The surface potential of the particle is assumed to be small compared to the thermal voltage scale. Additionally, the Debye length is assumed to be large compared to the particle size. These assumptions enable a linearization of the electrokinetic equations. The particle appears as a point charge in this thick-double-layer limit; the distribution of charge in the diffuse cloud surrounding it is determined by a balance of advection due to the particle motion, Brownian diffusion of ions, and electrostatic screening of the particle by the cloud. The ability of advection to deform the charge cloud from its equilibrium state is parametrized by a Péclet number, Pe. For weak advection (Pe ≪ 1), the cloud is only slightly deformed. In contrast, the cloud can be completely stripped from the particle at Pe ≫ 1; consequently, electrokinetic effects on the particle motion vanish in this regime. Therefore, in sedimentation the drag limits to Stokes' law for an uncharged particle as Pe → ∞. Likewise, the particle velocity for electrophoresis approaches Huckel's result. The strongly deformed cloud at large Pe is predicted to generate a concomitant increase in the sedimentation field in a dilute settling suspension.

Submillimeter-scale heterogeneity of labile phosphorus in sediments characterized by diffusive gradients in thin films and spatial analysis.

PubMed

Meng, Yuting; Ding, Shiming; Gong, Mengdan; Chen, Musong; Wang, Yan; Fan, Xianfang; Shi, Lei; Zhang, Chaosheng

2018-03-01

Sediments have a heterogeneous distribution of labile redox-sensitive elements due to a drastic downward transition from oxic to anoxic condition as a result of organic matter degradation. Characterization of the heterogeneous nature of sediments is vital for understanding of small-scale biogeochemical processes. However, there are limited reports on the related specialized methodology. In this study, the monthly distributions of labile phosphorus (P), a redox-sensitive limiting nutrient, were measured in the eutrophic Lake Taihu by Zr-oxide diffusive gradients in thin films (Zr-oxide DGT) on a two-dimensional (2D) submillimeter level. Geographical information system (GIS) techniques were used to visualize the labile P distribution at such a micro-scale, showing that the DGT-labile P was low in winter and high in summer. Spatial analysis methods, including semivariogram and Moran's I, were used to quantify the spatial variation of DGT-labile P. The distribution of DGT-labile P had clear submillimeter-scale spatial patterns with significant spatial autocorrelation during the whole year and displayed seasonal changes. High values of labile P with strong spatial variation were observed in summer, while low values of labile P with relatively uniform spatial patterns were detected in winter, demonstrating the strong influences of temperature on the mobility and spatial distribution of P in sediment profiles. Copyright © 2017 Elsevier Ltd. All rights reserved.

Constraints on cosmological dark matter annihilation from the Fermi-LAT isotropic diffuse gamma-ray measurement

DOE PAGES

Abdo, A. A.; Ackermann, M.; Ajello, M.; ...

2010-04-01

The first published Fermi large area telescope (Fermi-LAT) measurement of the isotropic diffuse gamma-ray emission is in good agreement with a single power law, and is not showing any signature of a dominant contribution from dark matter sources in the energy range from 20 to 100 GeV. Here, we use the absolute size and spectral shape of this measured flux to derive cross section limits on three types of generic dark matter candidates: annihilating into quarks, charged leptons and monochromatic photons. Predicted gamma-ray fluxes from annihilating dark matter are strongly affected by the underlying distribution of dark matter, and bymore » using different available results of matter structure formation we assess these uncertainties. We also quantify how the dark matter constraints depend on the assumed conventional backgrounds and on the Universe's transparency to high-energy gamma-rays. In reasonable background and dark matter structure scenarios (but not in all scenarios we consider) it is possible to exclude models proposed to explain the excess of electrons and positrons measured by the Fermi-LAT and PAMELA experiments. Derived limits also start to probe cross sections expected from thermally produced relics (e.g. in minimal supersymmetry models) annihilating predominantly into quarks. Finally, for the monochromatic gamma-ray signature, the current measurement constrains only dark matter scenarios with very strong signals.« less

FRACTIONAL PEARSON DIFFUSIONS.

PubMed

Leonenko, Nikolai N; Meerschaert, Mark M; Sikorskii, Alla

2013-07-15

Pearson diffusions are governed by diffusion equations with polynomial coefficients. Fractional Pearson diffusions are governed by the corresponding time-fractional diffusion equation. They are useful for modeling sub-diffusive phenomena, caused by particle sticking and trapping. This paper provides explicit strong solutions for fractional Pearson diffusions, using spectral methods. It also presents stochastic solutions, using a non-Markovian inverse stable time change.

CONSTRAINING THE EMISSIVITY OF ULTRAHIGH ENERGY COSMIC RAYS IN THE DISTANT UNIVERSE WITH THE DIFFUSE GAMMA-RAY EMISSION

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

Wang Xiangyu; Liu Ruoyu; Aharonian, Felix

Ultrahigh cosmic rays (UHECRs) with energies {approx}> 10{sup 19} eV emitted at cosmological distances will be attenuated by cosmic microwave and infrared background radiation through photohadronic processes. Lower energy extragalactic cosmic rays ({approx}10{sup 18}-10{sup 19} eV) can only travel a linear distance smaller than {approx}Gpc in a Hubble time due to the diffusion if the extragalactic magnetic fields are as strong as nano-Gauss. These prevent us from directly observing most of the UHECRs in the universe, and thus the observed UHECR intensity reflects only the emissivity in the nearby universe within hundreds of Mpc. However, UHECRs in the distant universe,more » through interactions with the cosmic background photons, produce UHE electrons and gamma rays that in turn initiate electromagnetic cascades on cosmic background photons. This secondary cascade radiation forms part of the extragalactic diffuse GeV-TeV gamma-ray radiation and, unlike the original UHECRs, is observable. Motivated by new measurements of extragalactic diffuse gamma-ray background radiation by Fermi/Large Area Telescope, we obtained upper limit placed on the UHECR emissivity in the distant universe by requiring that the cascade radiation they produce not exceed the observed levels. By comparison with the gamma-ray emissivity of candidate UHECR sources (such as gamma-ray bursts (GRBs) and active galactic nuclei) at high redshifts, we find that the obtained upper limit for a flat proton spectrum is {approx_equal} 10{sup 1.5} times larger than the gamma-ray emissivity in GRBs and {approx_equal} 10 times smaller than the gamma-ray emissivity in BL Lac objects. In the case of iron nuclei composition, the derived upper limit of UHECR emissivity is a factor of 3-5 times higher. Robust upper limit on the cosmogenic neutrino flux is further obtained, which is marginally reachable by the Icecube detector and the next-generation detector JEM-EUSO.« less

Nucleation speed limit on remote fluid induced earthquakes

USGS Publications Warehouse

Parsons, Thomas E.; Akinci, Aybige; Malignini, Luca

2017-01-01

Earthquakes triggered by other remote seismic events are explained as a response to long-traveling seismic waves that temporarily stress the crust. However, delays of hours or days after seismic waves pass through are reported by several studies, which are difficult to reconcile with the transient stresses imparted by seismic waves. We show that these delays are proportional to magnitude and that nucleation times are best fit to a fluid diffusion process if the governing rupture process involves unlocking a magnitude-dependent critical nucleation zone. It is well established that distant earthquakes can strongly affect the pressure and distribution of crustal pore fluids. Earth’s crust contains hydraulically isolated, pressurized compartments in which fluids are contained within low-permeability walls. We know that strong shaking induced by seismic waves from large earthquakes can change the permeability of rocks. Thus, the boundary of a pressurized compartment may see its permeability rise. Previously confined, overpressurized pore fluids may then diffuse away, infiltrate faults, decrease their strength, and induce earthquakes. Magnitude-dependent delays and critical nucleation zone conclusions can also be applied to human-induced earthquakes.

Nucleation speed limit on remote fluid-induced earthquakes.

PubMed

Parsons, Tom; Malagnini, Luca; Akinci, Aybige

2017-08-01

Earthquakes triggered by other remote seismic events are explained as a response to long-traveling seismic waves that temporarily stress the crust. However, delays of hours or days after seismic waves pass through are reported by several studies, which are difficult to reconcile with the transient stresses imparted by seismic waves. We show that these delays are proportional to magnitude and that nucleation times are best fit to a fluid diffusion process if the governing rupture process involves unlocking a magnitude-dependent critical nucleation zone. It is well established that distant earthquakes can strongly affect the pressure and distribution of crustal pore fluids. Earth's crust contains hydraulically isolated, pressurized compartments in which fluids are contained within low-permeability walls. We know that strong shaking induced by seismic waves from large earthquakes can change the permeability of rocks. Thus, the boundary of a pressurized compartment may see its permeability rise. Previously confined, overpressurized pore fluids may then diffuse away, infiltrate faults, decrease their strength, and induce earthquakes. Magnitude-dependent delays and critical nucleation zone conclusions can also be applied to human-induced earthquakes.

Preparation and Physical Properties of Segmented Thermoelectric YBa2Cu3O7-x -Ca3Co4O9 Ceramics

NASA Astrophysics Data System (ADS)

Wannasut, P.; Keawprak, N.; Jaiban, P.; Watcharapasorn, A.

2018-01-01

Segmented thermoelectric ceramics are now well known for their high conversion efficiency and are currently being investigated in both basic and applied energy researches. In this work, the successful preparation of the segmented thermoelectric YBa2Cu3O7-x -Ca3Co4O9 (YBCO-CCO) ceramic by hot pressing method and the study on its physical properties were presented. Under the optimum hot pressing condition of 800 °C temperature, 1-hour holding time and 1-ton weight, the segmented YBCO-CCO sample showed two strongly connected layers with the relative density of about 96%. The X-ray diffraction (XRD) patterns indicated that each segment showed pure phase corresponding to each respective composition. Scanning electron microscopy (SEM) results confirmed the sharp interface and good adhesion between YBCO and CCO layers. Although the chemical analysis indicated the limited inter-layer diffusion near the interface, some elemental diffusion at the boundary was expected to be the source of this strong bonding.

Nucleation speed limit on remote fluid-induced earthquakes

PubMed Central

Parsons, Tom; Malagnini, Luca; Akinci, Aybige

2017-01-01

Earthquakes triggered by other remote seismic events are explained as a response to long-traveling seismic waves that temporarily stress the crust. However, delays of hours or days after seismic waves pass through are reported by several studies, which are difficult to reconcile with the transient stresses imparted by seismic waves. We show that these delays are proportional to magnitude and that nucleation times are best fit to a fluid diffusion process if the governing rupture process involves unlocking a magnitude-dependent critical nucleation zone. It is well established that distant earthquakes can strongly affect the pressure and distribution of crustal pore fluids. Earth’s crust contains hydraulically isolated, pressurized compartments in which fluids are contained within low-permeability walls. We know that strong shaking induced by seismic waves from large earthquakes can change the permeability of rocks. Thus, the boundary of a pressurized compartment may see its permeability rise. Previously confined, overpressurized pore fluids may then diffuse away, infiltrate faults, decrease their strength, and induce earthquakes. Magnitude-dependent delays and critical nucleation zone conclusions can also be applied to human-induced earthquakes. PMID:28845448

Shot noise at high temperatures

NASA Astrophysics Data System (ADS)

Gutman, D. B.; Gefen, Yuval

2003-07-01

We consider the possibility of measuring nonequilibrium properties of the current correlation functions at high temperatures (and small bias). Through the example of the third cumulant of the current (S3) we demonstrate that odd-order correlation functions represent nonequilibrium physics even at small external bias and high temperatures. We calculate S3=y(eV/T)e2I for a quasi-one-dimensional diffusive constriction. We calculate the scaling function y in two regimes: when the scattering processes are purely elastic and when the inelastic electron-electron scattering is strong. In both cases we find that y interpolates between two constants. In the low- (high-) temperature limit y is strongly (weakly) enhanced (suppressed) by the electron-electron scattering.

Effective rate constants for nanostructured heterogeneous catalysts

NASA Astrophysics Data System (ADS)

Hendy, Shaun; Gaston, Nicola; Zhang, Philip; Lund, Nat

2012-02-01

There is currently a high level of interest in the use of nanostructured materials for catalysis. For instance, gold, which is largely inert in the bulk, can exhibit strong catalytic activity when in nanoparticle form. With precious metal catalysts such as Pt and Pd in high demand, the use of these materials in nanoparticle form can also substantially reduce costs by exposure of more surface area for the same volume of material. When reactants are plentiful, the effective activity of a nanoparticulate catalyst will increase roughly with its surface area. However, under diffusion-limited conditions, the reactant must diffuse to active sites on the catalyst, so a high surface area and a high density of active sites may bring diminishing returns if reactant is consumed faster than it arrives. Here we apply a mathematical homogenisation approach to derive simple expressions for the effective reactivity of a nanostructured catalyst under diffusion limited conditions that relate the intrinsic rate constants of the surfaces presented by the catalyst to an effective rate constant. When highly active catalytic sites, such as step edges or other defects are present, we show that distinct limiting cases emerge depending on the degree of overlap of the reactant depletion zone about each site. In gases, the size of this depletion zone is approximately the mean free path, so the effective reactivity will depend on the structure of the catalyst on that scale. We discuss implications for the optimal design of nanoparticle catalysts.

Diffuse-charge dynamics of ionic liquids in electrochemical systems.

PubMed

Zhao, Hui

2011-11-01

We employ a continuum theory of solvent-free ionic liquids accounting for both short-range electrostatic correlations and steric effects (finite ion size) [Bazant et al., Phys. Rev. Lett. 106, 046102 (2011)] to study the response of a model microelectrochemical cell to a step voltage. The model problem consists of a 1-1 symmetric ionic liquid between two parallel blocking electrodes, neglecting any transverse transport phenomena. Matched asymptotic expansions in the limit of thin double layers are applied to analyze the resulting one-dimensional equations and study the overall charge-time relation in the weakly nonlinear regime. One important conclusion is that our simple scaling analysis suggests that the length scale √(λ*(D)l*(c)) accurately characterizes the double-layer structure of ionic liquids with strong electrostatic correlations where l*(c) is the electrostatic correlation length (in contrast, the Debye screening length λ*(D) is the primary double-layer length for electrolytes) and the response time of λ(D)(*3/2)L*/(D*l(c)(1/2)) (not λ*(D)L*/D* that is the primary charging time of electrolytes) is the correct charging time scale of ionic liquids with strong electrostatic correlations where D* is the diffusivity and L* is the separation length of the cell. With these two new scales, data of both electric potential versus distance from the electrode and the total diffuse charge versus time collapse onto each individual master curve in the presence of strong electrostatic correlations. In addition, the dependance of the total diffuse charge on steric effects, short-range correlations, and driving voltages is thoroughly examined. The results from the asymptotic analysis are compared favorably with those from full numerical simulations. Finally, the absorption of excess salt by the double layer creates a depletion region outside the double layer. Such salt depletion may bring a correction to the leading order terms and break down the weakly nonlinear analysis. A criterion which justifies the weakly nonlinear analysis is verified with numerical simulations.

Enhancement of Fe diffusion in ZnSe/S laser crystals under hot isostatic pressing

NASA Astrophysics Data System (ADS)

Gafarov, Ozarfar; Martinez, Alan; Fedorov, Vladimir; Mirov, Sergey

2017-02-01

Many organic molecules have strong and narrow absorption features in the middle Infrared (mid-IR) spectral range. The ability to directly probe absorption features of molecules enables numerous mid-IR applications in non-invasive medical diagnosis, industrial processing and process control, environmental monitoring, etc. Thus, there is a strong demand for lasers operating in mid-IR spectral range. Transition metal (TM) doped II-VI semiconductors such as Fe/Cr:ZnSe/S are the material of choice for fabrication of mid-IR gain media due to favorable combination of properties: a four level energy structure, absence of excited state absorption , broad mid-IR vibronic absorption and emission bands. Despite the significant progress in post-growth thermal diffusion technology of TM:II-VI fabrication there are still some difficulties associated with diffusion of certain TM's in these materials. In this work we address the issue of poor diffusion of Fe in ZnSe/S polycrystals. It is well known that with the temperature increase the diffusion rate of impurity also increases. However, simple application of high temperatures during the diffusion process is problematic for ZnSe/S crystals due to their strong sublimation. The sublimation processes can be suppressed by application of high pressures. Hot isostatic pressing was utilized as the means for simultaneous application of high temperatures (1300°C) and high pressures (1000atm, 3000atm). It was determined that diffusion coefficient of Fe was improved 13 and 14 fold in ZnSe and ZnS, respectively, as compared to the standard diffusion at 950°C. The difference in diffusion coefficients can be due to strong increase in the grain size of polycrystals.

A diffusion-based truncated projection artifact reduction method for iterative digital breast tomosynthesis reconstruction

PubMed Central

Lu, Yao; Chan, Heang-Ping; Wei, Jun; Hadjiiski, Lubomir M

2014-01-01

Digital breast tomosynthesis (DBT) has strong promise to improve sensitivity for detecting breast cancer. DBT reconstruction estimates the breast tissue attenuation using projection views (PVs) acquired in a limited angular range. Because of the limited field of view (FOV) of the detector, the PVs may not completely cover the breast in the x-ray source motion direction at large projection angles. The voxels in the imaged volume cannot be updated when they are outside the FOV, thus causing a discontinuity in intensity across the FOV boundaries in the reconstructed slices, which we refer to as the truncated projection artifact (TPA). Most existing TPA reduction methods were developed for the filtered backprojection method in the context of computed tomography. In this study, we developed a new diffusion-based method to reduce TPAs during DBT reconstruction using the simultaneous algebraic reconstruction technique (SART). Our TPA reduction method compensates for the discontinuity in background intensity outside the FOV of the current PV after each PV updating in SART. The difference in voxel values across the FOV boundary is smoothly diffused to the region beyond the FOV of the current PV. Diffusion-based background intensity estimation is performed iteratively to avoid structured artifacts. The method is applicable to TPA in both the forward and backward directions of the PVs and for any number of iterations during reconstruction. The effectiveness of the new method was evaluated by comparing the visual quality of the reconstructed slices and the measured discontinuities across the TPA with and without artifact correction at various iterations. The results demonstrated that the diffusion-based intensity compensation method reduced the TPA while preserving the detailed tissue structures. The visibility of breast lesions obscured by the TPA was improved after artifact reduction. PMID:23318346

Probing Exciton Diffusion and Dissociation in Single-Walled Carbon Nanotube-C60 Heterojunctions

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

Dowgiallo, Anne-Marie; Mistry, Kevin S.; Johnson, Justin C.

The efficiency of thin-film organic photovoltaic (OPV) devices relies heavily upon the transport of excitons to type-II heterojunction interfaces, where there is sufficient driving force for exciton dissociation and ultimately the formation of charge carriers. Semiconducting single-walled carbon nanotubes (SWCNTs) are strong near-infrared absorbers that form type-II heterojunctions with fullerenes such as C60. Although the efficiencies of SWCNT-fullerene OPV devices have climbed over the past few years, questions remain regarding the fundamental factors that currently limit their performance. In this study, we determine the exciton diffusion length in the C60 layer of SWCNT-C60 bilayer active layers using femtosecond transient absorptionmore » measurements. We demonstrate that hole transfer from photoexcited C60 molecules to SWCNTs can be tracked by the growth of narrow spectroscopic signatures of holes in the SWCNT 'reporter layer'. In bilayers with thick C60 layers, the SWCNT charge-related signatures display a slow rise over hundreds of picoseconds, reflecting exciton diffusion through the C60 layer to the interface. A model based on exciton diffusion with a Beer-Lambert excitation profile, as well as Monte Carlo simulations, gives the best fit to the data as a function of C60 layer thickness using an exciton diffusion length of approximately 5 nm.« less

Probing Exciton Diffusion and Dissociation in Single-Walled Carbon Nanotube-C(60) Heterojunctions.

PubMed

Dowgiallo, Anne-Marie; Mistry, Kevin S; Johnson, Justin C; Reid, Obadiah G; Blackburn, Jeffrey L

2016-05-19

The efficiency of thin-film organic photovoltaic (OPV) devices relies heavily upon the transport of excitons to type-II heterojunction interfaces, where there is sufficient driving force for exciton dissociation and ultimately the formation of charge carriers. Semiconducting single-walled carbon nanotubes (SWCNTs) are strong near-infrared absorbers that form type-II heterojunctions with fullerenes such as C60. Although the efficiencies of SWCNT-fullerene OPV devices have climbed over the past few years, questions remain regarding the fundamental factors that currently limit their performance. In this study, we determine the exciton diffusion length in the C60 layer of SWCNT-C60 bilayer active layers using femtosecond transient absorption measurements. We demonstrate that hole transfer from photoexcited C60 molecules to SWCNTs can be tracked by the growth of narrow spectroscopic signatures of holes in the SWCNT "reporter layer". In bilayers with thick C60 layers, the SWCNT charge-related signatures display a slow rise over hundreds of picoseconds, reflecting exciton diffusion through the C60 layer to the interface. A model based on exciton diffusion with a Beer-Lambert excitation profile, as well as Monte Carlo simulations, gives the best fit to the data as a function of C60 layer thickness using an exciton diffusion length of approximately 5 nm.

Anisotropic Mesoscale Eddy Transport in Ocean General Circulation Models

NASA Astrophysics Data System (ADS)

Reckinger, S. J.; Fox-Kemper, B.; Bachman, S.; Bryan, F.; Dennis, J.; Danabasoglu, G.

2014-12-01

Modern climate models are limited to coarse-resolution representations of large-scale ocean circulation that rely on parameterizations for mesoscale eddies. The effects of eddies are typically introduced by relating subgrid eddy fluxes to the resolved gradients of buoyancy or other tracers, where the proportionality is, in general, governed by an eddy transport tensor. The symmetric part of the tensor, which represents the diffusive effects of mesoscale eddies, is universally treated isotropically in general circulation models. Thus, only a single parameter, namely the eddy diffusivity, is used at each spatial and temporal location to impart the influence of mesoscale eddies on the resolved flow. However, the diffusive processes that the parameterization approximates, such as shear dispersion, potential vorticity barriers, oceanic turbulence, and instabilities, typically have strongly anisotropic characteristics. Generalizing the eddy diffusivity tensor for anisotropy extends the number of parameters to three: a major diffusivity, a minor diffusivity, and the principal axis of alignment. The Community Earth System Model (CESM) with the anisotropic eddy parameterization is used to test various choices for the newly introduced parameters, which are motivated by observations and the eddy transport tensor diagnosed from high resolution simulations. Simply setting the ratio of major to minor diffusivities to a value of five globally, while aligning the major axis along the flow direction, improves biogeochemical tracer ventilation and reduces global temperature and salinity biases. These effects can be improved even further by parameterizing the anisotropic transport mechanisms in the ocean.

Phosphorus-defect interactions during thermal annealing of ion implanted silicon

NASA Astrophysics Data System (ADS)

Keys, Patrick Henry

Ion implantation of dopant atoms into silicon generates nonequilibrium levels of crystal defects that can lead to the detrimental effects of transient enhanced diffusion (TED), incomplete dopant activation, and p-n junction leakage. In order to control these effects, it is vital to have a clear understanding of dopant-defect interactions and develop models that account for these interactions. This research focuses on experimentally investigating and modeling the clustering of phosphorus dopant atoms with silicon interstitials. Damage recovery of 40keV Si+ implants in phosphorus doped wells is experimentally analyzed. The effects of background phosphorus concentration, self implant dose, and anneal temperature are investigated. Phosphorus concentrations ranging from 2.0 x 1017 to 4.0 x 1019 cm-3 and Si+ doses ranging from 5.0 x 1013 cm-2 to 2.0 x 1014 cm-2 are studied during 650-800°C anneals. A dramatic reduction in the number of interstitials bound in {311} defects with increasing phosphorus background concentration is observed. It is suggested that the reduction of interstitials in {311} defects at high phosphorus concentrations is due to the formation of phosphorus-interstitial clusters (PICs). The critical concentration for clustering (approximately 1.0 x 1019 cm-3 at 750°C) is strongly temperature dependent and in close agreement with the kink concentration of phosphorus diffusion. Information gained from these "well experiments" is applied to the study of direct phosphorus implantation. An experimental study is conducted on 40keV phosphorus implanted to a dose of 1.0 x 1014 cm-2 during 650-800°C anneals. Electrically inactive PICs are shown to form at concentrations below the solid solubility limit due to high interstitial supersaturations. Data useful for developing a model to accurately predict phosphorus diffusion under nonequilibrium conditions are extracted from the experimental results. A cluster-mediated diffusion model is developed using the Florida Object Oriented Process Simulator (FLOOPS). The nucleation of defects is controlled by the diffusion-limited competition for excess interstitials between PICs and {311} clusters. The release of interstitials is driven by cluster dissolution. Modeling results show a strong correlation to those experimentally observed over a wide temporal and thermal domain using a single set of parameters. Improvements in process simulator accuracy are demonstrated with respect to dopant activation, TED, and dose loss.

Position-Dependent Dynamics Explain Pore-Averaged Diffusion in Strongly Attractive Adsorptive Systems.

PubMed

Krekelberg, William P; Siderius, Daniel W; Shen, Vincent K; Truskett, Thomas M; Errington, Jeffrey R

2017-12-12

Using molecular simulations, we investigate the relationship between the pore-averaged and position-dependent self-diffusivity of a fluid adsorbed in a strongly attractive pore as a function of loading. Previous work (Krekelberg, W. P.; Siderius, D. W.; Shen, V. K.; Truskett, T. M.; Errington, J. R. Connection between thermodynamics and dynamics of simple fluids in highly attractive pores. Langmuir 2013, 29, 14527-14535, doi: 10.1021/la4037327) established that pore-averaged self-diffusivity in the multilayer adsorption regime, where the fluid exhibits a dense film at the pore surface and a lower density interior pore region, is nearly constant as a function of loading. Here we show that this puzzling behavior can be understood in terms of how loading affects the fraction of particles that reside in the film and interior pore regions as well as their distinct dynamics. Specifically, the insensitivity of pore-averaged diffusivity to loading arises from the approximate cancellation of two factors: an increase in the fraction of particles in the higher diffusivity interior pore region with loading and a corresponding decrease in the particle diffusivity in that region. We also find that the position-dependent self-diffusivities scale with the position-dependent density. We present a model for predicting the pore-average self-diffusivity based on the position-dependent self-diffusivity, which captures the unusual characteristics of pore-averaged self-diffusivity in strongly attractive pores over several orders of magnitude.

Diffuse reflectance relations based on diffusion dipole theory for large absorption and reduced scattering

NASA Astrophysics Data System (ADS)

Bremmer, Rolf H.; van Gemert, Martin J. C.; Faber, Dirk J.; van Leeuwen, Ton G.; Aalders, Maurice C. G.

2013-08-01

Diffuse reflectance spectra are used to determine the optical properties of biological samples. In medicine and forensic science, the turbid objects under study often possess large absorption and/or scattering properties. However, data analysis is frequently based on the diffusion approximation to the radiative transfer equation, implying that it is limited to tissues where the reduced scattering coefficient dominates over the absorption coefficient. Nevertheless, up to absorption coefficients of 20 m at reduced scattering coefficients of 1 and 11.5 mm-1, we observed excellent agreement (r2=0.994) between reflectance measurements of phantoms and the diffuse reflectance equation proposed by Zonios et al. [Appl. Opt. 38, 6628-6637 (1999)], derived as an approximation to one of the diffusion dipole equations of Farrell et al. [Med. Phys. 19, 879-888 (1992)]. However, two parameters were fitted to all phantom experiments, including strongly absorbing samples, implying that the reflectance equation differs from diffusion theory. Yet, the exact diffusion dipole approximation at high reduced scattering and absorption also showed agreement with the phantom measurements. The mathematical structure of the diffuse reflectance relation used, derived by Zonios et al. [Appl. Opt. 38, 6628-6637 (1999)], explains this observation. In conclusion, diffuse reflectance relations derived as an approximation to the diffusion dipole theory of Farrell et al. can analyze reflectance ratios accurately, even for much larger absorption than reduced scattering coefficients. This allows calibration of fiber-probe set-ups so that the object's diffuse reflectance can be related to its absorption even when large. These findings will greatly expand the application of diffuse reflection spectroscopy. In medicine, it may allow the use of blue/green wavelengths and measurements on whole blood, and in forensic science, it may allow inclusion of objects such as blood stains and cloth at crime scenes.

Chemical Complexity in Local Diffuse and Translucent Clouds: Ubiquitous Linear C3H and CH3CN, a Detection of HC3N and an Upper Limit on the Abundance of CH2CN

NASA Astrophysics Data System (ADS)

Liszt, Harvey; Gerin, Maryvonne; Beasley, Anthony; Pety, Jerome

2018-04-01

We present Jansky Very Large Array observations of 20–37 GHz absorption lines from nearby Galactic diffuse molecular gas seen against four cosmologically distant compact radio continuum sources. The main new observational results are that l-C3H and CH3CN are ubiqitous in the local diffuse molecular interstellar medium at {\\text{}}{A}{{V}} ≲ 1, while HC3N was seen only toward B0415 at {\\text{}}{A}{{V}} > 4 mag. The linear/cyclic ratio is much larger in C3H than in C3H2 and the ratio CH3CN/HCN is enhanced compared to TMC-1, although not as much as toward the Horsehead Nebula. More consequentially, this work completes a long-term program assessing the abundances of small hydrocarbons (CH, C2H, linear and cyclic C3H and C3 {{{H}}}2, and C4H and C4H‑) and the CN-bearing species (CN, HCN, HNC, HC3N, HC5N, and CH3CN): their systematics in diffuse molecular gas are presented in detail here. We also observed but did not strongly constrain the abundances of a few oxygen-bearing species, most prominently HNCO. We set limits on the column density of CH2CN, such that the anion CH2CN‑ is only viable as a carrier of diffuse interstellar bands if the N(CH2CN)/N(CH2CN‑) abundance ratio is much smaller in this species than in any others for which the anion has been observed. We argue that complex organic molecules (COMS) are not present in clouds meeting a reasonable definition of diffuse molecular gas, i.e., {\\text{}}{A}{{V}} ≲ 1 mag. Based on observations obtained with the NRAO Jansky Very Large Array (VLA).

Diffusion of Conserved Charges in Relativistic Heavy Ion Collisions

NASA Astrophysics Data System (ADS)

Greif, Moritz; Fotakis, Jan. A.; Denicol, Gabriel S.; Greiner, Carsten

2018-06-01

We demonstrate that the diffusion currents do not depend only on gradients of their corresponding charge density, but that the different diffusion charge currents are coupled. This happens in such a way that it is possible for density gradients of a given charge to generate dissipative currents of another charge. Within this scheme, the charge diffusion coefficient is best viewed as a matrix, in which the diagonal terms correspond to the usual charge diffusion coefficients, while the off-diagonal terms describe the coupling between the different currents. In this Letter, we calculate for the first time the complete diffusion matrix for hot and dense nuclear matter, including baryon, electric, and strangeness charges. We find that the baryon diffusion current is strongly affected by baryon charge gradients but also by its coupling to gradients in strangeness. The electric charge diffusion current is found to be strongly affected by electric and strangeness gradients, whereas strangeness currents depend mostly on strange and baryon gradients.

High-temperature morphological evolution of lithographically introduced cavities in silicon carbide

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

Narushima, Takayuki; Glaeser, Andreas M.

2000-12-01

Internal cavities of controlled geometry and crystallography were introduced in 6H silicon carbide single crystals by combining lithographic methods, ion beam etching, and solid-state diffusion bonding. The morphological evolution of these internal cavities (negative crystals) in response to anneals of up to 128 h duration at 1900 degrees C was examined using optical microscopy. Surface energy anisotropy and faceting have a strong influence on both the geometric and kinetic characteristics of evolution. Decomposition of 12{bar 1}0 cavity edges into 101{bar 0} facets was observed after 16 h anneals, indicating that 12{bar 1}0 faces are not components of the Wulff shape.more » The shape evolution kinetics of penny-shaped cavities were also investigated. Experimentally observed evolution rates decreased much more rapidly with those predicted by a model in which surface diffusion is assumed to be rate-limiting. This suggests that the development of facets, and the associated loss of ledges and terraces during the initial stages of evolution results in an evolution process limited by the nucleation rate of attachment/detachment sites (ledges) on the facets.« less

Relativistic Electron Precipitation: An Observational Study.

DTIC Science & Technology

1980-01-01

al., 1970). These so-called "n + 1/2" waves (- n + 1/2) are found throughout the magnetosphere outside the plasmapause (Kennel et al., 1970; Shaw and...diffusion scattering one requires 2 L D~ . LSD - z ~.(21) 73 where aL = loss cone pitch angle D SD = coefficient for strong diffusion. Equation (20) can be...with substitutions yields a fluctuating field wave amplitude for strong electron diffusion: a." 0- x(23) and 00for f= LSD (24) LRo LRo + For ions

The role of external and internal mass transfer in the process of Cu2+ removal by natural mineral sorbents.

PubMed

Sljivić, M; Smiciklas, I; Plećas, I; Pejanović, S

2011-07-01

The kinetics of Cu2+ sorption on to zeolite, clay and diatomite was investigated as a function of initial metal concentrations. For consideration of the mass transfer phenomena, single resistance models based on both film and intraparticle diffusion were tested and compared. The obtained results suggested that the rate-limiting step in Cu2+ sorption strongly depended on the sorbent type, as well as on initial cation concentration. The decrease in external mass transfer coefficients with the increase in initial metal concentrations was in excellent agreement with expressions based on Sherwood and Schmidt dimensionless numbers. The internal diffusivities through zeolite particles were in the range 1.0 x 10(-11) to 1.0 x 10(-13) m2/min, depending on the Cu2+ concentration and the applied theoretical model.

The influence of polarizability and charge transfer on specific ion effects in the dynamics of aqueous salt solutions

NASA Astrophysics Data System (ADS)

Nguyen, Mary; Rick, Steven W.

2018-06-01

The diffusion rates for water molecules in salt solutions depend on the identity of the ions, as well as their concentration. Among the alkali metal ions, cesium and potassium increase and sodium strongly decreases the diffusion constant of water. The origin of the difference can be understood by examining the simulation results using different potential models. In this work, aqueous solutions of salts are simulated with a variety of models. Commonly used non-polarizable models, which otherwise reproduce many experimental properties, do not capture the trend in the diffusion constant, while models which include polarization and/or charge transfer interactions do. For the non-polarizable models, the diffusion constant decreases too strongly with salt concentration. The changes in the water diffusion constant with increasing salt concentration match the diffusion constant of the ion. The ion diffusion constant is dependent on the residence time for water in the ion solvation shell. The non-polarizable models over-estimate the residence time, relative to the translational diffusion constant and so tend to under-estimate the ion and water diffusion constants.

Self-diffusion in dense granular shear flows.

PubMed

Utter, Brian; Behringer, R P

2004-03-01

Diffusivity is a key quantity in describing velocity fluctuations in granular materials. These fluctuations are the basis of many thermodynamic and hydrodynamic models which aim to provide a statistical description of granular systems. We present experimental results on diffusivity in dense, granular shear flows in a two-dimensional Couette geometry. We find that self-diffusivities D are proportional to the local shear rate gamma; with diffusivities along the direction of the mean flow approximately twice as large as those in the perpendicular direction. The magnitude of the diffusivity is D approximately gamma;a(2), where a is the particle radius. However, the gradient in shear rate, coupling to the mean flow, and strong drag at the moving boundary lead to particle displacements that can appear subdiffusive or superdiffusive. In particular, diffusion appears to be superdiffusive along the mean flow direction due to Taylor dispersion effects and subdiffusive along the perpendicular direction due to the gradient in shear rate. The anisotropic force network leads to an additional anisotropy in the diffusivity that is a property of dense systems and has no obvious analog in rapid flows. Specifically, the diffusivity is suppressed along the direction of the strong force network. A simple random walk simulation reproduces the key features of the data, such as the apparent superdiffusive and subdiffusive behavior arising from the mean velocity field, confirming the underlying diffusive motion. The additional anisotropy is not observed in the simulation since the strong force network is not included. Examples of correlated motion, such as transient vortices, and Lévy flights are also observed. Although correlated motion creates velocity fields which are qualitatively different from collisional Brownian motion and can introduce nondiffusive effects, on average the system appears simply diffusive.

A generalized electrochemical aggregative growth mechanism.

PubMed

Ustarroz, Jon; Hammons, Joshua A; Altantzis, Thomas; Hubin, Annick; Bals, Sara; Terryn, Herman

2013-08-07

The early stages of nanocrystal nucleation and growth are still an active field of research and remain unrevealed. In this work, by the combination of aberration-corrected transmission electron microscopy (TEM) and electrochemical characterization of the electrodeposition of different metals, we provide a complete reformulation of the Volmer-Weber 3D island growth mechanism, which has always been accepted to explain the early stages of metal electrodeposition and thin-film growth on low-energy substrates. We have developed a Generalized Electrochemical Aggregative Growth Mechanism which mimics the atomistic processes during the early stages of thin-film growth, by incorporating nanoclusters as building blocks. We discuss the influence of new processes such as nanocluster self-limiting growth, surface diffusion, aggregation, and coalescence on the growth mechanism and morphology of the resulting nanostructures. Self-limiting growth mechanisms hinder nanocluster growth and favor coalescence driven growth. The size of the primary nanoclusters is independent of the applied potential and deposition time. The balance between nucleation, nanocluster surface diffusion, and coalescence depends on the material and the overpotential, and influences strongly the morphology of the deposits. A small extent of coalescence leads to ultraporous dendritic structures, large surface coverage, and small particle size. Contrarily, full recrystallization leads to larger hemispherical monocrystalline islands and smaller particle density. The mechanism we propose represents a scientific breakthrough from the fundamental point of view and indicates that achieving the right balance between nucleation, self-limiting growth, cluster surface diffusion, and coalescence is essential and opens new, exciting possibilities to build up enhanced supported nanostructures using nanoclusters as building blocks.

Two-dimensional nanoscale correlations in the strong negative thermal expansion material ScF 3

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

Handunkanda, Sahan U.; Occhialini, Connor A.; Said, Ayman H.

We present diffuse x-ray scattering data on the strong negative thermal expansion (NTE) material ScF3 and find that two-dimensional nanoscale correlations exist at momentum-space regions associated with possibly rigid rotations of the perovskite octahedra. We address the extent to which rigid octahedral motion describes the dynamical fluctuations behind NTE by generalizing a simple model supporting a single floppy mode that is often used to heuristically describe instances of NTE. We find this model has tendencies toward dynamic inhomogeneities and its application to recent and existing experimental data suggest an intricate link between the nanometer correlation length scale, the energy scalemore » for octahedral tilt fluctuations, and the coefficient of thermal expansion in ScF3. We then investigate the breakdown of the rigid limit and propose a resolution to an outstanding debate concerning the role of molecular rigidity in strong NTE materials.« less

Complex dynamics induced by strong confinement - From tracer diffusion in strongly heterogeneous media to glassy relaxation of dense fluids in narrow slits

NASA Astrophysics Data System (ADS)

Mandal, Suvendu; Spanner-Denzer, Markus; Leitmann, Sebastian; Franosch, Thomas

2017-08-01

We provide an overview of recent advances of the complex dynamics of particles in strong confinements. The first paradigm is the Lorentz model where tracers explore a quenched disordered host structure. Such systems naturally occur as limiting cases of binary glass-forming systems if the dynamics of one component is much faster than the other. For a certain critical density of the host structure the tracers undergo a localization transition which constitutes a critical phenomenon. A series of predictions in the vicinity of the transition have been elaborated and tested versus computer simulations. Analytical progress is achieved for small obstacle densities. The second paradigm is a dense strongly interacting liquid confined to a narrow slab. Then the glass transition depends nonmonotonically on the separation of the plates due to an interplay of local packing and layering. Very small slab widths allow to address certain features of the statics and dynamics analytically.

Transport coefficients and heat fluxes in non-equilibrium high-temperature flows with electronic excitation

NASA Astrophysics Data System (ADS)

Istomin, V. A.; Kustova, E. V.

2017-02-01

The influence of electronic excitation on transport processes in non-equilibrium high-temperature ionized mixture flows is studied. Two five-component mixtures, N 2 / N2 + / N / N + / e - and O 2 / O2 + / O / O + / e - , are considered taking into account the electronic degrees of freedom for atomic species as well as the rotational-vibrational-electronic degrees of freedom for molecular species, both neutral and ionized. Using the modified Chapman-Enskog method, the transport coefficients (thermal conductivity, shear viscosity and bulk viscosity, diffusion and thermal diffusion) are calculated in the temperature range 500-50 000 K. Thermal conductivity and bulk viscosity coefficients are strongly affected by electronic states, especially for neutral atomic species. Shear viscosity, diffusion, and thermal diffusion coefficients are not sensible to electronic excitation if the size of excited states is assumed to be constant. The limits of applicability for the Stokes relation are discussed; at high temperatures, this relation is violated not only for molecular species but also for electronically excited atomic gases. Two test cases of strongly non-equilibrium flows behind plane shock waves corresponding to the spacecraft re-entry (Hermes and Fire II) are simulated numerically. Fluid-dynamic variables and heat fluxes are evaluated in gases with electronic excitation. In inviscid flows without chemical-radiative coupling, the flow-field is weakly affected by electronic states; however, in viscous flows, their influence can be more important, in particular, on the convective heat flux. The contribution of different dissipative processes to the heat transfer is evaluated as well as the effect of reaction rate coefficients. The competition of diffusion and heat conduction processes reduces the overall effect of electronic excitation on the convective heating, especially for the Fire II test case. It is shown that reliable models of chemical reaction rates are of great importance for accurate predictions of the fluid dynamic variables and heat fluxes.

Diffusion of neon in white dwarf stars.

PubMed

Hughto, J; Schneider, A S; Horowitz, C J; Berry, D K

2010-12-01

Sedimentation of the neutron rich isotope 22Ne may be an important source of gravitational energy during the cooling of white dwarf stars. This depends on the diffusion constant for 22Ne in strongly coupled plasma mixtures. We calculate self-diffusion constants D(i) from molecular dynamics simulations of carbon, oxygen, and neon mixtures. We find that D(i) in a mixture does not differ greatly from earlier one component plasma results. For strong coupling (coulomb parameter Γ> few), D(i) has a modest dependence on the charge Z(i) of the ion species, D(i)∝Z(i)(-2/3). However, D(i) depends more strongly on Z(i) for weak coupling (smaller Γ). We conclude that the self-diffusion constant D(Ne) for 22Ne in carbon, oxygen, and neon plasma mixtures is accurately known so that uncertainties in D(Ne) should be unimportant for simulations of white dwarf cooling.

The role of intra-NAPL diffusion on mass transfer from MGP residuals

NASA Astrophysics Data System (ADS)

Shafieiyoun, Saeid; Thomson, Neil R.

2018-06-01

An experimental and computational study was performed to investigate the role of multi-component intra-NAPL diffusion on NAPL-water mass transfer. Molecular weight and the NAPL component concentrations were determined to be the most important parameters affecting intra-NAPL diffusion coefficients. Four NAPLs with different viscosities but the same quantified mass were simulated. For a spherical NAPL body, a combination of NAPL properties and interphase mass transfer rate can result in internal diffusion limitations. When the main intra-NAPL diffusion coefficients are in the range of self-diffusion coefficients (10-5 to 10-6 cm2/s), dissolution is not limited by internal diffusion except for high mass transfer rate coefficients (>180 cm/day). For a complex and relatively high viscous NAPL (>50 g/(cm s)), smaller intra-NAPL diffusion coefficients (<10-8) are expected and even low mass transfer rate coefficients ( 6 cm/day) can result in diffusion-limited dissolution.

Green-noise halftoning with dot diffusion

NASA Astrophysics Data System (ADS)

Lippens, Stefaan; Philips, Wilfried

2007-02-01

Dot diffusion is a halftoning technique that is based on the traditional error diffusion concept, but offers a high degree of parallel processing by its block based approach. Traditional dot diffusion however suffers from periodicity artifacts. To limit the visibility of these artifacts, we propose grid diffusion, which applies different class matrices for different blocks. Furthermore, in this paper we will discuss two approaches in the dot diffusion framework to generate green-noise halftone patterns. The first approach is based on output dependent feedback (hysteresis), analogous to the standard green-noise error diffusion techniques. We observe that the resulting halftones are rather coarse and highly dependent on the used dot diffusion class matrices. In the second approach we don't limit the diffusion to the nearest neighbors. This leads to less coarse halftones, compared to the first approach. The drawback is that it can only cope with rather limited cluster sizes. We can reduce these drawbacks by combining the two approaches.

Anomalous dimensionality dependence of diffusion in a rugged energy landscape: How pathological is one dimension?

NASA Astrophysics Data System (ADS)

Seki, Kazuhiko; Bagchi, Kaushik; Bagchi, Biman

2016-05-01

Diffusion in one dimensional rugged energy landscape (REL) is predicted to be pathologically different (from any higher dimension) with a much larger chance of encountering broken ergodicity [D. L. Stein and C. M. Newman, AIP Conf. Proc. 1479, 620 (2012)]. However, no quantitative study of this difference has been reported, despite the prevalence of multidimensional physical models in the literature (like a high dimensional funnel guiding protein folding/unfolding). Paradoxically, some theoretical studies of these phenomena still employ a one dimensional diffusion description for analytical tractability. We explore the dimensionality dependent diffusion on REL by carrying out an effective medium approximation based analytical calculations and compare them with the available computer simulation results. We find that at an intermediate level of ruggedness (assumed to have a Gaussian distribution), where diffusion is well-defined, the value of the effective diffusion coefficient depends on dimensionality and changes (increases) by several factors (˜5-10) in going from 1d to 2d. In contrast, the changes in subsequent transitions (like 2d to 3d and 3d to 4d and so on) are far more modest, of the order of 10-20% only. When ruggedness is given by random traps with an exponential distribution of barrier heights, the mean square displacement (MSD) is sub-diffusive (a well-known result), but the growth of MSD is described by different exponents in one and higher dimensions. The reason for such strong ruggedness induced retardation in the case of one dimensional REL is discussed. We also discuss the special limiting case of infinite dimension (d = ∞) where the effective medium approximation becomes exact and where theoretical results become simple. We discuss, for the first time, the role of spatial correlation in the landscape on diffusion of a random walker.

Anomalous dimensionality dependence of diffusion in a rugged energy landscape: How pathological is one dimension?

PubMed

Seki, Kazuhiko; Bagchi, Kaushik; Bagchi, Biman

2016-05-21

Diffusion in one dimensional rugged energy landscape (REL) is predicted to be pathologically different (from any higher dimension) with a much larger chance of encountering broken ergodicity [D. L. Stein and C. M. Newman, AIP Conf. Proc. 1479, 620 (2012)]. However, no quantitative study of this difference has been reported, despite the prevalence of multidimensional physical models in the literature (like a high dimensional funnel guiding protein folding/unfolding). Paradoxically, some theoretical studies of these phenomena still employ a one dimensional diffusion description for analytical tractability. We explore the dimensionality dependent diffusion on REL by carrying out an effective medium approximation based analytical calculations and compare them with the available computer simulation results. We find that at an intermediate level of ruggedness (assumed to have a Gaussian distribution), where diffusion is well-defined, the value of the effective diffusion coefficient depends on dimensionality and changes (increases) by several factors (∼5-10) in going from 1d to 2d. In contrast, the changes in subsequent transitions (like 2d to 3d and 3d to 4d and so on) are far more modest, of the order of 10-20% only. When ruggedness is given by random traps with an exponential distribution of barrier heights, the mean square displacement (MSD) is sub-diffusive (a well-known result), but the growth of MSD is described by different exponents in one and higher dimensions. The reason for such strong ruggedness induced retardation in the case of one dimensional REL is discussed. We also discuss the special limiting case of infinite dimension (d = ∞) where the effective medium approximation becomes exact and where theoretical results become simple. We discuss, for the first time, the role of spatial correlation in the landscape on diffusion of a random walker.

Antenatal dexamethasone before asphyxia promotes cystic neural injury in preterm fetal sheep by inducing hyperglycemia.

PubMed

Lear, Christopher A; Davidson, Joanne O; Mackay, Georgia R; Drury, Paul P; Galinsky, Robert; Quaedackers, Josine S; Gunn, Alistair J; Bennet, Laura

2018-04-01

Antenatal glucocorticoid therapy significantly improves the short-term systemic outcomes of prematurely born infants, but there is limited information available on their impact on neurodevelopmental outcomes in at-risk preterm babies exposed to perinatal asphyxia. Preterm fetal sheep (0.7 of gestation) were exposed to a maternal injection of 12 mg dexamethasone or saline followed 4 h later by asphyxia induced by 25 min of complete umbilical cord occlusion. In a subsequent study, fetuses received titrated glucose infusions followed 4 h later by asphyxia to examine the hypothesis that hyperglycemia mediated the effects of dexamethasone. Post-mortems were performed 7 days after asphyxia for cerebral histology. Maternal dexamethasone before asphyxia was associated with severe, cystic brain injury compared to diffuse injury after saline injection, with increased numbers of seizures, worse recovery of brain activity, and increased arterial glucose levels before, during, and after asphyxia. Glucose infusions before asphyxia replicated these adverse outcomes, with a strong correlation between greater increases in glucose before asphyxia and greater neural injury. These findings strongly suggest that dexamethasone exposure and hyperglycemia can transform diffuse injury into cystic brain injury after asphyxia in preterm fetal sheep.

Attempt to model laboratory-scale diffusion and retardation data.

PubMed

Hölttä, P; Siitari-Kauppi, M; Hakanen, M; Tukiainen, V

2001-02-01

Different approaches for measuring the interaction between radionuclides and rock matrix are needed to test the compatibility of experimental retardation parameters and transport models used in assessing the safety of the underground repositories for the spent nuclear fuel. In this work, the retardation of sodium, calcium and strontium was studied on mica gneiss, unaltered, moderately altered and strongly altered tonalite using dynamic fracture column method. In-diffusion of calcium into rock cubes was determined to predict retardation in columns. In-diffusion of calcium into moderately and strongly altered tonalite was interpreted using a numerical code FTRANS. The code was able to interprete in-diffusion of weakly sorbing calcium into the saturated porous matrix. Elution curves of calcium for the moderately and strongly altered tonalite fracture columns were explained adequately using FTRANS code and parameters obtained from in-diffusion calculations. In this paper, mass distribution ratio values of sodium, calcium and strontium for intact rock are compared to values, previously obtained for crushed rock from batch and crushed rock column experiments. Kd values obtained from fracture column experiments were one order of magnitude lower than Kd values from batch experiments.

Diffusive dynamics of nanoparticles in ultra-confined media

DOE PAGES

Jacob, Jack Deodato; Conrad, Jacinta; Krishnamoorti, Ramanan; ...

2015-08-10

Differential dynamic microscopy (DDM) was used to investigate the diffusive dynamics of nanoparticles of diameter 200 400 nm that were strongly confined in a periodic square array of cylindrical nanoposts. The minimum distance between posts was 1.3 5 times the diameter of the nanoparticles. The image structure functions obtained from the DDM analysis were isotropic and could be fit by a stretched exponential function. The relaxation time scaled diffusively across the range of wave vectors studied, and the corresponding scalar diffusivities decreased monotonically with increased confinement. The decrease in diffusivity could be described by models for hindered diffusion that accountedmore » for steric restrictions and hydrodynamic interactions. The stretching exponent decreased linearly as the nanoparticles were increasingly confined by the posts. Altogether, these results are consistent with a picture in which strongly confined nanoparticles experience a heterogeneous spatial environment arising from hydrodynamics and volume exclusion on time scales comparable to cage escape, leading to multiple relaxation processes and Fickian but non-Gaussian diffusive dynamics.« less

Innovation-diffusion: a geographical study of the transition of family limitation practice in Taiwan.

PubMed

Ting, T Y

1984-09-01

This paper uses map analysis to study the transition of family limitation practice in Taiwan between 1961-80. The innovation-diffusion perspective emphasizes that birth control, particularly contraception, is a recent innovation and is essentially new in human culture. The innovation-diffusion theory assumes that the decline of fertility began in a setting where there was no, or at most very limited, previous practice of birth control. The theory emphasizes the importance of the spread of information. It also assumes that innovation starts in metropolitan centers, diffuses to other urban places with some delay, and penetrates to rural areas still later. Innovation behavior also diffuses from 1 area to another which is culturally and linguistically similar. Although there was some urban to rural diffusion from the Taiwan family planning program, the government supported program provided services more evenly between urban and rural areas, thus somewhat limiting the diffusion effect from the program. For the diffusion of family practice in Taiwan, it is expected that the availability of of information about and means of family limitation practice may effect the rate of the increase of small m values -- an index of family limitation -- in an area. The case study of Pingtung county shows that the demand-side diffusion from urban to rural areas was important in the earlier decade of the transition of family plimitation practice, but distance from urban center was less important as practice became more uniform through diffusion. Ethnicity, whether or not the township was dominated by Hakka or Fukienese, also seems to have played an important role in determining the pace at which the local residents adopted family practice limitation. Hakka townships seem to have adopted family limitation practice more slowly than Fukienese townships about the same distance from the urban center. The map analysis of Pingtung county provides descriptive evidence to support the diffusion of family limitation from urban centers to distant areas, while ethnic variables like Hakka population tend to delay the adoption of family limitation practice. In general, the urban center had higher m values than the surrounding rural areas in Pingtung county and for areas other than the urban center the the level of m values is a negative function of the distance to the urban center.

Microstructural imaging of the human brain with a 'super-scanner': 10 key advantages of ultra-strong gradients for diffusion MRI.

PubMed

Jones, D K; Alexander, D C; Bowtell, R; Cercignani, M; Dell'Acqua, F; McHugh, D J; Miller, K L; Palombo, M; Parker, G J M; Rudrapatna, U S; Tax, C M W

2018-05-22

The key component of a microstructural diffusion MRI 'super-scanner' is a dedicated high-strength gradient system that enables stronger diffusion weightings per unit time compared to conventional gradient designs. This can, in turn, drastically shorten the time needed for diffusion encoding, increase the signal-to-noise ratio, and facilitate measurements at shorter diffusion times. This review, written from the perspective of the UK National Facility for In Vivo MR Imaging of Human Tissue Microstructure, an initiative to establish a shared 300 mT/m-gradient facility amongst the microstructural imaging community, describes ten advantages of ultra-strong gradients for microstructural imaging. Specifically, we will discuss how the increase of the accessible measurement space compared to a lower-gradient systems (in terms of Δ, b-value, and TE) can accelerate developments in the areas of 1) axon diameter distribution mapping; 2) microstructural parameter estimation; 3) mapping micro-vs macroscopic anisotropy features with gradient waveforms beyond a single pair of pulsed-gradients; 4) multi-contrast experiments, e.g. diffusion-relaxometry; 5) tractography and high-resolution imaging in vivo and 6) post mortem; 7) diffusion-weighted spectroscopy of metabolites other than water; 8) tumour characterisation; 9) functional diffusion MRI; and 10) quality enhancement of images acquired on lower-gradient systems. We finally discuss practical barriers in the use of ultra-strong gradients, and provide an outlook on the next generation of 'super-scanners'. Copyright © 2018. Published by Elsevier Inc.

Modeling the Impact of Heterogeneous Spatial Distribution of Soil Microbes on Pesticide Biodegradation at the Centimeter Scale

NASA Astrophysics Data System (ADS)

Babey, T.; De Dreuzy, J. R.; Pinheiro, M.; Garnier, P.; Vieublé-Gonod, L.; Rapaport, A.

2015-12-01

Micro-organisms and substrates may be heterogeneously distributed in soils. This repartition as well as transport mechanisms bringing them into contact are expected to impact the biodegradation rates. Pinheiro et al [2015] have measured in cm-large reconstructed soil cores the fate of an injection of 2,4-D pesticide for different injection conditions and initial distributions of soil pesticide degraders. Through the calibration of a reactive transport model of these experiments, we show that: i) biodegradation of diffusion-controlled pesticide fluxes is favored by a high Damköhler number (high reaction rate compared to flux rate); ii) abiotic sorption processes are negligible and do not interact strongly with biodegradation; iii) biodegradation is primarily governed by the initial repartition of pesticide and degraders for diffusion-controlled transport, as diffusion greatly limits the flux of pesticide reaching the microbial hotspot due to dilution. These results suggest that for biodegradation to be substantial, a spatial heterogeneity in the repartition of microbes and substrate has to be associated with intermittent and fast transport processes to mix them.

A Decision-Based Modified Total Variation Diffusion Method for Impulse Noise Removal

PubMed Central

Zhu, Qingxin; Song, Xiuli; Tao, Jinsong

2017-01-01

Impulsive noise removal usually employs median filtering, switching median filtering, the total variation L1 method, and variants. These approaches however often introduce excessive smoothing and can result in extensive visual feature blurring and thus are suitable only for images with low density noise. A new method to remove noise is proposed in this paper to overcome this limitation, which divides pixels into different categories based on different noise characteristics. If an image is corrupted by salt-and-pepper noise, the pixels are divided into corrupted and noise-free; if the image is corrupted by random valued impulses, the pixels are divided into corrupted, noise-free, and possibly corrupted. Pixels falling into different categories are processed differently. If a pixel is corrupted, modified total variation diffusion is applied; if the pixel is possibly corrupted, weighted total variation diffusion is applied; otherwise, the pixel is left unchanged. Experimental results show that the proposed method is robust to different noise strengths and suitable for different images, with strong noise removal capability as shown by PSNR/SSIM results as well as the visual quality of restored images. PMID:28536602

PIV measurements in a compact return diffuser under multi-conditions

NASA Astrophysics Data System (ADS)

Zhou, L.; Lu, W. G.; Shi, W. D.

2013-12-01

Due to the complex three-dimensional geometries of impellers and diffusers, their design is a delicate and difficult task. Slight change could lead to significant changes in hydraulic performance and internal flow structure. Conversely, the grasp of the pump's internal flow pattern could benefit from pump design improvement. The internal flow fields in a compact return diffuser have been investigated experimentally under multi-conditions. A special Particle Image Velocimetry (PIV) test rig is designed, and the two-dimensional PIV measurements are successfully conducted in the diffuser mid-plane to capture the complex flow patterns. The analysis of the obtained results has been focused on the flow structure in diffuser, especially under part-load conditions. The vortex and recirculation flow patterns in diffuser are captured and analysed accordingly. Strong flow separation and back flow appeared at the part-load flow rates. Under the design and over-load conditions, the flow fields in diffuser are uniform, and the flow separation and back flow appear at the part-load flow rates, strong back flow is captured at one diffuser passage under 0.2Qdes.

Cavity Mediated Manipulation of Distant Spin Currents Using a Cavity-Magnon-Polariton.

PubMed

Bai, Lihui; Harder, Michael; Hyde, Paul; Zhang, Zhaohui; Hu, Can-Ming; Chen, Y P; Xiao, John Q

2017-05-26

Using electrical detection of a strongly coupled spin-photon system comprised of a microwave cavity mode and two magnetic samples, we demonstrate the long distance manipulation of spin currents. This distant control is not limited by the spin diffusion length, instead depending on the interplay between the local and global properties of the coupled system, enabling systematic spin current control over large distance scales (several centimeters in this work). This flexibility opens the door to improved spin current generation and manipulation for cavity spintronic devices.

Invisibility cloaking in the diffusive-light limit (presentation video)

NASA Astrophysics Data System (ADS)

Schittny, Robert; Kadic, Muamer; Wegener, Martin

2014-09-01

Albert Einstein's theory of relativity imposes stringent limitations to making macroscopic objects invisible with respect to electromagnetic light waves propagating in vacuum. These limitations are not relevant though for propagation of light in diffusive media like fog or milk because the effective energy speed is significantly lower than in vacuum due to multiple scattering events. Here, by exploiting the close mathematical analogy between the electrostatic or near-field limit of optics on the one hand and light diffusion on the other hand, we design, fabricate, and characterize simple core-shell cloaking structures for diffusive light propagation in cylindrical and spherical geometry.

A Theoretical Understanding of Circular Polarization Memory in Random Media

NASA Astrophysics Data System (ADS)

Dark, Julia

Radiative transport theory describes the propagation of light in random media that absorb, scatter, and emit radiation. To describe the propagation of light, the full polarization state is quantified using the Stokes parameters. For the sake of mathematical convenience, the polarization state of light is often neglected leading to the scalar radiative transport equation for the intensity only. For scalar transport theory, there is a well-established body of literature on numerical and analytic approximations to the radiative transport equation. We extend the scalar theory to the vector radiative transport equation (vRTE). In particular, we are interested in the theoretical basis for a phenomena called circular polarization memory. Circular polarization memory is the physical phenomena whereby circular polarization retains its ellipticity and handedness when propagating in random media. This is in contrast to the propagation of linear polarization in random media, which depolarizes at a faster rate, and specular reflection of circular polarization, whereby the circular polarization handedness flips. We investigate two limits that are of known interest in the phenomena of circular polarization memory. The first limit we investigate is that of forward-peaked scattering, i.e. the limit where most scattering events occur in the forward or near-forward directions. The second limit we consider is that of strong scattering and weak absorption. In the forward-peaked scattering limit we approximate the vRTE by a system of partial differential equations motivated by the scalar Fokker-Planck approximation. We call the leading order approximation the vector Fokker-Planck approximation. The vector Fokker Planck approximation predicts that strongly forward-peaked media exhibit circular polarization memory where the strength of the effect can be calculated from the expansion of the scattering matrix in special functions. In addition, we find in this limit that total intensity, linear polarization, and circular polarization decouple. From this result we conclude, that in the Fokker-Planck limit the scalar approximation is an appropriate leading order approximation. In the strong scattering and weak absorbing limit the vector radiative transport equation can be analyzed using boundary layer theory. In this case, the problem of light scattering in an optically thick medium is reduced to a 1D vRTE near the boundary and a 3D diffusion equation in the interior. We develop and implement a numerical solver for the boundary layer problem by using a discrete ordinate solver in the boundary layer and a spectral method to solve the diffusion approximation in the interior. We implement the method in Fortran 95 with external dependencies on BLAS, LAPACK, and FFTW. By analyzing the spectrum of the discretized vRTE in the boundary layer, we are able to predict the presence of circular polarization memory in a given medium.

Perpendicular diffusion of a dilute beam of charged dust particles in a strongly coupled dusty plasma

NASA Astrophysics Data System (ADS)

Liu, Bin; Goree, J.

2014-06-01

The diffusion of projectiles drifting through a target of strongly coupled dusty plasma is investigated in a simulation. A projectile's drift is driven by a constant force F. We characterize the random walk of the projectiles in the direction perpendicular to their drift. The perpendicular diffusion coefficient Dp⊥ is obtained from the simulation data. The force dependence of Dp⊥ is found to be a power law in a high force regime, but a constant at low forces. A mean kinetic energy Wp for perpendicular motion is also obtained. The diffusion coefficient is found to increase with Wp with a linear trend at higher energies, but an exponential trend at lower energies.

Diffusion of Sticky Nanoparticles in a Polymer Melt: Crossover from Suppressed to Enhanced Transport

DOE PAGES

Carroll, Bobby; Bocharova, Vera; Carrillo, Jan-Michael Y.; ...

2018-03-09

The self-diffusion of a single large particle in a fluid is usually described by the classic Stokes–Einstein (SE) hydrodynamic relation. However, there are many fluids where the SE prediction for nanoparticles diffusion fails. These systems include diffusion of nanoparticles in porous media, in entangled and unentangled polymer melts and solutions, and protein diffusion in biological environments. A fundamental understanding of the microscopic parameters that govern nanoparticle diffusion is relevant to a wide range of applications. Here in this work, we present experimental measurements of the tracer diffusion coefficient of small and large nanoparticles that experience strong attractions with unentangled andmore » entangled polymer melt matrices. For the small nanoparticle system, a crossover from suppressed to enhanced diffusion is observed with increasing polymer molecular weight. We interpret these observations based on our theoretical and simulation insights of the preceding article (paper 1) as a result of a crossover from an effective hydrodynamic core–shell to a nonhydrodynamic vehicle mechanism of transport, with the latter strongly dependent on polymer–nanoparticle desorption time. In conclusion, a general zeroth-order qualitative picture for small sticky nanoparticle diffusion in polymer melts is proposed.« less

Diffusion of Sticky Nanoparticles in a Polymer Melt: Crossover from Suppressed to Enhanced Transport

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

Carroll, Bobby; Bocharova, Vera; Carrillo, Jan-Michael Y.

The self-diffusion of a single large particle in a fluid is usually described by the classic Stokes–Einstein (SE) hydrodynamic relation. However, there are many fluids where the SE prediction for nanoparticles diffusion fails. These systems include diffusion of nanoparticles in porous media, in entangled and unentangled polymer melts and solutions, and protein diffusion in biological environments. A fundamental understanding of the microscopic parameters that govern nanoparticle diffusion is relevant to a wide range of applications. Here in this work, we present experimental measurements of the tracer diffusion coefficient of small and large nanoparticles that experience strong attractions with unentangled andmore » entangled polymer melt matrices. For the small nanoparticle system, a crossover from suppressed to enhanced diffusion is observed with increasing polymer molecular weight. We interpret these observations based on our theoretical and simulation insights of the preceding article (paper 1) as a result of a crossover from an effective hydrodynamic core–shell to a nonhydrodynamic vehicle mechanism of transport, with the latter strongly dependent on polymer–nanoparticle desorption time. In conclusion, a general zeroth-order qualitative picture for small sticky nanoparticle diffusion in polymer melts is proposed.« less

A novel method for computing effective diffusivity: Application to helium implanted α-Fe thin films

NASA Astrophysics Data System (ADS)

Dunn, Aaron; Agudo-Merida, Laura; Martin-Bragado, Ignacio; McPhie, Mathieu; Cherkaoui, Mohammed; Capolungo, Laurent

2014-05-01

The effective diffusivity of helium in thin iron films is quantified using spatially resolved stochastic cluster dynamics and object kinetic Monte Carlo simulations. The roles of total displacement dose (in DPA), damage rate, helium to DPA ratio, layer thickness, and damage type (cascade damage vs Frenkel pair implantation) on effective He diffusivity are investigated. Helium diffusivity is found to decrease with increasing total damage and decreasing damage rate. Arrhenius plots show strongly increased helium diffusivity at high temperatures, high total implantation, and low implantation rates due to decreased vacancy and vacancy cluster concentrations. At low temperatures, effective diffusivity is weakly dependent on foil thickness while at high temperatures, narrower foils prevent defect accumulation by releasing all defects at the free surfaces. Helium to DPA ratio is not shown to strongly change helium diffusivity in the range of irradiation conditions simulated. Frenkel pair implantation is shown to cause higher effective diffusivity and more complex diffusion mechanisms than cascade implantation. The results of these simulations indicate that the differences in damage rates between implantation experiments and fission or fusion environments may result in differences in the final microstructure.

Theoretical Analysis of Drug Dissolution: I. Solubility and Intrinsic Dissolution Rate.

PubMed

Shekunov, Boris; Montgomery, Eda Ross

2016-09-01

The first-principles approach presented in this work combines surface kinetics and convective diffusion modeling applied to compounds with pH-dependent solubility and in different dissolution media. This analysis is based on experimental data available for approximately 100 compounds of pharmaceutical interest. Overall, there is a linear relationship between the drug solubility and intrinsic dissolution rate expressed through the total kinetic coefficient of dissolution and dimensionless numbers defining the mass transfer regime. The contribution of surface kinetics appears to be significant constituting on average ∼20% resistance to the dissolution flux in the compendial rotating disk apparatus at 100 rpm. The surface kinetics contribution becomes more dominant under conditions of fast laminar or turbulent flows or in cases when the surface kinetic coefficient may decrease as a function of solution composition or pH. Limitations of the well-known convective diffusion equation for rotating disk by Levich are examined using direct computational modeling with simultaneous dissociation and acid-base reactions in which intrinsic dissolution rate is strongly dependent on pH profile and solution ionic strength. It is shown that concept of diffusion boundary layer does not strictly apply for reacting/interacting species and that thin-film diffusion models cannot be used quantitatively in general case. Copyright © 2016. Published by Elsevier Inc.

Li dynamics in carbon-rich polymer-derived SiCN ceramics probed by NMR

NASA Astrophysics Data System (ADS)

Baek, Seung-Ho; Reinold, Lukas; Graczyk-Zajac, Magdalena; Riedel, Ralf; Hammerath, Franziska; Buechner, Bernd; Grafe, Hajo

2014-03-01

We report 7Li, 29Si, and 13C NMR studies of two different carbon-rich SiCN ceramics SiCN-1 and SiCN-3 derived from the preceramic polymers polyphenylvinylsilylcarbodiimide and polyphenylvinylsilazane, respectively. From the spectral analysis of the three nuclei at room temperature, we find that only the 13C spectrum is strongly influenced by Li insertion/extraction, suggesting that carbon phases are the major electrochemically active sites for Li storage. Temperature and Larmor frequency (ωL) dependences of the 7Li linewidth and spin-lattice relaxation rates T1-1 are described by an activated law with the activation energy EA of 0.31 eV and the correlation time τ0 in the high temperature limit of 1.3 ps. The 3 / 2 power law dependence of T1-1 on ωL which deviates from the standard Bloembergen, Purcell, and Pound (BPP) model implies that the Li motion on the μs timescale is governed by continuum diffusion mechanism rather than jump diffusion. On the other hand, the rotating frame relaxation rate T1ρ-1 results suggest that the slow motion of Li on the ms timescale may be affected by complex diffusion and/or non-diffusion processes.

Lithium dynamics in carbon-rich polymer-derived SiCN ceramics probed by nuclear magnetic resonance

NASA Astrophysics Data System (ADS)

Baek, Seung-Ho; Reinold, Lukas Mirko; Graczyk-Zajac, Magdalena; Riedel, Ralf; Hammerath, Franziska; Büchner, Bernd; Grafe, Hans-Joachim

2014-05-01

We report 7Li, 29Si, and 13C NMR studies of two different carbon-rich SiCN ceramics SiCN-1 and SiCN-3 derived from the preceramic polymers polyphenylvinylsilylcarbodiimide and polyphenylvinylsilazane, respectively. From the spectral analysis of the three nuclei, we find that only the 13C spectrum is strongly influenced by Li insertion/extraction, suggesting that carbon phases are the major electrochemically active sites for Li storage. Temperature (T) and Larmor frequency (ωL) dependences of the 7Li linewidth and spin-lattice relaxation rates T1-1 are described by an activated law with the activation energy EA of 0.31 eV and the correlation time τ0 in the high temperature limit of 1.3 ps. The 3 / 2 power law dependence of T1-1 on ωL which deviates from the standard Bloembergen, Purcell, and Pound (BPP) model implies that the Li motion on the μs timescale is governed by continuum diffusion mechanism rather than jump diffusion. On the other hand, the rotating frame relaxation rate T1ρ-1 results suggest that the slow motion of Li on the ms timescale may be affected by complex diffusion and/or non-diffusion processes.

Investigation of Termination in Free Radical Polymerization by Use of >Diffusion-Limited Interactions in Polymer Solutions

NASA Astrophysics Data System (ADS)

Wisnudel, Marc; Torkelson, John

1997-03-01

Termination between radicals has been simulated by use of phosphorescence quenching interactions, showing that segmental diffusion plays a strong role in the origin of autoacceleration or the gel effect. Quenching rate constants (k_q) were measured between benzil-labeled polymer as a function of anthracene-labeled polymer in polystyrene or polymethylmethacrylate solutions. Values of kq were obtained for interactions involving end- or center-labeled chains as a function of polymer MW and concentration. A large effect of label location was observed as interactions between center-labeled chains resulted in values of kq that were more MW-dependent and smaller in magnitude than those for interactions between end-labeled chains. For interactions between end-labeled chains at concentrations between 0 and 600 g/L, data show only very weak dependencies of kq on MW and concentration dependencies similar to that of segmental mobility. In addition, comparisons of kq data for interactions in PMMA-toluene solutions with termination rate constant (k_t) data for MMA polymerizations, showing weaker concentration dependencies for both kq and kt than translational diffusion coefficients in similar solutions, also indicate that segmental diffusion is important in termination.

Preface of "The Second Symposium on Border Zones Between Experimental and Numerical Application Including Solution Approaches By Extensions of Standard Numerical Methods"

NASA Astrophysics Data System (ADS)

Ortleb, Sigrun; Seidel, Christian

2017-07-01

In this second symposium at the limits of experimental and numerical methods, recent research is presented on practically relevant problems. Presentations discuss experimental investigation as well as numerical methods with a strong focus on application. In addition, problems are identified which require a hybrid experimental-numerical approach. Topics include fast explicit diffusion applied to a geothermal energy storage tank, noise in experimental measurements of electrical quantities, thermal fluid structure interaction, tensegrity structures, experimental and numerical methods for Chladni figures, optimized construction of hydroelectric power stations, experimental and numerical limits in the investigation of rain-wind induced vibrations as well as the application of exponential integrators in a domain-based IMEX setting.

Angiofibroma-like perineurioma. Report of a case.

PubMed

Zámečník, Michal; Mukenšnabl, Petr; Chlumská, Alena

2013-04-01

We report an unusual perineurioma with numerous vessels, showing a strong similarity with angiofibroma. A 2,5 x 2 x 2 cm subcutaneous/dermal tumor occurred in 58-ys-old male in the left brachial region. Histologically, it was composed of haphazardly arranged bland spindle cells and it contained prominent vasculature. In rare foci, the tumor cells showed thin bipolar processes and an onion-like perivascular whorling pattern. Immunohistochemically, expression of perineural cell markers EMA, claudin-1 and CD34 was limited to perivascular foci and to rare cells among the vessels. In addition, the tumor expressed CD10 diffusely. Our finding indicates that diagnosis of perineurioma should be considered also by tumors with an "angiofibromatous" morphology. Especially soft tissue angiofibroma, which often express EMA (perineural cell marker), shows a strong resemblance to angiofibroma-like perineurioma.

An empirical correction for moderate multiple scattering in super-heterodyne light scattering.

PubMed

Botin, Denis; Mapa, Ludmila Marotta; Schweinfurth, Holger; Sieber, Bastian; Wittenberg, Christopher; Palberg, Thomas

2017-05-28

Frequency domain super-heterodyne laser light scattering is utilized in a low angle integral measurement configuration to determine flow and diffusion in charged sphere suspensions showing moderate to strong multiple scattering. We introduce an empirical correction to subtract the multiple scattering background and isolate the singly scattered light. We demonstrate the excellent feasibility of this simple approach for turbid suspensions of transmittance T ≥ 0.4. We study the particle concentration dependence of the electro-kinetic mobility in low salt aqueous suspension over an extended concentration regime and observe a maximum at intermediate concentrations. We further use our scheme for measurements of the self-diffusion coefficients in the fluid samples in the absence or presence of shear, as well as in polycrystalline samples during crystallization and coarsening. We discuss the scope and limits of our approach as well as possible future applications.

Retention of ion-implanted-xenon in olivine: Dependence on implantation dose

NASA Technical Reports Server (NTRS)

Melcher, C. L.; Tombrello, T. A.; Burnett, D. S.

1982-01-01

The diffusion of Xe in olivine, a major mineral in both meteorites and lunar samples, was studied. Xe ions were implanted at 200 keV into single-crystal synthetic-forsterite targets and the depth profiles were measured by alpha particle backscattering before and after annealing for 1 hour at temperatures up to 1500 C. The fraction of implanted Xe retained following annealing was strongly dependent on the implantation dose. Maximum retention of 100% occurred for an implantion dose of 3 x 10 to the 15th power Xe ions/sq cm. Retention was less at lower doses, with (approximately more than or = 50% loss at one hundred trillion Xe ions/sq cm. Taking the diffusion coefficient at this dose as a lower limit, the minimum activation energy necessary for Xe retention in a 10 micrometer layer for ten million years was calculated as a function of metamorphic temperature.

Spin interactions in Graphene-Single Molecule Magnets Hybrids

NASA Astrophysics Data System (ADS)

Cervetti, Christian; Rettori, Angelo; Pini, Maria Gloria; Cornia, Andrea; Repollés, Aña; Luis, Fernando; Rauschenbach, Stephan; Dressel, Martin; Kern, Klaus; Burghard, Marko; Bogani, Lapo

2014-03-01

Graphene is a potential component of novel spintronics devices owing to its long spin diffusion length. Besides its use as spin-transport channel, graphene can be employed for the detection and manipulation of molecular spins. This requires an appropriate coupling between the sheets and the single molecular magnets (SMM). Here, we present a comprehensive characterization of graphene-Fe4 SMM hybrids. The Fe4 clusters are anchored non-covalently to the graphene following a diffusion-limited assembly and can reorganize into random networks when subjected to slightly elevated temperature. Molecules anchored on graphene sheets show unaltered static magnetic properties, whilst the quantum dynamics is profoundly modulated. Interaction with Dirac fermions becomes the dominant spin-relaxation channel, with observable effects produced by graphene phonons and reduced dipolar interactions. Coupling to graphene drives the spins over Villain's threshold, allowing the first observation of strongly-perturbative tunneling processes. Preliminary spin-transport experiments at low-temperature are further presented.

Measuring restriction sizes using diffusion weighted magnetic resonance imaging: a review.

PubMed

Martin, Melanie

2013-01-01

This article reviews a new concept in magnetic resonance as applied to cellular and biological systems. Diffusion weighted magnetic resonance imaging can be used to infer information about restriction sizes of samples being measured. The measurements rely on the apparent diffusion coefficient changing with diffusion times as measurements move from restricted to free diffusion regimes. Pulsed gradient spin echo (PGSE) measurements are limited in the ability to shorten diffusion times and thus are limited in restriction sizes which can be probed. Oscillating gradient spin echo (OGSE) measurements could provide shorter diffusion times so smaller restriction sizes could be probed.

Optimal Network Modularity for Information Diffusion

NASA Astrophysics Data System (ADS)

Nematzadeh, Azadeh; Ferrara, Emilio; Flammini, Alessandro; Ahn, Yong-Yeol

2014-08-01

We investigate the impact of community structure on information diffusion with the linear threshold model. Our results demonstrate that modular structure may have counterintuitive effects on information diffusion when social reinforcement is present. We show that strong communities can facilitate global diffusion by enhancing local, intracommunity spreading. Using both analytic approaches and numerical simulations, we demonstrate the existence of an optimal network modularity, where global diffusion requires the minimal number of early adopters.

Density PDFs of diffuse gas in the Milky Way

NASA Astrophysics Data System (ADS)

Berkhuijsen, E. M.; Fletcher, A.

2012-09-01

The probability distribution functions (PDFs) of the average densities of the diffuse ionized gas (DIG) and the diffuse atomic gas are close to lognormal, especially when lines of sight at |b| < 5∘ and |b|≥ 5∘ are considered separately. Our results provide strong support for the existence of a lognormal density PDF in the diffuse ISM, consistent with a turbulent origin of density structure in the diffuse gas.

Neutral evolution in a biological population as diffusion in phenotype space: reproduction with local mutation but without selection.

PubMed

Lawson, Daniel John; Jensen, Henrik Jeldtoft

2007-03-02

The process of "evolutionary diffusion," i.e., reproduction with local mutation but without selection in a biological population, resembles standard diffusion in many ways. However, evolutionary diffusion allows the formation of localized peaks that undergo drift, even in the infinite population limit. We relate a microscopic evolution model to a stochastic model which we solve fully. This allows us to understand the large population limit, relates evolution to diffusion, and shows that independent local mutations act as a diffusion of interacting particles taking larger steps.

Optimization of the short-circuit current in an InP nanowire array solar cell through opto-electronic modeling.

PubMed

Chen, Yang; Kivisaari, Pyry; Pistol, Mats-Erik; Anttu, Nicklas

2016-09-23

InP nanowire arrays with axial p-i-n junctions are promising devices for next-generation photovoltaics, with a demonstrated efficiency of 13.8%. However, the short-circuit current in such arrays does not match their absorption performance. Here, through combined optical and electrical modeling, we study how the absorption of photons and separation of the resulting photogenerated electron-hole pairs define and limit the short-circuit current in the nanowires. We identify how photogenerated minority carriers in the top n segment (i.e. holes) diffuse to the ohmic top contact where they recombine without contributing to the short-circuit current. In our modeling, such contact recombination can lead to a 60% drop in the short-circuit current. To hinder such hole diffusion, we include a gradient doping profile in the n segment to create a front surface barrier. This approach leads to a modest 5% increase in the short-circuit current, limited by Auger recombination with increased doping. A more efficient approach is to switch the n segment to a material with a higher band gap, like GaP. Then, a much smaller number of holes is photogenerated in the n segment, strongly limiting the amount that can diffuse and disappear into the top contact. For a 500 nm long top segment, the GaP approach leads to a 50% higher short-circuit current than with an InP top segment. Such a long top segment could facilitate the fabrication and contacting of nanowire array solar cells. Such design schemes for managing minority carriers could open the door to higher performance in single- and multi-junction nanowire-based solar cells.

Turbulent eddy diffusion models in exposure assessment - Determination of the eddy diffusion coefficient.

PubMed

Shao, Yuan; Ramachandran, Sandhya; Arnold, Susan; Ramachandran, Gurumurthy

2017-03-01

The use of the turbulent eddy diffusion model and its variants in exposure assessment is limited due to the lack of knowledge regarding the isotropic eddy diffusion coefficient, D T . But some studies have suggested a possible relationship between D T and the air changes per hour (ACH) through a room. The main goal of this study was to accurately estimate D T for a range of ACH values by minimizing the difference between the concentrations measured and predicted by eddy diffusion model. We constructed an experimental chamber with a spatial concentration gradient away from the contaminant source, and conducted 27 3-hr long experiments using toluene and acetone under different air flow conditions (0.43-2.89 ACHs). An eddy diffusion model accounting for chamber boundary, general ventilation, and advection was developed. A mathematical expression for the slope based on the geometrical parameters of the ventilation system was also derived. There is a strong linear relationship between D T and ACH, providing a surrogate parameter for estimating D T in real-life settings. For the first time, a mathematical expression for the relationship between D T and ACH has been derived that also corrects for non-ideal conditions, and the calculated value of the slope between these two parameters is very close to the experimentally determined value. The values of D T obtained from the experiments are generally consistent with values reported in the literature. They are also independent of averaging time of measurements, allowing for comparison of values obtained from different measurement settings. These findings make the use of turbulent eddy diffusion models for exposure assessment in workplace/indoor environments more practical.

Scaling of chaos in strongly nonlinear lattices.

PubMed

Mulansky, Mario

2014-06-01

Although it is now understood that chaos in complex classical systems is the foundation of thermodynamic behavior, the detailed relations between the microscopic properties of the chaotic dynamics and the macroscopic thermodynamic observations still remain mostly in the dark. In this work, we numerically analyze the probability of chaos in strongly nonlinear Hamiltonian systems and find different scaling properties depending on the nonlinear structure of the model. We argue that these different scaling laws of chaos have definite consequences for the macroscopic diffusive behavior, as chaos is the microscopic mechanism of diffusion. This is compared with previous results on chaotic diffusion [M. Mulansky and A. Pikovsky, New J. Phys. 15, 053015 (2013)], and a relation between microscopic chaos and macroscopic diffusion is established.

Increasing M 2(dobdc) Loading in Selective Mixed-Matrix Membranes: A Rubber Toughening Approach

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

Smith, Zachary P.; Bachman, Jonathan E.; Li, Tao

Mixed-matrix membranes (MMMs) were formed by incorporating M 2(dobdc) (M = Mg, Ni; dobdc 4– = 2,5-dioxido-1,4-benzenedicarboxylate) metal–organic framework (MOF) nanoparticles in a series of poly(ether-imide) copolymers. Addition of the MOF nanoparticles improved the permeability of H 2, N 2, CH 4, and CO 2 relative to the pure copolymer by increasing gas solubility and, in most cases, diffusivity. More limited improvements in diffusivity were observed for the more strongly adsorbing gases. Because of such transport considerations, improvements in permeability and selectivity were most pronounced for H 2/CH 4 and H 2/N 2 separations. Incorporation of a greater ether contentmore » within the copolymers led to the formation of defect-free MMMs by physically sealing polymer–MOF interfacial defects, allowing higher MOF loadings to be achieved. For Mg 2(dobdc), selective, defect-free films could be formed with loadings of up to 51 wt %. However, at these high loadings, films became weak and brittle. The mechanical properties of the composite materials were therefore quantified by tensile tests and compared to those of the neat polymers used commercially for membrane film formation. High contents of flexible ether units and small MOF nanoparticle sizes were found to be necessary to form strong and ductile MMMs, although clear trade-offs exist between transport performance, MOF loading, and mechanical properties. In conclusion, these trade-offs are critically examined to evaluate the current limitations and potential benefits to forming M 2(dobdc) MMMs using this rubber toughening approach.« less

Increasing M 2(dobdc) Loading in Selective Mixed-Matrix Membranes: A Rubber Toughening Approach

DOE PAGES

Smith, Zachary P.; Bachman, Jonathan E.; Li, Tao; ...

2018-01-30

Mixed-matrix membranes (MMMs) were formed by incorporating M 2(dobdc) (M = Mg, Ni; dobdc 4– = 2,5-dioxido-1,4-benzenedicarboxylate) metal–organic framework (MOF) nanoparticles in a series of poly(ether-imide) copolymers. Addition of the MOF nanoparticles improved the permeability of H 2, N 2, CH 4, and CO 2 relative to the pure copolymer by increasing gas solubility and, in most cases, diffusivity. More limited improvements in diffusivity were observed for the more strongly adsorbing gases. Because of such transport considerations, improvements in permeability and selectivity were most pronounced for H 2/CH 4 and H 2/N 2 separations. Incorporation of a greater ether contentmore » within the copolymers led to the formation of defect-free MMMs by physically sealing polymer–MOF interfacial defects, allowing higher MOF loadings to be achieved. For Mg 2(dobdc), selective, defect-free films could be formed with loadings of up to 51 wt %. However, at these high loadings, films became weak and brittle. The mechanical properties of the composite materials were therefore quantified by tensile tests and compared to those of the neat polymers used commercially for membrane film formation. High contents of flexible ether units and small MOF nanoparticle sizes were found to be necessary to form strong and ductile MMMs, although clear trade-offs exist between transport performance, MOF loading, and mechanical properties. In conclusion, these trade-offs are critically examined to evaluate the current limitations and potential benefits to forming M 2(dobdc) MMMs using this rubber toughening approach.« less

Phase Transition in Opinion Diffusion in Social Networks

DTIC Science & Technology

2012-05-01

the opinions of social agents diffuse in a network under a so-called hard-interaction model, in which the agents inter- act more strongly with...gent behavior. Index Terms— opinion diffusion , opinion dynamics, social net- works, phase transition, herding. 1. INTRODUCTION The study of the

Post-mortem inference of the human hippocampal connectivity and microstructure using ultra-high field diffusion MRI at 11.7 T.

PubMed

Beaujoin, Justine; Palomero-Gallagher, Nicola; Boumezbeur, Fawzi; Axer, Markus; Bernard, Jeremy; Poupon, Fabrice; Schmitz, Daniel; Mangin, Jean-François; Poupon, Cyril

2018-06-01

The human hippocampus plays a key role in memory management and is one of the first structures affected by Alzheimer's disease. Ultra-high magnetic resonance imaging provides access to its inner structure in vivo. However, gradient limitations on clinical systems hinder access to its inner connectivity and microstructure. A major target of this paper is the demonstration of diffusion MRI potential, using ultra-high field (11.7 T) and strong gradients (750 mT/m), to reveal the extra- and intra-hippocampal connectivity in addition to its microstructure. To this purpose, a multiple-shell diffusion-weighted acquisition protocol was developed to reach an ultra-high spatio-angular resolution with a good signal-to-noise ratio. The MRI data set was analyzed using analytical Q-Ball Imaging, Diffusion Tensor Imaging (DTI), and Neurite Orientation Dispersion and Density Imaging models. High Angular Resolution Diffusion Imaging estimates allowed us to obtain an accurate tractography resolving more complex fiber architecture than DTI models, and subsequently provided a map of the cross-regional connectivity. The neurite density was akin to that found in the histological literature, revealing the three hippocampal layers. Moreover, a gradient of connectivity and neurite density was observed between the anterior and the posterior part of the hippocampus. These results demonstrate that ex vivo ultra-high field/ultra-high gradients diffusion-weighted MRI allows the mapping of the inner connectivity of the human hippocampus, its microstructure, and to accurately reconstruct elements of the polysynaptic intra-hippocampal pathway using fiber tractography techniques at very high spatial/angular resolutions.

Dynamics of proteins: Light scattering study of dilute and dense colloidal suspensions of eye lens homogenates

NASA Astrophysics Data System (ADS)

Giannopoulou, A.; Aletras, A. J.; Pharmakakis, N.; Papatheodorou, G. N.; Yannopoulos, S. N.

2007-11-01

We report a dynamic light scattering study on protein suspensions of bovine lens homogenates at conditions (pH and ionic strength) similar to the physiological ones. Light scattering data were collected at two temperatures, 20 and 37°C, over a wide range of concentrations from the very dilute limit up to the dense regime approaching the physiological lens concentration. A comparison with experimental data from intact bovine lenses was advanced, revealing differences between dispersions and lenses at similar concentrations. In the dilute regime, two scattering entities were detected and identified with the long-time self-diffusion modes of α-crystallins and their aggregates, which naturally exist in lens nucleus. Upon increasing protein concentration, significant changes in time correlation function were observed starting at ˜75mgml-1, where a new mode originating from collective diffusive motions becomes visible. Self-diffusion coefficients are temperature insensitive, whereas the collective diffusion coefficient depends strongly on temperature revealing a reduction of the net repulsive interparticle forces with decreasing temperature. While there are no rigorous theoretical approaches on particle diffusion properties for multicomponent, nonideal hard sphere polydispersed systems, as the suspensions studied here, a discussion of the volume fraction dependence of the long-time self-diffusion coefficient in the context of existing theoretical approaches was undertaken. This study is purported to provide some insight into the complex light scattering pattern of intact lenses and the interactions between the constituent proteins that are responsible for lens transparency. This would lead to understand basic mechanisms of specific protein interactions that lead to lens opacification (cataract) under pathological conditions.

Predicting the weathering of fuel and oil spills: A diffusion-limited evaporation model.

PubMed

Kotzakoulakis, Konstantinos; George, Simon C

2018-01-01

The majority of the evaporation models currently available in the literature for the prediction of oil spill weathering do not take into account diffusion-limited mass transport and the formation of a concentration gradient in the oil phase. The altered surface concentration of the spill caused by diffusion-limited transport leads to a slower evaporation rate compared to the predictions of diffusion-agnostic evaporation models. The model presented in this study incorporates a diffusive layer in the oil phase and predicts the diffusion-limited evaporation rate. The information required is the composition of the fluid from gas chromatography or alternatively the distillation data. If the density or a single viscosity measurement is available the accuracy of the predictions is higher. Environmental conditions such as water temperature, air pressure and wind velocity are taken into account. The model was tested with synthetic mixtures, petroleum fuels and crude oils with initial viscosities ranging from 2 to 13,000 cSt. The tested temperatures varied from 0 °C to 23.4 °C and wind velocities from 0.3 to 3.8 m/s. The average absolute deviation (AAD) of the diffusion-limited model ranged between 1.62% and 24.87%. In comparison, the AAD of a diffusion-agnostic model ranged between 2.34% and 136.62% against the same tested fluids. Copyright © 2017 Elsevier Ltd. All rights reserved.

Grain boundary diffusion in olivine (Invited)

NASA Astrophysics Data System (ADS)

Marquardt, K.; Dohmen, R.

2013-12-01

Olivine is the main constituent of Earth's upper mantle. The individual mineral grains are separated by grain boundaries that have very distinct properties compared to those of single crystals and strongly affect large-scale physical and chemical properties of rocks, e.g. viscosity, electrical conductivity and diffusivity. Knowledge on the grain boundary physical and chemical properties, their population and distribution in polycrystalline materials [1] is a prerequisite to understand and model bulk (rock) properties, including their role as pathways for element transport [2] and the potential of grain boundaries as storage sites for incompatible elements [3]. Studies on selected and well characterized single grain boundaries are needed for a detailed understanding of the influence of varying grain boundaries. For instance, the dependence of diffusion on the grain boundary structure (defined by the lattice misfit) and width in silicates is unknown [2, 4], but limited experimental studies in material sciences indicate major effects of grain boundary orientation on diffusion rates. We characterized the effect of grain boundary orientation and temperature on element diffusion in forsterite grain boundaries by transmission electron microscopy (TEM).The site specific TEM-foils were cut using the focused ion beam technique (FIB). To study diffusion we prepared amorphous thin-films of Ni2SiO4 composition perpendicular to the grain boundary using pulsed laser deposition. Annealing (800-1450°C) leads to crystallization of the thin-film and Ni-Mg inter-diffuse into the crystal volume and along the grain boundary. The inter-diffusion profiles were measured using energy dispersive x-ray spectrometry in the TEM, standardized using the Cliff-Lorimer equation and EMPA measurements. We obtain volume diffusion coefficients that are comparable to Ni-Mg inter-diffusion rates in forsterite determined in previous studies at comparable temperatures, with similar activation energies. Grain boundary diffusion perpendicular to the dislocation lines of the small angle grain boundaries proved to be about an order of magnitude faster than volume diffusion, whereas diffusion in high angle grain boundaries is several orders of magnitude faster. We will discuss the variation of element diffusion rates with grain boundary orientation and the temperature- and/or time-induced transition from one diffusion regime to the next regime. This is done using time series experiments and two-dimensional grain boundary diffusion simulations. Finally, we will debate the differences between our data and other data sets that result from different experimental setups, conditions and analyses.

SODIUM CHLORIDE AND SELECTIVE DIFFUSION IN LIVING ORGANISMS.

PubMed

Loeb, J

1922-11-20

1. It is shown that NaCl acts like CaCl(2) or LaCl(3) in preventing the diffusion of strong acids through the membrane of the egg of Fundulus with this difference only that a M/8 solution of NaCl acts like a M/1,000 solution of CaCl(2) and like a M/30,000 solution of LaCl(3). 2. It is shown that these salts inhibit the diffusion of non-dissociated weak acid through the membrane of the Fundulus egg but slightly if at all. 3. Both NaCl and CaCl(2) accelerate the diffusion of dissociated strong alkali through the egg membrane of Fundulus and CaCl(2) is more efficient in this respect than NaCl. 4. It is shown that in moderate concentrations NaCl accelerates the rate of diffusion of KCl through the membrane of the egg of Fundulus while CaCl(2) does not.

Tracking Water Diffusion Fronts in a Highly Viscous Aerosol Particle

NASA Astrophysics Data System (ADS)

Bastelberger, Sandra; Krieger, Ulrich; Peter, Thomas

2016-04-01

Field measurements indicate that atmospheric secondary aerosol particles can be present in a highly viscous, glassy state [1]. In contrast to liquid state particles, the gas phase equilibration is kinetically limited and governed by condensed phase diffusion. In recent water diffusion experiments on highly viscous single aerosol particles levitated in an electrodynamic balance (EDB) we observed a characteristic shift behavior of the Mie whispering gallery modes (WGM) indicative of the changing radial structure of the particle, thus providing us with an experimental method to track the diffusion process inside the particle. When a highly viscous, homogeneous particle is exposed to an abrupt increase in relative humidity, the rapid gas phase diffusion and strong concentration dependence of the diffusion coefficient in the condensed phase lead to extremely steep water concentration gradients inside the particle, reminiscent of diffusion fronts. The resulting quasi step-like concentration profile motivates the introduction of a simple core-shell model describing the morphology of the non-equilibrium particle during humidification. The subsequent particle growth and reduction of the shell refractive index can be observed as red and blueshift behavior of the WGM, respectively. The shift pattern can be attributed to a core-shell radius ratio and particle radius derived from model calculations [2]. If supplemented with growth information obtained from the WGM redshift and thermodynamic equilibrium data, we can infer a comprehensive picture of the time evolution of the diffusion fronts in the framework of our core-shell model. The measured time dependent concentration profile is then compared with simulations solving the non-linear diffusion equation [3] [1] Virtanen, A., et al., Nature, 467, 824-827, 2010 [2] Kaiser, T., Schweiger, G., Computers in Physics, Vol. 7, No. 6, 682-686, Nov/Dec 1993 [3] Zobrist, B., Soonsin, V., Luo, B.P., Peter, T. et al., Phys. Chem. Chem. Phys., 13,3514-3526, 2011

A discontinuous Galerkin method with a bound preserving limiter for the advection of non-diffusive fields in solid Earth geodynamics

NASA Astrophysics Data System (ADS)

He, Ying; Puckett, Elbridge Gerry; Billen, Magali I.

2017-02-01

Mineral composition has a strong effect on the properties of rocks and is an essentially non-diffusive property in the context of large-scale mantle convection. Due to the non-diffusive nature and the origin of compositionally distinct regions in the Earth the boundaries between distinct regions can be nearly discontinuous. While there are different methods for tracking rock composition in numerical simulations of mantle convection, one must consider trade-offs between computational cost, accuracy or ease of implementation when choosing an appropriate method. Existing methods can be computationally expensive, cause over-/undershoots, smear sharp boundaries, or are not easily adapted to tracking multiple compositional fields. Here we present a Discontinuous Galerkin method with a bound preserving limiter (abbreviated as DG-BP) using a second order Runge-Kutta, strong stability-preserving time discretization method for the advection of non-diffusive fields. First, we show that the method is bound-preserving for a point-wise divergence free flow (e.g., a prescribed circular flow in a box). However, using standard adaptive mesh refinement (AMR) there is an over-shoot error (2%) because the cell average is not preserved during mesh coarsening. The effectiveness of the algorithm for convection-dominated flows is demonstrated using the falling box problem. We find that the DG-BP method maintains sharper compositional boundaries (3-5 elements) as compared to an artificial entropy-viscosity method (6-15 elements), although the over-/undershoot errors are similar. When used with AMR the DG-BP method results in fewer degrees of freedom due to smaller regions of mesh refinement in the neighborhood of the discontinuity. However, using Taylor-Hood elements and a uniform mesh there is an over-/undershoot error on the order of 0.0001%, but this error increases to 0.01-0.10% when using AMR. Therefore, for research problems in which a continuous field method is desired the DG-BP method can provide improved tracking of sharp compositional boundaries. For applications in which strict bound-preserving behavior is desired, use of an element that provides a divergence-free condition on the weak formulation (e.g., Raviart-Thomas) and an improved mesh coarsening scheme for the AMR are required.

Underdamped scaled Brownian motion: (non-)existence of the overdamped limit in anomalous diffusion.

PubMed

Bodrova, Anna S; Chechkin, Aleksei V; Cherstvy, Andrey G; Safdari, Hadiseh; Sokolov, Igor M; Metzler, Ralf

2016-07-27

It is quite generally assumed that the overdamped Langevin equation provides a quantitative description of the dynamics of a classical Brownian particle in the long time limit. We establish and investigate a paradigm anomalous diffusion process governed by an underdamped Langevin equation with an explicit time dependence of the system temperature and thus the diffusion and damping coefficients. We show that for this underdamped scaled Brownian motion (UDSBM) the overdamped limit fails to describe the long time behaviour of the system and may practically even not exist at all for a certain range of the parameter values. Thus persistent inertial effects play a non-negligible role even at significantly long times. From this study a general questions on the applicability of the overdamped limit to describe the long time motion of an anomalously diffusing particle arises, with profound consequences for the relevance of overdamped anomalous diffusion models. We elucidate our results in view of analytical and simulations results for the anomalous diffusion of particles in free cooling granular gases.

An experimental description of the flow in a centrifugal compressor from alternate stall to surge

NASA Astrophysics Data System (ADS)

Moënne-Loccoz, V.; Trébinjac, I.; Benichou, E.; Goguey, S.; Paoletti, B.; Laucher, P.

2017-08-01

The present paper gives the experimental results obtained in a centrifugal compressor stage designed and built by SAFRAN Helicopter Engines. The compressor is composed of inlet guide vanes, a backswept splittered unshrouded impeller, a splittered vaned radial diffuser and axial outlet guide vanes. Previous numerical simulations revealed a particular S-shape pressure rise characteristic at partial rotation speed and predicted an alternate flow pattern in the vaned radial diffuser at low mass flow rate. This alternate flow pattern involves two adjacent vane passages. One passage exhibits very low momentum and a low pressure recovery, whereas the adjacent passage has very high momentum in the passage inlet and diffuses efficiently. Experimental measurements confirm the S-shape of the pressure rise characteristic even if the stability limit experimentally occurs at higher mass flow than numerically predicted. At low mass flow the alternate stall pattern is confirmed thanks to the data obtained by high-frequency pressure sensors. As the compressor is throttled the path to instability has been registered and a first scenario of the surge inception is given. The compressor first experiences a steady alternate stall in the diffuser. As the mass flow decreases, the alternate stall amplifies and triggers the mild surge in the vaned diffuser. An unsteady behavior results from the interaction of the alternate stall and the mild surge. Finally, when the pressure gradient becomes too strong, the alternate stall blows away and the compressor enters into deep surge.

Fluorescence diffuse tomography for tumor detection and monitoring

NASA Astrophysics Data System (ADS)

Balalaeva, Irina V.; Orlova, Anna G.; Shirmanova, Marina V.; Kibraeva, Elena A.; Zagainova, Elena V.; Turchin, Ilya V.

2007-05-01

Strong light scattering and absorption limit visualization of the internal structure of biological tissue. Only special tools for turbid media imaging, such as optical diffuse tomography, enable noninvasive investigation of the internal biological tissues, including visualization and intravital monitoring of deep tumors. In this work the preliminary results of fluorescence diffuse tomography (FDT) of small animals are presented. Using of exogenous fluorophores, targeted specifically at tumor cells, and fluorescent proteins expressed endogenously can significantly increase the contrast of obtained images. Fluorescent compounds of different nature, such as sulphonated aluminium phthalocyanine (Photosens), red fluorescing proteins and CdTe/CdSe-core/shell nanocrystals (quantum dots) were applied. The animal was scanned in the transilluminative configuration by low-frequency modulated light (1 kHz) from Nd:YAG laser with second harmonic generation at the wavelength of 532 nm or semiconductor laser at the wavelength of 655 nm. Photosens was injected intravenously into linear mice with metastazing Lewis lung carcinoma in dose 4 mg/kg. Quantum dots (5x10 -11 M) or protein DsRed2 (1-5x10 -6 M) in glass capsules (inner diameter 2-3 mm) were placed inside the esophagus of 7-day-old hairless rats (18-20 g) to simulate marked tumors. Cells of HEK-293 Phoenix line, transitory transfected with Turbo-RFP protein gene, were injected hypodermically to immunodeficient mice. This work demonstrates potential capabilities of FDT method for detection and monitoring of deep fluorescent-labeled tumors in animal models. Strong advantages of fluorescent proteins and quantum dots over the traditional photosensitizer for FDT imaging are shown.

Neutral Evolution in a Biological Population as Diffusion in Phenotype Space: Reproduction with Local Mutation but without Selection

NASA Astrophysics Data System (ADS)

Lawson, Daniel John; Jensen, Henrik Jeldtoft

2007-03-01

The process of “evolutionary diffusion,” i.e., reproduction with local mutation but without selection in a biological population, resembles standard diffusion in many ways. However, evolutionary diffusion allows the formation of localized peaks that undergo drift, even in the infinite population limit. We relate a microscopic evolution model to a stochastic model which we solve fully. This allows us to understand the large population limit, relates evolution to diffusion, and shows that independent local mutations act as a diffusion of interacting particles taking larger steps.

Suitability of the first-order mass transfer concept for describing cyclic diffusive mass transfer in stagnant zones

NASA Astrophysics Data System (ADS)

Griffioen, Jasper

1998-10-01

The concept of first-order mass transfer between mobile and immobile regions, which mathematically simplifies the concept of Fickian diffusion in stagnant areas, has often been used to describe physical nonequilibrium transport of solutes into natural porous media. This study compares the two concepts, using analytical expressions describing cyclic mass transfer into and out of stagnant layers. The results show that the first-order mass transfer concept cannot describe continuous diffusion into the immobile zone during period of net outward diffusion if the immobile zone has not filled completely during the period of net inward diffusion. This sets phenomenological limitations to the first-order mass transfer concept when short periods of relative time are involved; these limitations have to be compared with the practical limitations to the Fickian diffusion concept.

Exercise-induced oxyhaemoglobin desaturation, ventilatory limitation and lung diffusing capacity in women during and after exercise.

PubMed

Walls, Justin; Maskrey, Michael; Wood-Baker, Richard; Stedman, Wade

2002-06-01

Arterial haemoglobin saturation during exercise in healthy young women [eight subjects mean (SEM) age 20.8 (1.8) years] was measured to confirm the theory that young women experience exercise-induced arterial hypoxaemia (EIAH) at a lower relative percentage of maximal oxygen uptake (VO(2max)) than has been documented in their male counterparts. To determine if flow limitation [the percentage of the tidal volume ( V(T)) that met or exceeded the boundary established by multiple maximal expiratory manoeuvres] and/or post-exercise lung diffusing capacity are linked to EIAH in women, and to investigate the influence of exercise intensity and duration on post-exercise carbon monoxide lung diffusing capacity ( D(L, CO)), these parameters were measured during and after three exercise tests (incremental test until exhaustion, 5 km run and 5 km run with sprint). All subjects experienced physiologically significant EIAH (a fall of more than 3% in oxygen saturation of arterial blood from levels at rest) and seven subjects experienced flow limitation during the VO(2max) protocol [mean (SD) 12.2 (8.8)% of V(T)]. Even though there was no significant relationship between aerobic capacity and the degree of flow limitation ( r=0.33, P>0.05), the flow limitation was related to absolute ventilation in the subjects studied ( r=0.82, P<0.05). There was no significant relationship between decrements in post exercise D(L, CO) and EIAH ( r=0.05, P>0.05), however there was a strong correlation between the extent of flow limitation (% of V(T)) and EIAH ( r=0.71). Significant decreases in D(L, CO) lasted for up to 16 h after each of the exercise tests ( P<0.05) and lasted for a further 8 h after the maximal test ( P<0.05). Exercise intensity was the main contributing factor to the observed decreases in post-exercise D(L, CO) with the percentage of VO(2max) attained during the various tests being significantly related to the fall in D(L, CO) for 1, 2, 3, 16 and 24 h after exercise ( P<0.05). As the appearance of flow limitation closely coincided with the appearance of EIAH, the results from the present study suggest that flow limitation is a contributing factor to EIAH in women although the exact mechanism remains unclear.

Background Error Correlation Modeling with Diffusion Operators

DTIC Science & Technology

2013-01-01

RESPONSIBLE PERSON 19b. TELEPHONE NUMBER (Include area code) 07-10-2013 Book Chapter Background Error Correlation Modeling with Diffusion Operators...normalization Unclassified Unclassified Unclassified UU 27 Max Yaremchuk (228) 688-5259 Reset Chapter 8 Background error correlation modeling with diffusion ...field, then a structure like this simulates enhanced diffusive transport of model errors in the regions of strong cur- rents on the background of

Modification of the parallel scattering mean free path of cosmic rays in the presence of adiabatic focusing

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

He, H.-Q.; Schlickeiser, R., E-mail: hqhe@mail.iggcas.ac.cn, E-mail: rsch@tp4.rub.de

The cosmic ray mean free path in a large-scale nonuniform guide magnetic field with superposed magnetostatic turbulence is calculated to clarify some conflicting results in the literature. A new, exact integro-differential equation for the cosmic-ray anisotropy is derived from the Fokker-Planck transport equation. A perturbation analysis of this integro-differential equation leads to an analytical expression for the cosmic ray anisotropy and the focused transport equation for the isotropic part of the cosmic ray distribution function. The derived parallel spatial diffusion coefficient and the associated cosmic ray mean free path include the effect of adiabatic focusing and reduce to the standardmore » forms in the limit of a uniform guide magnetic field. For the illustrative case of isotropic pitch angle scattering, the derived mean free path agrees with the earlier expressions of Beeck and Wibberenz, Bieber and Burger, Kota, and Litvinenko, but disagrees with the result of Shalchi. The disagreement with the expression of Shalchi is particularly strong in the limit of strong adiabatic focusing.« less

Fluctuation-controlled front propagation

NASA Astrophysics Data System (ADS)

Ridgway, Douglas Thacher

1997-09-01

A number of fundamental pattern-forming systems are controlled by fluctuations at the front. These problems involve the interaction of an infinite dimensional probability distribution with a strongly nonlinear, spatially extended pattern-forming system. We have examined fluctuation-controlled growth in the context of the specific problems of diffusion-limited growth and biological evolution. Mean field theory of diffusion-limited growth exhibits a finite time singularity. Near the leading edge of a diffusion-limited front, this leads to acceleration and blowup. This may be resolved, in an ad hoc manner, by introducing a cutoff below which growth is weakened or eliminated (8). This model, referred to as the BLT model, captures a number of qualitative features of global pattern formation in diffusion-limited aggregation: contours of the mean field match contours of averaged particle density in simulation, and the modified mean field theory can form dendritic features not possible in the naive mean field theory. The morphology transition between dendritic and non-dendritic global patterns requires that BLT fronts have a Mullins-Sekerka instability of the wavefront shape, in order to form concave patterns. We compute the stability of BLT fronts numerically, and compare the results to fronts without a cutoff. A significant morphological instability of the BLT fronts exists, with a dominant wavenumber on the scale of the front width. For standard mean field fronts, no instability is found. The naive and ad hoc mean field theories are continuum-deterministic models intended to capture the behavior of a discrete stochastic system. A transformation which maps discrete systems into a continuum model with a singular multiplicative noise is known, however numerical simulations of the continuum stochastic system often give mean field behavior instead of the critical behavior of the discrete system. We have found a new interpretation of the singular noise, based on maintaining the symmetry of the absorbing state, but which is unsuccessful at capturing the behavior of diffusion-limited growth. In an effort to find a simpler model system, we turned to modelling fitness increases in evolution. The work was motivated by an experiment on vesicular stomatitis virus, a short (˜9600bp) single-stranded RNA virus. A highly bottlenecked viral population increases in fitness rapidly until a certain point, after which the fitness increases at a slower rate. This is well modeled by a constant population reproducing and mutating on a smooth fitness landscape. Mean field theory of this system displays the same infinite propagation velocity blowup as mean field diffusion-limited aggregation. However, we have been able to make progress on a number of fronts. One is solving systems of moment equations, where a hierarchy of moments is truncated arbitrarily at some level. Good results for front propagation velocity are found with just two moments, corresponding to inclusion of the basic finite population clustering effect ignored by mean field theory. In addition, for small mutation rates, most of the population will be entirely on a single site or two adjacent sites, and the density of these cases can be described and solved. (Abstract shortened by UMI.)

Signatures of evanescent transport in ballistic suspended graphene-superconductor junctions

PubMed Central

Kumaravadivel, Piranavan; Du, Xu

2016-01-01

In Dirac materials, the low energy excitations behave like ultra-relativistic massless particles with linear energy dispersion. A particularly intriguing phenomenon arises with the intrinsic charge transport behavior at the Dirac point where the charge density approaches zero. In graphene, a 2-D Dirac fermion gas system, it was predicted that charge transport near the Dirac point is carried by evanescent modes, resulting in unconventional “pseudo-diffusive” charge transport even in the absence of disorder. In the past decade, experimental observation of this phenomenon remained challenging due to the presence of strong disorder in graphene devices which limits the accessibility of the low carrier density regime close enough to the Dirac point. Here we report transport measurements on ballistic suspended graphene-Niobium Josephson weak links that demonstrate a transition from ballistic to pseudo-diffusive like evanescent transport below a carrier density of ~1010 cm−2. Approaching the Dirac point, the sub-harmonic gap structures due to multiple Andreev reflections display a strong Fermi energy-dependence and become increasingly pronounced, while the normalized excess current through the superconductor-graphene interface decreases sharply. Our observations are in qualitative agreement with the long standing theoretical prediction for the emergence of evanescent transport mediated pseudo-diffusive transport in graphene. PMID:27080733

Deciphering the Dipole Anisotropy of Galactic Cosmic Rays.

PubMed

Ahlers, Markus

2016-10-07

Recent measurements of the dipole anisotropy in the arrival directions of Galactic cosmic rays (CRs) indicate a strong energy dependence of the dipole amplitude and phase in the TeV-PeV range. We argue here that these observations can be well understood within standard diffusion theory as a combined effect of (i) one or more local sources at Galactic longitude 120°≲l≲300° dominating the CR gradient below 0.1-0.3 PeV, (ii) the presence of a strong ordered magnetic field in our local environment, (iii) the relative motion of the solar system, and (iv) the limited reconstruction capabilities of ground-based observatories. We show that an excellent candidate of the local CR source responsible for the dipole anisotropy at 1-100 TeV is the Vela supernova remnant.

The wiper model: avalanche dynamics in an exclusion process

NASA Astrophysics Data System (ADS)

Politi, Antonio; Romano, M. Carmen

2013-10-01

The exclusion-process model (Ciandrini et al 2010 Phys. Rev. E 81 051904) describing traffic of particles with internal stepping dynamics reveals the presence of strong correlations in realistic regimes. Here we study such a model in the limit of an infinitely fast translocation time, where the evolution can be interpreted as a ‘wiper’ that moves to dry neighbouring sites. We trace back the existence of long-range correlations to the existence of avalanches, where many sites are dried at once. At variance with self-organized criticality, in the wiper model avalanches have a typical size equal to the logarithm of the lattice size. In the thermodynamic limit, we find that the hydrodynamic behaviour is a mixture of stochastic (diffusive) fluctuations and increasingly coherent periodic oscillations that are reminiscent of a collective dynamics.

Fréchet derivative with respect to the shape of a strongly convex nonscattering region in optical tomography

NASA Astrophysics Data System (ADS)

Hyvönen, Nuutti

2007-10-01

The aim of optical tomography is to reconstruct the optical properties inside a physical body, e.g. a neonatal head, by illuminating it with near-infrared light and measuring the outward flux of photons on the object boundary. Because a brain consists of strongly scattering tissue with imbedded cavities filled by weakly scattering cerebrospinal fluid, propagation of near-infrared photons in the human head can be treated by combining the diffusion approximation of the radiative transfer equation with geometrical optics to obtain the radiosity-diffusion forward model of optical tomography. At the moment, a disadvantage with the radiosity-diffusion model is that the locations of the transparent cavities must be known in advance in order to be able to reconstruct the physiologically interesting quantities, i.e., the absorption and the scatter in the strongly scattering brain tissue. In this work we show that the boundary measurement map of optical tomography is Fréchet differentiable with respect to the shape of a strongly convex nonscattering region. Using this result, we introduce a numerical algorithm for approximating an unknown nonscattering cavity by a ball if the background diffuse optical properties of the object are known. The functionality of the method is demonstrated through two-dimensional numerical experiments.

Analysis of Al diffusion processes in TiN barrier layers for the application in silicon solar cell metallization

NASA Astrophysics Data System (ADS)

Kumm, J.; Samadi, H.; Chacko, R. V.; Hartmann, P.; Wolf, A.

2016-07-01

An evaporated Al layer is known as an excellent rear metallization for highly efficient solar cells, but suffers from incompatibility with a common solder process. To enable solar cell-interconnection and module integration, in this work the Al layer is complemented with a solder stack of TiN/Ti/Ag or TiN/NiV/Ag, in which the TiN layer acts as an Al diffusion barrier. X-ray photoelectron spectroscopy measurements prove that diffusion of Al through the stack and the formation of an Al2O3 layer on the stack's surface are responsible for a loss of solderability after a strong post-metallization anneal, which is often mandatory to improve contact resistance and passivation quality. An optimization of the reactive TiN sputter process results in a densification of the TiN layer, which improves its barrier quality against Al diffusion. However, measurements with X-ray diffraction and scanning electron microscopy show that small grains with vertical grain boundaries persist, which still offer fast diffusion paths. Therefore, the concept of stuffing is introduced. By incorporating oxygen into the grain boundaries of the sputtered TiN layer, Al diffusion is strongly reduced as confirmed by secondary ion mass spectroscopy profiles. A quantitative analysis reveals a one order of magnitude lower Al diffusion coefficient for stuffed TiN layers. This metallization system maintains its solderability even after strong post-metallization annealing at 425 °C for 15 min. This paper thus presents an industrially feasible, conventionally solderable, and long-term stable metallization scheme for highly efficient silicon solar cells.

The operational stability of a centrifugal compressor and its dependence on the characteristics of the subcomponents

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

Hunziker, R.; Gyarmathy, G.

1994-04-01

A centrifugal compressor was tested with three different diffusers with circular-arc vanes. The vane inlet angle was varied from 15 to 30 deg. Detailed static wall pressure measurements show that the pressure field in the diffuser inlet is very sensitive to flow rate. The stability limit regularly occurred at the flow rate giving the maximum pressure rise for the overall stage. Mild surge arises as a dynamic instability of the compression system. The analysis of the pressure rise characteristic of each individual subcomponent (impeller, diffuser inlet, diffuser channel,...) reveals their contribution to the overall pressure rise. The diffuser channels playmore » an inherently destabilizing role while the impeller and the diffuser inlet are typically stabilizing. The stability limit was mainly determined by a change in the characteristic of the diffuser inlet. Further, the stability limit was found to be independent of the development of inducer-tip recirculation.« less

Transport properties in dilute UN (X ) solid solutions (X =Xe ,Kr )

NASA Astrophysics Data System (ADS)

Claisse, Antoine; Schuler, Thomas; Lopes, Denise Adorno; Olsson, Pär

2016-11-01

Uranium nitride (UN) is a candidate fuel for current GEN III fission reactors, for which it is investigated as an accident-tolerant fuel, as well as for future GEN IV reactors. In this study, we investigate the kinetic properties of gas fission products (Xe and Kr) in UN. Binding and migration energies are obtained using density functional theory, with an added Hubbard correlation to model f electrons, and the occupation matrix control scheme to avoid metastable states. These energies are then used as input for the self-consistent mean field method which enables to determine transport coefficients for vacancy-mediated diffusion of Xe and Kr on the U sublattice. The magnetic ordering of the UN structure is explicitly taken into account, for both energetic and transport properties. Solute diffusivities are compared with experimental measurements and the effect of various parameters on the theoretical model is carefully investigated. We find that kinetic correlations are very strong in this system, and that despite atomic migration anisotropy, macroscopic solute diffusivities show limited anisotropy. Our model indicates that the discrepancy between experimental measurements probably results from different irradiation conditions, and hence different defect concentrations.

Microscopic theory of topologically entangled fluids of rigid macromolecules

NASA Astrophysics Data System (ADS)

Sussman, Daniel M.; Schweizer, Kenneth S.

2011-06-01

We present a first-principles theory for the slow dynamics of a fluid of entangling rigid crosses of zero excluded volume based on a generalization of the dynamic mean-field approach of Szamel for infinitely thin nonrotating rods. The latter theory exactly includes topological constraints at the two-body collision level and self-consistently renormalizes an effective diffusion tensor to account for many-body effects. Remarkably, it predicts scaling laws consistent with the phenomenological reptation-tube predictions of Doi and Edwards for the long-time diffusion and the localization length in the heavily entangled limit. We generalize this approach to a different macromolecular architecture, infinitely thin three-dimensional crosses, and also extend the range of densities over which a dynamic localization length can be calculated for rods. Ideal gases of nonrotating crosses have recently received attention in computer simulations and are relevant as a simple model of both a strong-glass former and entangling star-branched polymers. Comparisons of our theory with these simulations reveal reasonable agreement for the magnitude and reduced density dependence of the localization length and also the self-diffusion constant if the consequences of local density fluctuations are taken into account.

Experience-dependent plasticity in white matter microstructure: reasoning training alters structural connectivity

PubMed Central

Mackey, Allyson P.; Whitaker, Kirstie J.; Bunge, Silvia A.

2012-01-01

Diffusion tensor imaging (DTI) techniques have made it possible to investigate white matter plasticity in humans. Changes in DTI measures, principally increases in fractional anisotropy (FA), have been observed following training programs as diverse as juggling, meditation, and working memory. Here, we sought to test whether three months of reasoning training could alter white matter microstructure. We recruited participants (n = 23) who were enrolled in a course to prepare for the Law School Admission Test (LSAT), a test that places strong demands on reasoning skills, as well as age- and IQ-matched controls planning to take the LSAT in the future (n = 22). DTI data were collected at two scan sessions scheduled three months apart. In trained participants but not controls, we observed decreases in radial diffusivity (RD) in white matter connecting frontal cortices, and in mean diffusivity (MD) within frontal and parietal lobe white matter. Further, participants exhibiting larger gains on the LSAT exhibited greater decreases in MD in the right internal capsule. In summary, reasoning training altered multiple measures of white matter structure in young adults. While the cellular underpinnings are unknown, these results provide evidence of experience-dependent white matter changes that may not be limited to myelination. PMID:22936899

Separation of copper ions from iron ions using PVA-g-(acrylic acid/N-vinyl imidazole) membranes prepared by radiation-induced grafting.

PubMed

Ajji, Zaki; Ali, Ali M

2010-01-15

Acrylic acid (AAc), N-vinyl imidazole (Azol) and their binary mixtures were graft copolymerized onto poly(vinyl alcohol) membranes using gamma irradiation. The ability of the grafted membranes to separate Cu ions from Fe ions was investigated with respect to the grafting yield and the pH of the feed solution. The data showed that the diffusion of copper ions from the feed compartment to the receiver compartment depends on the grafting yield of the membranes and the pH of the feed solution. To the contrary, iron ions did not diffuse through the membranes of all grafting yields. However, a limited amount of iron ions diffused in strong acidic medium. This study shows that the prepared membranes could be considered for the separation of copper ions from iron ions. The temperature of thermal decomposition of pure PVA-g-AAc/Azol membrane, PVA-g-AAc/Azol membrane containing copper ions, and PVA-g-AAc/Azol membrane containing iron ions were determined using TGA analyzer. It was shown that the presence of Cu and Fe ions increases the decomposition temperature, and the membranes bonded with iron ions are more stable than those containing copper ions.

Laser excited atomic fluorescence spectrometry as a powerful tool for analytical applications and spectroscopic studies

NASA Astrophysics Data System (ADS)

Gornushkin, Igor B.

1997-12-01

Laser-excited atomic fluorescence spectrometry (LEAFS) with a novel diffusive tube electrothermal atomizer (ETA) has been used for the study of atomization and diffusion processes and for the direct trace analysis of complex matrices. A novel ETA was a graphite tube sealed by two graphite electrodes. A sample was introduced into the tube and the furnace assembly was heated. The vaporized sample diffused through the hot graphite and the atomic fraction of the vapor was excited by a tunable dye laser above the tube. Temporal behavior of atomic fluorescence of Cu, Ag, and Ni atoms, diffused through the furnace tube, was studied at different temperatures; the values for activation energies and diffusion coefficients were derived on the basis of the diffusion/vaporization kinetic model. The femtogram/nanogram concentrations of silver were determined in coastal Atlantic water and soil samples. Use of the new ETA resulted in significant reduction of matrix interferences, ultra-low limits of detection, good accuracy and precision. LEAFS coupled with laser ablation (LA) was studied in terms of its analytical and spectroscopic potential. Low concentrations of lead (0.15 ppm-750 ppm) in metallic matrices (copper, brass, steel, and zinc) were measured in a low pressure argon atmosphere. No matrix effect was observed, providing a universal calibration curve for all samples. A limit of detection of 22 ppb (0.5 fg) was achieved. Also, the lifetime of the metastable 6p21D level of lead was measured and found to be in good agreement with the literature data. A simple open-air LA-LEAFS system was used for the determination of cobalt in solid matrices (graphite, soil, and steel). The fluorescence of cobalt was excited from a level which was already populated in the ablation plasma and was monitored at the Stokes-shifted wavelength. Detection limits in the ppb to ppm range and linearity over four orders of magnitude were achieved. The resonance shadowgraph technique has been developed for time-resolved imaging of laser-produced plasmas. The shadowgraphs were obtained by igniting the plasma on the lead or tin surface and by illuminating the plasma by a laser tuned in resonance with a strong atomic transition. UV-photodecomposition of lead and tin clusters was visualized. The evolution of the plasmas was studied at different pressures of argon. A shock wave produced by the laser ablation was monitored and its speed was measured.

Absence of a charge diffusion pole at finite energies in an exactly solvable interacting flat-band model in d dimensions

NASA Astrophysics Data System (ADS)

Phillips, Philip W.; Setty, Chandan; Zhang, Shuyi

2018-05-01

Motivated by recent bounds for charge diffusion in critical matter, we investigate the following question: What sets the scale for the velocity for diffusing degrees of freedom in a scale-invariant system? To make our statements precise, we analyze the diffusion pole in an exactly solvable model for a Mott transition in the presence of a long-range interaction term. To achieve scale invariance, we limit our discussion to the flat-band regime. We find in this limit that the diffusion pole, which would normally obtain at finite energy, is pushed to zero energy, resulting in a vanishing of the diffusion constant. This occurs even in the presence of interactions in certain limits, indicating the robustness of this result to the inclusion of a scale in the problem. Consequently, scale invariance precludes any reasonable definition of the diffusion constant. Nonetheless, we do find that a scale can be defined, albeit irrelevant to diffusion, which is the product of the squared band velocity and the density of states.

Diffusion of strongly magnetized cosmic ray particles in a turbulent medium

NASA Technical Reports Server (NTRS)

Ptuskin, V. S.

1985-01-01

Cosmic ray (CR) propagation in a turbulent medium is usually considered in the diffusion approximation. Here, the diffusion equation is obtained for strongly magnetized particles in the general form. The influence of a large-scale random magnetic field on CR propagation in interstellar medium is discussed. Cosmic rays are assumed to propagate in a medium with a regular field H and an ensemble of random MHD waves. The energy density of waves on scales smaller than the free path 1 of CR particles is small. The collision integral of the general form which describes interaction between relativistic particles and waves in the quasilinear approximation is used.

Catalytic conversion reactions in nanoporous systems with concentration-dependent selectivity: Statistical mechanical modeling

DOE PAGES

Garcia, Andres; Wang, Jing; Windus, Theresa L.; ...

2016-05-20

Statistical mechanical modeling is developed to describe a catalytic conversion reaction A → B c or B t with concentration-dependent selectivity of the products, B c or B t, where reaction occurs inside catalytic particles traversed by narrow linear nanopores. The associated restricted diffusive transport, which in the extreme case is described by single-file diffusion, naturally induces strong concentration gradients. Hence, by comparing kinetic Monte Carlo simulation results with analytic treatments, selectivity is shown to be impacted by strong spatial correlations induced by restricted diffusivity in the presence of reaction and also by a subtle clustering of reactants, A.

Diffusion phenomenon at the interface of Cu-brass under a strong gravitational field

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

Ogata, Yudai; Tokuda, Makoto; Januszko, Kamila

2015-03-28

To investigate diffusion phenomenon at the interface between Cu and brass under a strong gravitational field generated by ultracentrifuge apparatus, we performed gravity experiments on samples prepared by electroplating with interfaces normal and parallel to the direction of gravity. For the parallel-mode sample, for which sedimentation cannot occur thorough the interface, the concentration change was significant within the lower gravity region; many pores were observed in this region. Many vacancies arising from crystal strain due to the strong gravitational field moved into the lower gravity region, and enhanced the atoms mobilities. For the two normal-mode samples, which have interface normalmore » to the direction of gravity, the composition gradient of the brass-on-Cu sample was steeper than that for Cu-on-brass. This showed that the atoms of denser Cu diffuse in the direction of gravity, whereas Zn atoms diffuse in the opposite direction by sedimentation. The interdiffusion coefficients became higher in the Cu-on-brass sample, and became lower in the brass-on-Cu sample. This rise may be related to the behavior of the vacancies.« less

Nanoparticle Motion in Entangled Melts of Linear and Nonconcatenated Ring Polymers

PubMed Central

2017-01-01

The motion of nanoparticles (NPs) in entangled melts of linear polymers and nonconcatenated ring polymers are compared by large-scale molecular dynamics simulations. The comparison provides a paradigm for the effects of polymer architecture on the dynamical coupling between NPs and polymers in nanocomposites. Strongly suppressed motion of NPs with diameter d larger than the entanglement spacing a is observed in a melt of linear polymers before the onset of Fickian NP diffusion. This strong suppression of NP motion occurs progressively as d exceeds a and is related to the hopping diffusion of NPs in the entanglement network. In contrast to the NP motion in linear polymers, the motion of NPs with d > a in ring polymers is not as strongly suppressed prior to Fickian diffusion. The diffusion coefficient D decreases with increasing d much slower in entangled rings than in entangled linear chains. NP motion in entangled nonconcatenated ring polymers is understood through a scaling analysis of the coupling between NP motion and the self-similar entangled dynamics of ring polymers. PMID:28392603

A new search for primordial black hole evaporations using the Whipple gamma-ray telescope

NASA Astrophysics Data System (ADS)

Linton, E. T.; Atkins, R. W.; Badran, H. M.; Blaylock, G.; Boyle, P. J.; Buckley, J. H.; Byrum, K. L.; Carter-Lewis, D. A.; Celik, O.; Chow, Y. C. K.; Cogan, P.; Daniel, M. K.; Dowdall, C.; Falcone, A. D.; Fegan, D. J.; Fegan, S. J.; Finley, J. P.; Fortin, P.; Guiterrez, K. J.; Hall, J.; Hanna, D.; Holder, J.; Horan, D.; Hughes, S. B.; Humensky, T. B.; Jung, I.; Kenny, G. E.; Kertzman, M.; Kieda, D. B.; Kildea, J.; Knapp, J.; Krawczynski, H.; Lang, M. J.; LeBohec, S.; Maier, G.; Moriarty, P.; Ong, R. A.; Perkins, J. S.; Pizlo, F.; Pohl, M.; Quinn, J.; Ragan, K.; Rebillot, P. F.; Reynolds, P. T.; Sembroski, G. H.; Steele, D.; Swordy, S. P.; Valcarcel, L.; Wakely, S. P.; Weekes, T. C.; White, R. J.

2006-01-01

Stephen Hawking's prediction that black holes should radiate like black bodies has several important consequences, including the possibility of the detection of small (~1015 g) black holes created in the very early universe. The detection of such primordial black holes (PBHs) would be an important discovery, not only confirming Hawking's theory, but also providing valuable insights into the history of the early universe. A search through 5.5 years of archival data from the Whipple Atmospheric Cerenkov Telescope is made for TeV gamma-ray bursts on 1, 3, and 5 s timescales. On the basis of a null result from this direct search for PBH evaporations, an upper limit of 1.08 × 106 pc-3 yr-1 (99% CL) is set on the PBH evaporation rate in the local region of the galaxy, assuming the Standard Model of particle physics. This is more than a factor of two better than the previous limit at this energy range and includes longer timescales than have previously been explored. Comparison of this result with previous limits on the fraction of the critical density comprised by PBHs, Ωpbh, depends strongly on assumptions made about PBH clustering; in models predicting strong PBH clustering, the limit in this work could be as many as ten orders of magnitude more stringently than those set by diffuse MeV gamma-ray observations.

Underdamped scaled Brownian motion: (non-)existence of the overdamped limit in anomalous diffusion

PubMed Central

Bodrova, Anna S.; Chechkin, Aleksei V.; Cherstvy, Andrey G.; Safdari, Hadiseh; Sokolov, Igor M.; Metzler, Ralf

2016-01-01

It is quite generally assumed that the overdamped Langevin equation provides a quantitative description of the dynamics of a classical Brownian particle in the long time limit. We establish and investigate a paradigm anomalous diffusion process governed by an underdamped Langevin equation with an explicit time dependence of the system temperature and thus the diffusion and damping coefficients. We show that for this underdamped scaled Brownian motion (UDSBM) the overdamped limit fails to describe the long time behaviour of the system and may practically even not exist at all for a certain range of the parameter values. Thus persistent inertial effects play a non-negligible role even at significantly long times. From this study a general questions on the applicability of the overdamped limit to describe the long time motion of an anomalously diffusing particle arises, with profound consequences for the relevance of overdamped anomalous diffusion models. We elucidate our results in view of analytical and simulations results for the anomalous diffusion of particles in free cooling granular gases. PMID:27462008

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

Kaufman, H.R.

Bohm diffusion has been found to be approximately valid for many plasmas in strong magnetic fields. Assuming Bohm diffusion describes electron diffusion directly (H. R. Kaufman, AIAA J. {bold 23}, 78 (1985)), with an equal ion loss possible from the ambipolar field that is generated (F. F. Chen, {ital Introduction} {ital to} {ital Plasma} {ital Physics} (Plenum, New York, 1974), p. 169), an order-of-magnitude analysis can show why such electron diffusion should be expected.

Educational Policy Diffusion and Transfer: The Case of Armenia

ERIC Educational Resources Information Center

Karakhanyan, Susanna; van Veen, Klaas; Bergen, Theo

2011-01-01

This paper explores the quality of the implementation of the West European Bologna reforms in higher education in a post-soviet country. This process of policy diffusion is analysed using concepts of policy diffusion/transfer and innovation literature, attempting to combine both streams of literature. Despite strong motivation to improve the…

Copper Gas Diffusers For Purging Line-Focus Laser Welds

NASA Technical Reports Server (NTRS)

Fonteyne, Steve L.; Hosking, Timothy J.; Shelley, D. Mark

1996-01-01

Modified flow diffusers built for inert-gas purging of welds made with 5-kW CO(2) lasers operating with line-focus optics in conduction mode instead of with point-focus optics in customary keyhole mode. Diffusers made of copper components brazed together, robust enough to withstand strong reflections of line-focused laser energy.

Flow and diffusion of high-stakes test scores.

PubMed

Marder, M; Bansal, D

2009-10-13

We apply visualization and modeling methods for convective and diffusive flows to public school mathematics test scores from Texas. We obtain plots that show the most likely future and past scores of students, the effects of random processes such as guessing, and the rate at which students appear in and disappear from schools. We show that student outcomes depend strongly upon economic class, and identify the grade levels where flows of different groups diverge most strongly. Changing the effectiveness of instruction in one grade naturally leads to strongly nonlinear effects on student outcomes in subsequent grades.

Importance of Ion Packing on the Dynamics of Ionic Liquids during Micropore Charging.

PubMed

He, Yadong; Qiao, Rui; Vatamanu, Jenel; Borodin, Oleg; Bedrov, Dmitry; Huang, Jingsong; Sumpter, Bobby G

2016-01-07

Molecular simulations of the diffusion of EMIM(+) and TFSI(-) ions in slit-shaped micropores under conditions similar to those during charging show that in pores that accommodate only a single layer of ions, ions diffuse increasingly faster as the pore becomes charged (with diffusion coefficients even reaching ∼5 × 10(-9) m(2)/s), unless the pore becomes very highly charged. In pores wide enough to fit more than one layer of ions, ion diffusion is slower than in the bulk and changes modestly as the pore becomes charged. Analysis of these results revealed that the fast (or slow) diffusion of ions inside a micropore during charging is correlated most strongly with the dense (or loose) ion packing inside the pore. The molecular details of the ions and the precise width of the pores modify these trends weakly, except when the pore is so narrow that the ion conformation relaxation is strongly constrained by the pore walls.

Importance of Ion Packing on the Dynamics of Ionic Liquids during Micropore Charging

DOE PAGES

He, Yadong; Qiao, Rui; Vatamanu, Jenel; ...

2015-12-07

In molecular simulations of the diffusion of EMIM+ and TESI- ions in slit-shaped micropores under conditions similar to those during charging show that in pores that accommodate only a single layer of ions, ions diffuse increasingly faster as the pore becomes charged (with diffusion coefficients even reaching similar to 5 x 10 -9 m 2/s), unless the pore becomes very highly charged. In pores wide enough to fit more than one layer of ions, ion diffusion is slower than in the bulk and changes modestly as the pore becomes charged. Moreover, analysis of these results revealed that the fast (ormore » slow) diffusion of ions inside a micropore during charging is correlated most strongly with the dense (or loose) ion packing inside the pore. Finally, the molecular details of the ions and the precise width of the pores modify these trends weakly, except when the pore is so narrow that the ion conformation relaxation is strongly constrained by the pore walls.« less

On the role of humic acids' carboxyl groups in the binding of charged organic compounds.

PubMed

Smilek, Jiří; Sedláček, Petr; Kalina, Michal; Klučáková, Martina

2015-11-01

Interactions of humic acids (HAs) with two cationic dyes (methylene blue and rhodamine 6G) were studied using a unique combination of diffusion and partitioning studies in HAs, containing hydrogels and batch sorption experiments. In order to investigate the involvement of carboxyl groups of HAs in these interactions, all experiments were performed for both, the original lignite HAs and HAs with selectively methylated carboxyls. The results of the diffusion experiments confirm that the interactions between the solute and humic substances have a strong impact on the rate of diffusion process. Surprisingly, the effect is almost equally approved for original and methylated HAs. On the other hand, the results of batch sorption experiments show strong improvement of the sorption capacity (methylated HAs), which is explained by changed morphology of alkylated HAs. The comparison of the results of diffusion and adsorption experiments shows that the diffusion experiments simulate the transport of solutes in natural humics containing environment more reasonably. Copyright © 2015 Elsevier Ltd. All rights reserved.

A Computational Investigation of Sooting Limits of Spherical Diffusion Flames

NASA Technical Reports Server (NTRS)

Lecoustre, V. R.; Chao, B. H.; Sunderland, P. B.; Urban, D. L.; Stocker, D. P.; Axelbaum, R. L.

2007-01-01

Limiting conditions for soot particle inception in spherical diffusion flames were investigated numerically. The flames were modeled using a one-dimensional, time accurate diffusion flame code with detailed chemistry and transport and an optically thick radiation model. Seventeen normal and inverse flames were considered, covering a wide range of stoichiometric mixture fraction, adiabatic flame temperature, and residence time. These flames were previously observed to reach their sooting limits after 2 s of microgravity. Sooting-limit diffusion flames with residence times longer than 200 ms were found to have temperatures near 1190 K where C/O = 0.6, whereas flames with shorter residence times required increased temperatures. Acetylene was found to be a reasonable surrogate for soot precursor species in these flames, having peak mole fractions of about 0.01.

Effects of C/O Ratio and Temperature on Sooting Limits of Spherical Diffusion Flames

NASA Technical Reports Server (NTRS)

Lecoustre, V. R.; Sunderland, P. B.; Chao, B. H.; Urban, D. L.; Stocker, D. P.; Axelbaum, R. L.

2008-01-01

Limiting conditions for soot particle inception in spherical diffusion flames were investigated numerically. The flames were modeled using a one-dimensional, time accurate diffusion flame code with detailed chemistry and transport and an optically thick radiation model. Seventeen normal and inverse flames were considered, covering a wide range of stoichiometric mixture fraction, adiabatic flame temperature, residence time and scalar dissipation rate. These flames were previously observed to reach their sooting limits after 2 s of microgravity. Sooting-limit diffusion flames with scalar dissipation rate lower than 2/s were found to have temperatures near 1400 K where C/O = 0.51, whereas flames with greater scalar dissipation rate required increased temperatures. This finding was valid across a broad range of fuel and oxidizer compositions and convection directions.

Diffusion-limited mixing by incompressible flows

NASA Astrophysics Data System (ADS)

Miles, Christopher J.; Doering, Charles R.

2018-05-01

Incompressible flows can be effective mixers by appropriately advecting a passive tracer to produce small filamentation length scales. In addition, diffusion is generally perceived as beneficial to mixing due to its ability to homogenize a passive tracer. However we provide numerical evidence that, in cases where advection and diffusion are both actively present, diffusion may produce negative effects by limiting the mixing effectiveness of incompressible optimal flows. This limitation appears to be due to the presence of a limiting length scale given by a generalised Batchelor length (Batchelor 1959 J. Fluid Mech. 5 113–33). This length scale limitation may in turn affect long-term mixing rates. More specifically, we consider local-in-time flow optimisation under energy and enstrophy flow constraints with the objective of maximising the mixing rate. We observe that, for enstrophy-bounded optimal flows, the strength of diffusion may not impact the long-term mixing rate. For energy-constrained optimal flows, however, an increase in the strength of diffusion can decrease the mixing rate. We provide analytical lower bounds on mixing rates and length scales achievable under related constraints (point-wise bounded speed and rate-of-strain) by extending the work of Lin et al (2011 J. Fluid Mech. 675 465–76) and Poon (1996 Commun. PDE 21 521–39).

Heavy-tailed fractional Pearson diffusions.

PubMed

Leonenko, N N; Papić, I; Sikorskii, A; Šuvak, N

2017-11-01

We define heavy-tailed fractional reciprocal gamma and Fisher-Snedecor diffusions by a non-Markovian time change in the corresponding Pearson diffusions. Pearson diffusions are governed by the backward Kolmogorov equations with space-varying polynomial coefficients and are widely used in applications. The corresponding fractional reciprocal gamma and Fisher-Snedecor diffusions are governed by the fractional backward Kolmogorov equations and have heavy-tailed marginal distributions in the steady state. We derive the explicit expressions for the transition densities of the fractional reciprocal gamma and Fisher-Snedecor diffusions and strong solutions of the associated Cauchy problems for the fractional backward Kolmogorov equation.

ANALYTIC FORMS OF THE PERPENDICULAR DIFFUSION COEFFICIENT IN NRMHD TURBULENCE

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

Shalchi, A., E-mail: andreasm4@yahoo.com

2015-02-01

In the past different analytic limits for the perpendicular diffusion coefficient of energetic particles interacting with magnetic turbulence were discussed. These different limits or cases correspond to different transport modes describing how the particles are diffusing across the large-scale magnetic field. In the current paper we describe a new transport regime by considering the model of noisy reduced magnetohydrodynamic turbulence. We derive different analytic forms of the perpendicular diffusion coefficient, and while we do this, we focus on the aforementioned new transport mode. We show that for this turbulence model a small perpendicular diffusion coefficient can be obtained so thatmore » the latter diffusion coefficient is more than hundred times smaller than the parallel diffusion coefficient. This result is relevant to explain observations in the solar system where such small perpendicular diffusion coefficients have been reported.« less

Lattice Boltzmann scheme for mixture modeling: analysis of the continuum diffusion regimes recovering Maxwell-Stefan model and incompressible Navier-Stokes equations.

PubMed

Asinari, Pietro

2009-11-01

A finite difference lattice Boltzmann scheme for homogeneous mixture modeling, which recovers Maxwell-Stefan diffusion model in the continuum limit, without the restriction of the mixture-averaged diffusion approximation, was recently proposed [P. Asinari, Phys. Rev. E 77, 056706 (2008)]. The theoretical basis is the Bhatnagar-Gross-Krook-type kinetic model for gas mixtures [P. Andries, K. Aoki, and B. Perthame, J. Stat. Phys. 106, 993 (2002)]. In the present paper, the recovered macroscopic equations in the continuum limit are systematically investigated by varying the ratio between the characteristic diffusion speed and the characteristic barycentric speed. It comes out that the diffusion speed must be at least one order of magnitude (in terms of Knudsen number) smaller than the barycentric speed, in order to recover the Navier-Stokes equations for mixtures in the incompressible limit. Some further numerical tests are also reported. In particular, (1) the solvent and dilute test cases are considered, because they are limiting cases in which the Maxwell-Stefan model reduces automatically to Fickian cases. Moreover, (2) some tests based on the Stefan diffusion tube are reported for proving the complete capabilities of the proposed scheme in solving Maxwell-Stefan diffusion problems. The proposed scheme agrees well with the expected theoretical results.

Self-crowding of AMPA receptors in the excitatory postsynaptic density can effectuate anomalous receptor sub-diffusion.

PubMed

Gupta, Rahul

2018-02-01

AMPA receptors (AMPARs) and their associations with auxiliary transmembrane proteins are bulky structures with large steric-exclusion volumes. Hence, self-crowding of AMPARs, depending on the local density, may affect their lateral diffusion in the postsynaptic membrane as well as in the highly crowded postsynaptic density (PSD) at excitatory synapses. Earlier theoretical studies considered only the roles of transmembrane obstacles and the AMPAR-binding submembranous scaffold proteins in shaping receptor diffusion within PSD. Using lattice model of diffusion, the present study investigates the additional impacts of self-crowding on the anomalousity and effective diffusion coefficient (Deff) of AMPAR diffusion. A recursive algorithm for avoiding false self-blocking during diffusion simulation is also proposed. The findings suggest that high density of AMPARs in the obstacle-free membrane itself engenders strongly anomalous diffusion and severe decline in Deff. Adding transmembrane obstacles to the membrane accentuates the anomalousity arising from self-crowding due to the reduced free diffusion space. Contrarily, enhanced AMPAR-scaffold binding, either through increase in binding strength or scaffold density or both, ameliorates the anomalousity resulting from self-crowding. However, binding has differential impacts on Deff depending on the receptor density. Increase in binding causes consistent decrease in Deff for low and moderate receptor density. For high density, binding increases Deff as long as it reduces anomalousity associated with intense self-crowding. Given a sufficiently strong binding condition when diffusion acquires normal behavior, further increase in binding causes decrease in Deff. Supporting earlier experimental observations are mentioned and implications of present findings to the experimental observations on AMPAR diffusion are also drawn.

Determination of equilibrium and rate constants for complex formation by fluorescence correlation spectroscopy supplemented by dynamic light scattering and Taylor dispersion analysis.

PubMed

Zhang, Xuzhu; Poniewierski, Andrzej; Jelińska, Aldona; Zagożdżon, Anna; Wisniewska, Agnieszka; Hou, Sen; Hołyst, Robert

2016-10-04

The equilibrium and rate constants of molecular complex formation are of great interest both in the field of chemistry and biology. Here, we use fluorescence correlation spectroscopy (FCS), supplemented by dynamic light scattering (DLS) and Taylor dispersion analysis (TDA), to study the complex formation in model systems of dye-micelle interactions. In our case, dyes rhodamine 110 and ATTO-488 interact with three differently charged surfactant micelles: octaethylene glycol monododecyl ether C 12 E 8 (neutral), cetyltrimethylammonium chloride CTAC (positive) and sodium dodecyl sulfate SDS (negative). To determine the rate constants for the dye-micelle complex formation we fit the experimental data obtained by FCS with a new form of the autocorrelation function, derived in the accompanying paper. Our results show that the association rate constants for the model systems are roughly two orders of magnitude smaller than those in the case of the diffusion-controlled limit. Because the complex stability is determined by the dissociation rate constant, a two-step reaction mechanism, including the diffusion-controlled and reaction-controlled rates, is used to explain the dye-micelle interaction. In the limit of fast reaction, we apply FCS to determine the equilibrium constant from the effective diffusion coefficient of the fluorescent components. Depending on the value of the equilibrium constant, we distinguish three types of interaction in the studied systems: weak, intermediate and strong. The values of the equilibrium constant obtained from the FCS and TDA experiments are very close to each other, which supports the theoretical model used to interpret the FCS data.

Charging of a conducting sphere in a weakly ionized collisional plasma: Temporal dynamics and stationary state

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

Grach, V. S., E-mail: vsgrach@app.sci-nnov.ru; Garasev, M. A.

2015-07-15

We consider the interaction of a isolated conducting sphere with a collisional weakly ionized plasma in an external field. We assume that the plasma consists of two species of ions neglecting of electrons. We take into account charging of the sphere due to sedimentation of plasma ions on it, the field of the sphere charge and the space charge, as well as recombination and molecular diffusion. The nonstationary problem of interaction of the sphere with the surrounding plasma is solved numerically. The temporal dynamics of the sphere charge and plasma perturbations is analyzed, as well as the properties of themore » stationary state. It is shown that the duration of transient period is determined by the recombination time and by the reverse conductivity of ions. The temporal dynamics of the sphere charge and plasma perturbations is determined by the intensity of recombination processes relative to the influence of the space charge field and diffusion. The stationary absolute value of the sphere charge increases linearly with the external electric field, decreases with the relative intensity of recombination processes and increases in the presence of substantial diffusion. The scales of the perturbed region in the plasma are determined by the radius of the sphere, the external field, the effect of diffusion, and the relative intensity of recombination processes. In the limiting case of the absence of molecular diffusion and a strong external field, the properties of the stationary state coincide with those obtained earlier as a result of approximate solution.« less

From conservative to reactive transport under diffusion-controlled conditions

NASA Astrophysics Data System (ADS)

Babey, Tristan; de Dreuzy, Jean-Raynald; Ginn, Timothy R.

2016-05-01

We assess the possibility to use conservative transport information, such as that contained in transit time distributions, breakthrough curves and tracer tests, to predict nonlinear fluid-rock interactions in fracture/matrix or mobile/immobile conditions. Reference simulated data are given by conservative and reactive transport simulations in several diffusive porosity structures differing by their topological organization. Reactions includes nonlinear kinetically controlled dissolution and desorption. Effective Multi-Rate Mass Transfer models (MRMT) are calibrated solely on conservative transport information without pore topology information and provide concentration distributions on which effective reaction rates are estimated. Reference simulated reaction rates and effective reaction rates evaluated by MRMT are compared, as well as characteristic desorption and dissolution times. Although not exactly equal, these indicators remain very close whatever the porous structure, differing at most by 0.6% and 10% for desorption and dissolution. At early times, this close agreement arises from the fine characterization of the diffusive porosity close to the mobile zone that controls fast mobile-diffusive exchanges. At intermediate to late times, concentration gradients are strongly reduced by diffusion, and reactivity can be captured by a very limited number of rates. We conclude that effective models calibrated solely on conservative transport information like MRMT can accurately estimate monocomponent kinetically controlled nonlinear fluid-rock interactions. Their relevance might extend to more advanced biogeochemical reactions because of the good characterization of conservative concentration distributions, even by parsimonious models (e.g., MRMT with 3-5 rates). We propose a methodology to estimate reactive transport from conservative transport in mobile-immobile conditions.

Analysis of Al diffusion processes in TiN barrier layers for the application in silicon solar cell metallization

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

Kumm, J.; Samadi, H.; Chacko, R. V.

An evaporated Al layer is known as an excellent rear metallization for highly efficient solar cells, but suffers from incompatibility with a common solder process. To enable solar cell-interconnection and module integration, in this work the Al layer is complemented with a solder stack of TiN/Ti/Ag or TiN/NiV/Ag, in which the TiN layer acts as an Al diffusion barrier. X-ray photoelectron spectroscopy measurements prove that diffusion of Al through the stack and the formation of an Al{sub 2}O{sub 3} layer on the stack's surface are responsible for a loss of solderability after a strong post-metallization anneal, which is often mandatorymore » to improve contact resistance and passivation quality. An optimization of the reactive TiN sputter process results in a densification of the TiN layer, which improves its barrier quality against Al diffusion. However, measurements with X-ray diffraction and scanning electron microscopy show that small grains with vertical grain boundaries persist, which still offer fast diffusion paths. Therefore, the concept of stuffing is introduced. By incorporating oxygen into the grain boundaries of the sputtered TiN layer, Al diffusion is strongly reduced as confirmed by secondary ion mass spectroscopy profiles. A quantitative analysis reveals a one order of magnitude lower Al diffusion coefficient for stuffed TiN layers. This metallization system maintains its solderability even after strong post-metallization annealing at 425 °C for 15 min. This paper thus presents an industrially feasible, conventionally solderable, and long-term stable metallization scheme for highly efficient silicon solar cells.« less

The C4H radical and the diffuse interstellar bands. An ab initio study

NASA Technical Reports Server (NTRS)

Kolbuszewski, Marcin

1994-01-01

An ab initio study of the low-lying electronic states of C4H has been presented where the species studied has a chi(2)sigma(+) ground state and two low lying pi states. Based on the vertical and adiabatic excitation energies between those states it is suggested that the 4428 A diffuse interstellar band is not carried by C4H. The application of the particle in a box model shows strong coincidences between the strong DIB's and predicted wavelengths of pi-pi transitions in C(2n)H series. Based on those coincidences, it is suggested the C(2n)H species as good candidates for carriers of diffuse interstellar bands.

Dissociative diffusion mechanism in vacancy-rich materials according to mass action kinetics

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

Biderman, N. J.; Sundaramoorthy, R.; Haldar, Pradeep

We conducted two sets of diffusion-reaction numerical simulations using a finite difference method (FDM) in order to investigate fast impurity diffusion via interstitial sites in vacancy-rich materials such as Cu(In,Ga)Se 2 (CIGS) and Cu 2ZnSn(S, Se) 4 (CZTSSe or CZTS) via the dissociative diffusion mechanism where the interstitial diffuser ultimately reacts with a vacancy to produce a substitutional. The first set of simulations extends the standard interstitial-limited dissociative diffusion theory to vacancy-rich material conditions where vacancies are annihilated in large amounts, introducing non-equilibrium vacancy concentration profiles. The second simulation set explores the vacancy-limited dissociative diffusion where impurity incorporation increases themore » equilibrium vacancy concentration. In addition to diffusion profiles of varying concentrations and shapes that were obtained in all simulations, some of the profiles can be fitted with the constant- and limited-source solutions of Fick’s second law despite the non-equilibrium condition induced by the interstitial-vacancy reaction. The first set of simulations reveals that the dissociative diffusion coefficient in vacancy-rich materials is inversely proportional to the initial vacancy concentration. In the second set of numerical simulations, impurity-induced changes in the vacancy concentration lead to distinctive diffusion profile shapes. The simulation results are also compared with published data of impurity diffusion in CIGS. And according to the characteristic properties of diffusion profiles from the two set of simulations, experimental detection of the dissociative diffusion mechanism in vacancy-rich materials may be possible.« less

Dissociative diffusion mechanism in vacancy-rich materials according to mass action kinetics

DOE PAGES

Biderman, N. J.; Sundaramoorthy, R.; Haldar, Pradeep; ...

2016-05-13

We conducted two sets of diffusion-reaction numerical simulations using a finite difference method (FDM) in order to investigate fast impurity diffusion via interstitial sites in vacancy-rich materials such as Cu(In,Ga)Se 2 (CIGS) and Cu 2ZnSn(S, Se) 4 (CZTSSe or CZTS) via the dissociative diffusion mechanism where the interstitial diffuser ultimately reacts with a vacancy to produce a substitutional. The first set of simulations extends the standard interstitial-limited dissociative diffusion theory to vacancy-rich material conditions where vacancies are annihilated in large amounts, introducing non-equilibrium vacancy concentration profiles. The second simulation set explores the vacancy-limited dissociative diffusion where impurity incorporation increases themore » equilibrium vacancy concentration. In addition to diffusion profiles of varying concentrations and shapes that were obtained in all simulations, some of the profiles can be fitted with the constant- and limited-source solutions of Fick’s second law despite the non-equilibrium condition induced by the interstitial-vacancy reaction. The first set of simulations reveals that the dissociative diffusion coefficient in vacancy-rich materials is inversely proportional to the initial vacancy concentration. In the second set of numerical simulations, impurity-induced changes in the vacancy concentration lead to distinctive diffusion profile shapes. The simulation results are also compared with published data of impurity diffusion in CIGS. And according to the characteristic properties of diffusion profiles from the two set of simulations, experimental detection of the dissociative diffusion mechanism in vacancy-rich materials may be possible.« less

Intraparticle diffusion limitations in the hydrogenation of monounsaturated edible oils and their fatty acid methyl esters

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

Jonker, G.H.; Veldsink, J.W.; Beenackers, A.A.C.M.

1998-12-01

Intraparticle diffusion limitation in the hydrogenation and isomerization of fatty acid methyl esters (FAMEs) and edible oils (triacylglycerol, TAG) in porous nickel catalyst was investigated both under reactive and under inert conditions. Under reactive conditions, the diffusion coefficients were determined from the best fits of the model simulations applying the intrinsic reacting kinetics of monounsaturated FAME hydrogenation to experiments under diffusion limited conditions. Due to the absence of reaction (hydrogenation of double bonds), the obtained effective H{sub z} diffusion coefficient (D{sub e}) with the HPLC technique is volume averaged and thereby determined by the larger intercrystalline pores (<30% of themore » total pore volume) only. Moreover, D{sub e} measured under reaction conditions reflected the influence of the micropores, resulting in a 10-fold lower value.« less

Dynamic Patterns of Modern Epidemics

NASA Astrophysics Data System (ADS)

Brockmann, Dirk; Hufnagel, Lars; Geisel, Theo

2004-03-01

We investigate the effects of scale-free travelling of humans and their inhomogeneous geographic distribution on the dynamic patterns of spreading epidemics. Our approach combines the susceptible/infected/recovered paradigm for the infection dynamics with superdiffusive dispersion of individuals and their inhomogeneous spatial distribution. We show that scale-free motion of individuals and their variable spatial distribution leads to the absence of wavefronts in dynamic epidemic patterns which are typical for the limiting cases of ordinary diffusion and spatially homogeneous populations. Instead, patterns emerge with isolated hotspots on highly populated areas from which regional epidemic outbursts are triggered. Hotspot sizes are independent of the correlation length in the spatial distribution of individuals and occur on all scales. Our theory predicts that highly populated areas are reached by an epidemic in advance and must receive special attention in control measure strategies. Furthermore, our analysis predicts strong fluctuations in the time course of the total infection which cannot be accounted for by ordinary reaction-diffusion models for epidemics.

Thermal diffusivity and butterfly velocity in anisotropic Q-lattice models

NASA Astrophysics Data System (ADS)

Jeong, Hyun-Sik; Ahn, Yongjun; Ahn, Dujin; Niu, Chao; Li, Wei-Jia; Kim, Keun-Young

2018-01-01

We study a relation between the thermal diffusivity ( D T ) and two quantum chaotic properties, Lyapunov time (τ L ) and butterfly velocity ( v B ) in strongly correlated systems by using a holographic method. Recently, it was shown that E_i:={D}_{T,i}/({v}{^{B,i}}^2{τ}_L)(i=x,y) is universal in the sense that it is determined only by some scaling exponents of the IR metric in the low temperature limit regardless of the matter fields and ultraviolet data. Inspired by this observation, by analyzing the anisotropic IR scaling geometry carefully, we find the concrete expressions for E_i in terms of the critical dynamical exponents z i in each direction, E_i={z}_i/2({z}_i-1) . Furthermore, we find the lower bound of E_i is always 1 /2, which is not affected by anisotropy, contrary to the η/s case. However, there may be an upper bound determined by given fixed anisotropy.

Free Energy Landscape of Cellulose as a Driving Factor in the Mobility of Adsorbed Water.

PubMed

Kulasinski, Karol

2017-06-06

The diffusion coefficient of water adsorbed in hydrophilic porous materials, such as noncrystalline cellulose, depends on water activity. Faster diffusion at higher water concentrations is observed in experimental and modeling studies. In this paper, two asymptotic water concentrations, near-vacuum and fully saturated, are investigated at the surface of crystalline cellulose with molecular dynamics simulations. An increasing water concentration leads to significant changes in the free energy landscape due to perturbation of local electrostatic potential. Smoothening of strong energy minima, corresponding to sorption sites, and formation of layered structure facilitates water transport in the vicinity of cellulose. The determined transition probabilities and hydrogen bond stability reflect the changes in the energy landscape. As a result of a concentration increase, the emerging basins of attraction and spreading out of those existing in the diluted state lead to an increase in water entropy. Thermal fluctuations of cellulose are demonstrated to rearrange the landscape in the diluted limit, increase adsorbed water entropy, and decrease the water-cellulose H-bond lifetime.

Jet Launching in Resistive GR-MHD Black Hole–Accretion Disk Systems

NASA Astrophysics Data System (ADS)

Qian, Qian; Fendt, Christian; Vourellis, Christos

2018-05-01

We investigate the launching mechanism of relativistic jets from black hole sources, in particular the strong winds from the surrounding accretion disk. Numerical investigations of the disk wind launching—the simulation of the accretion–ejection transition—have so far almost only been done for nonrelativistic systems. From these simulations we know that resistivity, or magnetic diffusivity, plays an important role for the launching process. Here we extend this treatment to general relativistic magnetohydrodynamics (GR-MHD), applying the resistive GR-MHD code rHARM. Our model setup considers a thin accretion disk threaded by a large-scale open magnetic field. We run a series of simulations with different Kerr parameter, field strength, and diffusivity level. Indeed, we find strong disk winds with, however, mildly relativistic speed, the latter most probably due to our limited computational domain. Further, we find that magnetic diffusivity lowers the efficiency of accretion and ejection, as it weakens the efficiency of the magnetic lever arm of the disk wind. As a major driving force of the disk wind we disentangle the toroidal magnetic field pressure gradient; however, magnetocentrifugal driving may also contribute. Black hole rotation in our simulations suppresses the accretion rate owing to an enhanced toroidal magnetic field pressure that seems to be induced by frame dragging. Comparing the energy fluxes from the Blandford–Znajek-driven central spine and the surrounding disk wind, we find that the total electromagnetic energy flux is dominated by the total matter energy flux of the disk wind (by a factor of 20). The kinetic energy flux of the matter outflow is comparatively small and comparable to the Blandford–Znajek electromagnetic energy flux.

Sucralose Destabilization of Protein Structure.

PubMed

Chen, Lee; Shukla, Nimesh; Cho, Inha; Cohn, Erin; Taylor, Erika A; Othon, Christina M

2015-04-16

Sucralose is a commonly employed artificial sweetener that behaves very differently than its natural disaccharide counterpart, sucrose, in terms of its interaction with biomolecules. The presence of sucralose in solution is found to destabilize the native structure of two model protein systems: the globular protein bovine serum albumin and an enzyme staphylococcal nuclease. The melting temperature of these proteins decreases as a linear function of sucralose concentration. We correlate this destabilization to the increased polarity of the molecule. The strongly polar nature is manifested as a large dielectric friction exerted on the excited-state rotational diffusion of tryptophan using time-resolved fluorescence anisotropy. Tryptophan exhibits rotational diffusion proportional to the measured bulk viscosity for sucrose solutions over a wide range of concentrations, consistent with a Stokes-Einstein model. For sucralose solutions, however, the diffusion is dependent on the concentration, strongly diverging from the viscosity predictions, and results in heterogeneous rotational diffusion.

The importance of vertical resolution in the free troposphere for modeling intercontinental plumes

NASA Astrophysics Data System (ADS)

Zhuang, Jiawei; Jacob, Daniel J.; Eastham, Sebastian D.

2018-05-01

Chemical plumes in the free troposphere can preserve their identity for more than a week as they are transported on intercontinental scales. Current global models cannot reproduce this transport. The plumes dilute far too rapidly due to numerical diffusion in sheared flow. We show how model accuracy can be limited by either horizontal resolution (Δx) or vertical resolution (Δz). Balancing horizontal and vertical numerical diffusion, and weighing computational cost, implies an optimal grid resolution ratio (Δx / Δz)opt ˜ 1000 for simulating the plumes. This is considerably higher than current global models (Δx / Δz ˜ 20) and explains the rapid plume dilution in the models as caused by insufficient vertical resolution. Plume simulations with the Geophysical Fluid Dynamics Laboratory Finite-Volume Cubed-Sphere Dynamical Core (GFDL-FV3) over a range of horizontal and vertical grid resolutions confirm this limiting behavior. Our highest-resolution simulation (Δx ≈ 25 km, Δz ≈ 80 m) preserves the maximum mixing ratio in the plume to within 35 % after 8 days in strongly sheared flow, a drastic improvement over current models. Adding free tropospheric vertical levels in global models is computationally inexpensive and would also improve the simulation of water vapor.

Vortical ciliary flows actively enhance mass transport in reef corals.

PubMed

Shapiro, Orr H; Fernandez, Vicente I; Garren, Melissa; Guasto, Jeffrey S; Debaillon-Vesque, François P; Kramarsky-Winter, Esti; Vardi, Assaf; Stocker, Roman

2014-09-16

The exchange of nutrients and dissolved gasses between corals and their environment is a critical determinant of the growth of coral colonies and the productivity of coral reefs. To date, this exchange has been assumed to be limited by molecular diffusion through an unstirred boundary layer extending 1-2 mm from the coral surface, with corals relying solely on external flow to overcome this limitation. Here, we present direct microscopic evidence that, instead, corals can actively enhance mass transport through strong vortical flows driven by motile epidermal cilia covering their entire surface. Ciliary beating produces quasi-steady arrays of counterrotating vortices that vigorously stir a layer of water extending up to 2 mm from the coral surface. We show that, under low ambient flow velocities, these vortices, rather than molecular diffusion, control the exchange of nutrients and oxygen between the coral and its environment, enhancing mass transfer rates by up to 400%. This ability of corals to stir their boundary layer changes the way that we perceive the microenvironment of coral surfaces, revealing an active mechanism complementing the passive enhancement of transport by ambient flow. These findings extend our understanding of mass transport processes in reef corals and may shed new light on the evolutionary success of corals and coral reefs.

Cell adhesion to borate glasses by colloidal probe microscopy.

PubMed

Wiederhorn, Sheldon M; Chae, Young-Hun; Simon, Carl G; Cahn, Jackson; Deng, Yan; Day, Delbert

2011-05-01

The adhesion of osteoblast-like cells to silicate and borate glasses was measured in cell growth medium using colloidal probe microscopy. The probes consisted of silicate and borate glass spheres, 25-50 μm in diameter, attached to atomic force microscope cantilevers. Variables of the study included glass composition and time of contact of the cell to the glasses. Increasing the time of contact from 15 to 900 s increased the force of adhesion. The data could be plotted linearly on a log-log plot of adhesive force versus time. Of the seven glasses tested, five had slopes close to 0.5, suggesting a square root dependence of the adhesive force on the contact time. Such behavior can be interpreted as a diffusion limited process occurring during the early stages of cell attachment. We suggest that the rate limiting step in the adhesion process is the diffusion of integrins resident in the cell membrane to the area of cell attachment. Data presented in this paper support the hypothesis of Hench et al. that strong adhesion depends on the formation of a calcium phosphate reaction layer on the surfaces of the glass. Glasses that did not form a calcium phosphate layer exhibited a weaker adhesive force relative to those glasses that did form a calcium phosphate layer. Published by Elsevier Ltd.

A DYNAMIC DENSITY FUNCTIONAL THEORY APPROACH TO DIFFUSION IN WHITE DWARFS AND NEUTRON STAR ENVELOPES

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

Diaw, A.; Murillo, M. S.

2016-09-20

We develop a multicomponent hydrodynamic model based on moments of the Born–Bogolyubov–Green–Kirkwood–Yvon hierarchy equations for physical conditions relevant to astrophysical plasmas. These equations incorporate strong correlations through a density functional theory closure, while transport enters through a relaxation approximation. This approach enables the introduction of Coulomb coupling correction terms into the standard Burgers equations. The diffusive currents for these strongly coupled plasmas is self-consistently derived. The settling of impurities and its impact on cooling can be greatly affected by strong Coulomb coupling, which we show can be quantified using the direct correlation function.

Optimal resource diffusion for suppressing disease spreading in multiplex networks

NASA Astrophysics Data System (ADS)

Chen, Xiaolong; Wang, Wei; Cai, Shimin; Stanley, H. Eugene; Braunstein, Lidia A.

2018-05-01

Resource diffusion is a ubiquitous phenomenon, but how it impacts epidemic spreading has received little study. We propose a model that couples epidemic spreading and resource diffusion in multiplex networks. The spread of disease in a physical contact layer and the recovery of the infected nodes are both strongly dependent upon resources supplied by their counterparts in the social layer. The generation and diffusion of resources in the social layer are in turn strongly dependent upon the state of the nodes in the physical contact layer. Resources diffuse preferentially or randomly in this model. To quantify the degree of preferential diffusion, a bias parameter that controls the resource diffusion is proposed. We conduct extensive simulations and find that the preferential resource diffusion can change phase transition type of the fraction of infected nodes. When the degree of interlayer correlation is below a critical value, increasing the bias parameter changes the phase transition from double continuous to single continuous. When the degree of interlayer correlation is above a critical value, the phase transition changes from multiple continuous to first discontinuous and then to hybrid. We find hysteresis loops in the phase transition. We also find that there is an optimal resource strategy at each fixed degree of interlayer correlation under which the threshold reaches a maximum and the disease can be maximally suppressed. In addition, the optimal controlling parameter increases as the degree of inter-layer correlation increases.

Growth Mechanism of Lipid-Based Nanodiscs -- a Model Membrane for Studying Kinetics of Particle Coalescence

NASA Astrophysics Data System (ADS)

Nieh, Mu-Ping; Dizon, Anthony; Li, Ming; Hu, Andrew; Fan, Tai-Hsi

2012-02-01

Lipid-based nanodiscs composed of long- and short- chain lipids [namely, dimyristoyl phosphatidylcholine (DMPC), dimyristoyl phosphatidylglycerol (DMPG) and dihexanoyl phosphatidylcholine (DHPC)] constantly form at high lipid concentrations and at low temperatures (i.e., below the melting transition temperature of DMPC, TM). The initial size of these nanodiscs (at high total lipid concentration, CL> 20 wt.%) is relatively uniform and of similar dimension (according to dynamic light scattering and small angle neutron scattering experiments), seemingly independent of thermal history. Upon dilution, the nanodiscs slowly coalesce and grow in size with time irreversibly. Our preliminary result shows that the growth rate strongly depends on several parameters such as charge density, CL and temperature. We have also found that the nanodisc coalescence is a reaction limit instead of diffusion limit process through a time-resolved study.

South Australian Scleroderma Register: analysis of deceased patients.

PubMed

Bond, C; Pile, K D; McNeil, J D; Ahern, M J; Smith, M D; Cleland, L G; Roberts-Thomson, P J

1998-11-01

The demographic, clinical, pathological and serological features of 123 decreased patients with systemic sclerosis have been analysed. These patients consisted of all identified patients dying with this disease in South Australia between 1983 and 1996 inclusive. There were 85 females and 38 males, with the ratio of limited:diffuse:overlap disease subset being 9:5:1. Disease characteristics revealed that patients with the limited disease tended to be female with high frequencies of the centromere autoantibody, while patients with the diffuse disease had equal gender representation with the frequent presence of nucleolar, speckled or homogeneous antinuclear antibody. Mean duration of disease and mean age of death for the limited:diffuse:overlap subsets differed significantly between groups (p < 0.05) and were 16.5, 9.3 and 10.9 years and 71.9, 57.8 and 52.8 years respectively. Cumulative survival curves for the subsets differed highly significantly, with patients with the limited diseases dying more commonly from right heart failure (documented terminally in 25% of the centromere positive limited subset), cardiovascular disease or cancer, while patients with the diffuse subset died from respiratory failure, renal failure or cardiovascular disease. In conclusion, this retrospective analysis has revealed that scleroderma is a relatively common but clinically heterogeneous disorder. There are important clinical and prognostic implications in defining limited versus diffuse versus overlap disease.

Nanoparticle Motion in Entangled Melts of Linear and Nonconcatenated Ring Polymers [Nanoparticle Motion in Entangled Melts of Non-Concatenated Ring Polymers].

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

Ge, Ting; Kalathi, Jagannathan T.; Halverson, Jonathan D.

The motion of nanoparticles (NPs) in entangled melts of linear polymers and non-concatenated ring polymers are compared by large-scale molecular dynamics simulations. The comparison provides a paradigm for the effects of polymer architecture on the dynamical coupling between NPs and polymers in nanocomposites. Strongly suppressed motion of NPs with diameter d larger than the entanglement spacing a is observed in a melt of linear polymers before the onset of Fickian NP diffusion. This strong suppression of NP motion occurs progressively as d exceeds a, and is related to the hopping diffusion of NPs in the entanglement network. In contrast tomore » the NP motion in linear polymers, the motion of NPs with d > a in ring polymers is not as strongly suppressed prior to Fickian diffusion. The diffusion coefficient D decreases with increasing d much slower in entangled rings than in entangled linear chains. NP motion in entangled non-concatenated ring polymers is understood through a scaling analysis of the coupling between NP motion and the self-similar entangled dynamics of ring polymers.« less

Anomalous Thermal Diffusivity in Bad Metals

NASA Astrophysics Data System (ADS)

Zhang, Jiecheng; Levenson-Falk, Eli M.; Ramshaw, Brad J.; Bonn, Douglas A.; Liang, Ruixing; Hardy, Walter N.; Hartnoll, Sean A.; Kapitulnik, Aharon

Local measurements of thermal diffusivity are used to analyze the transport of heat in the bad metallic regime of several strongly correlated materials. In underdoped YBCO systems, we use the in-plane anisotropy to analyze transport in this system. Specifically, we find that the diffusivity anisotropy is comparable to reported values of the electrical resistivity anisotropy and drops sharply below the charge order transition, suggesting that both anisotropies have the same origin. We interpret our results through a strong electron-phonon scattering picture and find that both electronic and phononic contributions to the diffusivity exhibit a saturated scattering time of ℏ /kB T . Our results suggest that neither well-defined electron nor phonon quasiparticles are present in underdoped YBCO systems, and thermal transport exhibits a collective behavior of a ''soup'' of strongly coupled electrons and phonons which moves at a velocity that is smaller than the Fermi velocity, but larger than the speed of sound. We generalize this treatment to measurements of other bad metals and discuss its implications. Work supported by the Gordon and Betty Moore Foundation through the EPiQS Initiative, Grant GBMF4529, and by a Department of Energy Early Career Award (SAH).

Nanoparticle Motion in Entangled Melts of Linear and Nonconcatenated Ring Polymers [Nanoparticle Motion in Entangled Melts of Non-Concatenated Ring Polymers].

DOE PAGES

Ge, Ting; Kalathi, Jagannathan T.; Halverson, Jonathan D.; ...

2017-02-13

The motion of nanoparticles (NPs) in entangled melts of linear polymers and non-concatenated ring polymers are compared by large-scale molecular dynamics simulations. The comparison provides a paradigm for the effects of polymer architecture on the dynamical coupling between NPs and polymers in nanocomposites. Strongly suppressed motion of NPs with diameter d larger than the entanglement spacing a is observed in a melt of linear polymers before the onset of Fickian NP diffusion. This strong suppression of NP motion occurs progressively as d exceeds a, and is related to the hopping diffusion of NPs in the entanglement network. In contrast tomore » the NP motion in linear polymers, the motion of NPs with d > a in ring polymers is not as strongly suppressed prior to Fickian diffusion. The diffusion coefficient D decreases with increasing d much slower in entangled rings than in entangled linear chains. NP motion in entangled non-concatenated ring polymers is understood through a scaling analysis of the coupling between NP motion and the self-similar entangled dynamics of ring polymers.« less

Effect of Ion Streaming on Diffusion of Dust Grains in Dissipative System

NASA Astrophysics Data System (ADS)

Begum, M.; Das, N.

2018-01-01

The presence of strong electric fields in the sheath region of laboratory complex plasma induces an ion drift and perturbs the field around dust grains. The downstream focusing of ions leads to the formation of oscillatory kind of attractive wake potential which superimpose with the normal Debye-Hückel (DH) potential. The structural properties of complex plasma and diffusion coefficient of dust grains in the presence of such a wake potential have been investigated using Langevin dynamics simulation in the subsonic regime of ion flow. The study reveals that the diffusion of dust grains is strongly affected by the ion flow, so that the diffusion changes its character in the wake potential to the DH potential dominant regimes. The dependence of the diffusion coefficient on the parameters, such as the neutral pressure, dust grain size, ion flow velocity, and Coulomb coupling parameter, have been calculated for the subsonic regime by using the Green-Kubo expression, which is based on the integrated velocity autocorrelation function. It is found that the diffusion and the structural property of the system is intimately connected with the interaction potential and significantly get affected in the presence of ion flow in the subsonic regime.

Sparse and Adaptive Diffusion Dictionary (SADD) for recovering intra-voxel white matter structure.

PubMed

Aranda, Ramon; Ramirez-Manzanares, Alonso; Rivera, Mariano

2015-12-01

On the analysis of the Diffusion-Weighted Magnetic Resonance Images, multi-compartment models overcome the limitations of the well-known Diffusion Tensor model for fitting in vivo brain axonal orientations at voxels with fiber crossings, branching, kissing or bifurcations. Some successful multi-compartment methods are based on diffusion dictionaries. The diffusion dictionary-based methods assume that the observed Magnetic Resonance signal at each voxel is a linear combination of the fixed dictionary elements (dictionary atoms). The atoms are fixed along different orientations and diffusivity profiles. In this work, we present a sparse and adaptive diffusion dictionary method based on the Diffusion Basis Functions Model to estimate in vivo brain axonal fiber populations. Our proposal overcomes the following limitations of the diffusion dictionary-based methods: the limited angular resolution and the fixed shapes for the atom set. We propose to iteratively re-estimate the orientations and the diffusivity profile of the atoms independently at each voxel by using a simplified and easier-to-solve mathematical approach. As a result, we improve the fitting of the Diffusion-Weighted Magnetic Resonance signal. The advantages with respect to the former Diffusion Basis Functions method are demonstrated on the synthetic data-set used on the 2012 HARDI Reconstruction Challenge and in vivo human data. We demonstrate that improvements obtained in the intra-voxel fiber structure estimations benefit brain research allowing to obtain better tractography estimations. Hence, these improvements result in an accurate computation of the brain connectivity patterns. Copyright © 2015 Elsevier B.V. All rights reserved.

Robust sub-shot-noise measurement via Rabi-Josephson oscillations in bimodal Bose-Einstein condensates

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

Tikhonenkov, I.; Vardi, A.; Moore, M. G.

2011-06-15

Mach-Zehnder atom interferometry requires hold-time phase squeezing to attain readout accuracy below the standard quantum limit. This increases its sensitivity to phase diffusion, restoring shot-noise scaling of the optimal signal-to-noise ratio in the presence of interactions. The contradiction between the preparations required for readout accuracy and robustness to interactions is removed by monitoring Rabi-Josephson oscillations instead of relative-phase oscillations during signal acquisition. Optimizing the signal-to-noise ratio with a Gaussian squeezed input, we find that hold-time number squeezing satisfies both demands and that sub-shot-noise scaling is retained even for strong interactions.

Do Physicians' Financial Incentives Affect Medical Treatment and Patient Health?†

PubMed Central

Clemens, Jeffrey; Gottlieb, Joshua D.

2014-01-01

We investigate whether physicians' financial incentives influence health care supply, technology diffusion, and resulting patient outcomes. In 1997, Medicare consolidated the geographic regions across which it adjusts physician payments, generating area-specific price shocks. Areas with higher payment shocks experience significant increases in health care supply. On average, a 2 percent increase in payment rates leads to a 3 percent increase in care provision. Elective procedures such as cataract surgery respond much more strongly than less discretionary services. Non-radiologists expand their provision of MRIs, suggesting effects on technology adoption. We estimate economically small health impacts, albeit with limited precision. PMID:25170174

On N. Park's Analytical solution for steady state density- and mixing regime—dependent solute transport in a vertical soil column

NASA Astrophysics Data System (ADS)

Thiele, Michael

1998-04-01

Recently, Park [1996] presented an analytical solution for stationary one-dimensional solute transport in a variable-density fluid flow through a vertical soil column. He used the widespread Bear-Scheidegger dispersion model describing solute mixing as a sum of molecular diffusion and velocity-proportional mechanical dispersion effects. His closed-form implicit concentration and pressure distributions thus allow for a discussion of the combined impact of molecular diffusion and mechanical dispersion in a variable-density environment. Whereas Park only considered the example of vanishing molecular diffusion in detail, both phenomena are taken into account simultaneously in the present study in order to elucidate their different influences on concentration distribution characteristics. The boundary value problem dealt with herein is based on an upward inflow of high-density fluid of constant solute concentration and corresponding outflow of a lower constant concentration fluid at the upper end of the column when dispersivity does not change along the flow path. The thickness of the transition zone between the two fluids appeared to strongly depend on the prevailing share of the molecular diffusion and mechanical dispersion mechanisms. The latter can be characterized by a molecular Peclet number Pe, which here is defined as the ratio of the column outflow velocity multiplied by a characteristic pore size and the molecular diffusion coefficient. For very small values of Pe, when molecular diffusion represents the exclusive mixing process, density differences have no impact on transition zone thicknesses. A relative density-;dependent thickness increases with flow velocities (increasing Pe values) very rapidly compared to the density-independent case, and after having passed a maximum decreases asymptotically to a constant value for the large Peclet number limit when mechanical dispersion is the only mixing mechanism. Hence the special transport problem analyzed gives further evidence for the importance of simultaneously considering molecular diffusion and mechanical dispersion in gravity-affected solute transport in porous media.

Diffusion and sorption of organic micropollutants in biofilms with varying thicknesses.

PubMed

Torresi, Elena; Polesel, Fabio; Bester, Kai; Christensson, Magnus; Smets, Barth F; Trapp, Stefan; Andersen, Henrik R; Plósz, Benedek Gy

2017-10-15

Solid-liquid partitioning is one of the main fate processes determining the removal of micropollutants in wastewater. Little is known on the sorption of micropollutants in biofilms, where molecular diffusion may significantly influence partitioning kinetics. In this study, the diffusion and the sorption of 23 micropollutants were investigated in novel moving bed biofilm reactor (MBBR) carriers with controlled biofilm thickness (50, 200 and 500 μm) using targeted batch experiments (initial concentration = 1 μg L -1 , for X-ray contrast media 15 μg L -1 ) and mathematical modelling. We assessed the influence of biofilm thickness and density on the dimensionless effective diffusivity coefficient f (equal to the biofilm-to-aqueous diffusivity ratio) and the distribution coefficient K d,eq (L g -1 ). Sorption was significant only for eight positively charged micropollutants (atenolol, metoprolol, propranolol, citalopram, venlafaxine, erythromycin, clarithromycin and roxithromycin), revealing the importance of electrostatic interactions with solids. Sorption equilibria were likely not reached within the duration of batch experiments (4 h), particularly for the thickest biofilm, requiring the calculation of the distribution coefficient K d,eq based on the approximation of the asymptotic equilibrium concentration (t > 4 h). K d,eq values increased with increasing biofilm thickness for all sorptive micropollutants (except atenolol), possibly due to higher porosity and accessible surface area in the thickest biofilm. Positive correlations between K d,eq and micropollutant properties (polarity and molecular size descriptors) were identified but not for all biofilm thicknesses, thus confirming the challenge of improving predictive sorption models for positively charged compounds. A diffusion-sorption model was developed and calibrated against experimental data, and estimated f values also increased with increasing biofilm thickness. This indicates that diffusion in thin biofilms may be strongly limited (f ≪ 0.1) by the high biomass density (reduced porosity). Copyright © 2017 Elsevier Ltd. All rights reserved.

Quantitative relationship between the octanol/water partition coefficient and the diffusion limitation of the exchange between adipose and blood

PubMed Central

2010-01-01

Background The goal of physiologically based pharmacokinetics (PBPK) is to predict drug kinetics from an understanding of the organ/blood exchange. The standard approach is to assume that the organ is "flow limited" which means that the venous blood leaving the organ equilibrates with the well-stirred tissue compartment. Although this assumption is valid for most solutes, it has been shown to be incorrect for several very highly fat soluble compounds which appear to be "diffusion limited". This paper describes the physical basis of this adipose diffusion limitation and its quantitative dependence on the blood/water (Kbld-wat) and octanol/water (Kow) partition coefficient. Methods Experimental measurements of the time dependent rat blood and adipose concentration following either intravenous or oral input were used to estimate the "apparent" adipose perfusion rate (FA) assuming that the tissue is flow limited. It is shown that the ratio of FA to the anatomic perfusion rate (F) provides a measure of the diffusion limitation. A quantitative relationship between this diffusion limitation and Kbld-wat and Kow is derived. This analysis was applied to previously published data, including the Oberg et. al. measurements of the rat plasma and adipose tissue concentration following an oral dose of a mixture of 13 different polychlorinated biphenyls. Results Solutes become diffusion limited at values of log Kow greater than about 5.6, with the adipose-blood exchange rate reduced by a factor of about 30 for a solute with a log Kow of 7.36. Quantitatively, a plot of FA/F versus Kow is well described assuming an adipose permeability-surface area product (PS) of 750/min. This PS corresponds to a 0.14 micron aqueous layer separating the well-stirred blood from the adipose lipid. This is approximately equal to the thickness of the rat adipose capillary endothelium. Conclusions These results can be used to quantitate the adipose-blood diffusion limitation as a function of Kow. This is especially important for the highly fat soluble persistent organic chemicals (e.g. polychlorinated biphenyls, dioxins) whose pharmacokinetics are primarily determined by the adipose-blood exchange kinetics. PMID:20055995

Quantitative relationship between the octanol/water partition coefficient and the diffusion limitation of the exchange between adipose and blood.

PubMed

Levitt, David G

2010-01-07

The goal of physiologically based pharmacokinetics (PBPK) is to predict drug kinetics from an understanding of the organ/blood exchange. The standard approach is to assume that the organ is "flow limited" which means that the venous blood leaving the organ equilibrates with the well-stirred tissue compartment. Although this assumption is valid for most solutes, it has been shown to be incorrect for several very highly fat soluble compounds which appear to be "diffusion limited". This paper describes the physical basis of this adipose diffusion limitation and its quantitative dependence on the blood/water (Kbld-wat) and octanol/water (Kow) partition coefficient. Experimental measurements of the time dependent rat blood and adipose concentration following either intravenous or oral input were used to estimate the "apparent" adipose perfusion rate (FA) assuming that the tissue is flow limited. It is shown that the ratio of FA to the anatomic perfusion rate (F) provides a measure of the diffusion limitation. A quantitative relationship between this diffusion limitation and Kbld-wat and Kow is derived. This analysis was applied to previously published data, including the Oberg et. al. measurements of the rat plasma and adipose tissue concentration following an oral dose of a mixture of 13 different polychlorinated biphenyls. Solutes become diffusion limited at values of log Kow greater than about 5.6, with the adipose-blood exchange rate reduced by a factor of about 30 for a solute with a log Kow of 7.36. Quantitatively, a plot of FA/F versus Kow is well described assuming an adipose permeability-surface area product (PS) of 750/min. This PS corresponds to a 0.14 micron aqueous layer separating the well-stirred blood from the adipose lipid. This is approximately equal to the thickness of the rat adipose capillary endothelium. These results can be used to quantitate the adipose-blood diffusion limitation as a function of Kow. This is especially important for the highly fat soluble persistent organic chemicals (e.g. polychlorinated biphenyls, dioxins) whose pharmacokinetics are primarily determined by the adipose-blood exchange kinetics.

Cadmium and Zn availability as affected by pH manipulation and its assessment by soil extraction, DGT and indicator plants.

PubMed

Muhammad, Iqbal; Puschenreiter, Markus; Wenzel, Walter W

2012-02-01

Manipulation of soil pH by soil additives and / or rhizosphere processes may enhance the efficiency of metal phytoextraction. Here we report on the effect of nitric acid additions to four polluted soils on Cd and Zn concentrations in soil solution (C(soln)) and 0.005M Ca(NO(3))(2) extracts, and related changes in the diffusive fluxes and resupply of the metals as assessed by diffusive gradients in thin films (DGT). The responses of these chemical indicators of bioavailability were compared to metal uptake in two indicator plant species, common dandelion (Taraxacum officinale F.H. Wigg) and narrow leaf plantain (Plantago lanceolata L.) grown for 75days in a pot experiment. Lowering soil pH increased C(soln), the 0.005M Ca(NO(3))(2)-soluble fractions and the DGT-measured Cd and Zn concentrations (C(DGT)) in the experimental soils. This was associated with enhanced uptake of Cd and Zn on soils acidified to pH 4.5 whereas plants did not survive at pH 3.5. Toxicity along with decreased kinetics of metal resupply (calculated by the 2D DIFS model) in the strong acidification treatment suggests that moderate acidification is more appropriate to enhance the phytoextraction process. Each of the chemical indicators of bioavailability predicted well (R(2)>0.70) the Cd and Zn concentrations in plantain shoots but due to metal toxicity not for dandelion. Concentration factors, i.e. the ratio between metal concentrations in shoots and in soil solution (CF) indicate that Cd and Zn uptake in plantain was not limited by diffusion which may explain that DGT did not perform better than C(soln). However, DGT is expected to predict plant uptake better in diffusion-limited conditions such as in the rhizosphere of metal-accumulating phytoextraction crops. Copyright © 2011 Elsevier B.V. All rights reserved.

Large-Time Behavior of Solutions to Vlasov-Poisson-Fokker-Planck Equations: From Evanescent Collisions to Diffusive Limit

NASA Astrophysics Data System (ADS)

Herda, Maxime; Rodrigues, L. Miguel

2018-03-01

The present contribution investigates the dynamics generated by the two-dimensional Vlasov-Poisson-Fokker-Planck equation for charged particles in a steady inhomogeneous background of opposite charges. We provide global in time estimates that are uniform with respect to initial data taken in a bounded set of a weighted L^2 space, and where dependencies on the mean-free path τ and the Debye length δ are made explicit. In our analysis the mean free path covers the full range of possible values: from the regime of evanescent collisions τ → ∞ to the strongly collisional regime τ → 0. As a counterpart, the largeness of the Debye length, that enforces a weakly nonlinear regime, is used to close our nonlinear estimates. Accordingly we pay a special attention to relax as much as possible the τ -dependent constraint on δ ensuring exponential decay with explicit τ -dependent rates towards the stationary solution. In the strongly collisional limit τ → 0, we also examine all possible asymptotic regimes selected by a choice of observation time scale. Here also, our emphasis is on strong convergence, uniformity with respect to time and to initial data in bounded sets of a L^2 space. Our proofs rely on a detailed study of the nonlinear elliptic equation defining stationary solutions and a careful tracking and optimization of parameter dependencies of hypocoercive/hypoelliptic estimates.

Use of Inert Gases to Study the Interaction of Blood Flow and Diffusion during Passive Absorption from the Gastrointestinal Tract of the Rat

PubMed Central

Levitt, Michael D.; Levitt, David G.

1973-01-01

Measurement of the relative absorption rates of inert gases (H2, He, CH4, SF6, and 133Xe) was used to investigate the interaction between diffusion and blood flow during passive absorption from the stomach, small bowel, and colon of the rat. If uptake is blood flow limited, the gases should be absorbed in proportion to their solubilities in blood, but if diffusion limited, uptake should be proportional to the diffusion rate of the gases in mucosal tissues. The observed absorption data were fitted to a series of models of interaction between perfusion and diffusion. A simple model accurately predicted the absorption rates of the gases from all segments of bowel. In this model, gas is absorbed into two distinct blood flows: one which flows in proximity to the lumen and completely equilibrates with the lumen, and a second which is sufficiently rapid and distant from the lumen that its gas uptake is entirely diffusion limited. The fraction of the total absorption attributable to the equilibrating flow can be readily calculated and equalled 93%, 77%, and 33% for the small bowel, colon, and stomach, respectively. Thus the rate of passive absorption of gases from the small bowel is limited almost entirely by the blood flow to the mucosa, and absorption from the stomach is largely limited by the diffusion rate of the gases. The flow which equilibrates with the lumen can be quantitated, and this flow may provide a useful measure of “effective” mucosal blood flow. Images PMID:4719667

The rate of the deoxygenation reaction limits myoglobin- and hemoglobin-facilitated O₂ diffusion in cells.

PubMed

Endeward, Volker

2012-05-01

A mathematical model describing facilitation of O(2) diffusion by the diffusion of myoglobin and hemoglobin is presented. The equations are solved numerically by a finite-difference method for the conditions as they prevail in cardiac and skeletal muscle and in red cells without major simplifications. It is demonstrated that, in the range of intracellular diffusion distances, the degree of facilitation is limited by the rate of the chemical reaction between myglobin or hemoglobin and O(2). The results are presented in the form of relationships between the degree of facilitation and the length of the diffusion path on the basis of the known kinetics of the oxygenation-deoxygenation reactions. It is concluded that the limitation by reaction kinetics reduces the maximally possible facilitated oxygen diffusion in cardiomyoctes by ∼50% and in skeletal muscle fibers by ∼ 20%. For human red blood cells, a reduction of facilitated O(2) diffusion by 36% is obtained in agreement with previous reports. This indicates that, especially in cardiomyocytes and red cells, chemical equilibrium between myoglobin or hemoglobin and O(2) is far from being established, an assumption that previously has often been made. Although the "O(2) transport function" of myoglobin in cardiac muscle cells thus is severely limited by the chemical reaction kinetics, and to a lesser extent also in skeletal muscle, it is noteworthy that the speed of release of O(2) from MbO(2), the "storage function," is not limited by the reaction kinetics under physiological conditions.

Self-crowding of AMPA receptors in the excitatory postsynaptic density can effectuate anomalous receptor sub-diffusion

PubMed Central

Gupta, Rahul

2018-01-01

AMPA receptors (AMPARs) and their associations with auxiliary transmembrane proteins are bulky structures with large steric-exclusion volumes. Hence, self-crowding of AMPARs, depending on the local density, may affect their lateral diffusion in the postsynaptic membrane as well as in the highly crowded postsynaptic density (PSD) at excitatory synapses. Earlier theoretical studies considered only the roles of transmembrane obstacles and the AMPAR-binding submembranous scaffold proteins in shaping receptor diffusion within PSD. Using lattice model of diffusion, the present study investigates the additional impacts of self-crowding on the anomalousity and effective diffusion coefficient (Deff) of AMPAR diffusion. A recursive algorithm for avoiding false self-blocking during diffusion simulation is also proposed. The findings suggest that high density of AMPARs in the obstacle-free membrane itself engenders strongly anomalous diffusion and severe decline in Deff. Adding transmembrane obstacles to the membrane accentuates the anomalousity arising from self-crowding due to the reduced free diffusion space. Contrarily, enhanced AMPAR-scaffold binding, either through increase in binding strength or scaffold density or both, ameliorates the anomalousity resulting from self-crowding. However, binding has differential impacts on Deff depending on the receptor density. Increase in binding causes consistent decrease in Deff for low and moderate receptor density. For high density, binding increases Deff as long as it reduces anomalousity associated with intense self-crowding. Given a sufficiently strong binding condition when diffusion acquires normal behavior, further increase in binding causes decrease in Deff. Supporting earlier experimental observations are mentioned and implications of present findings to the experimental observations on AMPAR diffusion are also drawn. PMID:29444074

Microfabricated valveless devices for thermal bioreactions based on diffusion-limited evaporation.

PubMed

Wang, Fang; Yang, Ming; Burns, Mark A

2008-01-01

Microfluidic devices that reduce evaporative loss during thermal bioreactions such as PCR without microvalves have been developed by relying on the principle of diffusion-limited evaporation. Both theoretical and experimental results demonstrate that the sample evaporative loss can be reduced by more than 20 times using long narrow diffusion channels on both sides of the reaction region. In order to further suppress the evaporation, the driving force for liquid evaporation is reduced by two additional techniques: decreasing the interfacial temperature using thermal isolation and reducing the vapor concentration gradient by replenishing water vapor in the diffusion channels. Both thermal isolation and vapor replenishment techniques can limit the sample evaporative loss to approximately 1% of the reaction content.

Magnetorotational instability in protoplanetary discs

NASA Astrophysics Data System (ADS)

Salmeron, Raquel; Wardle, Mark

2005-07-01

We investigate the linear growth and vertical structure of the magnetorotational instability (MRI) in weakly ionized, stratified accretion discs. The magnetic field is initially vertical and dust grains are assumed to have settled towards the mid-plane, so charges are carried by electrons and ions only. Solutions are obtained at representative radial locations from the central protostar for different choices of the initial magnetic field strength, sources of ionization, disc structure and configuration of the conductivity tensor. The MRI is active over a wide range of magnetic field strengths and fluid conditions in low-conductivity discs. Moreover, no evidence was found of a low-limit field strength below which unstable modes do not exist. For the minimum-mass solar nebula model, incorporating cosmic ray ionization, perturbations grow at 1 au for B<~ 8 G. For a significant subset of these strengths (200mG <~B<~ 5G), the maximum growth rate is of the order of the ideal magnetohydrodynamic (MHD) rate (0.75Ω). Hall conductivity modifies the structure and growth rate of global unstable modes at 1 au for all magnetic field strengths that support MRI. As a result, at this radius, modes obtained with a full conductivity tensor grow faster and are active over a more extended cross-section of the disc than perturbations in the ambipolar diffusion limit. For relatively strong fields (e.g. B>~ 200 mG), ambipolar diffusion alters the envelope shapes of the unstable modes, which peak at an intermediate height, instead of being mostly flat as modes in the Hall limit are in this region of parameter space. Similarly, when cosmic rays are assumed to be excluded from the disc by the winds emitted by the magnetically active protostar, unstable modes grow at this radius for B<~ 2 G. For strong fields, perturbations exhibit a kink at the height where X-ray ionization becomes active. Finally, for R= 5 au (10 au), unstable modes exist for B<~ 800 mG (B<~ 250 mG) and the maximum growth rate is close to the ideal-MHD rate for 20 <~B<~ 500 mG (2 <~B<~ 50 mG). Similarly, perturbations incorporating Hall conductivity have a higher wavenumber and grow faster than solutions in the ambipolar diffusion limit for B<~ 100 mG (B<~ 10 mG). Unstable modes grow even at the mid-plane for B>~ 100 mG (B~ 1 mG), but for weaker fields, a small dead region exists. This study shows that, despite the low magnetic coupling, the magnetic field is dynamically important for a large range of fluid conditions and field strengths in protostellar discs. An example of such magnetic activity is the generation of MRI unstable modes, which are supported at 1 au for field strengths up to a few gauss. Hall diffusion largely determines the structure and growth rate of these perturbations for all studied radii. At radii of order 1 au, in particular, it is crucial to incorporate the full conductivity tensor in the analysis of this instability and more generally in studies of the dynamics of astrophysical discs.

Hydrogen atom kinetics in capacitively coupled plasmas

NASA Astrophysics Data System (ADS)

Nunomura, Shota; Katayama, Hirotaka; Yoshida, Isao

2017-05-01

Hydrogen (H) atom kinetics has been investigated in capacitively coupled very high frequency (VHF) discharges at powers of 16-780 mW cm-2 and H2 gas pressures of 0.1-2 Torr. The H atom density has been measured using vacuum ultra violet absorption spectroscopy (VUVAS) with a micro-discharge hollow cathode lamp as a VUV light source. The measurements have been performed in two different electrode configurations of discharges: conventional parallel-plate diode and triode with an intermediate mesh electrode. We find that in the triode configuration, the H atom density is strongly reduced across the mesh electrode. The H atom density varies from ˜1012 cm-3 to ˜1010 cm-3 by crossing the mesh with 0.2 mm in thickness and 36% in aperture ratio. The fluid model simulations for VHF discharge plasmas have been performed to study the H atom generation, diffusion and recombination kinetics. The simulations suggest that H atoms are generated in the bulk plasma, by the electron impact dissociation (e + H2 \\to e + 2H) and the ion-molecule reaction (H2 + + H2 \\to {{{H}}}3+ + H). The diffusion of H atoms is strongly limited by a mesh electrode, and thus the mesh geometry influences the spatial distribution of the H atoms. The loss of H atoms is dominated by the surface recombination.

Moving Upstream in U.S. Hospital Care Toward Investments in Population Health.

PubMed

Begun, James W; Potthoff, Sandra

The root causes for most health outcomes are often collectively referred to as the social determinants of health. Hospitals and health systems now must decide how much to "move upstream," or invest in programs that directly affect the social determinants of health. Moving upstream in healthcare delivery requires an acceptance of responsibility for the health of populations. We examine responses of 950 nonfederal, general hospitals in the United States to the 2015 American Hospital Association Population Health Survey to identify characteristics that distinguish those hospitals that are most aligned with population health and most engaged in addressing social determinants of health. Those "upstream" hospitals are significantly more likely to be large, not-for-profit, metropolitan, teaching-affiliated, and members of systems. Internally, the more upstream hospitals are more likely to organize their population health activities with strong executive-level involvement, full-time-equivalent support, and coordination at the system level.The characteristics differentiating hospitals strongly involved in population health and upstream activity are not unlike those characteristics associated with diffusion of many innovations in hospitals. These hospitals may be the early adopters in a diffusion process that will eventually include most hospitals or, at least, most not-for-profit hospitals. Alternatively, the population health and social determinants movements could be transient or could be limited to a small portion of hospitals such as those identified here, with distinctive patient populations, missions, and resources.

Diffusion of excitons in materials for optoelectronic device applications

NASA Astrophysics Data System (ADS)

Singh, Jai; Narayan, Monishka Rita; Ompong, David

2015-06-01

The diffusion of singlet excitonsis known to occur through the Förster resonance energy transfer (FRET) mechanism and that of singlet and triplet excitonscan occur through the Dexter carrier transfer mechanism. It is shown here that if a material possesses the strong exciton-spin-orbit-photon interaction then triplet excitonscan also be transported /diffused through a mechanism like FRET. The theory is applicable to the diffusion of excitonsin optoelectronic devices like organic solar cells, organic light emitting devices and inorganic scintillators.

Thermal diffusion in partially ionized gases - The case of unequal temperatures. [in solar chromosphere

NASA Technical Reports Server (NTRS)

Geiss, J.; Burgi, A.

1987-01-01

Previous calculations of thermal diffusion coefficients in partially ionized gases are extended to the case of unequal neutral and ion temperatures and/or temperature gradients. Formulas are derived for the general case of a major gas as well as for minor atoms and ions. Strong enhancements of minor-ion thermal diffusion coefficients over their values in the fully ionized gas are found when the degree of ionization in the main gas is relatively low. However, compared to the case of equal temperatures, the enhancements are less strong when the neutrals are cooler than the ions. The specific case of the H-H(+) mixture, which is important in the study of solar and stellar atmospheres, is discussed as an application.

Strongly anomalous diffusion in sheared magnetic configurations

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

Vanden Eijnden, E.; Balescu, R.

1996-03-01

The statistical behavior of magnetic lines in a sheared magnetic configuration with reference surface {ital x}=0 is investigated within the framework of the kinetic theory. A Liouville equation is associated with the equations of motion of the stochastic magnetic lines. After averaging over an ensemble of realizations, it yields a convection-diffusion equation within the quasilinear approximation. The diffusion coefficients are space dependent and peaked around the reference surface {ital x}=0. Due to the shear, the diffusion of lines away from the reference surface is slowed down. The behavior of the lines is asymptotically strongly non-Gaussian. The reference surface acts likemore » an attractor around which the magnetic lines spread with an effective subdiffusive behavior. Comparison is also made with more usual treatments based on the study of the first two moments equations. For sheared systems, it is explicitly shown that the Corrsin approximation assumed in the latter approach is no longer valid. It is also concluded that the diffusion coefficients cannot be derived from the mean square displacement of the magnetic lines in an inhomogeneous medium. {copyright} {ital 1996 American Institute of Physics.}« less

Calculations of hydrogen diffusivity in Zr-based alloys: Influence of alloying elements and effect of stress

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

Yu, J.; Jiang, C.; Zhang, Y.

This report summarizes the progress on modeling hydrogen diffusivity in Zr-based alloys. The presence of hydrogen (H) can detrimentally affect the mechanical properties of many metals and alloys. To mitigate these detrimental effects requires fundamental understanding of the thermodynamics and kinetics governing H pickup and hydride formation. In this work, we focus on H diffusion in Zr-based alloys by studying the effects of alloying elements and stress, factors that have been shown to strongly affect H pickup and hydride formation in nuclear fuel claddings. A recently developed accelerated kinetic Monte Carlo method is used for the study. It is foundmore » that for the alloys considered here, H diffusivity depends weakly on composition, with negligible effect at high temperatures in the range of 600-1200 K. Therefore, the small variation in compositions of these alloys is likely not a major cause of the very different H pickup rates. In contrast, stress strongly affects H diffusivity. This effect needs to be considered for studying hydride formation and delayed hydride cracking.« less

Communication: Diverse nanoscale cluster dynamics: Diffusion of 2D epitaxial clusters

NASA Astrophysics Data System (ADS)

Lai, King C.; Evans, James W.; Liu, Da-Jiang

2017-11-01

The dynamics of nanoscale clusters can be distinct from macroscale behavior described by continuum formalisms. For diffusion of 2D clusters of N atoms in homoepitaxial systems mediated by edge atom hopping, macroscale theory predicts simple monotonic size scaling of the diffusion coefficient, DN ˜ N-β, with β = 3/2. However, modeling for nanoclusters on metal(100) surfaces reveals that slow nucleation-mediated diffusion displaying weak size scaling β < 1 occurs for "perfect" sizes Np = L2 and L(L+1) for integer L = 3,4,… (with unique square or near-square ground state shapes), and also for Np+3, Np+4,…. In contrast, fast facile nucleation-free diffusion displaying strong size scaling β ≈ 2.5 occurs for sizes Np+1 and Np+2. DN versus N oscillates strongly between the slowest branch (for Np+3) and the fastest branch (for Np+1). All branches merge for N = O(102), but macroscale behavior is only achieved for much larger N = O(103). This analysis reveals the unprecedented diversity of behavior on the nanoscale.

Effect of thermal diffusion on the stability of strongly tilted mantle plume tails

NASA Astrophysics Data System (ADS)

Kerr, R. C.; MéRiaux, C.; Lister, J. R.

2008-09-01

The effect of thermal diffusion on the stability of strongly tilted mantle plume tails is explored by investigating experimentally and numerically the gravitational instability of a rising horizontal cylindrical region of buoyant viscous fluid. At large viscosity ratios, we find that the instability is unaffected by diffusion when the Rayleigh number Ra is greater than about 300. When Ra is less than 300, diffusion significantly increases the time for instability, as the rising fluid region needs to grow substantially by entrainment before it becomes unstable. When Ra is less than about 140 and the rise height available H is less than about 40 times the cylinder radius, the rising region of fluid is unable to grow sufficiently and instability is prevented. When our results are applied to the Earth, we predict that thermal diffusion will stabilize plume tails in both the upper and lower mantle. We also predict that some of the buoyancy flux in mantle plumes is lost during ascent to form downstream thermal wakes in any larger-scale mantle flow.

Extreme water loss and abiotic O2 buildup on planets throughout the habitable zones of M dwarfs.

PubMed

Luger, R; Barnes, R

2015-02-01

We show that terrestrial planets in the habitable zones of M dwarfs older than ∼1 Gyr could have been in runaway greenhouses for several hundred million years following their formation due to the star's extended pre-main sequence phase, provided they form with abundant surface water. Such prolonged runaway greenhouses can lead to planetary evolution divergent from that of Earth. During this early runaway phase, photolysis of water vapor and hydrogen/oxygen escape to space can lead to the loss of several Earth oceans of water from planets throughout the habitable zone, regardless of whether the escape is energy-limited or diffusion-limited. We find that the amount of water lost scales with the planet mass, since the diffusion-limited hydrogen escape flux is proportional to the planet surface gravity. In addition to undergoing potential desiccation, planets with inefficient oxygen sinks at the surface may build up hundreds to thousands of bar of abiotically produced O2, resulting in potential false positives for life. The amount of O2 that builds up also scales with the planet mass; we find that O2 builds up at a constant rate that is controlled by diffusion: ∼5 bar/Myr on Earth-mass planets and up to ∼25 bar/Myr on super-Earths. As a result, some recently discovered super-Earths in the habitable zone such as GJ 667Cc could have built up as many as 2000 bar of O2 due to the loss of up to 10 Earth oceans of water. The fate of a given planet strongly depends on the extreme ultraviolet flux, the duration of the runaway regime, the initial water content, and the rate at which oxygen is absorbed by the surface. In general, we find that the initial phase of high luminosity may compromise the habitability of many terrestrial planets orbiting low-mass stars.

Computer simulation of viscous fingering in Sierpinski carpet

NASA Astrophysics Data System (ADS)

Ju-ping, Tian; Kai-lun, Yao

1998-09-01

A new method-mapping dilation method is proposed in this paper to construct Sierpinski carpet. Viscous fingering (VF) in Sierpinski carpet, based on the assumption that bond radii are beta distribution, is investigated by means of successive over-relaxation techniques. The topology and the geometry of the porous media have a strong effect on displacement processes. In the Sierpinski network, the VF pattern of porous media in the limit M → ∞ is found to be similar to the diffusion-limited-aggregation pattern. The fractal dimension for VF in fractal space is calculated and the fractal dimension D can be reasonably regarded as a useful parameter to evaluate the sweep efficiencies and oil recoveries. We have also found that the geometry of the porous medium also has strong effects on the displacement processes and the structure of the VF. Moreover, we find that the sweep efficiency of the displacement processes mainly depends upon the length of the network system and also on the viscosity ratio M. This shows that the current method can be used to solve VF problems in complex structures if the structures are self-similar, or they can be reduced to a self-similar structure.

Diffusive processes in a stochastic magnetic field

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

Wang, H.; Vlad, M.; Vanden Eijnden, E.

1995-05-01

The statistical representation of a fluctuating (stochastic) magnetic field configuration is studied in detail. The Eulerian correlation functions of the magnetic field are determined, taking into account all geometrical constraints: these objects form a nondiagonal matrix. The Lagrangian correlations, within the reasonable Corrsin approximation, are reduced to a single scalar function, determined by an integral equation. The mean square perpendicular deviation of a geometrical point moving along a perturbed field line is determined by a nonlinear second-order differential equation. The separation of neighboring field lines in a stochastic magnetic field is studied. We find exponentiation lengths of both signs describing,more » in particular, a decay (on the average) of any initial anisotropy. The vanishing sum of these exponentiation lengths ensures the existence of an invariant which was overlooked in previous works. Next, the separation of a particle`s trajectory from the magnetic field line to which it was initially attached is studied by a similar method. Here too an initial phase of exponential separation appears. Assuming the existence of a final diffusive phase, anomalous diffusion coefficients are found for both weakly and strongly collisional limits. The latter is identical to the well known Rechester-Rosenbluth coefficient, which is obtained here by a more quantitative (though not entirely deductive) treatment than in earlier works.« less

Molecular dynamics simulations reveal that water diffusion between graphene oxide layers is slow

DOE PAGES

Devanathan, Ram; Chase-Woods, Dylan; Shin, Yongsoon; ...

2016-07-08

Membranes made of stacked layers of graphene oxide (GO) hold the tantalizing promise of revolutionizing desalination and water filtration if selective transport of molecules can be controlled. We present the findings of a molecular dynamics simulation study of water intercalated between GO layers that have a C/O ratio of 4. We simulated a range of hydration levels from 1 wt.% to 23.3 wt.% water. The interlayer spacing increased upon hydration from 0.8 nm to 1.1 nm. We also synthesized GO membranes that showed an increase in spacing from about 0.7 nm to 0.8 nm and an increase in mass ofmore » about 14% on hydration. Water diffusion through GO layers is an order of magnitude slower than that in bulk water, because of strong hydrogen bonded interactions. Most of the water molecules are bound to OH groups even at the highest hydration level. We observed large water clusters that could span graphitic regions, oxidized regions and holes that have been experimentally observed in GO. As a result, slow interlayer diffusion can be consistent with experimentally observed water transport in GO if holes lead to a shorter path length than previously assumed and sorption serves as a key rate-limiting step.« less

Particle-Image Velocimetry in Microgravity Laminar Jet Diffusion Flames

NASA Technical Reports Server (NTRS)

Sunderland, P. B.; Greenberg, P. S.; Urban, D. L.; Wernet, M. P.; Yanis, W.

1999-01-01

This paper discusses planned velocity measurements in microgravity laminar jet diffusion flames. These measurements will be conducted using Particle-Image Velocimetry (PIV) in the NASA Glenn 2.2-second drop tower. The observations are of fundamental interest and may ultimately lead to improved efficiency and decreased emissions from practical combustors. The velocity measurements will support the evaluation of analytical and numerical combustion models. There is strong motivation for the proposed microgravity flame configuration. Laminar jet flames are fundamental to combustion and their study has contributed to myriad advances in combustion science, including the development of theoretical, computational and diagnostic combustion tools. Nonbuoyant laminar jet flames are pertinent to the turbulent flames of more practical interest via the laminar flamelet concept. The influence of gravity on these flames is deleterious: it complicates theoretical and numerical modeling, introduces hydrodynamic instabilities, decreases length scales and spatial resolution, and limits the variability of residence time. Whereas many normal-gravity laminar jet diffusion flames have been thoroughly examined (including measurements of velocities, temperatures, compositions, sooting behavior and emissive and absorptive properties), measurements in microgravity gas-jet flames have been less complete and, notably, have included only cursory velocity measurements. It is envisioned that our velocity measurements will fill an important gap in the understanding of nonbuoyant laminar jet flames.

Effect of Pore Clogging on Kinetics of Lead Uptake by Clinoptilolite.

PubMed

Inglezakis; Diamandis; Loizidou; Grigoropoulou

1999-07-01

The kinetics of lead-sodium ion exchange using pretreated natural clinoptilolite are investigated, more specifically the influence of agitation (0, 210, and 650 rpm) on the limiting step of the overall process, for particle sizes of 0.63-0.8 and 0.8-1 mm at ambient temperature and initial lead solutions of 500 mg l-1 without pH adjustment. The isotopic exchange model is found to fit the ion exchange process. Particle diffusion is shown to be the controlling step for both particle sizes under agitation, while in the absence of agitation film diffusion is shown to control. The ion exchange process effective diffusion coefficients are calculated and found to depend strongly on particle size in the case of agitation at 210 rpm and only slightly on particle size at 650 rpm. Lead uptake rates are higher for smaller particles only at rigorous agitation, while at mild agitation the results are reversed. These facts are due to partial clogging of the pores of the mineral during the grinding process. This is verified through comparison of lead uptake rates for two samples of the same particle size, one of which is rigorously washed for a certain time before being exposed to the ion exchange. Copyright 1999 Academic Press.

Synchrotron-based analysis of chromium distributions in multicrystalline silicon for solar cells

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

Jensen, Mallory Ann; Hofstetter, Jasmin; Morishige, Ashley E.

Chromium (Cr) can degrade silicon wafer-based solar cell efficiencies at concentrations as low as 10(10) cm(-3). In this contribution, we employ synchrotron-based X-ray fluorescence microscopy to study chromium distributions in multicrystalline silicon in as-grown material and after phosphorous diffusion. We complement quantified precipitate size and spatial distribution with interstitial Cr concentration and minority carrier lifetime measurements to provide insight into chromium gettering kinetics and offer suggestions for minimizing the device impacts of chromium. We observe that Cr-rich precipitates in as-grown material are generally smaller than iron-rich precipitates and that Cri point defects account for only one-half of the total Crmore » in the as-grown material. This observation is consistent with previous hypotheses that Cr transport and CrSi2 growth are more strongly diffusion-limited during ingot cooling. We apply two phosphorous diffusion gettering profiles that both increase minority carrier lifetime by two orders of magnitude and reduce [Cr-i] by three orders of magnitude to approximate to 10(10) cm(-3). Some Cr-rich precipitates persist after both processes, and locally high [Cri] after the high-temperature process indicates that further optimization of the chromium gettering profile is possible. (C) 2015 AIP Publishing LLC.« less

Reduce Confusion about Diffusion.

ERIC Educational Resources Information Center

Hebrank, Mary R.

1997-01-01

Presents activities that allow students to explore the fundamental but poorly understood concept of diffusion by appealing to their kinesthetic senses first, then challenging their analytical skills as they try to deduce the mathematical principle involved. Presents a computer simulation of diffusion and discusses diffusion's limitations and…

Class of self-limiting growth models in the presence of nonlinear diffusion

NASA Astrophysics Data System (ADS)

Kar, Sandip; Banik, Suman Kumar; Ray, Deb Shankar

2002-06-01

The source term in a reaction-diffusion system, in general, does not involve explicit time dependence. A class of self-limiting growth models dealing with animal and tumor growth and bacterial population in a culture, on the other hand, are described by kinetics with explicit functions of time. We analyze a reaction-diffusion system to study the propagation of spatial front for these models.

Unsteady Extinction of Opposed Jet Ethylene/Methane HIFiRE Surrogate Fuel Mixtures vs Air

NASA Technical Reports Server (NTRS)

Vaden, Sarah N.; Debes, Rachel L.; Lash, E. Lara; Burk, Rachel S.; Boyd, C. Merritt; Wilson, Lloyd G.; Pellett, Gerald L.

2009-01-01

A unique idealized study of the subject fuel vs. air systems was conducted using an Oscillatory-input Opposed Jet Burner (OOJB) system and a newly refined analysis. Extensive dynamic-extinction measurements were obtained on unanchored (free-floating) laminar Counter Flow Diffusion Flames (CFDFs) at 1-atm, stabilized by steady input velocities (e.g., U(sub air)) and perturbed by superimposed in-phase sinusoidal velocity inputs at fuel and air nozzle exits. Ethylene (C2H4) and methane (CH4), and intermediate 64/36 and 15/85 molar percent mixtures were studied. The latter gaseous surrogates were chosen earlier to mimic ignition and respective steady Flame Strengths (FS = U(sub air)) of vaporized and cracked, and un-cracked, JP-7 "like" kerosene for a Hypersonic International Flight Research Experimentation (HIFiRE) scramjet. For steady idealized flameholding, the 100% C2H4 flame is respectively approx. 1.3 and approx.2.7 times stronger than a 64/36 mix and CH4; but is still 12.0 times weaker than a 100% H2-air flame. Limited Hot-Wire (HW) measurements of velocity oscillations at convergent-nozzle exits, and more extensive Probe Microphone (PM) measurements of acoustic pressures, were used to normalize Dynamic FSs, which decayed linearly with pk/pk U(sub air) (velocity magnitude, HW), and also pk/pk P (pressure magnitude, PM). Thus Dynamic Flame Weakening (DFW) is defined as % decrease in FS per Pascal of pk/pk P oscillation, namely, DFW = -100 d(U(sub air)/U(sub air),0Hz)/d(pkpk P). Key findings are: (1) Ethylene flames are uniquely strong and resilient to extinction by oscillating inflows below 150 Hz; (2) Methane flames are uniquely weak; (3) Ethylene / methane surrogate flames are disproportionately strong with respect to ethylene content; and (4) Flame weakening is consistent with limited published results on forced unsteady CFDFs. Thus from 0 to approx. 10 Hz and slightly higher, lagging diffusive responses of key species led to progressive phase lags (relative to inputs) in the oscillating flames, and caused maximum weakening. At 20 to 150 Hz, diffusion-rate-limited effects diminished, causing flames to "regain strengnth," and eventually become completely insensitive beyond 300 Hz. Detailed mechanistic understanding is needed. Overall, ethylene flames are remarkably resilient to dynamic extinction by oscillating inflows. They are the strongest, with the notable exception of H2. For HIFiRE tests, the 64%/36% surrogate disproportionally retains the high dynamic FS of ethylene, so the potential for loss of scramjet flameholding (flameout) due to low frequency oscillations is significantly mitigated.

Exact Markov chains versus diffusion theory for haploid random mating.

PubMed

Tyvand, Peder A; Thorvaldsen, Steinar

2010-05-01

Exact discrete Markov chains are applied to the Wright-Fisher model and the Moran model of haploid random mating. Selection and mutations are neglected. At each discrete value of time t there is a given number n of diploid monoecious organisms. The evolution of the population distribution is given in diffusion variables, to compare the two models of random mating with their common diffusion limit. Only the Moran model converges uniformly to the diffusion limit near the boundary. The Wright-Fisher model allows the population size to change with the generations. Diffusion theory tends to under-predict the loss of genetic information when a population enters a bottleneck. 2010 Elsevier Inc. All rights reserved.

Sodium diffusion in 4H-SiC

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

Linnarsson, M. K., E-mail: marga@kth.se; Hallén, A.

Sodium diffusion has been studied in p-type 4H-SiC. Heat treatments have been performed from 1200 °C to 1800 °C for 1 min to 4 h. Secondary ion mass spectrometry has been used to measure the sodium distribution. We show that sodium has a considerable mobility at 1200 °C in p-type 4H-SiC. On the other hand for sodium atoms trapped at suitable sites the mobility is limited up to 1800 °C. Trap limited diffusion kinetics is suggested and an effective diffusivity has been extracted with an activation energy of 4 eV for sodium diffusion in p-type 4H-SiC.

Development of a High Angular Resolution Diffusion Imaging Human Brain Template

PubMed Central

Varentsova, Anna; Zhang, Shengwei; Arfanakis, Konstantinos

2014-01-01

Brain diffusion templates contain rich information about the microstructure of the brain, and are used as references in spatial normalization or in the development of brain atlases. The accuracy of diffusion templates constructed based on the diffusion tensor (DT) model is limited in regions with complex neuronal micro-architecture. High angular resolution diffusion imaging (HARDI) overcomes limitations of the DT model and is capable of resolving intravoxel heterogeneity. However, when HARDI is combined with multiple-shot sequences to minimize image artifacts, the scan time becomes inappropriate for human brain imaging. In this work, an artifact-free HARDI template of the human brain was developed from low angular resolution multiple-shot diffusion data. The resulting HARDI template was produced in ICBM-152 space based on Turboprop diffusion data, was shown to resolve complex neuronal micro-architecture in regions with intravoxel heterogeneity, and contained fiber orientation information consistent with known human brain anatomy. PMID:24440528

A diffusion-limited reaction model for self-propagating Al/Pt multilayers with quench limits

NASA Astrophysics Data System (ADS)

Kittell, D. E.; Yarrington, C. D.; Hobbs, M. L.; Abere, M. J.; Adams, D. P.

2018-04-01

A diffusion-limited reaction model was calibrated for Al/Pt multilayers ignited on oxidized silicon, sapphire, and tungsten substrates, as well as for some Al/Pt multilayers ignited as free-standing foils. The model was implemented in a finite element analysis code and used to match experimental burn front velocity data collected from several years of testing at Sandia National Laboratories. Moreover, both the simulations and experiments reveal well-defined quench limits in the total Al + Pt layer (i.e., bilayer) thickness. At these limits, the heat generated from atomic diffusion is insufficient to support a self-propagating wave front on top of the substrates. Quench limits for reactive multilayers are seldom reported and are found to depend on the thermal properties of the individual layers. Here, the diffusion-limited reaction model is generalized to allow for temperature- and composition-dependent material properties, phase change, and anisotropic thermal conductivity. Utilizing this increase in model fidelity, excellent overall agreement is shown between the simulations and experimental results with a single calibrated parameter set. However, the burn front velocities of Al/Pt multilayers ignited on tungsten substrates are over-predicted. Possible sources of error are discussed and a higher activation energy (from 41.9 kJ/mol.at. to 47.5 kJ/mol.at.) is shown to bring the simulations into agreement with the velocity data observed on tungsten substrates. This higher activation energy suggests an inhibited diffusion mechanism present at lower heating rates.

Ionic-Electronic Ambipolar Transport in Metal Halide Perovskites: Can Electronic Conductivity Limit Ionic Diffusion?

PubMed

Kerner, Ross A; Rand, Barry P

2018-01-04

Ambipolar transport describes the nonequilibrium, coupled motion of positively and negatively charged particles to ensure that internal electric fields remain small. It is commonly invoked in the semiconductor community where the motion of excess electrons and holes drift and diffuse together. However, the concept of ambipolar transport is not limited to semiconductor physics. Materials scientists working on ion conducting ceramics understand ambipolar transport dictates the coupled diffusion of ions and the rate is limited by the ion with the lowest diffusion coefficient. In this Perspective, we review a third application of ambipolar transport relevant to mixed ionic-electronic conducting materials for which the motion of ions is expected to be coupled to electronic carriers. In this unique situation, the ambipolar diffusion model has been successful at explaining the photoenhanced diffusion of metal ions in chalcogenide glasses and other properties of materials. Recent examples of photoenhanced phenomena in metal halide perovskites are discussed and indicate that mixed ionic-electronic ambipolar transport is similarly important for a deep understanding of these emerging materials.

Uphill diffusion in multicomponent mixtures.

PubMed

Krishna, Rajamani

2015-05-21

Molecular diffusion is an omnipresent phenomena that is important in a wide variety of contexts in chemical, physical, and biological processes. In the majority of cases, the diffusion process can be adequately described by Fick's law that postulates a linear relationship between the flux of any species and its own concentration gradient. Most commonly, a component diffuses down the concentration gradient. The major objective of this review is to highlight a very wide variety of situations that cause the uphill transport of one constituent in the mixture. Uphill diffusion may occur in multicomponent mixtures in which the diffusion flux of any species is strongly coupled to that of its partner species. Such coupling effects often arise from strong thermodynamic non-idealities. For a quantitative description we need to use chemical potential gradients as driving forces. The transport of ionic species in aqueous solutions is coupled with its partner ions because of the electro-neutrality constraints; such constraints may accelerate or decelerate a specific ion. When uphill diffusion occurs, we observe transient overshoots during equilibration; the equilibration process follows serpentine trajectories in composition space. For mixtures of liquids, alloys, ceramics and glasses the serpentine trajectories could cause entry into meta-stable composition zones; such entry could result in phenomena such as spinodal decomposition, spontaneous emulsification, and the Ouzo effect. For distillation of multicomponent mixtures that form azeotropes, uphill diffusion may allow crossing of distillation boundaries that are normally forbidden. For mixture separations with microporous adsorbents, uphill diffusion can cause supra-equilibrium loadings to be achieved during transient uptake within crystals; this allows the possibility of over-riding adsorption equilibrium for achieving difficult separations.

Constraints on dark matter annihilations from diffuse gamma-ray emission in the Galaxy

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

Tavakoli, Maryam; Evoli, Carmelo; Cholis, Ilias

2014-01-01

Recent advances in γ-ray cosmic ray, infrared and radio astronomy have allowed us to develop a significantly better understanding of the galactic medium properties in the last few years. In this work using the DRAGON code, that numerically solves the CR propagation equation and calculating γ-ray emissivities in a 2-dimensional grid enclosing the Galaxy, we study in a self consistent manner models for the galactic diffuse γ-ray emission. Our models are cross-checked to both the available CR and γ-ray data. We address the extend to which dark matter annihilations in the Galaxy can contribute to the diffuse γ-ray flux towardsmore » different directions on the sky. Moreover we discuss the impact that astrophysical uncertainties of non DM nature, have on the derived γ-ray limits. Such uncertainties are related to the diffusion properties on the Galaxy, the interstellar gas and the interstellar radiation field energy densities. Light ∼ 10 GeV dark matter annihilating dominantly to hadrons is more strongly constrained by γ-ray observations towards the inner parts of the Galaxy and influenced the most by assumptions of the gas distribution; while TeV scale DM annihilating dominantly to leptons has its tightest constraints from observations towards the galactic center avoiding the galactic disk plane, with the main astrophysical uncertainty being the radiation field energy density. In addition, we present a method of deriving constraints on the dark matter distribution profile from the diffuse γ-ray spectra. These results critically depend on the assumed mass of the dark matter particles and the type of its end annihilation products.« less

Influence of surface wettability on transport mechanisms governing water droplet evaporation.

PubMed

Pan, Zhenhai; Weibel, Justin A; Garimella, Suresh V

2014-08-19

Prediction and manipulation of the evaporation of small droplets is a fundamental problem with importance in a variety of microfluidic, microfabrication, and biomedical applications. A vapor-diffusion-based model has been widely employed to predict the interfacial evaporation rate; however, its scope of applicability is limited due to incorporation of a number of simplifying assumptions of the physical behavior. Two key transport mechanisms besides vapor diffusion-evaporative cooling and natural convection in the surrounding gas-are investigated here as a function of the substrate wettability using an augmented droplet evaporation model. Three regimes are distinguished by the instantaneous contact angle (CA). In Regime I (CA ≲ 60°), the flat droplet shape results in a small thermal resistance between the liquid-vapor interface and substrate, which mitigates the effect of evaporative cooling; upward gas-phase natural convection enhances evaporation. In Regime II (60 ≲ CA ≲ 90°), evaporative cooling at the interface suppresses evaporation with increasing contact angle and counterbalances the gas-phase convection enhancement. Because effects of the evaporative cooling and gas-phase convection mechanisms largely neutralize each other, the vapor-diffusion-based model can predict the overall evaporation rates in this regime. In Regime III (CA ≳ 90°), evaporative cooling suppresses the evaporation rate significantly and reverses entirely the direction of natural convection induced by vapor concentration gradients in the gas phase. Delineation of these counteracting mechanisms reconciles previous debate (founded on single-surface experiments or models that consider only a subset of the governing transport mechanisms) regarding the applicability of the classic vapor-diffusion model. The vapor diffusion-based model cannot predict the local evaporation flux along the interface for high contact angle (CA ≥ 90°) when evaporative cooling is strong and the temperature gradient along the interface determines the peak local evaporation flux.

Effect of EMIC Wave Normal Angle Distribution on Relativistic Electron Scattering

NASA Technical Reports Server (NTRS)

Gamayunov, K. V.; Khazanov, G. V.

2006-01-01

The flux level of outer-zone relativistic electrons (above 1 MeV) is extremely variable during geomagnetic storms, and controlled by a competition between acceleration and loss. Precipitation of these electrons due to resonant pitch-angle scattering by electromagnetic ion cyclotron (EMIC) waves is considered one of the major loss mechanisms. This mechanism was suggested in early theoretical studies more than three decades ago. However, direct experimental evidence of the wave role in relativistic electrons precipitation is difficult to obtain because of lack of concurrent measurements of precipitating electrons at low altitudes and the waves in a magnetically conjugate equatorial region. Recently, the data from balloon-borne X-ray instruments provided indirect but strong evidence on an efficiency of the EMIC wave induced loss for the outer-zone relativistic electrons. These observations stimulated theoretical studies that, particularly, demonstrated that EMIC wave induced pitch-angle diffusion of MeV electrons can operate in the strong diffusion limit and this mechanism can compete with relativistic electron depletion caused by the Dst effect during the initial and main phases of storm. Although an effectiveness of relativistic electron scattering by EMIC waves depends strongly on the wave spectral properties, the most favorable assumptions regarding wave characteristics has been made in all previous theoretical studies. Particularly, only quasi field-aligned EMIC waves have been considered as a driver for relativistic electron loss. At the same time, there is growing experimental and theoretical evidence that these waves can be highly oblique; EMIC wave energy can occupy not only the region of generation, i.e. the region of small wave normal angles, but also the entire wave normal angle region, and even only the region near 90 degrees. The latter can dramatically change he effectiveness of relativistic electron scattering by EMIC waves. In the present study, we calculate the pitch-angle diffusion coefficients using the typical wave normal distributions obtained from our self-consistent ring current-EMIC wave model, and try to quantify the effect of EMIC wave normal angle characteristics on relativistic electron scattering.

Effects of sorption competition on caesium diffusion through compacted argillaceous rock

NASA Astrophysics Data System (ADS)

Jakob, Andreas; Pfingsten, Wilfried; Van Loon, Luc

2009-05-01

We carried out a small-scale laboratory diffusion experiment on a disk-like sample of Opalinus clay from the Mont Terri underground laboratory (Switzerland) using 134Cs as tracer. A through-diffusion phase was followed by an out-diffusion phase where the tracer taken up by the sample was released again. Since the tracer concentration at both boundaries was monitored, careful mass-balance considerations were feasible. A first analysis of the experimental data was done in the frame of a single-species model accounting only for transport and non-linear sorption of caesium. The model could match the data of the through-diffusion phase, however only, when strongly reducing the sorption data based on batch sorption experiments. Yet, such a procedure was in strong contradiction with sorption measurements performed on dispersed and compacted systems. In addition, predictions concerning tracer out-diffusion and mass-balance considerations clearly revealed the shortcomings of this type of model. In a second attempt we applied a multi-species transport model where now the whole water chemistry and a sorption model for caesium were considered. First, the value for the diffusion coefficient was fixed to the best-fit value of the single-species model. But again, the sorption site densities had to be reduced strongly albeit the reduction factor was smaller. Only when fixing the sorption site densities to those values of the sorption model and letting the effective diffusion coefficient D e free for the adjustment, could through-diffusion data be reasonably well fitted and out-diffusion as well as mass-balances be predicted in a satisfying manner. The main results are: (1) The best-fit could be achieved with a value for D e of 1.8 × 10 -10 m 2 s -1 which is rather high but corroborated by results of a molecular modelling study. (2) If caesium arrives in the Opalinus clay sample potassium and sodium (calcium etc.) ions are released and caesium ions are sorbed. The released cations diffuse to lower concentration regions according to their individual concentration gradients. Since locally the cation concentration for potassium, (sodium and calcium) is increased, sorption of these cations is also locally enhanced, affecting in return the sorption behaviour of migrating caesium. Consequently, the sorption process of caesium in such diffusion experiments cannot be addressed by a non-linear isotherm formalism any longer. (3) A reasonable analysis of such single tracer diffusion experiments therefore requires the combined description of transport (diffusion) and sorption of many cations and the whole complex water chemistry of the system. Thus, single-species models can only be applied with care in the considered concentration ranges.

Probabilistic migration modelling focused on functional barrier efficiency and low migration concepts in support of risk assessment.

PubMed

Brandsch, Rainer

2017-10-01

Migration modelling provides reliable migration estimates from food-contact materials (FCM) to food or food simulants based on mass-transfer parameters like diffusion and partition coefficients related to individual materials. In most cases, mass-transfer parameters are not readily available from the literature and for this reason are estimated with a given uncertainty. Historically, uncertainty was accounted for by introducing upper limit concepts first, turning out to be of limited applicability due to highly overestimated migration results. Probabilistic migration modelling gives the possibility to consider uncertainty of the mass-transfer parameters as well as other model inputs. With respect to a functional barrier, the most important parameters among others are the diffusion properties of the functional barrier and its thickness. A software tool that accepts distribution as inputs and is capable of applying Monte Carlo methods, i.e., random sampling from the input distributions of the relevant parameters (i.e., diffusion coefficient and layer thickness), predicts migration results with related uncertainty and confidence intervals. The capabilities of probabilistic migration modelling are presented in the view of three case studies (1) sensitivity analysis, (2) functional barrier efficiency and (3) validation by experimental testing. Based on the predicted migration by probabilistic migration modelling and related exposure estimates, safety evaluation of new materials in the context of existing or new packaging concepts is possible. Identifying associated migration risk and potential safety concerns in the early stage of packaging development is possible. Furthermore, dedicated material selection exhibiting required functional barrier efficiency under application conditions becomes feasible. Validation of the migration risk assessment by probabilistic migration modelling through a minimum of dedicated experimental testing is strongly recommended.

Vertical distribution and composition of phytoplankton under the influence of an upper mixed layer.

PubMed

Ryabov, Alexei B; Rudolf, Lars; Blasius, Bernd

2010-03-07

The vertical distribution of phytoplankton is of fundamental importance for the dynamics and structure of aquatic communities. Here, using an advection-reaction-diffusion model, we investigate the distribution and competition of phytoplankton species in a water column, in which inverse resource gradients of light and a nutrient can limit growth of the biomass. This problem poses a challenge for ecologists, as the location of a production layer is not fixed, but rather depends on many internal parameters and environmental factors. In particular, we study the influence of an upper mixed layer (UML) in this system and show that it leads to a variety of dynamic effects: (i) Our model predicts alternative density profiles with a maximum of biomass either within or below the UML, thereby the system may be bistable or the relaxation from an unstable state may require a long-lasting transition. (ii) Reduced mixing in the deep layer can induce oscillations of the biomass; we show that a UML can sustain these oscillations even if the diffusivity is less than the critical mixing for a sinking phytoplankton population. (iii) A UML can strongly modify the outcome of competition between different phytoplankton species, yielding bistability both in the spatial distribution and in the species composition. (iv) A light limited species can obtain a competitive advantage if the diffusivity in the deep layers is reduced below a critical value. This yields a subtle competitive exclusion effect, where the oscillatory states in the deep layers are displaced by steady solutions in the UML. Finally, we present a novel graphical approach for deducing the competition outcome and for the analysis of the role of a UML in aquatic systems. 2009 Elsevier Ltd. All rights reserved.

Thermal effects in tissues induced by interstitial irradiation of near infrared laser with a cylindrical diffuser

NASA Astrophysics Data System (ADS)

Le, Kelvin; Johsi, Chet; Figueroa, Daniel; Goddard, Jessica; Li, Xiaosong; Towner, Rheal A.; Saunders, Debra; Smith, Nataliya; Liu, Hong; Hode, Tomas; Nordquist, Robert E.; Chen, Wei R.

2011-03-01

Laser immunotherapy (LIT), using non-invasive laser irradiation, has resulted in promising outcomes in the treatment of late-stage cancer patients. However, the tissue absorption of laser light limits the clinical applications of LIT in patients with dark skin, or with deep tumors. The present study is designed to investigate the thermal effects of interstitial irradiation using an 805-nm laser with a cylindrical diffuser, in order to overcome the limitations of the non-invasive mode of treatment. Cow liver and rat tumors were irradiated using interstitial fiber. The temperature increase was monitored by thermocouples that were inserted into the tissue at different sites around the cylinder fiber. Three-dimensional temperature distribution in target tissues during and after interstitial laser irradiation was also determined by Proton Resonance Frequency. The preliminary results showed that the output power of laser and the optical parameters of the target tissue determined the light distribution in the tissue. The temperature distributions varied in the tissue according to the locations relative to the active tip of the cylindrical diffuser. The temperature increase is strongly related to the laser power and irradiation time. Our results using thermocouples and optical sensors indicated that the PRF method is reliable and accurate for temperature determination. Although the inhomogeneous biological tissues could result in temperature fluctuation, the temperature trend still can be reliable enough for the guidance of interstitial irradiation. While this study provides temperature profiles in tumor tissue during interstitial irradiation, the biological effects of the irradiation remain unclear. Future studies will be needed, particularly in combination with the application of immunostimulant for inducing tumor-specific immune responses in the treatment of metastatic tumors.

Experimental measurement of self-diffusion in a strongly coupled plasma

DOE PAGES

Strickler, Trevor S.; Langin, Thomas K.; McQuillen, Paul; ...

2016-05-17

Here, we present a study of the collisional relaxation of ion velocities in a strongly coupled, ultracold neutral plasma on short time scales compared to the inverse collision rate. The measured average velocity of a tagged population of ions is shown to be equivalent to the ion-velocity autocorrelation function. We thus gain access to fundamental aspects of the single-particle dynamics in strongly coupled plasmas and to the ion self-diffusion constant under conditions where experimental measurements have been lacking. Nonexponential decay towards equilibrium of the average velocity heralds non-Markovian dynamics that are not predicted by traditional descriptions of weakly coupled plasmas.more » This demonstrates the utility of ultracold neutral plasmas for studying the effects of strong coupling on collisional processes, which is of interest for dense laboratory and astrophysical plasmas.« less

Molecular-level understanding of protein adsorption at the interface between water and a strongly interacting uncharged solid surface.

PubMed

Penna, Matthew J; Mijajlovic, Milan; Biggs, Mark J

2014-04-09

Although protein adsorption on solids is of immense relevance, experimental limitations mean there is still a remarkable lack of understanding of the adsorption mechanism, particularly at a molecular level. By subjecting 240+ molecular dynamics simulations of two peptide/water/solid surface systems to statistical analysis, a generalized molecular level mechanism for peptide adsorption has been identified for uncharged surfaces that interact strongly with the solution phase. This mechanism is composed of three phases: (1) biased diffusion of the peptide from the bulk phase toward the surface; (2) anchoring of the peptide to the water/solid interface via interaction of a hydrophilic group with the water adjacent to the surface or a strongly interacting hydrophobic group with the surface; and (3) lockdown of the peptide on the surface via a slow, stepwise and largely sequential adsorption of its residues, which we term 'statistical zippering'. The adsorption mechanism is dictated by the existence of water layers adjacent to the solid and orientational ordering therein. By extending the solid into the solution by ~8 Å and endowing it with a charged character, the water layers ensure the peptide feels the effect of the solid at a range well beyond the dispersion force that arises from it, thus inducing biased diffusion from afar. The charging of the interface also facilitates anchoring of the peptide near the surface via one of its hydrophilic groups, allowing it time it would otherwise not have to rearrange and lockdown. Finally, the slowness of the lockdown process is dictated by the need for the peptide groups to replace adjacent tightly bound interfacial water.

Resolving Overlimiting Current Mechanisms in Microchannel-Nanochannel Interface Devices

NASA Astrophysics Data System (ADS)

Yossifon, Gilad; Leibowitz, Neta; Liel, Uri; Schiffbauer, Jarrod; Park, Sinwook

2015-11-01

We present results demonstrating the space charge-mediated transition between classical, diffusion-limited current and surface-conduction dominant over-limiting currents in a shallow micro-nanochannel device. The extended space charge layer develops at the depleted micro-nanochannel entrance at high current and is correlated with a distinctive maximum in the dc resistance. Experimental results for a shallow surface-conduction dominated system are compared with theoretical models, allowing estimates of the effective surface charge at high voltage to be obtained. Further, we extend the study to microchannels of moderate to large depths where the role of various electro-convection mechanisms becomes dominant. In particular, electro-osmotic of the second kind and electro-osmotic instability (EOI) which competes each other at geometrically heterogeneous (e.g. undulated nanoslot interface, array of nanoslots) nanoslot devices. Also, these effects are also shown to be strongly modulated by the non-ideal permselectivity of the nanochannel.

Heavy doping of CdTe single crystals by Cr ion implantation

NASA Astrophysics Data System (ADS)

Popovych, Volodymyr D.; Böttger, Roman; Heller, Rene; Zhou, Shengqiang; Bester, Mariusz; Cieniek, Bogumil; Mroczka, Robert; Lopucki, Rafal; Sagan, Piotr; Kuzma, Marian

2018-03-01

Implantation of bulk CdTe single crystals with high fluences of 500 keV Cr+ ions was performed to achieve Cr concentration above the equilibrium solubility limit of this element in CdTe lattice. The structure and composition of the implanted samples were studied using secondary ion mass spectrometry (SIMS), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis, X-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS) to characterize the incorporation of chromium into the host lattice and to investigate irradiation-induced damage build-up. It was found that out-diffusion of Cr atoms and sputtering of the targets alter the depth distribution and limit concentration of the projectile ions in the as-implanted samples. Appearance of crystallographically oriented, metallic α-Cr nanoparticles inside CdTe matrix was found after implantation, as well as a strong disorder at the depth far beyond the projected range of the implanted ions.

Limitations of signal averaging due to temporal correlation in laser remote-sensing measurements.

PubMed

Menyuk, N; Killinger, D K; Menyuk, C R

1982-09-15

Laser remote sensing involves the measurement of laser-beam transmission through the atmosphere and is subject to uncertainties caused by strong fluctuations due primarily to speckle, glint, and atmospheric-turbulence effects. These uncertainties are generally reduced by taking average values of increasing numbers of measurements. An experiment was carried out to directly measure the effect of signal averaging on back-scattered laser return signals from a diffusely reflecting target using a direct-detection differential-absorption lidar (DIAL) system. The improvement in accuracy obtained by averaging over increasing numbers of data points was found to be smaller than that predicted for independent measurements. The experimental results are shown to be in excellent agreement with a theoretical analysis which considers the effect of temporal correlation. The analysis indicates that small but long-term temporal correlation severely limits the improvement available through signal averaging.

Heat and momentum transfer for magnetoconvection in a vertical external magnetic field

NASA Astrophysics Data System (ADS)

Zürner, Till; Liu, Wenjun; Krasnov, Dmitry; Schumacher, Jörg

2016-11-01

The scaling theory of Grossmann and Lohse for the turbulent heat and momentum transfer is extended to the magnetoconvection case in the presence of a (strong) vertical magnetic field. The comparison with existing laboratory experiments and direct numerical simulations in the quasistatic limit allows to restrict the parameter space to very low Prandtl and magnetic Prandtl numbers and thus to reduce the number of unknown parameters in the model. Also included is the Chandrasekhar limit for which the outer magnetic induction field B is large enough such that convective motion is suppressed and heat is transported by diffusion. Our theory identifies four distinct regimes of magnetoconvection which are distinguished by the strength of the outer magnetic field and the level of turbulence in the flow, respectively. LIMTECH Research Alliance and Research Training Group GK 1567 on Lorentz Force Velocimetry, funded by the Deutsche Forschungsgemeinschaft.

Lévy flight with absorption: A model for diffusing diffusivity with long tails

NASA Astrophysics Data System (ADS)

Jain, Rohit; Sebastian, K. L.

2017-03-01

We consider diffusion of a particle in rearranging environment, so that the diffusivity of the particle is a stochastic function of time. In our previous model of "diffusing diffusivity" [Jain and Sebastian, J. Phys. Chem. B 120, 3988 (2016), 10.1021/acs.jpcb.6b01527], it was shown that the mean square displacement of particle remains Fickian, i.e., ∝T at all times, but the probability distribution of particle displacement is not Gaussian at all times. It is exponential at short times and crosses over to become Gaussian only in a large time limit in the case where the distribution of D in that model has a steady state limit which is exponential, i.e., πe(D ) ˜e-D /D0 . In the present study, we model the diffusivity of a particle as a Lévy flight process so that D has a power-law tailed distribution, viz., πe(D ) ˜D-1 -α with 0 <α <1 . We find that in the short time limit, the width of displacement distribution is proportional to √{T }, implying that the diffusion is Fickian. But for long times, the width is proportional to T1 /2 α which is a characteristic of anomalous diffusion. The distribution function for the displacement of the particle is found to be a symmetric stable distribution with a stability index 2 α which preserves its shape at all times.

Determination of the diffusion coefficient of hydrogen ion in hydrogels.

PubMed

Schuszter, Gábor; Gehér-Herczegh, Tünde; Szűcs, Árpád; Tóth, Ágota; Horváth, Dezső

2017-05-17

The role of diffusion in chemical pattern formation has been widely studied due to the great diversity of patterns emerging in reaction-diffusion systems, particularly in H + -autocatalytic reactions where hydrogels are applied to avoid convection. A custom-made conductometric cell is designed to measure the effective diffusion coefficient of a pair of strong electrolytes containing sodium ions or hydrogen ions with a common anion. This together with the individual diffusion coefficient for sodium ions, obtained from PFGSE-NMR spectroscopy, allows the determination of the diffusion coefficient of hydrogen ions in hydrogels. Numerical calculations are also performed to study the behavior of a diffusion-migration model describing ionic diffusion in our system. The method we present for one particular case may be extended for various hydrogels and diffusing ions (such as hydroxide) which are relevant e.g. for the development of pH-regulated self-healing mechanisms and hydrogels used for drug delivery.

Lithium diffusion in polyether ether ketone and polyimide stimulated by in situ electron irradiation and studied by the neutron depth profiling method

NASA Astrophysics Data System (ADS)

Vacik, J.; Hnatowicz, V.; Attar, F. M. D.; Mathakari, N. L.; Dahiwale, S. S.; Dhole, S. D.; Bhoraskar, V. N.

2014-10-01

Diffusion of lithium from a LiCl aqueous solution into polyether ether ketone (PEEK) and polyimide (PI) assisted by in situ irradiation with 6.5 MeV electrons was studied by the neutron depth profiling method. The number of the Li atoms was found to be roughly proportional to the diffusion time. Regardless of the diffusion time, the measured depth profiles in PEEK exhibit a nearly exponential form, indicating achievement of a steady-state phase of a diffusion-reaction process specified in the text. The form of the profiles in PI is more complex and it depends strongly on the diffusion time. For the longer diffusion time, the profile consists of near-surface bell-shaped part due to Fickian-like diffusion and deeper exponential part.

A convergent diffusion and social marketing approach for disseminating proven approaches to physical activity promotion.

PubMed

Dearing, James W; Maibach, Edward W; Buller, David B

2006-10-01

Approaches from diffusion of innovations and social marketing are used here to propose efficient means to promote and enhance the dissemination of evidence-based physical activity programs. While both approaches have traditionally been conceptualized as top-down, center-to-periphery, centralized efforts at social change, their operational methods have usually differed. The operational methods of diffusion theory have a strong relational emphasis, while the operational methods of social marketing have a strong transactional emphasis. Here, we argue for a convergence of diffusion of innovation and social marketing principles to stimulate the efficient dissemination of proven-effective programs. In general terms, we are encouraging a focus on societal sectors as a logical and efficient means for enhancing the impact of dissemination efforts. This requires an understanding of complex organizations and the functional roles played by different individuals in such organizations. In specific terms, ten principles are provided for working effectively within societal sectors and enhancing user involvement in the processes of adoption and implementation.

Self-diffusion imaging by spin echo in Earth's magnetic field.

PubMed

Mohoric, A; Stepisnik, J; Kos, M; Planinsi

1999-01-01

The NMR of the Earth's magnetic field is used for diffusion-weighted imaging of phantoms. Due to a weak Larmor field, care needs to be taken regarding the use of the usual high field assumption in calculating the effect of the applied inhomogeneous magnetic field. The usual definition of the magnetic field gradient must be replaced by a generalized formula valid when the strength of a nonuniform magnetic field and a Larmor field are comparable (J. Stepisnik, Z. Phys. Chem. 190, 51-62 (1995)). It turns out that the expression for spin echo attenuation is identical to the well-known Torrey formula only when the applied nonuniform field has a proper symmetry. This kind of problem may occur in a strong Larmor field as well as when the slow diffusion rate of particles needs an extremely strong gradient to be applied. The measurements of the geomagnetic field NMR demonstrate the usefulness of the method for diffusion and flow-weighted imaging. Copyright 1999 Academic Press.

Repeated-cascade theory of strong turbulence in a magnetized plasma

NASA Technical Reports Server (NTRS)

Tchen, C. M.

1976-01-01

A two-dimensional Navier-Stokes equation of vorticity in fluid turbulence is used to model drift turbulence in a plasma with a strong constant magnetic field and a constant mean density gradient. The nonlinear eddy diffusivity is described by a time-integrated Lagrangian correlation of velocities, and the repeated-cascade method is employed to choose the rank accounting for nearest-neighbor interactions, to calculate the Lagrangian correlation, and to close the correlation hierarchy. As a result, the diffusivity becomes dependent on the plasma's induced diffusion and is represented by a memory chain that is cut off by similarity and inertial randomization. Spectral laws relating the kinetic-energy spectrum to the -5, -5/2, -3, and -11 powers of wavenumber are derived for the velocity subranges of production, approach to inertia, inertia, and dissipation, respectively. It is found that the diffusivity is proportional to some inverse power of the magnetic field, that power being 1, 2/3, 5/6, and 2, respectively, for the four velocity subranges.

Fractional Dynamics of Single File Diffusion in Dusty Plasma Ring

NASA Astrophysics Data System (ADS)

Muniandy, S. V.; Chew, W. X.; Asgari, H.; Wong, C. S.; Lim, S. C.

2011-11-01

Single file diffusion (SFD) refers to the constrained motion of particles in quasi-one-dimensional channel such that the particles are unable to pass each other. Possible SFD of charged dust confined in biharmonic annular potential well with screened Coulomb interaction is investigated. Transition from normal diffusion to anomalous sub-diffusion behaviors is observed. Deviation from SFD's mean square displacement scaling behavior of 1/2-exponent may occur in strongly interacting systems. A phenomenological model based on fractional Langevin equation is proposed to account for the anomalous SFD behavior in dusty plasma ring.

Characterization of single-file diffusion in one-dimensional dusty plasma

NASA Astrophysics Data System (ADS)

Theisen, W. L.; Sheridan, T. E.

2010-11-01

Single-file diffusion occurs in one-dimensional systems when particles cannot pass each other and the mean-squared displacement (msd) of these particles increases with time t. Diffusive processes that follow Ficks law predict that the msd increases as t, however, single-file diffusion is sub-Fickean meaning that the msd is predicted to increase as t^1/2. One-dimensional dusty plasma rings have been created under strongly coupled, over-damped conditions. Particle position data from these rings will be analyzed to determine the scaling of the msd with time. Results will be compared with predictions of single-file diffusion theory.

Self-diffusion and conductivity in an ultracold strongly coupled plasma: Calculation by the method of molecular dynamics

NASA Astrophysics Data System (ADS)

Zelener, B. B.; Zelener, B. V.; Manykin, E. A.; Bronin, S. Ya; Bobrov, A. A.; Khikhlukha, D. R.

2018-01-01

We present results of calculations by the method of molecular dynamics of self-diffusion and conductivity of electron and ion components of ultracold plasma in a comparison with available theoretical and experimental data. For the ion self-diffusion coefficient, good agreement was obtained with experiments on ultracold plasma. The results of the calculation of self-diffusion also agree well with other calculations performed for the same values of the coupling parameter, but at high temperatures. The difference in the results of the conductivity calculations on the basis of the current autocorrelation function and on the basis of the diffusion coefficient is discussed.

Effect of rotational diffusion in an orientational potential well on the point spread function of electric dipole emitters.

PubMed

Stallinga, Sjoerd

2015-02-01

A study is presented of the point spread function (PSF) of electric dipole emitters that go through a series of absorption-emission cycles while the dipole orientation is changing due to rotational diffusion within the constraint of an orientational potential well. An analytical expression for the PSF is derived valid for arbitrary orientational potential wells in the limit of image acquisition times much larger than the rotational relaxation time. This framework is used to study the effects of the direction of incidence, polarization, and degree of coherence of the illumination. In the limit of fast rotational diffusion on the scale of the fluorescence lifetime the illumination influences only the PSF height, not its shape. In the limit of slow rotational diffusion on the scale of the fluorescence lifetime there is a significant effect on the PSF shape as well, provided the illumination is (partially) coherent. For oblique incidence, illumination asymmetries can arise in the PSF that give rise to position offsets in localization based on Gaussian spot fitting. These asymmetries persist in the limit of free diffusion in a zero orientational potential well.

Integrating flood modelling in a hydrological catchment model: flow approximations and spatial resolution.

NASA Astrophysics Data System (ADS)

van den Bout, Bastian; Jetten, Victor

2017-04-01

Within hydrological models, flow approximations are commonly used to reduce computation time. The validity of these approximations is strongly determined by flow height, flow velocity, the spatial resolution of the model, and by the manner in which flow routing is implemented. The assumptions of these approximations can furthermore limit emergent behavior, and influence flow behavior under space-time scaling. In this presentation, the validity and performance of the kinematic, diffusive and dynamic flow approximations are investigated for use in a catchment-based flood model. Particularly, the validity during flood events and for varying spatial resolutions is investigated. The OpenLISEM hydrological model is extended to implement these flow approximations and channel flooding based on dynamic flow. The kinematic routing uses a predefined converging flow network, the diffusive and dynamic routing uses a 2D flow solution over a DEM. The channel flow in all cases is a 1D kinematic wave approximation. The flow approximations are used to recreate measured discharge in three catchments of different size in China, Spain and Italy, among which is the hydrograph of the 2003 flood event in the Fella river basin (Italy). Furthermore, spatial resolutions are varied for the flood simulation in order to investigate the influence of spatial resolution on these flow approximations. Results show that the kinematic, diffusive and dynamic flow approximation provide least to highest accuracy, respectively, in recreating measured temporal variation of the discharge. Kinematic flow, which is commonly used in hydrological modelling, substantially over-estimates hydrological connectivity in the simulations with a spatial resolution of below 30 meters. Since spatial resolutions of models have strongly increased over the past decades, usage of routed kinematic flow should be reconsidered. In the case of flood events, spatial modelling of kinematic flow substantially over-estimates hydrological connectivity and flow concentration, leading to significant errors. The combination of diffusive or dynamic overland flow and dynamic channel flooding provides high accuracy in recreating the 2003 Fella river flood event. Finally, flow approximations substantially influenced the predictive potential of the (flash) flood model.

Stability and Behaviors of Methane/Propane and Hydrogen Micro Flames

NASA Astrophysics Data System (ADS)

Yoshimoto, Takamitsu; Kinoshita, Koichiro; Kitamura, Hideki; Tanigawa, Ryoichi

The flame stability limits essentially define the fundamental operation of the combustion system. Recently the micro diffusion flame has been remarked. The critical conditions of the flame stability limit are highly dependent on nozzle diameter, species of fuel and so on. The micro diffusion flame of Methane/Propane and Hydrogen is formed by using the micro-scale nozzle of which inner diameter is less than 1mm. The configurations and behaviors of the flame are observed directly and visualized by the high speed video camera The criteria of stability limits are proposed for the micro diffusion flame. The objectives of the present study are to get further understanding of lifting/blow-off for the micro diffusion flame. The results obtained are as follows. (1) The behaviors of the flames are classified into some regions for each diffusion flame. (2) The micro diffusion flame of Methane/Propane cannot be sustained, when the nozzle diameter is less than 0.14 mm. (3) The diffusion flame cannot be sustained below the critical fuel flow rate. (4) The minimum flow which is formed does not depends on the average jet velocity, but on the fuel flow rate. (5) the micro flame is laminar. The flame length is decided by fuel flow rate.

A diffusion-limited reaction model for self-propagating Al/Pt multilayers with quench limits

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

Kittell, David E.; Yarrington, Cole D.; Hobbs, M. L.

A diffusion-limited reaction model was calibrated for Al/Pt multilayers ignited on oxidized silicon, sapphire, and tungsten substrates, as well as for some Al/Pt multilayers ignited as free-standing foils. The model was implemented in a finite element analysis code and used to match experimental burn front velocity data collected from several years of testing at Sandia National Laboratories. Moreover, both the simulations and experiments reveal well-defined quench limits in the total Al + Pt layer (i.e., bilayer) thickness. At these limits, the heat generated from atomic diffusion is insufficient to support a self-propagating wave front on top of the substrates. Quenchmore » limits for reactive multilayers are seldom reported and are found to depend on the thermal properties of the individual layers. Here, the diffusion-limited reaction model is generalized to allow for temperature- and composition-dependent material properties, phase change, and anisotropic thermal conductivity. Utilizing this increase in model fidelity, excellent overall agreement is shown between the simulations and experimental results with a single calibrated parameter set. However, the burn front velocities of Al/Pt multilayers ignited on tungsten substrates are over-predicted. Finally, possible sources of error are discussed and a higher activation energy (from 41.9 kJ/mol.at. to 47.5 kJ/mol.at.) is shown to bring the simulations into agreement with the velocity data observed on tungsten substrates. Finally, this higher activation energy suggests an inhibited diffusion mechanism present at lower heating rates.« less

A diffusion-limited reaction model for self-propagating Al/Pt multilayers with quench limits

DOE PAGES

Kittell, David E.; Yarrington, Cole D.; Hobbs, M. L.; ...

2018-04-14

A diffusion-limited reaction model was calibrated for Al/Pt multilayers ignited on oxidized silicon, sapphire, and tungsten substrates, as well as for some Al/Pt multilayers ignited as free-standing foils. The model was implemented in a finite element analysis code and used to match experimental burn front velocity data collected from several years of testing at Sandia National Laboratories. Moreover, both the simulations and experiments reveal well-defined quench limits in the total Al + Pt layer (i.e., bilayer) thickness. At these limits, the heat generated from atomic diffusion is insufficient to support a self-propagating wave front on top of the substrates. Quenchmore » limits for reactive multilayers are seldom reported and are found to depend on the thermal properties of the individual layers. Here, the diffusion-limited reaction model is generalized to allow for temperature- and composition-dependent material properties, phase change, and anisotropic thermal conductivity. Utilizing this increase in model fidelity, excellent overall agreement is shown between the simulations and experimental results with a single calibrated parameter set. However, the burn front velocities of Al/Pt multilayers ignited on tungsten substrates are over-predicted. Finally, possible sources of error are discussed and a higher activation energy (from 41.9 kJ/mol.at. to 47.5 kJ/mol.at.) is shown to bring the simulations into agreement with the velocity data observed on tungsten substrates. Finally, this higher activation energy suggests an inhibited diffusion mechanism present at lower heating rates.« less

Diffusion archeology for diffusion progression history reconstruction.

PubMed

Sefer, Emre; Kingsford, Carl

2016-11-01

Diffusion through graphs can be used to model many real-world processes, such as the spread of diseases, social network memes, computer viruses, or water contaminants. Often, a real-world diffusion cannot be directly observed while it is occurring - perhaps it is not noticed until some time has passed, continuous monitoring is too costly, or privacy concerns limit data access. This leads to the need to reconstruct how the present state of the diffusion came to be from partial diffusion data. Here, we tackle the problem of reconstructing a diffusion history from one or more snapshots of the diffusion state. This ability can be invaluable to learn when certain computer nodes are infected or which people are the initial disease spreaders to control future diffusions. We formulate this problem over discrete-time SEIRS-type diffusion models in terms of maximum likelihood. We design methods that are based on submodularity and a novel prize-collecting dominating-set vertex cover (PCDSVC) relaxation that can identify likely diffusion steps with some provable performance guarantees. Our methods are the first to be able to reconstruct complete diffusion histories accurately in real and simulated situations. As a special case, they can also identify the initial spreaders better than the existing methods for that problem. Our results for both meme and contaminant diffusion show that the partial diffusion data problem can be overcome with proper modeling and methods, and that hidden temporal characteristics of diffusion can be predicted from limited data.

Diffusion archeology for diffusion progression history reconstruction

PubMed Central

Sefer, Emre; Kingsford, Carl

2015-01-01

Diffusion through graphs can be used to model many real-world processes, such as the spread of diseases, social network memes, computer viruses, or water contaminants. Often, a real-world diffusion cannot be directly observed while it is occurring — perhaps it is not noticed until some time has passed, continuous monitoring is too costly, or privacy concerns limit data access. This leads to the need to reconstruct how the present state of the diffusion came to be from partial diffusion data. Here, we tackle the problem of reconstructing a diffusion history from one or more snapshots of the diffusion state. This ability can be invaluable to learn when certain computer nodes are infected or which people are the initial disease spreaders to control future diffusions. We formulate this problem over discrete-time SEIRS-type diffusion models in terms of maximum likelihood. We design methods that are based on submodularity and a novel prize-collecting dominating-set vertex cover (PCDSVC) relaxation that can identify likely diffusion steps with some provable performance guarantees. Our methods are the first to be able to reconstruct complete diffusion histories accurately in real and simulated situations. As a special case, they can also identify the initial spreaders better than the existing methods for that problem. Our results for both meme and contaminant diffusion show that the partial diffusion data problem can be overcome with proper modeling and methods, and that hidden temporal characteristics of diffusion can be predicted from limited data. PMID:27821901

Vortex motion and flux-flow resistivity in dirty multiband superconductors

NASA Astrophysics Data System (ADS)

Silaev, Mihail; Vargunin, Artjom

2016-12-01

The conductivity of vortex lattices in multiband superconductors with high concentration of impurities is calculated based on microscopic kinetic theory at temperatures significantly smaller than the critical one. Both the limits of high and low fields are considered, when the magnetic induction is close to or much smaller than the critical field strength Hc 2, respectively. It is shown that in contrast to single-band superconductors, the resistive properties are not universal but depend on the pairing constants and ratios of diffusivities in different bands. The low-field magnetoresistance can strongly exceed the Bardeen-Stephen estimation in a quantitative agreement with experimental data for the two-band superconductor MgB2.

Steady state whistler turbulence and stability of thermal barriers in tandem mirrors

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

Litwin, C.; Sudan, R.N.

The effect of the whistler turbulence on anisotropic electrons in a thermal barrier is examined. The electron distribution function is derived self-consistently by solving the steady state quasilinear diffusion equation. Saturated amplitudes are computed using the resonance broadening theory or convective stabilization. Estimated power levels necessary for sustaining the steady state of a strongly anisotropic electron population are found to exceed by orders of magnitude the estimates based on Fokker--Planck calculations for the range of parameters of tandem mirror (TMX-U and MFTF-B) experiments (Nucl. Fusion 25, 1205 (1985)). Upper limits on the allowed degree of anisotropy for existing power densitiesmore » are calculated.« less

The Marriage of Gas and Dust

NASA Astrophysics Data System (ADS)

Price, D. J.; Laibe, G.

2015-10-01

Dust-gas mixtures are the simplest example of a two fluid mixture. We show that when simulating such mixtures with particles or with particles coupled to grids a problem arises due to the need to resolve a very small length scale when the coupling is strong. Since this is occurs in the limit when the fluids are well coupled, we show how the dust-gas equations can be reformulated to describe a single fluid mixture. The equations are similar to the usual fluid equations supplemented by a diffusion equation for the dust-to-gas ratio or alternatively the dust fraction. This solves a number of numerical problems as well as making the physics clear.

Folding, But Not Surface Area Expansion, Is Associated with Cellular Morphological Maturation in the Fetal Cerebral Cortex

PubMed Central

Studholme, Colin; Frias, Antonio E.

2017-01-01

Altered macroscopic anatomical characteristics of the cerebral cortex have been identified in individuals affected by various neurodevelopmental disorders. However, the cellular developmental mechanisms that give rise to these abnormalities are not understood. Previously, advances in image reconstruction of diffusion magnetic resonance imaging (MRI) have made possible high-resolution in utero measurements of water diffusion anisotropy in the fetal brain. Here, diffusion anisotropy within the developing fetal cerebral cortex is longitudinally characterized in the rhesus macaque, focusing on gestation day (G85) through G135 of the 165 d term. Additionally, for subsets of animals characterized at G90 and G135, immunohistochemical staining was performed, and 3D structure tensor analyses were used to identify the cellular processes that most closely parallel changes in water diffusion anisotropy with cerebral cortical maturation. Strong correlations were found between maturation of dendritic arbors on the cellular level and the loss of diffusion anisotropy with cortical development. In turn, diffusion anisotropy changes were strongly associated both regionally and temporally with cortical folding. Notably, the regional and temporal dependence of diffusion anisotropy and folding were distinct from the patterns observed for cerebral cortical surface area expansion. These findings strengthen the link proposed in previous studies between cellular-level changes in dendrite morphology and noninvasive diffusion MRI measurements of the developing cerebral cortex and support the possibility that, in gyroencephalic species, structural differentiation within the cortex is coupled to the formation of gyri and sulci. SIGNIFICANCE STATEMENT Abnormal brain morphology has been found in populations with neurodevelopmental disorders. However, the mechanisms linking cellular level and macroscopic maturation are poorly understood, even in normal brains. This study contributes new understanding to this subject using serial in utero MRI measurements of rhesus macaque fetuses, from which macroscopic and cellular information can be derived. We found that morphological differentiation of dendrites was strongly associated both regionally and temporally with folding of the cerebral cortex. Interestingly, parallel associations were not observed with cortical surface area expansion. These findings support the possibility that perturbed morphological differentiation of cells within the cortex may underlie abnormal macroscopic characteristics of individuals affected by neurodevelopmental disorders. PMID:28069920

Influence of compositional complexity on interdiffusion in Ni-containing concentrated solid-solution alloys

DOE PAGES

Jin, Ke; Zhang, Chuan; Zhang, Fan; ...

2018-03-07

To investigate the compositional effects on thermal-diffusion kinetics in concentrated solid-solution alloys, interdiffusion in seven diffusion couples with alloys from binary to quinary is systematically studied. The alloys with higher compositional complexity exhibit in general lower diffusion coefficients against homologous temperature, however, an exception is found that diffusion in NiCoFeCrPd is faster than in NiCoFeCr and NiCoCr. While the derived diffusion parameters suggest that diffusion in medium and high entropy alloys is overall more retarded than in pure metals and binary alloys, they strongly depend on specific constituents. The comparative features are captured by computational thermodynamics approaches using a self-consistentmore » database.« less

Out-diffusion of deep donors in nitrogen-doped silicon and the diffusivity of vacancies

NASA Astrophysics Data System (ADS)

Voronkov, V. V.; Falster, R.

2012-07-01

A strong resistivity increase in annealed nitrogen-doped silicon samples was reported long ago—but has remained not fully understood. It is now shown that the complicated evolution of the resistivity depth profiles observed can be reproduced by a simple model based on the out-diffusion of some relevant species. Two versions of such an approach were analyzed: (A) out-diffusion of deep donors treated as VN (off-centre substitutional nitrogen), (B) out-diffusion of vacancies (V) and interstitial trimers (N3) produced by dissociation of VN3. Version B, although more complicated, is attractive due to a coincidence of the deduced vacancy diffusivity DV at 1000 °C with the value extrapolated from low-temperature data by Watkins.

relationship between diffusion tensor imaging findings and cognitive outcomes following adult traumatic brain injury: A meta-analysis.

PubMed

Wallace, E J; Mathias, J L; Ward, L

2018-05-24

Cognitive impairments are common following a traumatic brain injury (TBI) and frequently result from white matter (WM) damage. This damage can be quantified using diffusion tensor imaging (DTI), which measures the directionality (fractional anisotropy: FA) and amount (mean diffusivity/apparent diffusion coefficient: MD/ADC) of water diffusion in WM, with high FA and low MD/ADC thought to indicate greater WM integrity. However, the relationship between DTI and cognitive outcomes is currently unclear. The data from 20 studies that examined the relationship between WM integrity (measured using DTI) and cognition (categorised into seven domains) following mild-severe adult TBI were meta-analysed. Overall, high FA and low MD/ADC in most brain regions was associated with better cognitive performance, with memory and attention most strongly related to DTI findings. Specifically, memory and/or attention were very strongly related to DTI findings in the corpus callosum, fornix, internal capsule, arcuate and uncinate fasciculi. However, most findings were based on single studies and therefore await replication. Larger-scale, longitudinal studies are now needed to determine the predictive utility of DTI. Copyright © 2018. Published by Elsevier Ltd.

Dependence of Ion Dynamics on the Polymer Chain Length in Poly(ethylene oxide)-Based Polymer Electrolytes.

PubMed

Chattoraj, Joyjit; Knappe, Marisa; Heuer, Andreas

2015-06-04

It is known from experiments that in the polymer electrolyte system, which contains poly(ethylene oxide) chains (PEO), lithium-cations (Li(+)), and bis(trifluoromethanesulfonyl)imide-anions (TFSI(-)), the cation and the anion diffusion and the ionic conductivity exhibit a similar chain-length dependence: with increasing chain length, they start dropping steadily, and later, they saturate to constant values. These results are surprising because Li-cations are strongly correlated with the polymer chains, whereas TFSI-anions do not have such bonding. To understand this phenomenon, we perform molecular dynamics simulations of this system for four different polymer chain lengths. The diffusion results obtained from our simulations display excellent agreement with the experimental data. The cation transport model based on the Rouse dynamics can successfully quantify the Li-diffusion results, which correlates Li diffusion with the polymer center-of-mass motion and the polymer segmental motion. The ionic conductivity as a function of the chain length is then estimated based on the chain-length-dependent ion diffusion, which shows a temperature-dependent deviation for short chain lengths. We argue that in the first regime, counterion correlations modify the conductivity, whereas for the long chains, the system behaves as a strong electrolyte.

Communication: Diverse nanoscale cluster dynamics: Diffusion of 2D epitaxial clusters

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

Lai, King C.; Evans, James W.; Liu, Da -Jiang

The dynamics of nanoscale clusters can be distinct from macroscale behavior described by continuum formalisms. For diffusion of 2D clusters of N atoms in homoepitaxial systems mediated by edge atom hopping, macroscale theory predicts simple monotonic size scaling of the diffusion coefficient, D N ~ N –β, with β = 3/2. However, modeling for nanoclusters on metal(100) surfaces reveals that slow nucleation-mediated diffusion displaying weak size scaling β < 1 occurs for “perfect” sizes N p = L 2 and L(L+1) for integer L = 3,4,… (with unique square or near-square ground state shapes), and also for N p+3, Nmore » p+4,…. In contrast, fast facile nucleation-free diffusion displaying strong size scaling β ≈ 2.5 occurs for sizes N p+1 and N p+2. D N versus N oscillates strongly between the slowest branch (for N p+3) and the fastest branch (for N p+1). All branches merge for N = O(10 2), but macroscale behavior is only achieved for much larger N = O(10 3). Here, this analysis reveals the unprecedented diversity of behavior on the nanoscale.« less

Communication: Diverse nanoscale cluster dynamics: Diffusion of 2D epitaxial clusters

DOE PAGES

Lai, King C.; Evans, James W.; Liu, Da -Jiang

2017-11-27

The dynamics of nanoscale clusters can be distinct from macroscale behavior described by continuum formalisms. For diffusion of 2D clusters of N atoms in homoepitaxial systems mediated by edge atom hopping, macroscale theory predicts simple monotonic size scaling of the diffusion coefficient, D N ~ N –β, with β = 3/2. However, modeling for nanoclusters on metal(100) surfaces reveals that slow nucleation-mediated diffusion displaying weak size scaling β < 1 occurs for “perfect” sizes N p = L 2 and L(L+1) for integer L = 3,4,… (with unique square or near-square ground state shapes), and also for N p+3, Nmore » p+4,…. In contrast, fast facile nucleation-free diffusion displaying strong size scaling β ≈ 2.5 occurs for sizes N p+1 and N p+2. D N versus N oscillates strongly between the slowest branch (for N p+3) and the fastest branch (for N p+1). All branches merge for N = O(10 2), but macroscale behavior is only achieved for much larger N = O(10 3). Here, this analysis reveals the unprecedented diversity of behavior on the nanoscale.« less

Chemistry in motion: tiny synthetic motors.

PubMed

Colberg, Peter H; Reigh, Shang Yik; Robertson, Bryan; Kapral, Raymond

2014-12-16

CONSPECTUS: Diffusion is the principal transport mechanism that controls the motion of solute molecules and other species in solution; however, the random walk process that underlies diffusion is slow and often nonspecific. Although diffusion is an essential mechanism for transport in the biological realm, biological systems have devised more efficient transport mechanisms using molecular motors. Most biological motors utilize some form of chemical energy derived from their surroundings to induce conformational changes in order to carry out specific functions. These small molecular motors operate in the presence of strong thermal fluctuations and in the regime of low Reynolds numbers, where viscous forces dominate inertial forces. Thus, their dynamical behavior is fundamentally different from that of macroscopic motors, and different mechanisms are responsible for the production of useful mechanical motion. There is no reason why our interest should be confined to the small motors that occur naturally in biological systems. Recently, micron and nanoscale motors that use chemical energy to produce directed motion by a number of different mechanisms have been made in the laboratory. These small synthetic motors also experience strong thermal fluctuations and operate in regimes where viscous forces dominate. Potentially, these motors could be directed to perform different transport tasks, analogous to those of biological motors, for both in vivo and in vitro applications. Although some synthetic motors execute conformational changes to effect motion, the majority do not, and, instead, they use other mechanisms to convert chemical energy into directed motion. In this Account, we describe how synthetic motors that operate by self-diffusiophoresis make use of a self-generated concentration gradient to drive motor motion. A description of propulsion by self-diffusiophoresis is presented for Janus particle motors comprising catalytic and noncatalytic faces. The properties of the dynamics of chemically powered motors are illustrated by presenting the results of particle-based simulations of sphere-dimer motors constructed from linked catalytic and noncatalytic spheres. The geometries of both Janus and sphere-dimer motors with asymmetric catalytic activity support the formation of concentration gradients around the motors. Because directed motion can occur only when the system is not in equilibrium, the nature of the environment and the role it plays in motor dynamics are described. Rotational Brownian motion also acts to limit directed motion, and it has especially strong effects for very small motors. We address the following question: how small can motors be and still exhibit effects due to propulsion, even if only to enhance diffusion? Synthetic motors have the potential to transform the manner in which chemical dynamical processes are carried out for a wide range of applications.

Diffusion of Antimicrobials Across Silicone Hydrogel Contact Lenses.

PubMed

Zambelli, Alison M; Brothers, Kimberly M; Hunt, Kristin M; Romanowski, Eric G; Nau, Amy C; Dhaliwal, Deepinder K; Shanks, Robert M Q

2015-09-01

To measure the diffusion of topical preparations of moxifloxacin, amphotericin B (AmB), and polyhexamethylene biguanide (PHMB) through silicone hydrogel (SH) contact lenses (CLs) in vitro. Using an in vitro model, the diffusion of three antimicrobials through SH CLs was measured. Diffused compounds were measured using a spectrophotometer at set time points over a period of 4 hr. The amount of each diffused antimicrobial was determined by comparing the experimental value with a standard curve. A biological assay was performed to validate the CL diffusion assay by testing antimicrobial activity of diffused material against lawns of susceptible bacteria (Staphylococcus epidermidis) and yeast (Saccharomyces cerevisiae). Experiments were repeated at least two times with a total of at least four independent replicates. Our data show detectable moxifloxacin and PHMB diffusion through SH CLs at 30 min, whereas AmB diffusion remained below the limit of detection within the 4-hr experimental period. In the biological assay, diffused moxifloxacin demonstrated microbial killing starting at 20 min on bacterial lawns, whereas PHMB and AmB failed to demonstrate killing on microbial lawns over the course of the 60-min experiment. In vitro diffusion assays demonstrate limited penetration of certain anti-infective agents through SH CLs. Further studies regarding the clinical benefit of using these agents along with bandage CL for corneal pathologic condition are warranted.

Population genetics inference for longitudinally-sampled mutants under strong selection.

PubMed

Lacerda, Miguel; Seoighe, Cathal

2014-11-01

Longitudinal allele frequency data are becoming increasingly prevalent. Such samples permit statistical inference of the population genetics parameters that influence the fate of mutant variants. To infer these parameters by maximum likelihood, the mutant frequency is often assumed to evolve according to the Wright-Fisher model. For computational reasons, this discrete model is commonly approximated by a diffusion process that requires the assumption that the forces of natural selection and mutation are weak. This assumption is not always appropriate. For example, mutations that impart drug resistance in pathogens may evolve under strong selective pressure. Here, we present an alternative approximation to the mutant-frequency distribution that does not make any assumptions about the magnitude of selection or mutation and is much more computationally efficient than the standard diffusion approximation. Simulation studies are used to compare the performance of our method to that of the Wright-Fisher and Gaussian diffusion approximations. For large populations, our method is found to provide a much better approximation to the mutant-frequency distribution when selection is strong, while all three methods perform comparably when selection is weak. Importantly, maximum-likelihood estimates of the selection coefficient are severely attenuated when selection is strong under the two diffusion models, but not when our method is used. This is further demonstrated with an application to mutant-frequency data from an experimental study of bacteriophage evolution. We therefore recommend our method for estimating the selection coefficient when the effective population size is too large to utilize the discrete Wright-Fisher model. Copyright © 2014 by the Genetics Society of America.

Effect of Drying Temperature on the Chemical Properties and Diffusivity of belimbi (averrhoa belimbi)

NASA Astrophysics Data System (ADS)

Shahari, N.; Jamil, N.; Rasmani, K. A.; Nursabrina

2015-09-01

In recent years, many dried fruit products have been developed in response to a strong demand by the customer. This type of fruit has a different composition and hence different moisture diffusivity (D). During drying, Fick's Law of diffusion, which describes the movement of liquid water was used to calculate this diffusivity. However diffusivity has strong effects on the material drying characteristics and these must be determined. In this paper, Fick's Law of diffusion with different kinds of boundary conditions was solve using separation of variable (SOV). In order to get the value of D, results obtained using SOV will be compared with the results from the drying of belimbi at temperature of 40°C, 50°C and 60°C. Although the results show that variation in the values of diffusivity for different temperatures is relatively small, but the variation in the total time required for drying is significantly bigger: between 3-7 hours. Its shown that diffusivity is an important measurement and should be considered in the modeling of the drying process. The chemical properties of belimbi slices in terms of vitamin C, total ash and antioxidant activity with different air temperatures and pretreatment were also investigated. Higher drying temperatures gives less drying time, a lower vitamin C and antioxidant activity but a greater total of ash, whilst pre-treatment can increased vitamin C and antioxidant activity. The results show that pre-treatment and the drying temperature are important variables to improve mass and heat transfer, as well as the belimbi chemical properties.

Development of a high angular resolution diffusion imaging human brain template.

PubMed

Varentsova, Anna; Zhang, Shengwei; Arfanakis, Konstantinos

2014-05-01

Brain diffusion templates contain rich information about the microstructure of the brain, and are used as references in spatial normalization or in the development of brain atlases. The accuracy of diffusion templates constructed based on the diffusion tensor (DT) model is limited in regions with complex neuronal micro-architecture. High angular resolution diffusion imaging (HARDI) overcomes limitations of the DT model and is capable of resolving intravoxel heterogeneity. However, when HARDI is combined with multiple-shot sequences to minimize image artifacts, the scan time becomes inappropriate for human brain imaging. In this work, an artifact-free HARDI template of the human brain was developed from low angular resolution multiple-shot diffusion data. The resulting HARDI template was produced in ICBM-152 space based on Turboprop diffusion data, was shown to resolve complex neuronal micro-architecture in regions with intravoxel heterogeneity, and contained fiber orientation information consistent with known human brain anatomy. Copyright © 2014 Elsevier Inc. All rights reserved.

Anomalous diffusion with linear reaction dynamics: from continuous time random walks to fractional reaction-diffusion equations.

PubMed

Henry, B I; Langlands, T A M; Wearne, S L

2006-09-01

We have revisited the problem of anomalously diffusing species, modeled at the mesoscopic level using continuous time random walks, to include linear reaction dynamics. If a constant proportion of walkers are added or removed instantaneously at the start of each step then the long time asymptotic limit yields a fractional reaction-diffusion equation with a fractional order temporal derivative operating on both the standard diffusion term and a linear reaction kinetics term. If the walkers are added or removed at a constant per capita rate during the waiting time between steps then the long time asymptotic limit has a standard linear reaction kinetics term but a fractional order temporal derivative operating on a nonstandard diffusion term. Results from the above two models are compared with a phenomenological model with standard linear reaction kinetics and a fractional order temporal derivative operating on a standard diffusion term. We have also developed further extensions of the CTRW model to include more general reaction dynamics.

Nature of self-diffusion in two-dimensional fluids

NASA Astrophysics Data System (ADS)

Choi, Bongsik; Han, Kyeong Hwan; Kim, Changho; Talkner, Peter; Kidera, Akinori; Lee, Eok Kyun

2017-12-01

Self-diffusion in a two-dimensional simple fluid is investigated by both analytical and numerical means. We investigate the anomalous aspects of self-diffusion in two-dimensional fluids with regards to the mean square displacement, the time-dependent diffusion coefficient, and the velocity autocorrelation function (VACF) using a consistency equation relating these quantities. We numerically confirm the consistency equation by extensive molecular dynamics simulations for finite systems, corroborate earlier results indicating that the kinematic viscosity approaches a finite, non-vanishing value in the thermodynamic limit, and establish the finite size behavior of the diffusion coefficient. We obtain the exact solution of the consistency equation in the thermodynamic limit and use this solution to determine the large time asymptotics of the mean square displacement, the diffusion coefficient, and the VACF. An asymptotic decay law of the VACF resembles the previously known self-consistent form, 1/(t\\sqrt{{ln}t}), however with a rescaled time.

The narrow pulse approximation and long length scale determination in xenon gas diffusion NMR studies of model porous media

NASA Technical Reports Server (NTRS)

Mair, R. W.; Sen, P. N.; Hurlimann, M. D.; Patz, S.; Cory, D. G.; Walsworth, R. L.

2002-01-01

We report a systematic study of xenon gas diffusion NMR in simple model porous media, random packs of mono-sized glass beads, and focus on three specific areas peculiar to gas-phase diffusion. These topics are: (i) diffusion of spins on the order of the pore dimensions during the application of the diffusion encoding gradient pulses in a PGSE experiment (breakdown of the narrow pulse approximation and imperfect background gradient cancellation), (ii) the ability to derive long length scale structural information, and (iii) effects of finite sample size. We find that the time-dependent diffusion coefficient, D(t), of the imbibed xenon gas at short diffusion times in small beads is significantly affected by the gas pressure. In particular, as expected, we find smaller deviations between measured D(t) and theoretical predictions as the gas pressure is increased, resulting from reduced diffusion during the application of the gradient pulse. The deviations are then completely removed when water D(t) is observed in the same samples. The use of gas also allows us to probe D(t) over a wide range of length scales and observe the long time asymptotic limit which is proportional to the inverse tortuosity of the sample, as well as the diffusion distance where this limit takes effect (approximately 1-1.5 bead diameters). The Pade approximation can be used as a reference for expected xenon D(t) data between the short and the long time limits, allowing us to explore deviations from the expected behavior at intermediate times as a result of finite sample size effects. Finally, the application of the Pade interpolation between the long and the short time asymptotic limits yields a fitted length scale (the Pade length), which is found to be approximately 0.13b for all bead packs, where b is the bead diameter. c. 2002 Elsevier Sciences (USA).

The narrow pulse approximation and long length scale determination in xenon gas diffusion NMR studies of model porous media.

PubMed

Mair, R W; Sen, P N; Hürlimann, M D; Patz, S; Cory, D G; Walsworth, R L

2002-06-01

We report a systematic study of xenon gas diffusion NMR in simple model porous media, random packs of mono-sized glass beads, and focus on three specific areas peculiar to gas-phase diffusion. These topics are: (i) diffusion of spins on the order of the pore dimensions during the application of the diffusion encoding gradient pulses in a PGSE experiment (breakdown of the narrow pulse approximation and imperfect background gradient cancellation), (ii) the ability to derive long length scale structural information, and (iii) effects of finite sample size. We find that the time-dependent diffusion coefficient, D(t), of the imbibed xenon gas at short diffusion times in small beads is significantly affected by the gas pressure. In particular, as expected, we find smaller deviations between measured D(t) and theoretical predictions as the gas pressure is increased, resulting from reduced diffusion during the application of the gradient pulse. The deviations are then completely removed when water D(t) is observed in the same samples. The use of gas also allows us to probe D(t) over a wide range of length scales and observe the long time asymptotic limit which is proportional to the inverse tortuosity of the sample, as well as the diffusion distance where this limit takes effect (approximately 1-1.5 bead diameters). The Padé approximation can be used as a reference for expected xenon D(t) data between the short and the long time limits, allowing us to explore deviations from the expected behavior at intermediate times as a result of finite sample size effects. Finally, the application of the Padé interpolation between the long and the short time asymptotic limits yields a fitted length scale (the Padé length), which is found to be approximately 0.13b for all bead packs, where b is the bead diameter. c. 2002 Elsevier Sciences (USA).

Consume, Modify, Share (CMS): The Interplay between Individual Decisions and Structural Network Properties in the Diffusion of Information

PubMed Central

Koren, Hila; Kaminer, Ido

2016-01-01

Widely used information diffusion models such as Independent Cascade Model, Susceptible Infected Recovered (SIR) and others fail to acknowledge that information is constantly subject to modification. Some aspects of information diffusion are best explained by network structural characteristics while in some cases strong influence comes from individual decisions. We introduce reinvention, the ability to modify information, as an individual level decision that affects the diffusion process as a whole. Based on a combination of constructs from the Diffusion of Innovations and the Critical Mass Theories, the present study advances the CMS (consume, modify, share) model which accounts for the interplay between network structure and human behavior and interactions. The model's building blocks include processes leading up to and following the formation of a critical mass of information adopters and disseminators. We examine the formation of an inflection point, information reach, sustainability of the diffusion process and collective value creation. The CMS model is tested on two directed networks and one undirected network, assuming weak or strong ties and applying constant and relative modification schemes. While all three networks are designed for disseminating new knowledge they differ in structural properties. Our findings suggest that modification enhances the diffusion of information in networks that support undirected connections and carries the biggest effect when information is shared via weak ties. Rogers' diffusion model and traditional information contagion models are fine tuned. Our results show that modifications not only contribute to a sustainable diffusion process, but also aid information in reaching remote areas of the network. The results point to the importance of cultivating weak ties, allowing reciprocal interaction among nodes and supporting the modification of information in promoting diffusion processes. These results have theoretical and practical implications for designing networks aimed at accelerating the creation and diffusion of information. PMID:27798636

Sign Crossover in All Maxwell-Stefan Diffusivities for Molten Salt LiF-BeF2: A Molecular Dynamics Study.

PubMed

Chakraborty, Brahmananda

2015-08-20

Applying Green-Kubo formalism and equilibrium molecular dynamics (MD) simulations, we have studied for the first time the dynamic correlation, Onsager coefficients, and Maxwell-Stefan (MS) diffusivities of molten salt LiF-BeF2, which is a potential candidate for a coolant in a high temperature reactor. We observe an unusual composition dependence and strikingly a crossover in sign for all the MS diffusivities at a composition of around 7% of LiF where the MS diffusivity between cation-anion pair (Đ(BeF) and Đ(LiF)) jumps from positive to negative value while the MS diffusivity between cation-cation pair (Đ(LiBe)) becomes positive from a negative value. Even though the negative MS diffusivities have been observed for electrolyte solutions between cation-cation pair, here we report negative MS diffusivity between cation-anion pair where Đ(BeF) shows a sharp rise around 66% of BeF2, reaches maximum value at 70% of BeF2, and then decreases almost exponentially with a sign change for BeF2 around 93%. For low mole fraction of LiF, Đ(BeF) follows the Debye-Huckel theory and rises with the square root of LiF mole fraction similar to the MS diffusivity between cation-anion pair in aqueous solution of electrolyte salt. Negative MS diffusivities while unusual are, however, shown to satisfy the non-negative entropy constraints at all thermodynamic states as required by the second law of thermodynamics. We have established a strong correlation between the structure and dynamics and predict that the formation of flouride polyanion network between Be and F ions and coulomb interaction is responsible for sharp variation of the MS diffusivities which controls the multicomponent diffusion phenomenon in LiF-BeF2 which has a strong impact on the performance of the reactor.

Consume, Modify, Share (CMS): The Interplay between Individual Decisions and Structural Network Properties in the Diffusion of Information.

PubMed

Koren, Hila; Kaminer, Ido; Raban, Daphne Ruth

2016-01-01

Widely used information diffusion models such as Independent Cascade Model, Susceptible Infected Recovered (SIR) and others fail to acknowledge that information is constantly subject to modification. Some aspects of information diffusion are best explained by network structural characteristics while in some cases strong influence comes from individual decisions. We introduce reinvention, the ability to modify information, as an individual level decision that affects the diffusion process as a whole. Based on a combination of constructs from the Diffusion of Innovations and the Critical Mass Theories, the present study advances the CMS (consume, modify, share) model which accounts for the interplay between network structure and human behavior and interactions. The model's building blocks include processes leading up to and following the formation of a critical mass of information adopters and disseminators. We examine the formation of an inflection point, information reach, sustainability of the diffusion process and collective value creation. The CMS model is tested on two directed networks and one undirected network, assuming weak or strong ties and applying constant and relative modification schemes. While all three networks are designed for disseminating new knowledge they differ in structural properties. Our findings suggest that modification enhances the diffusion of information in networks that support undirected connections and carries the biggest effect when information is shared via weak ties. Rogers' diffusion model and traditional information contagion models are fine tuned. Our results show that modifications not only contribute to a sustainable diffusion process, but also aid information in reaching remote areas of the network. The results point to the importance of cultivating weak ties, allowing reciprocal interaction among nodes and supporting the modification of information in promoting diffusion processes. These results have theoretical and practical implications for designing networks aimed at accelerating the creation and diffusion of information.

Using the graphene Moiré pattern for the trapping of C60 and homoepitaxy of graphene.

PubMed

Lu, Jiong; Yeo, Pei Shan Emmeline; Zheng, Yi; Yang, Zhiyong; Bao, Qiaoliang; Gan, Chee Kwan; Loh, Kian Ping

2012-01-24

The graphene Moiré superstructure offers a complex landscape of humps and valleys to molecules adsorbing and diffusing on it. Using C(60) molecules as the classic hard sphere analogue, we examine its assembly and layered growth on this corrugated landscape. At the monolayer level, the cohesive interactions of C(60) molecules adsorbing on the Moiré lattice freeze the molecular rotation of C(60) trapped in the valley sites, resulting in molecular alignment of all similarly trapped C(60) molecules at room temperature. The hierarchy of adsorption potential well on the Moiré lattice causes diffusion-limited dendritic growth of C(60) films, as opposed to isotropic growth observed on a smooth surface like graphite. Due to the strong binding energy of the C(60) film, part of the dentritic C(60) films polymerize at 850 K and act as solid carbon sources for graphene homoepitaxy. Our findings point to the possibility of using periodically corrugated graphene in molecular spintronics due to its ability to trap and align organic molecules at room temperature. © 2011 American Chemical Society

Energetic particle signatures of satellites and rings in Neptune's magnetosphere

NASA Technical Reports Server (NTRS)

Selesnick, R. S.; Stone, E. C.

1992-01-01

The cosmic ray system on Voyager 2 found a trapped radiation environment in Neptune's inner magnetosphere which is controlled primarily by absorption at the rings and satellite surfaces. The intensity of electrons with kinetic energies approximately greater than 1 MeV shows particularly strong and narrow signatures associated with absorption by the satellite 1989N1 at an orbital radius of 4.75 Neptune radii. Closer to the planet are several signatures of the inner satellites and rings. Absorption limits the intensity of the inner radiation belt sufficiently for the maximum intensity to occur outside the orbit of 1989N1 at a magnetic L shell of about 7. Radial profiles of the electron phase space density show that electrons diffuse inward from a source in the outer magnetosphere. Many of the inward-diffusing electrons are absorbed upon reaching a satellite orbital radius, but the finite absorption efficiency allows some of the electrons to pass by unaffected. The locations of the satellite and ring signatures also provide constraints on the nondipolar components of the planetary magnetic field.

New techniques for diffusing-wave spectroscopy

NASA Technical Reports Server (NTRS)

Mason, T. G.; Gang, HU; Krall, A. H.; Weitz, David A.

1994-01-01

We present two new types of measurements that can be made with diffusing-wave spectroscopy (DWS), a form of dynamic light scattering that applies in limit of strong multiple scattering. The first application is to measure the frequency-dependent linear viscoelastic moduli of complex fluids using light scattering. This is accomplished by measuring the mean square displacement of probe particles using DWS. Their response to thermal fluctuations is determined by the fluctuation-dissipation relation, and is controlled by the response of the surrounding complex fluid. This response can be described in terms of a memory function, which is directly related to the complex elastic modulus of the system. Thus by measuring the mean square displacement, we are able to determine the frequency dependent modulus. The second application is the measurement of shape fluctuations of scattering particles. This is achieved by generalizing the theory for DWS to incorporate the effects if amplitude fluctuations in the scattering intensity of the particles. We apply this new method to study the thermally induced fluctuations in the shape of spherical emulsion droplets whose geometry is controlled by surface tension.

A model of recovering the parameters of fast nonlocal heat transport in magnetic fusion plasmas

NASA Astrophysics Data System (ADS)

Kukushkin, A. B.; Kulichenko, A. A.; Sdvizhenskii, P. A.; Sokolov, A. V.; Voloshinov, V. V.

2017-12-01

A model is elaborated for interpreting the initial stage of the fast nonlocal transport events, which exhibit immediate response, in the diffusion time scale, of the spatial profile of electron temperature to its local perturbation, while the net heat flux is directed opposite to ordinary diffusion (i.e. along the temperature gradient). We solve the inverse problem of recovering the kernel of the integral equation, which describes nonlocal (superdiffusive) transport of energy due to emission and absorption of electromagnetic (EM) waves with long free path and strong reflection from the vacuum vessel’s wall. To allow for the errors of experimental data, we use the method based on the regularized (in the framework of an ill-posed problem, using the parametric models) approximation of available experimental data. The model is applied to interpreting the data from stellarator LHD and tokamak TFTR. The EM wave transport is considered here in the single-group approximation, however the limitations of the physics model enable us to identify the spectral range of the EM waves which might be responsible for the observed phenomenon.

Intrinsic diffusion sensitivity of the balanced steady-state free precession (bSSFP) imaging sequence.

PubMed

Bär, Sébastien; Weigel, Matthias; von Elverfeldt, Dominik; Hennig, Jürgen; Leupold, Jochen

2015-11-01

The purpose of this work was to analyze the intrinsic diffusion sensitivity of the balanced steady-state free precession (bSSFP) imaging sequence, meaning the observation of diffusion-induced attenuation of the bSSFP steady-state signal due to the imaging gradients. Although these diffusion effects are usually neglected for most clinical gradient systems, such strong gradient systems are employed for high resolution imaging of small animals or MR Microscopy. The impact on the bSSFP signal of the imaging gradients characterized by their b-values was analyzed with simulations and experiments at a 7T animal scanner using a gradient system with maximum gradient amplitude of approx. 700 mT/m. It was found that the readout gradients have a stronger impact on the attenuation than the phase encoding gradients. Also, as the PE gradients are varying with each repetition interval, the diffusion effects induce strong modulations of the bSSFP signal over the sequence repetition cycles depending on the phase encoding gradient table. It is shown that a signal gain can be obtained through a change of flip angle as a new optimal flip angle maximizing the signal can be defined. The dependency of the diffusion effects on relaxation times and b-values were explored with simulations. The attenuation increases with T2. In conclusion, diffusion attenuation of the bSSFP signal becomes significant for high resolution imaging voxel size (roughly < 100 μm) of long T2 substances. Copyright © 2015 John Wiley & Sons, Ltd.

The nanofluidic confinement apparatus: studying confinement-dependent nanoparticle behavior and diffusion

PubMed Central

Fringes, Stefan; Holzner, Felix

2018-01-01

The behavior of nanoparticles under nanofluidic confinement depends strongly on their distance to the confining walls; however, a measurement in which the gap distance is varied is challenging. Here, we present a versatile setup for investigating the behavior of nanoparticles as a function of the gap distance, which is controlled to the nanometer. The setup is designed as an open system that operates with a small amount of dispersion of ≈20 μL, permits the use of coated and patterned samples and allows high-numerical-aperture microscopy access. Using the tool, we measure the vertical position (termed height) and the lateral diffusion of 60 nm, charged, Au nanospheres as a function of confinement between a glass surface and a polymer surface. Interferometric scattering detection provides an effective particle illumination time of less than 30 μs, which results in lateral and vertical position detection accuracy ≈10 nm for diffusing particles. We found the height of the particles to be consistently above that of the gap center, corresponding to a higher charge on the polymer substrate. In terms of diffusion, we found a strong monotonic decay of the diffusion constant with decreasing gap distance. This result cannot be explained by hydrodynamic effects, including the asymmetric vertical position of the particles in the gap. Instead we attribute it to an electroviscous effect. For strong confinement of less than 120 nm gap distance, we detect the onset of subdiffusion, which can be correlated to the motion of the particles along high-gap-distance paths. PMID:29441273

Self-learning kinetic Monte Carlo simulations of diffusion in ferromagnetic α-Fe-Si alloys

NASA Astrophysics Data System (ADS)

Nandipati, Giridhar; Jiang, Xiujuan; Vemuri, Rama S.; Mathaudhu, Suveen; Rohatgi, Aashish

2018-01-01

Diffusion of Si atom and vacancy in the A2-phase of α-Fe-Si alloys in the ferromagnetic state, with and without magnetic order and in various temperature ranges, are studied using AKSOME, an on-lattice self-learning KMC code. Diffusion of the Si atom and the vacancy are studied in the dilute limit and up to 12 at.% Si, respectively, in the temperature range 350-700 K. Local Si neighborhood dependent activation energies for vacancy hops were calculated on-the-fly using a broken-bond model based on pairwise interaction. The migration barrier and prefactor for the Si diffusion in the dilute limit were obtained and found to agree with published data within the limits of uncertainty. Simulations results show that the prefactor and the migration barrier for the Si diffusion are approximately an order of magnitude higher, and a tenth of an electron-volt higher, respectively, in the magnetic disordered state than in the fully ordered state. However, the net result is that magnetic disorder does not have a significant effect on Si diffusivity within the range of parameters studied in this work. Nevertheless, with increasing temperature, the magnetic disorder increases and its effect on the Si diffusivity also increases. In the case of vacancy diffusion, with increasing Si concentration, its diffusion prefactor decreases while the migration barrier more or less remained constant and the effect of magnetic disorder increases with Si concentration. Important vacancy-Si/Fe atom exchange processes and their activation barriers were identified, and the effect of energetics on ordered phase formation in Fe-Si alloys are discussed.

Determination of diffusion coefficients and diffusion characteristics for chlorferon and diethylthiophosphate in Ca-alginate gel beads.

PubMed

Ha, Jiyeon; Engler, Cady R; Lee, Seung Jae

2008-07-01

Diffusion characteristics of chlorferon and diethylthiophosphate (DETP) in Ca-alginate gel beads were studied to assist in designing and operating bioreactor systems. Diffusion coefficients for chlorferon and DETP in Ca-alginate gel beads determined at conditions suitable for biodegradation studies were 2.70 x 10(-11) m(2)/s and 4.28 x 10(-11) m(2)/s, respectively. Diffusivities of chlorferon and DETP were influenced by several factors, including viscosity of the bulk solution, agitation speed, and the concentrations of diffusing substrate and immobilized cells. Diffusion coefficients increased with increasing agitation speed, probably due to poor mixing at low speed and some attrition of beads at high speeds. Diffusion coefficients also increased with decreasing substrate concentration. Increased cell concentration in the gel beads caused lower diffusivity. Theoretical models to predict diffusivities as a function of cell weight fraction overestimated the effective diffusivities for both chlorferon and DETP, but linear relations between effective diffusivity and cell weight fraction were derived from experimental data. Calcium-alginate gel beads with radii of 1.65-1.70 mm used in this study were not subject to diffusional limitations: external mass transfer resistances were negligible based on Biot number calculations and effectiveness factors indicated that internal mass transfer resistance was negligible. Therefore, the degradation rates of chlorferon and DETP inside Ca-alginate gel beads were reaction-limited. (c) 2007 Wiley Periodicals, Inc.

Diffusion kinetics of the ion exchange of benzocaine on sulfocationites

NASA Astrophysics Data System (ADS)

Al'tshuler, O. G.; Shkurenko, G. Yu.; Gorlov, A. A.; Al'tshuler, G. N.

2016-06-01

The theory of the ion exchange kinetics on strong acid cationites with the participation of weak electrolytes is discussed. The kinetics of desorption of benzocaine in the protonated and molecular forms from strong acid cationites, sulfonated polycalixarene, and KU-23 30/100 sulfocationite, is studied experimentally. It is shown that the flow of protonated benzocaine from cationite upon desorption proceeding by the ion-exchange mechanism is more intense than upon desorption of nonionized benzocaine molecules. It is established that the diffusion coefficient of benzocaine cations is (1.21 ± 0.23) × 10-12 m2/s in KU-23 30/100 sulfocation and (0.65 ± 0.06) × 10-13 m2/s in sulfonated polycalixarene, while the diffusion coefficient of benzocaine molecules is (0.65 ± 0.15) × 10-14 m2/s in sulfonated polycalixarene.

Modeling diffusion control on organic matter decomposition in unsaturated soil pore space

NASA Astrophysics Data System (ADS)

Vogel, Laure; Pot, Valérie; Garnier, Patricia; Vieublé-Gonod, Laure; Nunan, Naoise; Raynaud, Xavier; Chenu, Claire

2014-05-01

Soil Organic Matter decomposition is affected by soil structure and water content, but field and laboratory studies about this issue conclude to highly variable outcomes. Variability could be explained by the discrepancy between the scale at which key processes occur and the measurements scale. We think that physical and biological interactions driving carbon transformation dynamics can be best understood at the pore scale. Because of the spatial disconnection between carbon sources and decomposers, the latter rely on nutrient transport unless they can actively move. In hydrostatic case, diffusion in soil pore space is thus thought to regulate biological activity. In unsaturated conditions, the heterogeneous distribution of water modifies diffusion pathways and rates, thus affects diffusion control on decomposition. Innovative imaging and modeling tools offer new means to address these effects. We have developed a new model based on the association between a 3D Lattice-Boltzmann Model and an adimensional decomposition module. We designed scenarios to study the impact of physical (geometry, saturation, decomposers position) and biological properties on decomposition. The model was applied on porous media with various morphologies. We selected three cubic images of 100 voxels side from µCT-scanned images of an undisturbed soil sample at 68µm resolution. We used LBM to perform phase separation and obtained water phase distributions at equilibrium for different saturation indices. We then simulated the diffusion of a simple soluble substrate (glucose) and its consumption by bacteria. The same mass of glucose was added as a pulse at the beginning of all simulations. Bacteria were placed in few voxels either regularly spaced or concentrated close to or far from the glucose source. We modulated physiological features of decomposers in order to weight them against abiotic conditions. We could evidence several effects creating unequal substrate access conditions for decomposers, hence inducing contrasted decomposition kinetics: position of bacteria relative to the substrate diffusion pathways, diffusion rate and hydraulic connectivity between bacteria and substrate source, local substrate enrichment due to restricted mass transfer. Physiological characteristics had a strong impact on decomposition only when glucose diffused easily but not when diffusion limitation prevailed. This suggests that carbon dynamics should not be considered to derive from decomposers' physiology alone but rather from the interactions of biological and physical processes at the microscale.

Stefan-Maxwell Relations and Heat Flux with Anisotropic Transport Coefficients for Ionized Gases in a Magnetic Field with Application to the Problem of Ambipolar Diffusion

NASA Astrophysics Data System (ADS)

Kolesnichenko, A. V.; Marov, M. Ya.

2018-01-01

The defining relations for the thermodynamic diffusion and heat fluxes in a multicomponent, partially ionized gas mixture in an external electromagnetic field have been obtained by the methods of the kinetic theory. Generalized Stefan-Maxwell relations and algebraic equations for anisotropic transport coefficients (the multicomponent diffusion, thermal diffusion, electric and thermoelectric conductivity coefficients as well as the thermal diffusion ratios) associated with diffusion-thermal processes have been derived. The defining second-order equations are derived by the Chapman-Enskog procedure using Sonine polynomial expansions. The modified Stefan-Maxwell relations are used for the description of ambipolar diffusion in the Earth's ionospheric plasma (in the F region) composed of electrons, ions of many species, and neutral particles in a strong electromagnetic field.

Passive Rocket Diffuser Theory: A Re-Examination of Minimum Second Throat Size

NASA Technical Reports Server (NTRS)

Jones, Daniel R.

2016-01-01

Second-throat diffusers serve to isolate rocket engines from the effects of ambient back pressure during testing without using active control systems. Among the most critical design parameters is the relative area of the diffuser throat to that of the nozzle throat. A smaller second throat is generally desirable because it decreases the stagnation-to-ambient pressure ratio the diffuser requires for nominal operation. There is a limit, however. Below a certain size, the second throat can cause pressure buildup within the diffuser and prevent it from reaching the start condition that protects the nozzle from side-load damage. This paper presents a method for improved estimation of the minimum second throat area which enables diffuser start. The new 3-zone model uses traditional quasi-one-dimensional compressible flow theory to approximate the structure of two distinct diffuser flow fields observed in Computational Fluid Dynamics (CFD) simulations and combines them to provide a less-conservative estimate of the second throat size limit. It is unique among second throat sizing methods in that it accounts for all major conical nozzle and second throat diffuser design parameters within its limits of application. The performance of the 3-zone method is compared to the historical normal shock and force balance methods, and verified against a large number of CFD simulations at specific heat ratios of 1.4 and 1.25. Validation is left as future work, and the model is currently intended to function only as a first-order design tool.

Thermal diffusivity and adiabatic limit temperature characterization of consolidate granular expanded perlite using the flash method

NASA Astrophysics Data System (ADS)

Raefat, Saad; Garoum, Mohammed; Laaroussi, Najma; Thiam, Macodou; Amarray, Khaoula

2017-07-01

In this work experimental investigation of apparent thermal diffusivity and adiabatic limit temperature of expanded granular perlite mixes has been made using the flash technic. Perlite granulates were sieved to produce essentially three characteristic grain sizes. The consolidated samples were manufactured by mixing controlled proportions of the plaster and water. The effect of the particle size on the diffusivity was examined. The inverse estimation of the diffusivity and the adiabatic limit temperature at the rear face as well as the heat losses coefficients were performed using several numerical global minimization procedures. The function to be minimized is the quadratic distance between the experimental temperature rise at the rear face and the analytical model derived from the one dimension heat conduction. It is shown that, for all granulometry tested, the estimated parameters lead to a good agreement between the mathematical model and experimental data.

A mass diffusion-based interpretation of the effect of total solids content on solid-state anaerobic digestion of cellulosic biomass.

PubMed

Xu, Fuqing; Wang, Zhi-Wu; Tang, Li; Li, Yebo

2014-09-01

In solid-state anaerobic digestion (SS-AD) of cellulosic biomass, the volumetric methane production rate has often been found to increase with the increase in total solids (TS) content until a threshold is reached, and then to decrease. This phenomenon cannot be explained by conventional understanding derived from liquid anaerobic digestion. This study proposed that the high TS content-caused mass diffusion limitation may be responsible for the observed methane production deterioration. Based on this hypothesis, a new SS-AD model was developed by taking into account the mass diffusion limitation and hydrolysis inhibition. The good agreement between model simulation and the experimental as well as literature data verified that the observed reduction in volumetric methane production rate could be ascribed to hydrolysis inhibition as a result of the mass diffusion limitation in SS-AD. Copyright © 2014 Elsevier Ltd. All rights reserved.

Global solutions to a class of multi-species reaction-diffusion systems with cross-diffusions arising in population dynamics

NASA Astrophysics Data System (ADS)

Wen, Zijuan; Fu, Shengmao

2009-08-01

In this paper, an n-species strongly coupled cooperating diffusive system is considered in a bounded smooth domain, subject to homogeneous Neumann boundary conditions. Employing the method of energy estimates, we obtain some conditions on the diffusion matrix and inter-specific cooperatives to ensure the global existence and uniform boundedness of a nonnegative solution. The globally asymptotical stability of the constant positive steady state is also discussed. As a consequence, all the results hold true for multi-species Lotka-Volterra type competition model and prey-predator model.

Diffusion length variation and proton damage coefficients for InP/In(x)Ga(1-x)As/GaAs solar cells

NASA Technical Reports Server (NTRS)

Jain, R. K.; Weinberg, I.; Flood, D. J.

1993-01-01

Indium phosphide solar cells are more radiation resistant than gallium arsenide and silicon solar cells, and their growth by heteroepitaxy offers additional advantages leading to the development of lighter, mechanically strong and cost-effective cells. Changes in heteroepitaxial InP cell efficiency under 0.5 and 3 MeV proton irradiations are explained by the variation in the minority-carrier diffusion length. The base diffusion length versus proton fluence is calculated by simulating the cell performance. The diffusion length damage coefficient K(L) is plotted as a function of proton fluence.

Kramers turnover: From energy diffusion to spatial diffusion using metadynamics

PubMed Central

Tiwary, Pratyush; Berne, B. J.

2016-01-01

We consider the rate of transition for a particle between two metastable states coupled to a thermal environment for various magnitudes of the coupling strength using the recently proposed infrequent metadynamics approach [P. Tiwary and M. Parrinello, Phys. Rev. Lett. 111, 230602 (2013)]. We are interested in understanding how this approach for obtaining rate constants performs as the dynamics regime changes from energy diffusion to spatial diffusion. Reassuringly, we find that the approach works remarkably well for various coupling strengths in the strong coupling regime, and to some extent even in the weak coupling regime. PMID:27059558

Monte Carlo simulation of the back-diffusion of electrons in nitrogen

NASA Astrophysics Data System (ADS)

Radmilović-Radjenović, M.; Nina, A.; Nikitović, Ž.

2009-01-01

In this paper, the process of back-diffusion in nitrogen is studied by means of Monte Carlo simulations. In particular we analyze the influence of different aspects of back-diffusion in order to simplify the models of plasma displays, low pressure gas breakdown and detectors of high energy particles. The obtained simulation results show that the escape coefficient depends strongly on the reflection coefficient and the initial energy of electrons. It was also found that the back-diffusion range and number of collisions before returning to the cathode in nitrogen are smaller than those in argon for similar conditions.

Using Magnetic Fields to Control Convection during Protein Crystallization: Analysis and Validation Studies

NASA Technical Reports Server (NTRS)

Ramachandran, N.; Leslie, F. W.

2004-01-01

The effect of convection during the crystallization of proteins is not very well understood. In a gravitational field, convection is caused by crystal sedimentation and by solutal buoyancy induced flow and these can lead to crystal imperfections. While crystallization in microgravity can approach diffusion limited growth conditions (no convection), terrestrially strong magnetic fields can be used to control fluid flow and sedimentation effects. In this work, we develop the analysis for magnetic flow control and test the predictions using analog experiments. Specifically, experiments on solutal convection in a paramagnetic fluid were conducted in a strong magnetic field gradient using a dilute solution of Manganese Chloride. The observed flows indicate that the magnetic field can completely counter the settling effects of gravity locally and are consistent with the theoretical predictions presented. This phenomenon suggests that magnetic fields may be useful in mimicking the microgravity environment of space for some crystal growth ana biological applications where fluid convection is undesirable.

Strong exchange and magnetic blocking in N₂³⁻-radical-bridged lanthanide complexes.

PubMed

Rinehart, Jeffrey D; Fang, Ming; Evans, William J; Long, Jeffrey R

2011-05-22

Single-molecule magnets approach the ultimate size limit for spin-based devices. These complexes can retain spin information over long periods of time at low temperature, suggesting possible applications in high-density information storage, quantum computing and spintronics. Notably, the success of most such applications hinges upon raising the inherent molecular spin-inversion barrier. Although recent advances have shown the viability of lanthanide-containing complexes in generating large barriers, weak or non-existent magnetic exchange coupling allows fast relaxation pathways that mitigate the full potential of these species. Here, we show that the diffuse spin of an N(2)(3-) radical bridge can lead to exceptionally strong magnetic exchange in dinuclear Ln(III) (Ln = Gd, Dy) complexes. The Gd(III) congener exhibits the strongest magnetic coupling yet observed for that ion, while incorporation of the high-anisotropy Dy(III) ion gives rise to a molecule with a record magnetic blocking temperature of 8.3 K at a sweep rate of 0.08 T s(-1).

Lossy radial diffusion of relativistic Jovian electrons. [calculation of synchrotron radiation and electron radiation for Jupiter

NASA Technical Reports Server (NTRS)

Barbosa, D. D.; Coroniti, F. V.

1976-01-01

The radial diffusion equation with synchrotron losses was solved by the Laplace transform method for near-equatorially mirroring relativistic electrons. The evolution of a power law distribution function was found and the characteristics of synchrotron burn-off are stated in terms of explicit parameters for an arbitrary diffusion coefficient. Emissivity from the radiation belts of Jupiter was studied. Asymptotic forms for the distribution in the strong synchrotron loss regime are provided.

Reconnection Diffusion in Turbulent Fluids and Its Implications for Star Formation

NASA Astrophysics Data System (ADS)

Lazarian, A.

2014-05-01

Astrophysical fluids are turbulent a fact which changes the dynamics of many key processes, including magnetic reconnection. Fast reconnection of magnetic field in turbulent fluids allows the field to change its topology and connections. As a result, the traditional concept of magnetic fields being frozen into the plasma is no longer applicable. Plasma associated with a given magnetic field line at one instant is distributed along a different set of magnetic field lines at the next instant. This diffusion of plasmas and magnetic field is enabled by reconnection and therefore is termed "reconnection diffusion". The astrophysical implications of this concept include heat transfer in plasmas, advection of heavy elements in interstellar medium, magnetic field generation etc. However, the most dramatic implications of the concept are related to the star formation process. The reason is that magnetic fields are dynamically important for most of the stages of star formation. The existing theory of star formation has been developed ignoring the possibility of reconnection diffusion. Instead, it appeals to the decoupling of mass and magnetic field arising from neutrals drifting in respect to ions entrained on magnetic field lines, i.e. through the process that is termed "ambipolar diffusion". The predictions of ambipolar diffusion and reconnection diffusion are very different. For instance, if the ionization of media is high, ambipolar diffusion predicts that the coupling of mass and magnetic field is nearly perfect. At the same time, reconnection diffusion is independent of the ionization but depends on the scale of the turbulent eddies and on the turbulent velocities. In the paper we explain the physics of reconnection diffusion both from macroscopic and microscopic points of view, i.e. appealing to the reconnection of flux tubes and to the diffusion of magnetic field lines. We make use of the Lazarian and Vishniac (Astrophys. J. 517:700, 1999) theory of magnetic reconnection and show that this theory is applicable to the partially ionized gas. We quantify the reconnection diffusion rate both for weak and strong MHD turbulence and address the problem of reconnection diffusion acting together with ambipolar diffusion. In addition, we provide a criterion for correctly representing the magnetic diffusivity in simulations of star formation. We discuss the intimate relation between the processes of reconnection diffusion, field wandering and turbulent mixing of a magnetized media and show that the role of the plasma effects is limited to "breaking up lines" on small scales and does not affect the rate of reconnection diffusion. We address the existing observational results and demonstrate how reconnection diffusion can explain the puzzles presented by observations, in particular, the observed higher magnetization of cloud cores in comparison with the magnetization of envelopes. We also outline a possible set of observational tests of the reconnection diffusion concept and discuss how the application of the new concept changes our understanding of star formation and its numerical modeling. Finally, we outline the differences of the process of reconnection diffusion and the process of accumulation of matter along magnetic field lines that is frequently invoked to explain the results of numerical simulations.

Experimental and theoretical comparison of Sb, As, and P diffusion mechanisms and doping in CdTe

NASA Astrophysics Data System (ADS)

Colegrove, E.; Yang, J.-H.; Harvey, S. P.; Young, M. R.; Burst, J. M.; Duenow, J. N.; Albin, D. S.; Wei, S.-H.; Metzger, W. K.

2018-02-01

Fundamental material doping challenges have limited CdTe electro-optical applications. In this work, the As atomistic diffusion mechanisms in CdTe are examined by spatially resolving dopant incorporation in both single-crystalline and polycrystalline CdTe over a range of experimental conditions. Density-functional theory calculations predict experimental activation energies and indicate that As diffuses slowly through the Te sublattice and quickly along GBs similar to Sb. Because of its atomic size and associated defect chemistry, As does not have a fast interstitial diffusion component similar to P. Experiments to incorporate and activate P, As, and Sb in polycrystalline CdTe are conducted to examine if ex situ Group V doping can overcome historic polycrystalline doping limits. The distinct P, As, and Sb diffusion characteristics create different strategies for increasing hole density. Because fast interstitial diffusion is prominent for P, less aggressive diffusion conditions followed by Cd overpressure to relocate the Group V element to the Te lattice site is effective. For larger atoms, slower diffusion through the Te sublattice requires more aggressive diffusion, however further activation is not always necessary. Based on the new physical understanding, we have obtained greater than 1016 cm-3 hole density in polycrystalline CdTe films by As and P diffusion.

Experimental and theoretical comparison of Sb, As, and P diffusion mechanisms and doping in CdTe

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

Colegrove, E.; Yang, J-H; Harvey, S. P.

Fundamental material doping challenges have limited CdTe electro-optical applications. In this work, the As atomistic diffusion mechanisms in CdTe are examined by spatially resolving dopant incorporation in both single-crystalline and polycrystalline CdTe over a range of experimental conditions. Density-functional theory calculations predict experimental activation energies and indicate As diffuses slowly through the Te sublattice and quickly along GBs similar to Sb. Because of its atomic size and associated defect chemistry, As does not have a fast interstitial diffusion component similar to P. Experiments to incorporate and activate P, As, and Sb in polycrystalline CdTe are conducted to examine if ex-situmore » Group V doping can overcome historic polycrystalline doping limits. The distinct P, As, and Sb diffusion characteristics create different strategies for increasing hole density. Because fast interstitial diffusion is prominent for P, less aggressive diffusion conditions followed by Cd overpressure to relocate the Group V element to the Te lattice site is effective. For larger atoms, slower diffusion through the Te sublattice requires more aggressive diffusion, however further activation is not always necessary. Based on the new physical understanding, we have obtained greater than 10^16 cm^-3 hole density in polycrystalline CdTe films by As and P diffusion.« less

Experimental and theoretical comparison of Sb, As, and P diffusion mechanisms and doping in CdTe

DOE PAGES

Colegrove, E.; Yang, J-H; Harvey, S. P.; ...

2018-01-29

Fundamental material doping challenges have limited CdTe electro-optical applications. In this work, the As atomistic diffusion mechanisms in CdTe are examined by spatially resolving dopant incorporation in both single-crystalline and polycrystalline CdTe over a range of experimental conditions. Density-functional theory calculations predict experimental activation energies and indicate As diffuses slowly through the Te sublattice and quickly along GBs similar to Sb. Because of its atomic size and associated defect chemistry, As does not have a fast interstitial diffusion component similar to P. Experiments to incorporate and activate P, As, and Sb in polycrystalline CdTe are conducted to examine if ex-situmore » Group V doping can overcome historic polycrystalline doping limits. The distinct P, As, and Sb diffusion characteristics create different strategies for increasing hole density. Because fast interstitial diffusion is prominent for P, less aggressive diffusion conditions followed by Cd overpressure to relocate the Group V element to the Te lattice site is effective. For larger atoms, slower diffusion through the Te sublattice requires more aggressive diffusion, however further activation is not always necessary. Based on the new physical understanding, we have obtained greater than 10^16 cm^-3 hole density in polycrystalline CdTe films by As and P diffusion.« less

A "desperation-reaction" model of medical diffusion.

PubMed Central

Warner, K E

1975-01-01

Knowledge about the adoption and diffusion of innovations is briefly reviewed. A model is then proposed to explain how certain innovations, intended to address dire medical problems, might diffuse in a manner not previously reported, with extensive diffusion occurring during what would be a period of small-scale experimentation and limited adoption in the conventional innovation-diffusion environment. The model is illustrated with findings from a case study of the diffusion of drug therapies for four types of leukemia. Possible implications of "desperation-reaction" diffusion are suggested. PMID:1065622

Self-diffusion studies by intra- and inter-molecular spin-lattice relaxometry using field-cycling: Liquids, plastic crystals, porous media, and polymer segments.

PubMed

Kimmich, Rainer; Fatkullin, Nail

2017-08-01

Field-cycling NMR relaxometry is a well-established technique for probing molecular dynamics in a frequency range from typically a few kHz up to several tens of MHz. For the interpretation of relaxometry data, it is quite often assumed that the spin-lattice relaxation process is of an intra-molecular nature so that rotational fluctuations dominate. However, dipolar interactions as the main type of couplings between protons and other dipolar species without quadrupole moments can imply appreciable inter-molecular contributions. These fluctuate due to translational displacements and to a lesser degree also by rotational reorientations in the short-range limit. The analysis of the inter-molecular proton spin-lattice relaxation rate thus permits one to evaluate self-diffusion variables such as the diffusion coefficient or the mean square displacement on a time scale from nanoseconds to several hundreds of microseconds. Numerous applications to solvents, plastic crystals and polymers will be reviewed. The technique is of particular interest for polymer dynamics since inter-molecular spin-lattice relaxation diffusometry bridges the time scales of quasi-elastic neutron scattering and field-gradient NMR diffusometry. This is just the range where model-specific intra-coil mechanisms are assumed to occur. They are expected to reveal themselves by characteristic power laws for the time-dependence of the mean-square segment displacement. These can be favorably tested on this basis. Results reported in the literature will be compared with theoretical predictions. On the other hand, there is a second way for translational diffusion phenomena to affect the spin-lattice relaxation dispersion. If rotational diffusion of molecules is restricted, translational diffusion properties can be deduced even from molecular reorientation dynamics detected by intra-molecular spin-lattice relaxation. This sort of scenario will be relevant for adsorbates on surfaces or polymer segments under entanglement and chain connectivity constraints. Under such conditions, reorientations will be correlated with translational displacements leading to the so-called RMTD relaxation process (reorientation mediated by translational displacements). Applications to porous glasses, protein solutions, lipid bilayers, and clays will be discussed. Finally, we will address the intriguing fact that the various time limits of the segment mean-square displacement of polymers in some cases perfectly reproduce predictions of the tube/reptation model whereas the reorientation dynamics suggests strongly deviating power laws. Copyright © 2017 Elsevier B.V. All rights reserved.

Hybrid colored noise process with space-dependent switching rates

NASA Astrophysics Data System (ADS)

Bressloff, Paul C.; Lawley, Sean D.

2017-07-01

A fundamental issue in the theory of continuous stochastic process is the interpretation of multiplicative white noise, which is often referred to as the Itô-Stratonovich dilemma. From a physical perspective, this reflects the need to introduce additional constraints in order to specify the nature of the noise, whereas from a mathematical perspective it reflects an ambiguity in the formulation of stochastic differential equations (SDEs). Recently, we have identified a mechanism for obtaining an Itô SDE based on a form of temporal disorder. Motivated by switching processes in molecular biology, we considered a Brownian particle that randomly switches between two distinct conformational states with different diffusivities. In each state, the particle undergoes normal diffusion (additive noise) so there is no ambiguity in the interpretation of the noise. However, if the switching rates depend on position, then in the fast switching limit one obtains Brownian motion with a space-dependent diffusivity of the Itô form. In this paper, we extend our theory to include colored additive noise. We show that the nature of the effective multiplicative noise process obtained by taking both the white-noise limit (κ →0 ) and fast switching limit (ɛ →0 ) depends on the order the two limits are taken. If the white-noise limit is taken first, then we obtain Itô, and if the fast switching limit is taken first, then we obtain Stratonovich. Moreover, the form of the effective diffusion coefficient differs in the two cases. The latter result holds even in the case of space-independent transition rates, where one obtains additive noise processes with different diffusion coefficients. Finally, we show that yet another form of multiplicative noise is obtained in the simultaneous limit ɛ ,κ →0 with ɛ /κ2 fixed.

Structure and Early Soot Oxidation Properties of Laminar Diffusion Flames

NASA Technical Reports Server (NTRS)

El-Leathy, A. M.; Xu, F.; Faeth, G. M.

2001-01-01

Soot is an important unsolved problem of combustion science because it is present in most hydrocarbon-fueled flames and current understanding of the reactive and physical properties of soot in flame environments is limited. This lack of understanding affects progress toward developing reliable predictions of flame radiation properties, reliable predictions of flame pollutant emission properties and reliable methods of computational combustion, among others. Motivated by these observations, the present investigation extended past studies of soot formation in this laboratory, to consider soot oxidation in laminar diffusion flames using similar methods. Early work showed that O2 was responsible for soot oxidation in high temperature O2-rich environments. Subsequent work in high temperature flame environments having small O2 concentrations, however, showed that soot oxidation rates substantially exceeded estimates based on the classical O2 oxidation rates of Nagle and Strickland-Constable and suggests that radicals such as O and OH might be strong contributors to soot oxidation for such conditions. Neoh et al. subsequently made observations in premixed flames, supported by later work, that showed that OH was responsible for soot oxidation at these conditions with a very reasonable collision efficiency of 0.13. Subsequent studies in diffusion flames, however, were not in agreement with the premixed flame studies: they agreed that OH played a dominant role in soot oxidation in flames, but found collision efficiencies that varied with flame conditions and were not in good agreement with each other or with Neoh et al. One explanation for these discrepancies is that optical scattering and extinction properties were used to infer soot structure properties for the studies that have not been very successful for representing the optical properties of soot. Whatever the source of the problem, however, these differences among observations of soot oxidation in premixed and diffusion flames clearly must be resolved. Motivated by these findings, the present study undertook measurements of soot and flame properties within the soot oxidation region of some typical laminar diffusion flames and exploited the new measurements to identify soot oxidation mechanisms for these conditions. Present considerations were limited to the early stages of soot oxidation (carbon consumption less than 70%) where reactions at the surface of primary soot particles dominate the process, rather than the later stages when particle porosity and internal particle oxidation become important as discussed by Neoh et al.

Preferential flow, diffuse flow, and perching in an interbedded fractured-rock unsaturated zone

NASA Astrophysics Data System (ADS)

Nimmo, John R.; Creasey, Kaitlyn M.; Perkins, Kim S.; Mirus, Benjamin B.

2017-03-01

Layers of strong geologic contrast within the unsaturated zone can control recharge and contaminant transport to underlying aquifers. Slow diffuse flow in certain geologic layers, and rapid preferential flow in others, complicates the prediction of vertical and lateral fluxes. A simple model is presented, designed to use limited geological site information to predict these critical subsurface processes in response to a sustained infiltration source. The model is developed and tested using site-specific information from the Idaho National Laboratory in the Eastern Snake River Plain (ESRP), USA, where there are natural and anthropogenic sources of high-volume infiltration from floods, spills, leaks, wastewater disposal, retention ponds, and hydrologic field experiments. The thick unsaturated zone overlying the ESRP aquifer is a good example of a sharply stratified unsaturated zone. Sedimentary interbeds are interspersed between massive and fractured basalt units. The combination of surficial sediments, basalts, and interbeds determines the water fluxes through the variably saturated subsurface. Interbeds are generally less conductive, sometimes causing perched water to collect above them. The model successfully predicts the volume and extent of perching and approximates vertical travel times during events that generate high fluxes from the land surface. These developments are applicable to sites having a thick, geologically complex unsaturated zone of substantial thickness in which preferential and diffuse flow, and perching of percolated water, are important to contaminant transport or aquifer recharge.

Signaling cascades modulate the speed of signal propagation through space.

PubMed

Govern, Christopher C; Chakraborty, Arup K

2009-01-01

Cells are not mixed bags of signaling molecules. As a consequence, signals must travel from their origin to distal locations. Much is understood about the purely diffusive propagation of signals through space. Many signals, however, propagate via signaling cascades. Here, we show that, depending on their kinetics, cascades speed up or slow down the propagation of signals through space, relative to pure diffusion. We modeled simple cascades operating under different limits of Michaelis-Menten kinetics using deterministic reaction-diffusion equations. Cascades operating far from enzyme saturation speed up signal propagation; the second mobile species moves more quickly than the first through space, on average. The enhanced speed is due to more efficient serial activation of a downstream signaling module (by the signaling molecule immediately upstream in the cascade) at points distal from the signaling origin, compared to locations closer to the source. Conversely, cascades operating under saturated kinetics, which exhibit zero-order ultrasensitivity, can slow down signals, ultimately localizing them to regions around the origin. Signal speed modulation may be a fundamental function of cascades, affecting the ability of signals to penetrate within a cell, to cross-react with other signals, and to activate distant targets. In particular, enhanced speeds provide a way to increase signal penetration into a cell without needing to flood the cell with large numbers of active signaling molecules; conversely, diminished speeds in zero-order ultrasensitive cascades facilitate strong, but localized, signaling.

Preferential flow, diffuse flow, and perching in an interbedded fractured-rock unsaturated zone

USGS Publications Warehouse

Nimmo, John R.; Creasey, Kaitlyn M; Perkins, Kimberlie; Mirus, Benjamin B.

2017-01-01

Layers of strong geologic contrast within the unsaturated zone can control recharge and contaminant transport to underlying aquifers. Slow diffuse flow in certain geologic layers, and rapid preferential flow in others, complicates the prediction of vertical and lateral fluxes. A simple model is presented, designed to use limited geological site information to predict these critical subsurface processes in response to a sustained infiltration source. The model is developed and tested using site-specific information from the Idaho National Laboratory in the Eastern Snake River Plain (ESRP), USA, where there are natural and anthropogenic sources of high-volume infiltration from floods, spills, leaks, wastewater disposal, retention ponds, and hydrologic field experiments. The thick unsaturated zone overlying the ESRP aquifer is a good example of a sharply stratified unsaturated zone. Sedimentary interbeds are interspersed between massive and fractured basalt units. The combination of surficial sediments, basalts, and interbeds determines the water fluxes through the variably saturated subsurface. Interbeds are generally less conductive, sometimes causing perched water to collect above them. The model successfully predicts the volume and extent of perching and approximates vertical travel times during events that generate high fluxes from the land surface. These developments are applicable to sites having a thick, geologically complex unsaturated zone of substantial thickness in which preferential and diffuse flow, and perching of percolated water, are important to contaminant transport or aquifer recharge.

Numerical Simulation of the Diffusion Processes in Nanoelectrode Arrays Using an Axial Neighbor Symmetry Approximation.

PubMed

Peinetti, Ana Sol; Gilardoni, Rodrigo S; Mizrahi, Martín; Requejo, Felix G; González, Graciela A; Battaglini, Fernando

2016-06-07

Nanoelectrode arrays have introduced a complete new battery of devices with fascinating electrocatalytic, sensitivity, and selectivity properties. To understand and predict the electrochemical response of these arrays, a theoretical framework is needed. Cyclic voltammetry is a well-fitted experimental technique to understand the undergoing diffusion and kinetics processes. Previous works describing microelectrode arrays have exploited the interelectrode distance to simulate its behavior as the summation of individual electrodes. This approach becomes limited when the size of the electrodes decreases to the nanometer scale due to their strong radial effect with the consequent overlapping of the diffusional fields. In this work, we present a computational model able to simulate the electrochemical behavior of arrays working either as the summation of individual electrodes or being affected by the overlapping of the diffusional fields without previous considerations. Our computational model relays in dividing a regular electrode array in cells. In each of them, there is a central electrode surrounded by neighbor electrodes; these neighbor electrodes are transformed in a ring maintaining the same active electrode area than the summation of the closest neighbor electrodes. Using this axial neighbor symmetry approximation, the problem acquires a cylindrical symmetry, being applicable to any diffusion pattern. The model is validated against micro- and nanoelectrode arrays showing its ability to predict their behavior and therefore to be used as a designing tool.

Simulation study of pO2 distribution in induced tumour masses and normal tissues within a microcirculation environment.

PubMed

Li, Mao; Li, Yan; Wen, Peng Paul

2014-01-01

The biological microenvironment is interrupted when tumour masses are introduced because of the strong competition for oxygen. During the period of avascular growth of tumours, capillaries that existed play a crucial role in supplying oxygen to both tumourous and healthy cells. Due to limitations of oxygen supply from capillaries, healthy cells have to compete for oxygen with tumourous cells. In this study, an improved Krogh's cylinder model which is more realistic than the previously reported assumption that oxygen is homogeneously distributed in a microenvironment, is proposed to describe the process of the oxygen diffusion from a capillary to its surrounding environment. The capillary wall permeability is also taken into account. The simulation study is conducted and the results show that when tumour masses are implanted at the upstream part of a capillary and followed by normal tissues, the whole normal tissues suffer from hypoxia. In contrast, when normal tissues are ahead of tumour masses, their pO2 is sufficient. In both situations, the pO2 in the whole normal tissues drops significantly due to the axial diffusion at the interface of normal tissues and tumourous cells. As the existence of the axial oxygen diffusion cannot supply the whole tumour masses, only these tumourous cells that are near the interface can be partially supplied, and have a small chance to survive.

Function-specific and Enhanced Brain Structural Connectivity Mapping via Joint Modeling of Diffusion and Functional MRI.

PubMed

Chu, Shu-Hsien; Parhi, Keshab K; Lenglet, Christophe

2018-03-16

A joint structural-functional brain network model is presented, which enables the discovery of function-specific brain circuits, and recovers structural connections that are under-estimated by diffusion MRI (dMRI). Incorporating information from functional MRI (fMRI) into diffusion MRI to estimate brain circuits is a challenging task. Usually, seed regions for tractography are selected from fMRI activation maps to extract the white matter pathways of interest. The proposed method jointly analyzes whole brain dMRI and fMRI data, allowing the estimation of complete function-specific structural networks instead of interactively investigating the connectivity of individual cortical/sub-cortical areas. Additionally, tractography techniques are prone to limitations, which can result in erroneous pathways. The proposed framework explicitly models the interactions between structural and functional connectivity measures thereby improving anatomical circuit estimation. Results on Human Connectome Project (HCP) data demonstrate the benefits of the approach by successfully identifying function-specific anatomical circuits, such as the language and resting-state networks. In contrast to correlation-based or independent component analysis (ICA) functional connectivity mapping, detailed anatomical connectivity patterns are revealed for each functional module. Results on a phantom (Fibercup) also indicate improvements in structural connectivity mapping by rejecting false-positive connections with insufficient support from fMRI, and enhancing under-estimated connectivity with strong functional correlation.

The mechanistic basis of internal conductance: a theoretical analysis of mesophyll cell photosynthesis and CO2 diffusion.

PubMed

Tholen, Danny; Zhu, Xin-Guang

2011-05-01

Photosynthesis is limited by the conductance of carbon dioxide (CO(2)) from intercellular spaces to the sites of carboxylation. Although the concept of internal conductance (g(i)) has been known for over 50 years, shortcomings in the theoretical description of this process may have resulted in a limited understanding of the underlying mechanisms. To tackle this issue, we developed a three-dimensional reaction-diffusion model of photosynthesis in a typical C(3) mesophyll cell that includes all major components of the CO(2) diffusion pathway and associated reactions. Using this novel systems model, we systematically and quantitatively examined the mechanisms underlying g(i). Our results identify the resistances of the cell wall and chloroplast envelope as the most significant limitations to photosynthesis. In addition, the concentration of carbonic anhydrase in the stroma may also be limiting for the photosynthetic rate. Our analysis demonstrated that higher levels of photorespiration increase the apparent resistance to CO(2) diffusion, an effect that has thus far been ignored when determining g(i). Finally, we show that outward bicarbonate leakage through the chloroplast envelope could contribute to the observed decrease in g(i) under elevated CO(2). Our analysis suggests that physiological and anatomical features associated with g(i) have been evolutionarily fine-tuned to benefit CO(2) diffusion and photosynthesis. The model presented here provides a novel theoretical framework to further analyze the mechanisms underlying diffusion processes in the mesophyll.

The Mechanistic Basis of Internal Conductance: A Theoretical Analysis of Mesophyll Cell Photosynthesis and CO2 Diffusion1[W][OA

PubMed Central

Tholen, Danny; Zhu, Xin-Guang

2011-01-01

Photosynthesis is limited by the conductance of carbon dioxide (CO2) from intercellular spaces to the sites of carboxylation. Although the concept of internal conductance (gi) has been known for over 50 years, shortcomings in the theoretical description of this process may have resulted in a limited understanding of the underlying mechanisms. To tackle this issue, we developed a three-dimensional reaction-diffusion model of photosynthesis in a typical C3 mesophyll cell that includes all major components of the CO2 diffusion pathway and associated reactions. Using this novel systems model, we systematically and quantitatively examined the mechanisms underlying gi. Our results identify the resistances of the cell wall and chloroplast envelope as the most significant limitations to photosynthesis. In addition, the concentration of carbonic anhydrase in the stroma may also be limiting for the photosynthetic rate. Our analysis demonstrated that higher levels of photorespiration increase the apparent resistance to CO2 diffusion, an effect that has thus far been ignored when determining gi. Finally, we show that outward bicarbonate leakage through the chloroplast envelope could contribute to the observed decrease in gi under elevated CO2. Our analysis suggests that physiological and anatomical features associated with gi have been evolutionarily fine-tuned to benefit CO2 diffusion and photosynthesis. The model presented here provides a novel theoretical framework to further analyze the mechanisms underlying diffusion processes in the mesophyll. PMID:21441385

Universal time-dependent dispersion properties for diffusion in a one-dimensional critically tilted potential

NASA Astrophysics Data System (ADS)

Guérin, T.; Dean, D. S.

2017-01-01

We consider the time-dependent dispersion properties of overdamped tracer particles diffusing in a one-dimensional periodic potential under the influence of an additional constant tilting force F . The system is studied in the region where the force is close to the critical value Fc at which the barriers separating neighboring potential wells disappear. We show that, when F crosses the critical value, the shape of the mean-square displacement (MSD) curves is strongly modified. We identify a diffusive regime at intermediate-time scales with an effective diffusion coefficient which is much larger than the late-time diffusion coefficient for F >Fc , whereas for F

Observations of diffusion-limited aggregation-like patterns by atmospheric plasma jet

NASA Astrophysics Data System (ADS)

Chiu, Ching-Yang; Chu, Hong-Yu

2017-11-01

We report on the observations of diffusion-limited aggregation-like patterns during the thin film removal process by an atmospheric plasma jet. The fractal patterns are found to have various structures like dense branching and tree-like patterns. The determination of surface morphology reveals that the footprints of discharge bursts are not as random as expected. We propose a diffusion-limited aggregation model with a few extra requirements by analogy with the experimental results, and thereby present the beauty of nature. We show that the model simulates not only the shapes of the patterns similar to the experimental observations, but also the growing sequences of fluctuating, oscillatory, and zigzag traces.

Fermi LAT Search for Dark Matter in Gamma-Ray Lines and the Inclusive Photon Spectrum

NASA Technical Reports Server (NTRS)

Ackermann, M.; Ajello, M.; Albert, A.; Baldini, L.; Barbiellini, G.; Bechtol, K.; Bellazzini, R.; Berenji, B.; Blandford, R. D.; Bloom, E. D.;

Fermi LAT search for dark matter in gamma-ray lines and the inclusive photon spectrum

DOE PAGES

Ackermann, M.

2012-07-05

Dark matter particle annihilation or decay can produce monochromatic gamma-ray lines and contribute to the diffuse gamma-ray background. Furthermore, we present the flux upper limits for gamma-ray spectral lines from 7 to 200 GeV and for the diffuse gamma-ray background from 4.8 GeV to 264 GeV obtained from two years of Fermi Large Area Telescope data integrated over most of the sky. Here, we give cross-section upper limits and decay lifetime lower limits for dark matter models that produce gamma-ray lines or contribute to the diffuse spectrum, including models proposed as explanations of the PAMELA and Fermi cosmic-ray data.

Estimating the Diffusion Coefficients of Sugars Using Diffusion Experiments in Agar-Gel and Computer Simulations.

PubMed

Miyamoto, Shuichi; Atsuyama, Kenji; Ekino, Keisuke; Shin, Takashi

2018-01-01

The isolation of useful microbes is one of the traditional approaches for the lead generation in drug discovery. As an effective technique for microbe isolation, we recently developed a multidimensional diffusion-based gradient culture system of microbes. In order to enhance the utility of the system, it is favorable to have diffusion coefficients of nutrients such as sugars in the culture medium beforehand. We have, therefore, built a simple and convenient experimental system that uses agar-gel to observe diffusion. Next, we performed computer simulations-based on random-walk concepts-of the experimental diffusion system and derived correlation formulas that relate observable diffusion data to diffusion coefficients. Finally, we applied these correlation formulas to our experimentally-determined diffusion data to estimate the diffusion coefficients of sugars. Our values for these coefficients agree reasonably well with values published in the literature. The effectiveness of our simple technique, which has elucidated the diffusion coefficients of some molecules which are rarely reported (e.g., galactose, trehalose, and glycerol) is demonstrated by the strong correspondence between the literature values and those obtained in our experiments.

Skin welding using pulsed laser radiation and a dye

NASA Astrophysics Data System (ADS)

Fried, Nathaniel M.; Walsh, Joseph T., Jr.

1998-07-01

Previous skin welding studies have used continuous wave (CW) delivery of radiation. However, heat diffusion during irradiation prevents strong welds from being achieved without creating large zones of thermal damage to surrounding tissue. This damage may prevent normal wound healing. Strong welds and minimal thermal damage can be achieved by introducing a dye and delivering the radiation in a pulsed mode. Two-cm-long, full-thickness incisions were made in guinea pig skin. India ink was used as an absorber, and egg white albumin was used as an adhesive. A 5-mm-diameter spot of CW, 1.06-micrometer Nd:YAG laser radiation was scanned over the weld site, producing 100 millisecond pulses. The cooling time between scans and number of scans was varied. Thermal damage zones were measured using a transmission polarizing microscope to identify birefringence changes in tissue. Tensile strengths were measured using a tensiometer. For pulsed welding and long cooling times, weld strengths of 2.4 kg/cm2 were measured, and thermal damage to the epidermis was limited to approximately 500 micrometers. With CW welding, comparable weld strengths resulted in approximately 2700 micrometer of thermal damage. CW laser radiation weld strengths were only 0.6 kg/cm2 when thermal damage in the epidermis was limited to approximately 500 micrometers.

Diffusion of Antimicrobials Across Silicone Hydrogel Contact Lenses

PubMed Central

Zambelli, Alison M.; Brothers, Kimberly M.; Hunt, Kristin M.; Romanowski, Eric G.; Nau, Amy C.; Dhaliwal, Deepinder K.; Shanks, Robert M. Q.

2014-01-01

Objectives To measure the diffusion of topical preparations of moxifloxacin, amphotericin B (AmB), and polyhexamethylene biguanide (PHMB) through silicone hydrogel (SH) contact lenses in vitro. Methods Using an in vitro model, the diffusion of three antimicrobials through SH contact lenses was measured. Diffused compounds were measured using a spectrophotometer at set time points over a period of four hours. The amount of each diffused antimicrobial was determined by comparing the experimental value to a standard curve. A biological assay was performed to validate the contact lens diffusion assay by testing antimicrobial activity of diffused material against lawns of susceptible bacteria (Staphylococcus epidermidis) and yeast (Saccharomyces cerevisiae). Experiments were repeated at least two times with a total of at least 4 independent replicates. Results Our data show detectable moxifloxacin and PHMB diffusion through SH contact lenses at 30 minutes, while amphotericin B diffusion remained below the limit of detection within the 4 hour experimental period. In the biological assay, diffused moxifloxacin demonstrated microbial killing starting at 20 minutes on bacterial lawns, whereas PHMB and amphotericin B failed to demonstrate killing on microbial lawns over the course of the 60 minute experiment. Conclusions In vitro diffusion assays demonstrate limited penetration of certain anti-infective agents through silicone hydrogel contact lenses. Further studies regarding the clinical benefit of using these agents along with bandage contact lens use for corneal pathology are warranted. PMID:25806673

Strain-Rate-Free Diffusion Flames: Initiation, Properties, and Quenching

NASA Technical Reports Server (NTRS)

Fendell, Francis; Rungaldier, Harald; Gokoglu, Suleyman; Schultz, Donald

1997-01-01

For about a half century, the stabilization of a steady planar deflagration on a heat-sink-type flat-flame burner has been of extraordinary service for the theoretical modeling and diagnostic probing of combusting gaseous mixtures. However, most engineering devices and most unwanted fire involve the burning of initially unmixed reactants. The most vigorous burning of initially separated gaseous fuel and oxidizer is the diffusion flame. In this useful idealization (limiting case), the reactants are converted to product at a mathematically thin interface, so no interpenetration of fuel and oxidizer occurs. This limit is of practical importance because it often characterizes the condition of optimal performance (and sometimes environmentally objectionable operation) of a combustor. A steady planar diffusion flame is most closely approached in the laboratory in the counterflow apparatus. The utility of this simple-strain-rate flow for the modeling and probing of diffusion flames was noted by Pandya and Weinberg 35 years ago, though only in the last decade or so has its use become internationally common place. However, typically, as the strain rate a is reduced below about 20 cm(exp -1), and the diffusion-flame limit (reaction rate much faster than the flow rate) is approached, the burning is observed to become unstable in earth gravity. The advantageous steady planar flow is not available in the diffusion-flame limit in earth gravity. This is unfortunate because the typical spatial scale in a counterflow is (k/a)(sup 1/2), where k denotes a characteristic diffusion coefficient; thus, the length scale becomes large, and the reacting flow is particularly amenable to diagnostic probing, as the diffusion-flame limit is approached. The disruption of planar symmetry is owing the fact that, as the strain rate a decreases, the residence time (l/a) of the throughput in the counterflow burner increases. Observationally, when the residence time exceeds about 50 msec, the inevitably present convective (Rayleigh-Benard) instabilities, associated with hot-under-cold (flame-under-fresh-reactant) stratification of fluid in a gravitational field, have time to grow to finite amplitude during transit of the burner.

Measuring and Overcoming Limits of the Saffman-Delbrück Model for Soap Film Viscosities

PubMed Central

Vivek, Skanda; Weeks, Eric R.

2015-01-01

We observe tracer particles diffusing in soap films to measure the two-dimensional (2D) viscous properties of the films. Saffman-Delbrück type models relate the single-particle diffusivity to parameters of the film (such as thickness h) for thin films, but the relation breaks down for thicker films. Notably, the diffusivity is faster than expected for thicker films, with the crossover at h/d = 5.2 ± 0.9 using the tracer particle diameter d. This indicates a crossover from purely 2D diffusion to diffusion that is more three-dimensional. We demonstrate that measuring the correlations of particle pairs as a function of their separation overcomes the limitations of the Saffman-Delbrück model and allows one to measure the viscosity of a soap film for any thickness. PMID:25822262

Measuring and overcoming limits of the Saffman-Delbrück model for soap film viscosities.

PubMed

Vivek, Skanda; Weeks, Eric R

2015-01-01

We observe tracer particles diffusing in soap films to measure the two-dimensional (2D) viscous properties of the films. Saffman-Delbrück type models relate the single-particle diffusivity to parameters of the film (such as thickness h) for thin films, but the relation breaks down for thicker films. Notably, the diffusivity is faster than expected for thicker films, with the crossover at h/d = 5.2 ± 0.9 using the tracer particle diameter d. This indicates a crossover from purely 2D diffusion to diffusion that is more three-dimensional. We demonstrate that measuring the correlations of particle pairs as a function of their separation overcomes the limitations of the Saffman-Delbrück model and allows one to measure the viscosity of a soap film for any thickness.

From elemental tellurium to Ge2Sb2Te5 melts: High temperature dynamic and relaxation properties in relationship with the possible fragile to strong transition

NASA Astrophysics Data System (ADS)

Flores-Ruiz, H.; Micoulaut, M.

2018-01-01

We investigate the dynamic properties of Ge-Sb-Te phase change melts using first principles molecular dynamics with a special emphasis on the effect of tellurium composition on melt dynamics. From structural models and trajectories established previously [H. Flores-Ruiz et al., Phys. Rev. B 92, 134205 (2015)], we calculate the diffusion coefficients for the different species, the activation energies for diffusion, the Van Hove correlation, and the intermediate scattering functions able to substantiate the dynamics and relaxation behavior of the liquids as a function of temperature and composition that is also compared to experiment whenever possible. We find that the diffusion is mostly Arrhenius-like and that the addition of Ge/Sb atoms leads to a global decrease of the jump probability and to an increase in activated dynamics for diffusion. Relaxation behavior is analyzed and used in order to evaluate the possibility of a fragile to strong transition that is evidenced from the calculated high fragility (M = 129) of Ge2Sb2Te5 at high temperatures.

Diffusion of Mg dopant in metal-organic vapor-phase epitaxy grown GaN and AlxGa1-xN

NASA Astrophysics Data System (ADS)

Köhler, K.; Gutt, R.; Wiegert, J.; Kirste, L.

2013-02-01

Diffusion of the p-type dopant Mg in GaN and AlxGa1-xN which is accompanied by segregation and affected by transient effects in metal-organic vapor-phase epitaxy reactors is investigated. We have grown 110 nm thick Mg doped GaN and Al0.1Ga0.9N layers on top of undoped GaN and Al0.1Ga0.9N layers, respectively, in a temperature range between 925 °C and 1050 °C where we placed special emphasis on the lower temperature limit without diffusion to allow separation of Mg transients, diffusion, and segregation. Hereby, AlxGa1-xN layers enable monitoring of the resolution limit by secondary ion mass spectrometry analyses for the respective samples; therefore, thin AlxGa1-xN marker layers are incorporated in the thick GaN layers. We found an upper limit of 1.25 × 1019 cm-3 for diffusing Mg atoms in both sample types. Owing to the marked influence of Mg segregation in Al0.1Ga0.9N, diffusion is only seen by using a GaN cap on top of the Al0.1Ga0.9N layer sequence. Diffusion in Al0.1Ga0.9N is shown to be increased by about 25%-30% compared to GaN. Post growth annealing experiments under conditions equivalent to those used for growth of the Mg doped samples showed negligible diffusion. Comparing the results to well established findings on other doped III-V compounds, diffusion is explained by an interstitial-substitutional mechanism with a diffusion coefficient, which is concentration dependent. Analysis of the temperature dependent diffusivity revealed an activation energy of 5.0 eV for GaN:Mg and 5.2 eV for Al0.1Ga0.9N:Mg.

Causal Diffusion and the Survival of Charge Fluctuations

NASA Astrophysics Data System (ADS)

Abdel-Aziz, Mohamed; Gavin, Sean

2004-10-01

Diffusion may obliterate fluctuation signals of the QCD phase transition in nuclear collisions at SPS and RHIC energies. We propose a hyperbolic diffusion equation to study the dissipation of net charge fluctuations [1]. This equation is needed in a relativistic context, because the classic parabolic diffusion equation violates causality. We find that causality substantially limits the extent to which diffusion can dissipate these fluctuations. [1] M. Abdel-Aziz and S. Gavin, nucl-th/0404058

Understanding molecular structure dependence of exciton diffusion in conjugated small molecules

NASA Astrophysics Data System (ADS)

Li, Zi; Zhang, Xu; Woellner, Cristiano F.; Lu, Gang

2014-04-01

First-principles simulations are carried out to understand molecular structure dependence of exciton diffusion in a series of small conjugated molecules arranged in a disordered, crystalline, and blend structure. Exciton diffusion length (LD), lifetime, and diffusivity in four diketopyrrolopyrrole derivatives are calculated and the results compare very well with experimental values. The correlation between exciton diffusion and molecular structure is examined in detail. In the disordered molecule structure, a longer backbone length leads to a shorter exciton lifetime and a higher exciton diffusivity, but it does not change LD substantially. Removal of the end alkyl chains or the extra branch on the side alkyl chains reduces LD. In the crystalline structure, exciton diffusion exhibits a strong anisotropy whose origin can be elucidated from the intermolecular transition density interaction point of view. In the blend structure, LD increases with the crystalline ratios, which are estimated and consistent with the experimental results.

The continuous similarity model of bulk soil-water evaporation

NASA Technical Reports Server (NTRS)

Clapp, R. B.

1983-01-01

The continuous similarity model of evaporation is described. In it, evaporation is conceptualized as a two stage process. For an initially moist soil, evaporation is first climate limited, but later it becomes soil limited. During the latter stage, the evaporation rate is termed evaporability, and mathematically it is inversely proportional to the evaporation deficit. A functional approximation of the moisture distribution within the soil column is also included in the model. The model was tested using data from four experiments conducted near Phoenix, Arizona; and there was excellent agreement between the simulated and observed evaporation. The model also predicted the time of transition to the soil limited stage reasonably well. For one of the experiments, a third stage of evaporation, when vapor diffusion predominates, was observed. The occurrence of this stage was related to the decrease in moisture at the surface of the soil. The continuous similarity model does not account for vapor flow. The results show that climate, through the potential evaporation rate, has a strong influence on the time of transition to the soil limited stage. After this transition, however, bulk evaporation is independent of climate until the effects of vapor flow within the soil predominate.

Performance of external and internal coil configurations for prostate investigations at 7 Tesla

PubMed Central

Metzger, Gregory J.; van de Moortele, Pierre-Francois; Akgun, Can; Snyder, Carl J.; Moeller, Steen; Strupp, John; Andersen, Peter; Shrivastava, Devashish; Vaughan, Tommy; Ugurbil, Kamil; Adriany, Gregor

2010-01-01

Three different coil configurations were evaluated through simulation and experimentally to determine safe operating limits and evaluate subject size dependent performance for prostate imaging at 7 Tesla. The coils included a transceiver endorectal coil (trERC), a 16 channel transceiver external surface array (trESA) and a trESA combined with a receive-only ERC (trESA+roERC). While the transmit B1 (B1+) homogeneity was far superior for the trESA, the maximum achievable B1+ is subject size dependent and limited by transmit chain losses and amplifier performance. For the trERC, limitations in transmit homogeneity greatly compromised image quality and limited coverage of the prostate. Despite these challenges, the high peak B1+ close to the trERC and subject size independent performance provides potential advantages especially for spectroscopic localization where high bandwidth RF pulses are required. On the receive side, the combined trESA+roERC provided the highest SNR and improved homogeneity over the trERC resulting in better visualization of the prostate and surrounding anatomy. In addition, the parallel imaging performance of the trESA+roERC holds strong promise for diffusion weighted imaging and dynamic contrast enhanced MRI. PMID:20740657

Self-Diffusion of Drops in a Dilute Sheared Emulsion

NASA Technical Reports Server (NTRS)

Loewenberg, Michael; Hinch, E. J.

1996-01-01

Self-diffusion coefficients that describe cross-flow migration of non-Brownian drops in a dilute sheared emulsion were obtained by trajectory calculations. A boundary integral formulation was used to describe pairwise interactions between deformable drops; interactions between undeformed drops were described with mobility functions for spherical drops. The results indicate that drops have large anisotropic self-diffusivities which depend strongly on the drop viscosity and modestly on the shear-rate. Pairwise interactions between drops in shear-flow do not appreciably promote drop breakup.

Emergence of microstructure and oxygen diffusion in yttrium-stabilized cubic zirconia

NASA Astrophysics Data System (ADS)

Yang, C.; Trachenko, K.; Hull, S.; Todorov, I. T.; Dove, M. T.

2018-05-01

Large-scale molecular dynamics simulations have been used to study the microstructure in Y-doped ZrO2. From simulations performed as a function of composition the dependence of microstructure on composition is quantified, showing how it is formed from two coexisting phases, and the transformation to the stabilized cubic form is observed at higher concentrations of yttrium and higher temperatures. The effect of composition and temperature on oxygen diffusion is also studied, showing strong correlations between microstructure and diffusion.

Anisotropic diffusion in mesh-free numerical magnetohydrodynamics

NASA Astrophysics Data System (ADS)

Hopkins, Philip F.

2017-04-01

We extend recently developed mesh-free Lagrangian methods for numerical magnetohydrodynamics (MHD) to arbitrary anisotropic diffusion equations, including: passive scalar diffusion, Spitzer-Braginskii conduction and viscosity, cosmic ray diffusion/streaming, anisotropic radiation transport, non-ideal MHD (Ohmic resistivity, ambipolar diffusion, the Hall effect) and turbulent 'eddy diffusion'. We study these as implemented in the code GIZMO for both new meshless finite-volume Godunov schemes (MFM/MFV). We show that the MFM/MFV methods are accurate and stable even with noisy fields and irregular particle arrangements, and recover the correct behaviour even in arbitrarily anisotropic cases. They are competitive with state-of-the-art AMR/moving-mesh methods, and can correctly treat anisotropic diffusion-driven instabilities (e.g. the MTI and HBI, Hall MRI). We also develop a new scheme for stabilizing anisotropic tensor-valued fluxes with high-order gradient estimators and non-linear flux limiters, which is trivially generalized to AMR/moving-mesh codes. We also present applications of some of these improvements for SPH, in the form of a new integral-Godunov SPH formulation that adopts a moving-least squares gradient estimator and introduces a flux-limited Riemann problem between particles.

Statistical variances of diffusional properties from ab initio molecular dynamics simulations

NASA Astrophysics Data System (ADS)

He, Xingfeng; Zhu, Yizhou; Epstein, Alexander; Mo, Yifei

2018-12-01

Ab initio molecular dynamics (AIMD) simulation is widely employed in studying diffusion mechanisms and in quantifying diffusional properties of materials. However, AIMD simulations are often limited to a few hundred atoms and a short, sub-nanosecond physical timescale, which leads to models that include only a limited number of diffusion events. As a result, the diffusional properties obtained from AIMD simulations are often plagued by poor statistics. In this paper, we re-examine the process to estimate diffusivity and ionic conductivity from the AIMD simulations and establish the procedure to minimize the fitting errors. In addition, we propose methods for quantifying the statistical variance of the diffusivity and ionic conductivity from the number of diffusion events observed during the AIMD simulation. Since an adequate number of diffusion events must be sampled, AIMD simulations should be sufficiently long and can only be performed on materials with reasonably fast diffusion. We chart the ranges of materials and physical conditions that can be accessible by AIMD simulations in studying diffusional properties. Our work provides the foundation for quantifying the statistical confidence levels of diffusion results from AIMD simulations and for correctly employing this powerful technique.

Radiation hydrodynamics of triggered star formation: the effect of the diffuse radiation field

NASA Astrophysics Data System (ADS)

Haworth, Thomas J.; Harries, Tim J.

2012-02-01

We investigate the effect of including diffuse field radiation when modelling the radiatively driven implosion of a Bonnor-Ebert sphere (BES). Radiation-hydrodynamical calculations are performed by using operator splitting to combine Monte Carlo photoionization with grid-based Eulerian hydrodynamics that includes self-gravity. It is found that the diffuse field has a significant effect on the nature of radiatively driven collapse which is strongly coupled to the strength of the driving shock that is established before impacting the BES. This can result in either slower or more rapid star formation than expected using the on-the-spot approximation depending on the distance of the BES from the source object. As well as directly compressing the BES, stronger shocks increase the thickness and density in the shell of accumulated material, which leads to short, strong, photoevaporative ejections that reinforce the compression whenever it slows. This happens particularly effectively when the diffuse field is included as rocket motion is induced over a larger area of the shell surface. The formation and evolution of 'elephant trunks' via instability is also found to vary significantly when the diffuse field is included. Since the perturbations that seed instabilities are smeared out elephant trunks form less readily and, once formed, are exposed to enhanced thermal compression.

Stochastic theory of photon flow in homogeneous and heterogeneous anisotropic biological and artificial material

NASA Astrophysics Data System (ADS)

Miller, Steven D.

1995-05-01

Standard Monte Carlo methods used in photon diffusion score absorbed photons or statistical weight deposited within voxels comprising a mesh. An alternative approach to a stochastic description is considered for rapid surface flux calculations and finite medias. Matrix elements are assigned to a spatial lattice whose function is to score vector intersections of scattered photons making transitions into either the forward or back solid angle half spaces. These complete matrix elements can be related to the directional fluxes within the lattice space. This model differentiates between ballistic, quasi-ballistic, and highly diffuse photon contributions, and effectively models the subsurface generation of a scattered light flux from a ballistic source. The connection between a path integral and diffusion is illustrated. Flux perturbations can be effectively illustrated for tissue-tumor-tissue and for 3 layer systems with strong absorption in one or more layers. For conditions where the diffusion theory has difficulties such as strong absorption, highly collimated sources, small finite volumes, and subsurface regions, the computation time of the algorithm is rapid with good accuracy and compliments other description of photon diffusion. The model has the potential to do computations relevant to photodynamic therapy (PDT) and analysis of laser beam interaction with tissues.

Diffusion Influenced Adsorption Kinetics.

PubMed

Miura, Toshiaki; Seki, Kazuhiko

2015-08-27

When the kinetics of adsorption is influenced by the diffusive flow of solutes, the solute concentration at the surface is influenced by the surface coverage of solutes, which is given by the Langmuir-Hinshelwood adsorption equation. The diffusion equation with the boundary condition given by the Langmuir-Hinshelwood adsorption equation leads to the nonlinear integro-differential equation for the surface coverage. In this paper, we solved the nonlinear integro-differential equation using the Grünwald-Letnikov formula developed to solve fractional kinetics. Guided by the numerical results, analytical expressions for the upper and lower bounds of the exact numerical results were obtained. The upper and lower bounds were close to the exact numerical results in the diffusion- and reaction-controlled limits, respectively. We examined the validity of the two simple analytical expressions obtained in the diffusion-controlled limit. The results were generalized to include the effect of dispersive diffusion. We also investigated the effect of molecular rearrangement of anisotropic molecules on surface coverage.

Nature of self-diffusion in two-dimensional fluids

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

Choi, Bongsik; Han, Kyeong Hwan; Kim, Changho

Self-diffusion in a two-dimensional simple fluid is investigated by both analytical and numerical means. We investigate the anomalous aspects of self-diffusion in two-dimensional fluids with regards to the mean square displacement, the time-dependent diffusion coefficient, and the velocity autocorrelation function (VACF) using a consistency equation relating these quantities. Here, we numerically confirm the consistency equation by extensive molecular dynamics simulations for finite systems, corroborate earlier results indicating that the kinematic viscosity approaches a finite, non-vanishing value in the thermodynamic limit, and establish the finite size behavior of the diffusion coefficient. We obtain the exact solution of the consistency equation in the thermodynamic limit and use this solution to determine the large time asymptotics of the mean square displacement, the diffusion coefficient, and the VACF. An asymptotic decay law of the VACF resembles the previously known self-consistent form, 1/(more » $$t\\sqrt{In t)}$$ however with a rescaled time.« less

Single-molecule diffusion and conformational dynamics by spatial integration of temporal fluctuations

PubMed Central

Serag, Maged F.; Abadi, Maram; Habuchi, Satoshi

2014-01-01

Single-molecule localization and tracking has been used to translate spatiotemporal information of individual molecules to map their diffusion behaviours. However, accurate analysis of diffusion behaviours and including other parameters, such as the conformation and size of molecules, remain as limitations to the method. Here, we report a method that addresses the limitations of existing single-molecular localization methods. The method is based on temporal tracking of the cumulative area occupied by molecules. These temporal fluctuations are tied to molecular size, rates of diffusion and conformational changes. By analysing fluorescent nanospheres and double-stranded DNA molecules of different lengths and topological forms, we demonstrate that our cumulative-area method surpasses the conventional single-molecule localization method in terms of the accuracy of determined diffusion coefficients. Furthermore, the cumulative-area method provides conformational relaxation times of structurally flexible chains along with diffusion coefficients, which together are relevant to work in a wide spectrum of scientific fields. PMID:25283876

The impact of physiological crowding on the diffusivity of membrane bound proteins.

PubMed

Houser, Justin R; Busch, David J; Bell, David R; Li, Brian; Ren, Pengyu; Stachowiak, Jeanne C

2016-02-21

Diffusion of transmembrane and peripheral membrane-bound proteins within the crowded cellular membrane environment is essential to diverse biological processes including cellular signaling, endocytosis, and motility. Nonetheless we presently lack a detailed understanding of the influence of physiological levels of crowding on membrane protein diffusion. Utilizing quantitative in vitro measurements, here we demonstrate that the diffusivities of membrane bound proteins follow a single linearly decreasing trend with increasing membrane coverage by proteins. This trend holds for homogenous protein populations across a range of protein sizes and for heterogeneous mixtures of proteins of different sizes, such that protein diffusivity is controlled by the total coverage of the surrounding membrane. These results demonstrate that steric exclusion within the crowded membrane environment can fundamentally limit the diffusive rate of proteins, regardless of their size. In cells this "speed limit" could be modulated by changes in local membrane coverage, providing a mechanism for tuning the rate of molecular interaction and assembly.

Nature of self-diffusion in two-dimensional fluids

DOE PAGES

Choi, Bongsik; Han, Kyeong Hwan; Kim, Changho; ...

2017-12-18

Self-diffusion in a two-dimensional simple fluid is investigated by both analytical and numerical means. We investigate the anomalous aspects of self-diffusion in two-dimensional fluids with regards to the mean square displacement, the time-dependent diffusion coefficient, and the velocity autocorrelation function (VACF) using a consistency equation relating these quantities. Here, we numerically confirm the consistency equation by extensive molecular dynamics simulations for finite systems, corroborate earlier results indicating that the kinematic viscosity approaches a finite, non-vanishing value in the thermodynamic limit, and establish the finite size behavior of the diffusion coefficient. We obtain the exact solution of the consistency equation in the thermodynamic limit and use this solution to determine the large time asymptotics of the mean square displacement, the diffusion coefficient, and the VACF. An asymptotic decay law of the VACF resembles the previously known self-consistent form, 1/(more » $$t\\sqrt{In t)}$$ however with a rescaled time.« less

Compositional and Ionic-Size Controls on the Diffusion of Divalent Cations in Garnet: Insights from Atomistic Simulations

NASA Astrophysics Data System (ADS)

Carlson, W. D.

2012-12-01

Divalent cations in garnet (Mg, Fe, Mn, Ca) diffuse at rates that depend strongly on the host-crystal composition and on the ionic radius of the diffusant. Understanding of the nanoscale basis for these behaviors comes from atomistic simulations that calculate energies in the static limit for the defects and transition-state configurations associated with each diffusive step. Diffusion of divalent cations requires (a) creation of a cation-vacancy defect in a dodecahedral site and of a charge-compensating oxygen-vacancy defect that may or may not be in close spatial association; (b) except in the case of self-diffusion, creation of an impurity defect in which a foreign atom replaces the normal atom in a dodecahedral site adjacent to the vacancy; and (c) during the diffusive process, motion of the diffusing atom to a 'saddlepoint' position that represents the transition-state configuration. Comparisons of the system's energy in these various states, in structures of different composition and for ions of different ionic size, allows assessment of the nanoscale controls on diffusion kinetics. Molecular-statics calculations quantify defect energies and identify the transition-state configuration: the maximum energy along the diffusion path between two adjacent dodecahedral sites results when the diffusing ion is surrounded symmetrically by the six oxygen atoms that lie between the two sites. Across the range of end-member compositions, self-diffusion coefficients measured at identical conditions, and the tracer diffusivity of a single ion measured at identical conditions, can each vary by five orders of magnitude or more. Measured activation energies for these motions, however, are all equivalent to within ±6%. Calculated activation energies are in agreement with observations, in that they vary by only ±10%. Calculated vacancy-formation energies, on the other hand, are significantly larger in expanded structures; for example, that energy is greater for Prp than for Grs by ~ 470 kJ/mol. Thus in expanded structures, much higher vacancy concentrations can be produced at the same energetic cost, greatly enhancing rates of diffusion. The primary explanation for the more rapid diffusion of divalent cations in structures with larger cell dimensions therefore comes not from reduced saddlepoint strain energies in more compliant structures, but instead from the smaller energy required to create vacancy defects. Diffusivities of divalent cations exhibit a curious parabolic dependence on ionic size: for each structure, an optimally-sized ion exists, close in size to the dominant ion, that exhibits the fastest diffusion. Larger ions — and enigmatically, smaller ions — both diffuse more slowly. Calculated impurity-defect energies show that undersized impurity ions are bound more tightly in their sites, but the effects are too small in comparison to corresponding reductions in strain energy for the transition-state configuration to account for observed rate differences. Calculated vacancy-association energies reveal a slight tendency for vacancies to associate preferentially with larger impurity ions, but again the effect appears to be too small to provide a full explanation for observed behaviors.

Pathways for diffusion in the potential energy landscape of the network glass former SiO2

NASA Astrophysics Data System (ADS)

Niblett, S. P.; Biedermann, M.; Wales, D. J.; de Souza, V. K.

2017-10-01

We study the dynamical behaviour of a computer model for viscous silica, the archetypal strong glass former, and compare its diffusion mechanism with earlier studies of a fragile binary Lennard-Jones liquid. Three different methods of analysis are employed. First, the temperature and time scale dependence of the diffusion constant is analysed. Negative correlation of particle displacements influences transport properties in silica as well as in fragile liquids. We suggest that the difference between Arrhenius and super-Arrhenius diffusive behaviour results from competition between the correlation time scale and the caging time scale. Second, we analyse the dynamics using a geometrical definition of cage-breaking transitions that was proposed previously for fragile glass formers. We find that this definition accurately captures the bond rearrangement mechanisms that control transport in open network liquids, and reproduces the diffusion constants accurately at low temperatures. As the same method is applicable to both strong and fragile glass formers, we can compare correlation time scales in these two types of systems. We compare the time spent in chains of correlated cage breaks with the characteristic caging time and find that correlations in the fragile binary Lennard-Jones system persist for an order of magnitude longer than those in the strong silica system. We investigate the origin of the correlation behaviour by sampling the potential energy landscape for silica and comparing it with the binary Lennard-Jones model. We find no qualitative difference between the landscapes, but several metrics suggest that the landscape of the fragile liquid is rougher and more frustrated. Metabasins in silica are smaller than those in binary Lennard-Jones and contain fewer high-barrier processes. This difference probably leads to the observed separation of correlation and caging time scales.

Effect of short wavelength illumination on the characteristic bulk diffusion length in ribbon silicon solar cells

NASA Technical Reports Server (NTRS)

Ho, C. T.; Mathias, J. D.

1981-01-01

The influence of short wavelength light on the characteristic bulk minority carrier diffusion length of the ribbon silicon photovoltaic cell has been investigated. We have measured the intensity and wavelength dependence of the diffusion length in an EFG ribbon cell, and compared it with a standard Czochralski grown silicon cell. While the various short wavelength illuminations have shown no influence on the diffusion length in the CZ cell, the diffusion lengths in the ribbon cell exhibit a strong dependence on the volume generation rate as well as on the wavelength of the superimposed lights. We have concluded that the trap-filling phenomenon at various depths in the bulk neutral region of the cell is consistent with the experimental observation.

Novel surface diffusion characteristics for a robust pentacene derivative on Au(1 1 1) surfaces

NASA Astrophysics Data System (ADS)

Miller, Ryan A.; Larson, Amanda; Pohl, Karsten

2017-06-01

Molecular dynamics simulations have been performed in both the ab initio and classical mechanics frameworks of 5,6,7-trithiapentacene-13-one (TTPO) molecules on flat Au(1 1 1) surfaces. Results show new surface diffusion characteristics including a strong preference for the molecule to align its long axis parallel to the sixfold Au(1 1 1) symmetry directions and subsequently diffuse along these close-packed directions, and a calculated activation energy for diffusion of 0.142 eV, about four times larger than that for pure pentacene on Au. The temperature-dependent diffusion coefficients were calculated to help quantify the molecular mobility during the experimentally observed process of forming self-assembled monolayers on gold electrodes.

Fabrication and characterization of cylindrical light diffusers comprised of shape memory polymer.

PubMed

Small, Ward; Buckley, Patrick R; Wilson, Thomas S; Loge, Jeffrey M; Maitland, Kristen D; Maitland, Duncan J

2008-01-01

We developed a technique for constructing light diffusing devices comprised of a flexible shape memory polymer (SMP) cylindrical diffuser attached to the tip of an optical fiber. The devices are fabricated by casting an SMP rod over the cleaved tip of an optical fiber and media blasting the SMP rod to create a light diffusing surface. The axial and polar emission profiles and circumferential (azimuthal) uniformity are characterized for various blasting pressures, nozzle-to-sample distances, and nozzle translation speeds. The diffusers are generally strongly forward-directed and consistently withstand over 8 W of incident IR laser light without suffering damage when immersed in water. These devices are suitable for various endoluminal and interstitial biomedical applications.

Fabrication and characterization of cylindrical light diffusers comprised of shape memory polymer

PubMed Central

Small, Ward; Buckley, Patrick R.; Wilson, Thomas S.; Loge, Jeffrey M.; Maitland, Kristen D.; Maitland, Duncan J.

2009-01-01

We developed a technique for constructing light diffusing devices comprised of a flexible shape memory polymer (SMP) cylindrical diffuser attached to the tip of an optical fiber. The devices are fabricated by casting an SMP rod over the cleaved tip of an optical fiber and media blasting the SMP rod to create a light diffusing surface. The axial and polar emission profiles and circumferential (azimuthal) uniformity are characterized for various blasting pressures, nozzle-to-sample distances, and nozzle translation speeds. The diffusers are generally strongly forward-directed and consistently withstand over 8 W of incident IR laser light without suffering damage when immersed in water. These devices are suitable for various endoluminal and interstitial biomedical applications. PMID:18465981

Full three-dimensional morphology evolution of amorphous thin films for atomic layer deposition

NASA Astrophysics Data System (ADS)

Jin, Lingpeng; Li, Yawei; Hu, Zhigao; Chu, Junhao

2018-04-01

We introduce a Monte Carlo model based on random deposition and diffusion limited aggregation in order to study the morphological evolution of deposition of nanofilm, which is difficult to carry out by the experimental methods. The instantaneous evolution of morphology and the corresponding parameters are observed when employing a novel perspective, modeling the aggregation of nanoscale units. Despite simplifying the chemical details, the simulation results qualitatively describe experiments with bulky precursors, and the strong dependence of growth rate on steric hindrance is obtained. Moreover, the well know behavior that the delay before steady growth is accurately predicted and analyzed based solely on modeling. Through this work, the great influence of steric hindrance on the initial stage of ALD is described.

Fourier mode analysis of slab-geometry transport iterations in spatially periodic media

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

Larsen, E; Zika, M

1999-04-01

We describe a Fourier analysis of the diffusion-synthetic acceleration (DSA) and transport-synthetic acceleration (TSA) iteration schemes for a spatially periodic, but otherwise arbitrarily heterogeneous, medium. Both DSA and TSA converge more slowly in a heterogeneous medium than in a homogeneous medium composed of the volume-averaged scattering ratio. In the limit of a homogeneous medium, our heterogeneous analysis contains eigenvalues of multiplicity two at ''resonant'' wave numbers. In the presence of material heterogeneities, error modes corresponding to these resonant wave numbers are ''excited'' more than other error modes. For DSA and TSA, the iteration spectral radius may occur at these resonantmore » wave numbers, in which case the material heterogeneities most strongly affect iterative performance.« less

Satellite sweeping of electrons at Neptune and Uranus

NASA Technical Reports Server (NTRS)

Cooper, John F.

1990-01-01

Knowledge of satellite sweeping parameters at Neptune and Uranus, and of their functional dependences on particle energy and pitch angle, can be critical in the proper identification of parent absorbers for observed absorption signatures in regions where OTD (offset, tilted dipole) models are valid representations of the measured magnetic fields. In this paper, critical electron energies are calculated for longitudinal drift resonance, snowplow (i.e., strong) absorption, leapfrog, and corkscrew effects, using a reduced version of OTD that neglects nonaxial dipole offsets. Earlier analytic work on sweeping rates is extended to give the radial dependence of these rates within the minimum-L region and to set limits on diffusion of electrons with the simplifying approximation that leapfrog effects are ignored.

Universal time-dependent dispersion properties for diffusion in a one-dimensional critically tilted potential.

PubMed

Guérin, T; Dean, D S

2017-01-01

We consider the time-dependent dispersion properties of overdamped tracer particles diffusing in a one-dimensional periodic potential under the influence of an additional constant tilting force F. The system is studied in the region where the force is close to the critical value F_{c} at which the barriers separating neighboring potential wells disappear. We show that, when F crosses the critical value, the shape of the mean-square displacement (MSD) curves is strongly modified. We identify a diffusive regime at intermediate-time scales with an effective diffusion coefficient which is much larger than the late-time diffusion coefficient for F>F_{c}, whereas for F

Global stability and pattern formation in a nonlocal diffusive Lotka-Volterra competition model

NASA Astrophysics Data System (ADS)

Ni, Wenjie; Shi, Junping; Wang, Mingxin

2018-06-01

A diffusive Lotka-Volterra competition model with nonlocal intraspecific and interspecific competition between species is formulated and analyzed. The nonlocal competition strength is assumed to be determined by a diffusion kernel function to model the movement pattern of the biological species. It is shown that when there is no nonlocal intraspecific competition, the dynamics properties of nonlocal diffusive competition problem are similar to those of classical diffusive Lotka-Volterra competition model regardless of the strength of nonlocal interspecific competition. Global stability of nonnegative constant equilibria are proved using Lyapunov or upper-lower solution methods. On the other hand, strong nonlocal intraspecific competition increases the system spatiotemporal dynamic complexity. For the weak competition case, the nonlocal diffusive competition model may possess nonconstant positive equilibria for some suitably large nonlocal intraspecific competition coefficients.

Channel waveguides in glass via silver-sodium field-assisted ion exchange

NASA Technical Reports Server (NTRS)

Forrest, K.; Pagano, S. J.; Viehmann, W.

1986-01-01

Multimode channel waveguides have been formed in sodium aluminosilicate glass by field-assisted diffusion of Ag(+) ions from vacuum-evaporated Ag films. The two-dimensional refractive index profiles of the waveguides were controlled by varying the diffusion time, the diffusion temperature, and the electric field strength. Estimates of the diffusion rate through a strip aperture were obtained, assuming the electric field was strong 120-240 V/mm. The maximum change in refractive index in the sodium aluminosilicate glasses was estimated near 65 percent of the change in soda-lime silicate glass. The physical properties of the glasses are given in a table.

Diffusion length variation in 0.5- and 3-MeV-proton-irradiated, heteroepitaxial indium phosphide solar cells

NASA Technical Reports Server (NTRS)

Jain, Raj K.; Weinberg, Irving; Flood, Dennis J.

1993-01-01

Indium phosphide (InP) solar cells are more radiation resistant than gallium arsenide (GaAs) and silicon (Si) solar cells, and their growth by heteroepitaxy offers additional advantages leading to the development of light weight, mechanically strong, and cost-effective cells. Changes in heteroepitaxial InP cell efficiency under 0.5- and 3-MeV proton irradiations have been explained by the variation in the minority-carrier diffusion length. The base diffusion length versus proton fluence was calculated by simulating the cell performance. The diffusion length damage coefficient, K(sub L), was also plotted as a function of proton fluence.

Mass Transfer in a Nanoscale Material Enhanced by an Opposing Flux

NASA Astrophysics Data System (ADS)

Chmelik, Christian; Bux, Helge; Caro, Jürgen; Heinke, Lars; Hibbe, Florian; Titze, Tobias; Kärger, Jörg

2010-02-01

Diffusion is known to be quantified by measuring the rate of molecular fluxes in the direction of falling concentration. In contrast with intuition, considering methanol diffusion in a novel type of nanoporous material (MOF ZIF-8), this rate has now been found to be enhanced rather than slowed down by an opposing flux of labeled molecules. In terms of the key quantities of random particle movement, this result means that the self-diffusivity exceeds the transport diffusivity. It is rationalized by considering the strong intermolecular interaction and the dominating role of intercage hopping in mass transfer in the systems under study.

Effect of horizontal molecular orientation on triplet-exciton diffusion in amorphous organic films

NASA Astrophysics Data System (ADS)

Sawabe, T.; Takasu, I.; Yonehara, T.; Ono, T.; Yoshida, J.; Enomoto, S.; Amemiya, I.; Adachi, C.

2012-09-01

Triplet harvesting is a candidate technology for highly efficient and long-life white OLEDs, where green or red phosphorescent emitters are activated by the triplet-excitons diffused from blue fluorescent emitters. We examined two oxadiazole-based electron transport materials with different horizontal molecular orientation as a triplet-exciton diffusion layer (TDL) in triplet-harvesting OLEDs. The device characteristics and the transient electroluminescent analyses of the red phosphorescent emitter showed that the triplet-exciton diffusion was more effective in the highly oriented TDL. The results are ascribed to the strong orbital overlap between the oriented molecules, which provides rapid electron exchange (Dexter energy transfer) in the TDL.

Ratchet effect for nanoparticle transport in hair follicles.

PubMed

Radtke, Matthias; Patzelt, Alexa; Knorr, Fanny; Lademann, Jürgen; Netz, Roland R

2017-07-01

The motion of a single rigid nanoparticle inside a hair follicle is investigated by means of Brownian dynamics simulations. The cuticular hair structure is modeled as a periodic asymmetric ratchet-shaped surface. Induced by oscillating radial hair motion we find directed nanoparticle transport into the hair follicle with maximal velocity at a specific optimal frequency and an optimal particle size. We observe flow reversal when switching from radial to axial oscillatory hair motion. We also study the diffusion behavior and find strongly enhanced diffusion for axial motion with a diffusivity significantly larger than for free diffusion. Copyright © 2016 Elsevier B.V. All rights reserved.

Symmetry and diffusivity of the interstitial hydrogen shallow-donor center in In 2O 3

DOE PAGES

Weiser, Philip; Qin, Ying; Yin, Weikai; ...

2016-11-16

Uniaxial stress experiments performed for the 3306 cm -1 vibrational line assigned to the interstitial-hydrogen, shallow-donor center in In 2O 3 reveal its symmetry and transition- moment direction. The defect alignment that can be produced by a [001] stress applied at 165 K is due to a process that is also a hydrogen- diffusion jump, providing a microscopic determination of the diffusion constant for H in In 2O 3 and its mechanism. Lastly, our experimental results strongly complement theoretical predictions for the structure and diffusion of the interstitial hydrogen donor center in In 2O 3.

Oxygen chemical diffusion in hypo-stoichiometric MOX

NASA Astrophysics Data System (ADS)

Kato, Masato; Morimoto, Kyoichi; Tamura, Tetsuya; Sunaoshi, Takeo; Konashi, Kenji; Aono, Shigenori; Kashimura, Motoaki

2009-06-01

Kinetics of the oxygen-to-metal ratio change in (U 0.8Pu 0.2)O 2-x and (U 0.7Pu 0.3)O 2-x was evaluated in the temperature range of 1523-1623 K using a thermo-gravimetric technique. The oxygen chemical diffusion coefficients were decided as a function of temperature from the kinetics of the reduction process under a hypo-stoichiometric composition. The diffusion coefficient of (U 0.7Pu 0.3)O 2-x was smaller than that of (U 0.8Pu 0.2)O 2-x. No strong dependence was observed for the diffusion coefficient on the O/M variation of samples.

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

Pezeshki, Alan M.; Sacci, Robert L.; Delnick, Frank M.

Here, an improved method for quantitative measurement of the charge transfer, finite diffusion, and ohmic overpotentials in redox flow batteries using electrochemical impedance spectroscopy is presented. The use of a pulse dampener in the hydraulic circuit enables the collection of impedance spectra at low frequencies with a peristaltic pump, allowing the measurement of finite diffusion resistances at operationally relevant flow rates. This method is used to resolve the rate-limiting processes for the V 2+/V 3+ redox couple on carbon felt and carbon paper electrodes in the vanadium redox flow battery. Carbon felt was limited by both charge transfer and ohmicmore » resistance, while carbon paper was limited by charge transfer, finite diffusion, and ohmic resistances. The influences of vanadium concentration and flow field design also are quantified.« less

Multiple Scattering Effects on Pulse Propagation in Optically Turbid Media.

NASA Astrophysics Data System (ADS)

Joelson, Bradley David

The effects of multiple scattering in a optically turbid media is examined for an impulse solution to the radiative transfer equation for a variety of geometries and phase functions. In regions where the complexities of the phase function proved too cumbersome for analytic methods Monte Carlo techniques were developed to describe the entire scalar radiance distribution. The determination of a general spread function is strongly dependent on geometry and particular regions where limits can be placed on the variables of the problem. Hence, the general spread function is first simplified by considering optical regions which reduce the complexity of the variable dependence. First, in the small-angle limit we calculate some contracted spread functions along with their moments and then use Monte Carlo techniques to establish the limitations imposed by the small-angle approximation in planar geometry. The point spread function (PSF) for a spherical geometry is calculated for the full angular spread in the forward direction of ocean waters using Monte Carlo methods in the optically thin and moderate depths and analytic methods in the diffusion domain. The angular dependence of the PSF for various ocean waters is examined for a range of optical parameters. The analytic method used in the diffusion calculation is justified by examining the angular dependence of the radiance of a impulse solution in a planar geometry for a prolongated Henyey-Greenstein phase function of asymmetry factor approximately equal to that of the ocean phase functions. The Legendre moments of the radiance are examined in order to examine the viability of the diffusion approximation which assumes a linearly anisotropic angular distribution for the radiance. A realistic lidar calculation is performed for a variety of ocean waters to determine the effects of multiple scattering on the determination of the speed of sound by using the range gated frequency spectrum of the lidar signal. It is shown that the optical properties of the ocean help to ensure single scatter form for the frequency spectra of the lidar signal. This spectra can then be used to compute the speed of sound and backscatter probability.

Extreme Water Loss and Abiotic O2 Buildup on Planets Throughout the Habitable Zones of M Dwarfs

PubMed Central

Barnes, R.

2015-01-01

Abstract We show that terrestrial planets in the habitable zones of M dwarfs older than ∼1 Gyr could have been in runaway greenhouses for several hundred million years following their formation due to the star's extended pre-main sequence phase, provided they form with abundant surface water. Such prolonged runaway greenhouses can lead to planetary evolution divergent from that of Earth. During this early runaway phase, photolysis of water vapor and hydrogen/oxygen escape to space can lead to the loss of several Earth oceans of water from planets throughout the habitable zone, regardless of whether the escape is energy-limited or diffusion-limited. We find that the amount of water lost scales with the planet mass, since the diffusion-limited hydrogen escape flux is proportional to the planet surface gravity. In addition to undergoing potential desiccation, planets with inefficient oxygen sinks at the surface may build up hundreds to thousands of bar of abiotically produced O2, resulting in potential false positives for life. The amount of O2 that builds up also scales with the planet mass; we find that O2 builds up at a constant rate that is controlled by diffusion: ∼5 bar/Myr on Earth-mass planets and up to ∼25 bar/Myr on super-Earths. As a result, some recently discovered super-Earths in the habitable zone such as GJ 667Cc could have built up as many as 2000 bar of O2 due to the loss of up to 10 Earth oceans of water. The fate of a given planet strongly depends on the extreme ultraviolet flux, the duration of the runaway regime, the initial water content, and the rate at which oxygen is absorbed by the surface. In general, we find that the initial phase of high luminosity may compromise the habitability of many terrestrial planets orbiting low-mass stars. Key Words: Astrobiology—Biosignatures—Extrasolar terrestrial planets—Habitability—Planetary atmospheres. Astrobiology 15, 119–143. PMID:25629240

Causal electric charge diffusion and balance functions in relativistic heavy-ion collisions

NASA Astrophysics Data System (ADS)

Kapusta, Joseph I.; Plumberg, Christopher

2018-01-01

We study the propagation and diffusion of electric charge fluctuations in high-energy heavy-ion collisions using the Cattaneo form for the dissipative part of the electric current. As opposed to the ordinary diffusion equation this form limits the speed at which charge can propagate. Including the noise term in the current, which arises uniquely from the fluctuation-dissipation theorem, we calculate the balance functions for charged hadrons in a simple 1+1-dimensional Bjorken hydrodynamical model. Limiting the speed of propagation of charge fluctuations increases the height and reduces the width of these balance functions when plotted versus rapidity. We also estimate the numerical value of the associated diffusion time constant from anti-de Sitter-space/conformal-field theory.

Phase-field modeling of two-dimensional crystal growth with anisotropic diffusion.

PubMed

Meca, Esteban; Shenoy, Vivek B; Lowengrub, John

2013-11-01

In the present article, we introduce a phase-field model for thin-film growth with anisotropic step energy, attachment kinetics, and diffusion, with second-order (thin-interface) corrections. We are mainly interested in the limit in which kinetic anisotropy dominates, and hence we study how the expected shape of a crystallite, which in the long-time limit is the kinetic Wulff shape, is modified by anisotropic diffusion. We present results that prove that anisotropic diffusion plays an important, counterintuitive role in the evolving crystal shape, and we add second-order corrections to the model that provide a significant increase in accuracy for small supersaturations. We also study the effect of different crystal symmetries and discuss the influence of the deposition rate.

Solution of a cauchy problem for a diffusion equation in a Hilbert space by a Feynman formula

NASA Astrophysics Data System (ADS)

Remizov, I. D.

2012-07-01

The Cauchy problem for a class of diffusion equations in a Hilbert space is studied. It is proved that the Cauchy problem in well posed in the class of uniform limits of infinitely smooth bounded cylindrical functions on the Hilbert space, and the solution is presented in the form of the so-called Feynman formula, i.e., a limit of multiple integrals against a gaussian measure as the multiplicity tends to infinity. It is also proved that the solution of the Cauchy problem depends continuously on the diffusion coefficient. A process reducing an approximate solution of an infinite-dimensional diffusion equation to finding a multiple integral of a real function of finitely many real variables is indicated.

Conductive polymer layers to limit transfer of fuel reactants to catalysts of fuel cells to reduce reactant crossover

DOEpatents

Stanis, Ronald J.; Lambert, Timothy N.

2016-12-06

An apparatus of an aspect includes a fuel cell catalyst layer. The fuel cell catalyst layer is operable to catalyze a reaction involving a fuel reactant. A fuel cell gas diffusion layer is coupled with the fuel cell catalyst layer. The fuel cell gas diffusion layer includes a porous electrically conductive material. The porous electrically conductive material is operable to allow the fuel reactant to transfer through the fuel cell gas diffusion layer to reach the fuel cell catalyst layer. The porous electrically conductive material is also operable to conduct electrons associated with the reaction through the fuel cell gas diffusion layer. An electrically conductive polymer material is coupled with the fuel cell gas diffusion layer. The electrically conductive polymer material is operable to limit transfer of the fuel reactant to the fuel cell catalyst layer.

Multiphoton microscopy guides neurotrophin modification with poly(ethylene glycol) to enhance interstitial diffusion

NASA Astrophysics Data System (ADS)

Stroh, Mark; Zipfel, Warren R.; Williams, Rebecca M.; Ma, Shu Chin; Webb, Watt W.; Saltzman, W. Mark

2004-07-01

Brain-derived neurotrophic factor (BDNF) is a promising therapeutic agent for the treatment of neurodegenerative diseases. However, the limited distribution of this molecule after administration into the brain tissue considerably hampers its efficacy. Here, we show how multiphoton microscopy of fluorescently tagged BDNF in brain-tissue slices provides a useful and rapid screening method for examining the diffusion of large molecules in tissues, and for studying the effects of chemical modifications-for example, conjugating with polyethylene glycol (PEG)-on the diffusion constant. This single variable, obtained by monitoring short-term diffusion in real time, can be effectively used for rational drug design. In this study on fluorescently tagged BDNF and BDNF-PEG, we identify slow diffusion as a major contributing factor to the limited penetration of BDNF, and demonstrate how chemical modification can be used to overcome this barrier.

Electrochemical properties of electrodes with different shapes and diffusion kinetic analysis of microbial fuel cells on ocean floor

NASA Astrophysics Data System (ADS)

Fu, Yubin; Liu, Jia; Su, Jia; Zhao, Zhongkai; Liu, Yang; Xu, Qian

2012-03-01

Microbial fuel cell (MFC) on the ocean floor is a kind of novel energy- harvesting device that can be developed to drive small instruments to work continuously. The shape of electrode has a great effect on the performance of the MFC. In this paper, several shapes of electrode and cell structure were designed, and their performance in MFC were compared in pairs: Mesh (cell-1) vs. flat plate (cell-2), branch (cell-3) vs. cylinder (cell-4), and forest (cell-5) vs. disk (cell-6) FC. Our results showed that the maximum power densities were 16.50, 14.20, 19.30, 15.00, 14.64, and 9.95 mWm-2 for cell-1, 2, 3, 4, 5 and 6 respectively. And the corresponding diffusion-limited currents were 7.16, 2.80, 18.86, 10.50, 18.00, and 6.900 mA. The mesh and branch anodes showed higher power densities and much higher diffusion-limited currents than the flat plate and the cylinder anodes respectively due to the low diffusion hindrance with the former anodes. The forest cathode improved by 47% of the power density and by 161% of diffusion-limited current than the disk cathode due to the former's extended solid/liquid/gas three-phase boundary. These results indicated that the shape of electrode is a major parameter that determining the diffusion-limited current of an MFC, and the differences in the electrode shape lead to the differences in cell performance. These results would be useful for MFC structure design in practical applications.

Double diffusion in the frontal zones of the Yellow and East China Seas in winter

NASA Astrophysics Data System (ADS)

Oh, K.; Lee, S.

2017-12-01

Where the cold, fresh water of the Yellow Sea (YS) and the warm, salty water of the East China Sea (ECS) meet, northern and southern fronts are formed in the southeastern YS and the northwestern ECS, respectively. Strong thermohaline fronts are formed on the northern front, and a strong thermocline and a temperature reversal phenomenon are represented in this front. To understand the water structure of this thermohaline zone, we examined double diffusion in the frontal zones in February 2003 using hydrographic data. In the northern front, the warm, salty Cheju Warm Current Water (CWCW) moved northwards along the bottom layer and the cold, fresh Yellow Sea Cold Water (YSCW) flowed southward in the upper layer. As a result, strong thermohaline fronts forms in the area where the two water masses met, and the slope was developed downward across the front. In this area, a strong thermocline and temperature reversal structures were present. The cold, fresh Korean Coastal Water (KCW) was also found in the upper layer near the thermocline, and has a low-temperature, low-salinity more than surrounding water. When cold, fresh water is located over warm, salty water, heat diffuses through the interface between the two water masses, and then the diffusive-convection can be expected to occur. On the other hand, when warm, salty water overlays cold, fresh water, heat in the upper layer is preferentially transferred downward, and the salt-fingering occurs. The diffusive-convection occurs predominantly in the northern thermohaline front, where the cold, fresh YSCW is situated above the warm, salty CWCW and has the effect of strengthening stratification, so that the water column maintains a physically stable structure. In addition, this phenomenon seems to play a role in maintaining the reversal structure. The salt-fingering occurs in upper layers of the northern front where the cold, fresh YSCW is located over the most cold, fresh KCW. Near the northern thermo-halocline zone, the salt-fingering occurs simultaneously with the diffusive-convection, because three water masses, YSCW, KCW and CWCW, interact in that area. Therefore, it can be seen that the water structure of the northern frontal zone in winter is influenced mainly by the cold, fresh YSCW, the most cold, fresh KCW, and the warm, salty CWCW.

Modelling nematode movement using time-fractional dynamics.

PubMed

Hapca, Simona; Crawford, John W; MacMillan, Keith; Wilson, Mike J; Young, Iain M

2007-09-07

We use a correlated random walk model in two dimensions to simulate the movement of the slug parasitic nematode Phasmarhabditis hermaphrodita in homogeneous environments. The model incorporates the observed statistical distributions of turning angle and speed derived from time-lapse studies of individual nematode trails. We identify strong temporal correlations between the turning angles and speed that preclude the case of a simple random walk in which successive steps are independent. These correlated random walks are appropriately modelled using an anomalous diffusion model, more precisely using a fractional sub-diffusion model for which the associated stochastic process is characterised by strong memory effects in the probability density function.

Sentiment Diffusion of Public Opinions about Hot Events: Based on Complex Network

PubMed Central

Hao, Xiaoqing; An, Haizhong; Zhang, Lijia; Li, Huajiao; Wei, Guannan

2015-01-01

To study the sentiment diffusion of online public opinions about hot events, we collected people’s posts through web data mining techniques. We calculated the sentiment value of each post based on a sentiment dictionary. Next, we divided those posts into five different orientations of sentiments: strongly positive (P), weakly positive (p), neutral (o), weakly negative (n), and strongly negative (N). These sentiments are combined into modes through coarse graining. We constructed sentiment mode complex network of online public opinions (SMCOP) with modes as nodes and the conversion relation in chronological order between different types of modes as edges. We calculated the strength, k-plex clique, clustering coefficient and betweenness centrality of the SMCOP. The results show that the strength distribution obeys power law. Most posts’ sentiments are weakly positive and neutral, whereas few are strongly negative. There are weakly positive subgroups and neutral subgroups with ppppp and ooooo as the core mode, respectively. Few modes have larger betweenness centrality values and most modes convert to each other with these higher betweenness centrality modes as mediums. Therefore, the relevant person or institutes can take measures to lead people’s sentiments regarding online hot events according to the sentiment diffusion mechanism. PMID:26462230

Modeling Issues and Results for Hydrogen Isotopes in NIF Materials

NASA Astrophysics Data System (ADS)

Grossman, Arthur A.; Doerner, R. P.; Luckhardt, S. C.; Seraydarian, R.; Sze, D.; Burnham, A.

1998-11-01

The TMAP4 (G. Longhurst, et al. INEL 1992) model of hydrogen isotope transport in solid materials includes a particle diffusion calculation with Fick's Law modified for Soret Effect (Thermal Diffusion or Thermomigration), coupled to heat transport calculations which are needed because of the strong temperature dependence of diffusivity. These TMAP4 calculations applied to NIF show that high temperatures approaching the melting point and strong thermal gradients of 10^6 K/cm are reached in the first micron of wall material during the SXR pulse. These strong thermal gradients can drive hydrogen isotope migration up or down the thermal gradient depending on the sign of the heat of transport (Soret coefficient) which depends on whether the material dissolves hydrogen endothermically or exothermically. Two candidates for NIF wall material-boron carbide and stainless steel are compared. Boron carbide dissolves hydrogen exothermically so it may drive Soret migration down the thermal gradient deeper into the material, although the thermal gradient is not as large and hydrogen is not as mobile as in stainless steel. Stainless steel dissolves hydrogen endothermically, with a negative Soret coefficient which can drive hydrogen up the thermal gradient and out of the wall.

Limiting diffusion current at rotating disk electrode with dense particle layer.

PubMed

Weroński, P; Nosek, M; Batys, P

2013-09-28

Exploiting the concept of diffusion permeability of multilayer gel membrane and porous multilayer we have derived a simple analytical equation for the limiting diffusion current at rotating disk electrode (RDE) covered by a thin layer with variable tortuosity and porosity, under the assumption of negligible convection in the porous film. The variation of limiting diffusion current with the porosity and tortuosity of the film can be described in terms of the equivalent thickness of stagnant solution layer, i.e., the average ratio of squared tortuosity to porosity. In case of monolayer of monodisperse spherical particles, the equivalent layer thickness is an algebraic function of the surface coverage. Thus, by means of cyclic voltammetry of RDE with a deposited particle monolayer we can determine the monolayer surface coverage. The effect of particle layer adsorbed on the surface of RDE increases non-linearly with surface coverage. We have tested our theoretical results experimentally by means of cyclic voltammetry measurements of limiting diffusion current at the glassy carbon RDE covered with a monolayer of 3 μm silica particles. The theoretical and experimental results are in a good agreement at the surface coverage higher than 0.7. This result suggests that convection in a monolayer of 3 μm monodisperse spherical particles is negligibly small, in the context of the coverage determination, in the range of very dense particle layers.

THE SPATIO-TEMPORAL EVOLUTION OF SOLAR FLARES OBSERVED WITH AIA/SDO: FRACTAL DIFFUSION, SUB-DIFFUSION, OR LOGISTIC GROWTH?

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

Aschwanden, Markus J., E-mail: aschwanden@lmsal.com

2012-09-20

We explore the spatio-temporal evolution of solar flares by fitting a radial expansion model r(t) that consists of an exponentially growing acceleration phase, followed by a deceleration phase that is parameterized by the generalized diffusion function r(t){proportional_to}{kappa}(t - t{sub 1}){sup {beta}/2}, which includes the logistic growth limit ({beta} = 0), sub-diffusion ({beta} = 0-1), classical diffusion ({beta} = 1), super-diffusion ({beta} = 1-2), and the linear expansion limit ({beta} = 2). We analyze all M- and X-class flares observed with Geostationary Operational Environmental Satellite and Atmospheric Imaging Assembly/Solar Dynamics Observatory (SDO) during the first two years of the SDO mission,more » amounting to 155 events. We find that most flares operate in the sub-diffusive regime ({beta} = 0.53 {+-} 0.27), which we interpret in terms of anisotropic chain reactions of intermittent magnetic reconnection episodes in a low plasma-{beta} corona. We find a mean propagation speed of v = 15 {+-} 12 km s{sup -1}, with maximum speeds of v{sub max} = 80 {+-} 85 km s{sup -1} per flare, which is substantially slower than the sonic speeds expected for thermal diffusion of flare plasmas. The diffusive characteristics established here (for the first time for solar flares) is consistent with the fractal-diffusive self-organized criticality model, which predicted diffusive transport merely based on cellular automaton simulations.« less

Excess oxygen limited diffusion and precipitation of iron in amorphous silicon dioxide

NASA Astrophysics Data System (ADS)

Leveneur, J.; Langlois, M.; Kennedy, J.; Metson, James B.

2017-10-01

In micro- and nano- electronic device fabrication, and particularly 3D designs, the diffusion of a metal into sublayers during annealing needs to be minimized as it is usually detrimental to device performance. Diffusion also causes the formation and growth of nanoprecipitates in solid matrices. In this paper, the diffusion behavior of low energy, low fluence, ion implanted iron into a thermally grown silicon oxide layer on silicon is investigated. Different ion beam analysis and imaging techniques were used. Magnetization measurements were also undertaken to provide evidence of nanocrystalline ordering. While standard vacuum furnace annealing and electron beam annealing lead to fast diffusion of the implanted species towards the Si/SiO2 interface, we show that furnace annealing in an oxygen rich atmosphere prevents the diffusion of iron that, in turn, limits the growth of the nanoparticles. The diffusion and particle growth is also greatly reduced when oxygen atoms are implanted in the SiO2 prior to Fe implantation, effectively acting as a diffusion barrier. The excess oxygen is hypothesized to trap Fe atoms and reduce their mean free path during the diffusion. Monte-Carlo simulations of the diffusion process which consider the random walk of Fe, Fick's diffusion of O atoms, Fe precipitation, and desorption of the SiO2 layer under the electron beam annealing were performed. Simulation results for the three preparation conditions are found in good agreement with the experimental data.

A comparative assessment of preclinical chemotherapeutic response of tumors using quantitative non-Gaussian diffusion MRI

PubMed Central

Xu, Junzhong; Li, Ke; Smith, R. Adam; Waterton, John C.; Zhao, Ping; Ding, Zhaohua; Does, Mark D.; Manning, H. Charles; Gore, John C.

2016-01-01

Background Diffusion-weighted MRI (DWI) signal attenuation is often not mono-exponential (i.e. non-Gaussian diffusion) with stronger diffusion weighting. Several non-Gaussian diffusion models have been developed and may provide new information or higher sensitivity compared with the conventional apparent diffusion coefficient (ADC) method. However the relative merits of these models to detect tumor therapeutic response is not fully clear. Methods Conventional ADC, and three widely-used non-Gaussian models, (bi-exponential, stretched exponential, and statistical model), were implemented and compared for assessing SW620 human colon cancer xenografts responding to barasertib, an agent known to induce apoptosis via polyploidy. Bayesian Information Criterion (BIC) was used for model selection among all three non-Gaussian models. Results All of tumor volume, histology, conventional ADC, and three non-Gaussian DWI models could show significant differences between control and treatment groups after four days of treatment. However, only the non-Gaussian models detected significant changes after two days of treatment. For any treatment or control group, over 65.7% of tumor voxels indicate the bi-exponential model is strongly or very strongly preferred. Conclusion Non-Gaussian DWI model-derived biomarkers are capable of detecting tumor earlier chemotherapeutic response of tumors compared with conventional ADC and tumor volume. The bi-exponential model provides better fitting compared with statistical and stretched exponential models for the tumor and treatment models used in the current work. PMID:27919785

A one dimensional moving bed biofilm reactor model for nitrification of municipal wastewaters.

PubMed

Barry, Ugo; Choubert, Jean-Marc; Canler, Jean-Pierre; Pétrimaux, Olivier; Héduit, Alain; Lessard, Paul

2017-08-01

This work presents a one-dimensional model of a moving bed bioreactor (MBBR) process designed for the removal of nitrogen from raw wastewaters. A comprehensive experimental strategy was deployed at a semi-industrial pilot-scale plant fed with a municipal wastewater operated at 10-12 °C, and surface loading rates of 1-2 g filtered COD/m 2 d and 0.4-0.55 g NH 4 -N/m 2 d. Data were collected on influent/effluent composition, and on measurement of key variables or parameters (biofilm mass and maximal thickness, thickness of the limit liquid layer, maximal nitrification rate, oxygen mass transfer coefficient). Based on time-course variations in these variables, the MBBR model was calibrated at two time-scales and magnitudes of dynamic conditions, i.e., short-term (4 days) calibration under dynamic conditions and long-term (33 days) calibration, and for three types of carriers. A set of parameters suitable for the conditions was proposed, and the calibrated parameter set is able to simulate the time-course change of nitrogen forms in the effluent of the MBBR tanks, under the tested operated conditions. Parameters linked to diffusion had a strong influence on how robustly the model is able to accurately reproduce time-course changes in effluent quality. Then the model was used to optimize the operations of MBBR layout. It was shown that the main optimization track consists of the limitation of the aeration supply without changing the overall performance of the process. Further work would investigate the influence of the hydrodynamic conditions onto the thickness of the limit liquid layer and the "apparent" diffusion coefficient in the biofilm parameters.

Identifying the limitations of conventional biofiltration of diffuse methane emissions at long-term operation.

PubMed

Gómez-Cuervo, S; Hernández, J; Omil, F

2016-08-01

There is growing international concern about the increasing levels of greenhouse gases in the atmosphere, particularly CO2 and methane. The emissions of methane derived from human activities are associated with large flows and very low concentrations, such as those emitted from landfills and wastewater treatment plants, among others. The present work was focused on the biological methane degradation at diffuse concentrations (0.2% vv(-1)) in a conventional biofilter using a mixture of compost, perlite and bark chips as carrier. An extensive characterization of the process was carried out at long-term operation (250 days) in a fully monitored pilot plant, achieving stable conditions during the entire period. Operational parameters such as waterings, nitrogen addition and inlet loads and contact time influences were evaluated. Obtained results indicate that empty bed residence times within 4-8 min are crucial to maximize elimination rates. Waterings and the type of nitrogen supplied in the nutrient solution (ammonia or nitrate) have a strong impact on the biofilter performance. The better results compatible with a stable operation were achieved using nitrate, with elimination capacities up to 7.6 ± 1.1 g CH4 m(-3 )h(-1). The operation at low inlet concentrations (IC) implied that removal rates obtained were quite limited (ranging 3-8 g CH4 m(-3 )h(-1)); however, these results could be significantly increased (up to 20.6 g CH4 m(-3) h(-1)) at higher IC, which indicates that the mass transfer from the gas to the liquid layer surrounding the biofilm is a key limitation of the process.

Size effects in non-linear heat conduction with flux-limited behaviors

NASA Astrophysics Data System (ADS)

Li, Shu-Nan; Cao, Bing-Yang

2017-11-01

Size effects are discussed for several non-linear heat conduction models with flux-limited behaviors, including the phonon hydrodynamic, Lagrange multiplier, hierarchy moment, nonlinear phonon hydrodynamic, tempered diffusion, thermon gas and generalized nonlinear models. For the phonon hydrodynamic, Lagrange multiplier and tempered diffusion models, heat flux will not exist in problems with sufficiently small scale. The existence of heat flux needs the sizes of heat conduction larger than their corresponding critical sizes, which are determined by the physical properties and boundary temperatures. The critical sizes can be regarded as the theoretical limits of the applicable ranges for these non-linear heat conduction models with flux-limited behaviors. For sufficiently small scale heat conduction, the phonon hydrodynamic and Lagrange multiplier models can also predict the theoretical possibility of violating the second law and multiplicity. Comparisons are also made between these non-Fourier models and non-linear Fourier heat conduction in the type of fast diffusion, which can also predict flux-limited behaviors.

Strong diffusion formulation of Markov chain ensembles and its optimal weaker reductions

NASA Astrophysics Data System (ADS)

Güler, Marifi

2017-10-01

Two self-contained diffusion formulations, in the form of coupled stochastic differential equations, are developed for the temporal evolution of state densities over an ensemble of Markov chains evolving independently under a common transition rate matrix. Our first formulation derives from Kurtz's strong approximation theorem of density-dependent Markov jump processes [Stoch. Process. Their Appl. 6, 223 (1978), 10.1016/0304-4149(78)90020-0] and, therefore, strongly converges with an error bound of the order of lnN /N for ensemble size N . The second formulation eliminates some fluctuation variables, and correspondingly some noise terms, within the governing equations of the strong formulation, with the objective of achieving a simpler analytic formulation and a faster computation algorithm when the transition rates are constant or slowly varying. There, the reduction of the structural complexity is optimal in the sense that the elimination of any given set of variables takes place with the lowest attainable increase in the error bound. The resultant formulations are supported by numerical simulations.

Ribosome surface properties may impose limits on the nature of the cytoplasmic proteome

PubMed Central

2017-01-01

Much of the molecular motion in the cytoplasm is diffusive, which possibly limits the tempo of processes. We studied the dependence of protein mobility on protein surface properties and ionic strength. We used surface-modified fluorescent proteins (FPs) and determined their translational diffusion coefficients (D) in the cytoplasm of Escherichia coli, Lactococcus lactis and Haloferax volcanii. We find that in E. coli D depends on the net charge and its distribution over the protein, with positive proteins diffusing up to 100-fold slower than negative ones. This effect is weaker in L. lactis and Hfx. volcanii due to electrostatic screening. The decrease in mobility is probably caused by interaction of positive FPs with ribosomes as shown in in vivo diffusion measurements and confirmed in vitro with purified ribosomes. Ribosome surface properties may thus limit the composition of the cytoplasmic proteome. This finding lays bare a paradox in the functioning of prokaryotic (endo)symbionts. PMID:29154755

Exact calculations of survival probability for diffusion on growing lines, disks, and spheres: The role of dimension.

PubMed

Simpson, Matthew J; Baker, Ruth E

2015-09-07

Unlike standard applications of transport theory, the transport of molecules and cells during embryonic development often takes place within growing multidimensional tissues. In this work, we consider a model of diffusion on uniformly growing lines, disks, and spheres. An exact solution of the partial differential equation governing the diffusion of a population of individuals on the growing domain is derived. Using this solution, we study the survival probability, S(t). For the standard non-growing case with an absorbing boundary, we observe that S(t) decays to zero in the long time limit. In contrast, when the domain grows linearly or exponentially with time, we show that S(t) decays to a constant, positive value, indicating that a proportion of the diffusing substance remains on the growing domain indefinitely. Comparing S(t) for diffusion on lines, disks, and spheres indicates that there are minimal differences in S(t) in the limit of zero growth and minimal differences in S(t) in the limit of fast growth. In contrast, for intermediate growth rates, we observe modest differences in S(t) between different geometries. These differences can be quantified by evaluating the exact expressions derived and presented here.

Materials Outgassing Rate Decay in Vacuum at Isothermal Conditions

NASA Technical Reports Server (NTRS)

Huang, Alvin Y.; Kastanas, George N.; Kramer, Leonard; Soares, Carlos E.; Mikatarian, Ronald R.

2016-01-01

As a laboratory for scientific research, the International Space Station has been in Low Earth Orbit for nearly 20 years and is expected to be on-orbit for another 10 years. The ISS has been maintaining a relatively pristine contamination environment for science payloads. Materials outgassing induced contamination is currently the dominant source for sensitive surfaces on ISS and modeling the outgassing rate decay over a 20 to 30 year period is challenging. Materials outgassing is described herein as a diffusion-reaction process using ASTM E 1559 rate data. The observation of -1/2 (diffusion) or non-integers (reaction limited) as rate decay exponents for common ISS materials indicate classical reaction kinetics is unsatisfactory in modeling materials outgassing. Non-randomness of reactant concentrations at the interface is the source of this deviation from classical reaction kinetics. A diffusion limited decay was adopted as the result of the correlation of the contaminant layer thicknesses on returned ISS hardware, the existence of high outgassing silicone exhibiting near diffusion limited decay, and the confirmation of non-depleted material after ten years in the Low Earth Orbit.Keywords: Materials Outgassing, ASTM E 1559, Reaction Kinetics, Diffusion, Space Environments Effects, Contamination

Radionuclide migration in clayrock host formations for deep geological disposal of radioactive waste: advances in process understanding and up-scaling methods resulting from the EC integrated project `Funmig

NASA Astrophysics Data System (ADS)

Altmann, S.; Tournassat, C.; Goutelard, F.; Parneix, J. C.; Gimmi, T.; Maes, N.

2009-04-01

One of the ‘pillars' supporting Safety Cases for deep geological disposal of radioactive waste in clayrock formations is the knowledge base regarding radionuclide (Rn) retention by sorption and diffusion-driven transport which is why the EC integrated project ‘Funmig' focused a major part of its effort on advancing understanding of these two macroscopic phenomena. This talk presents some of the main results of this four year effort (2005-2008). One of the keys to understanding diffusion-driven transport of anionic and cationic radionuclide species in clayrocks lies in a detailed understanding of the phenomena governing Rn total concentration and speciation (dissolved, adsorbed) in the different types of pore spaces present in highly-compacted masses of permanently charged clay minerals. Work carried out on a specifically synthesized montmorillonite (a model for the clay mineral fraction in clayrocks) led to development, and preliminary experimental validation, of a conceptually coherent set of theoretical models (molecular dynamics, electrostatic double layer, thermodynamic) describing dissolved ion and water solvent behavior in this material. This work, complemented by the existing state of the art, provides a sound theoretical basis for explaining such important phenomena as anion exclusion, cation exchange and the diffusion behavior of anions, weakly sorbing cations and water tracers. Concerning the behavior of strongly sorbing and/or redox-reactive radionuclides in clay systems, project research improved understanding of the nature of sorption reactions and sorbed species structure for key radioelements, or analogues (U, Se, Eu, Sm, Yb, Nd) on the basal surfaces and in the interlayers of synthetic or purified clay minerals. A probable mechanism for Se(IV) retention by reduction to Se° in Fe2+-containing clays was brought to light; this same process was also studied on the Callovo-Oxfordien clayrock targeted by the French radwaste management program. The migration of most radionuclides in clayrocks, in particular the actinides, is limited by their strong sorption on rock mineral surfaces. Much effort was devoted in Funmig to improving understanding of this process on the clayrocks being studied in the Swiss, Belgian and French radwaste management programs. Specific attention was focused on (i) elucidating the effect of dissolved organic matter on Am(III), Th(IV), Eu(III) sorption on clayrock surfaces and (ii) determining the link between Kd measured on dispersed rock systems and the Kd operant in intact rock volumes, i.e. during diffusion. Regarding the latter question, results indicate that Kd values for ‘dispersed' and ‘intact' materials are quite similar for certain elements (Na, Sr, Cs, Co). On the other hand, Kd values obtained by modeling results of diffusion experiments involving strongly sorbing elements as Cs, Co and Eu were always significantly smaller than those predicted based on sorption data measured in corresponding batch systems. This is an area where additional research is being planned. A major effort was devoted to improving understanding of the effects of small-scale (m to cm) clayrock structure and large-scale (dm to hm) mineralogical composition on radionuclide diffusion-retention. The program focusing on the small-scale produced a method for simulating the results of tracer diffusion in an intact rock based on the actual rock microstructure of the rock sample to be used in the diffusion experiment. This model was used to predict / inverse model the spatial distribution of highly sorbing tracers (Eu, Cu). This overall approach is also being used to understand how changes in mineralogical composition can affect the values of macroscopic diffusion parameters (De, tortuosity, anisiotropy). At a much larger scale, the results of (i) a geostatistical analysis of clayrock mineralogical variability and (ii) measurements of De and Kd dependence on mineralogy for Cs and Cl, were combined to create models of parameter variability at the formation scale. These models were used to evaluate the effects of formation scale heterogeneity on predictive modeling of radionuclide migration. Measurements and modeling of natural tracer profiles were also carried out in order to evaluate the diffusion characteristics at geological time and space scales.

Bringing diffuse X-ray scattering into focus

DOE PAGES

Wall, Michael E.; Wolff, Alexander M.; Fraser, James S.

2018-02-16

X-ray crystallography is experiencing a renaissance as a method for probing the protein conformational ensemble. The inherent limitations of Bragg analysis, however, which only reveals the mean structure, have given way to a surge in interest in diffuse scattering, which is caused by structure variations. Diffuse scattering is present in all macromolecular crystallography experiments. Recent studies are shedding light on the origins of diffuse scattering in protein crystallography, and provide clues for leveraging diffuse scattering to model protein motions with atomic detail.

Bringing diffuse X-ray scattering into focus

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

Wall, Michael E.; Wolff, Alexander M.; Fraser, James S.

X-ray crystallography is experiencing a renaissance as a method for probing the protein conformational ensemble. The inherent limitations of Bragg analysis, however, which only reveals the mean structure, have given way to a surge in interest in diffuse scattering, which is caused by structure variations. Diffuse scattering is present in all macromolecular crystallography experiments. Recent studies are shedding light on the origins of diffuse scattering in protein crystallography, and provide clues for leveraging diffuse scattering to model protein motions with atomic detail.

Cathodic electrodeposition of p-CuSCN nanorod and its dye-sensitized photocathodic property

NASA Astrophysics Data System (ADS)

Sun, Lina; Ichinose, Keigo; Sekiya, Tomohiro; Sugiura, Takashi; Yoshida, Tsukasa

Mechanism of cathodic electrodeposition of CuSCN from ethanolic solutions containing Cu2+ and SCN- was studied in detail. Job's plot for the absorption spectra of mixed solution in various Cu2+: SCN- ratios have revealed the presence of [Cu(SCN)2]0 as a soluble species responsible to the electrode process in SCN- rich solutions. From Levich analysis of diffusion limited current employing a rotating disc electrode (RDE), diffusion coefficients of 5.2 × 10-6 cm2 s-1 and 3.0 × 10-6 cm2 s-1 in ethanol at 298 K were determined for [Cu(SCN)2]0 and [Cu(SCN)]+, respectively. Morphology as well as crystallographic orientation of the product films significantly changed by the composition of the electrolytic baths. When the bath contains excess of Cu2+ and mixed solvent up to 50% ethanol content to water was used, strong anisotropic crystal growth along the c-axis was observed. When electrolysis was carried out under stationary conditions, the nanorod structures in high aspect ratios could be obtained, due to the limited transport of the active species to the tip of the rods. When rhodamine B was adsorbed onto such CuSCN as a sensitizer, dye-sensitized photocathodic current was observed with an incident photon to current conversion efficiency (IPCE) of 4.4% at the absorption maximum, suggesting its usefulness as the hole conducting electrode in construction of nanostructured solar cells.

METAL DIFFUSION IN SMOOTHED PARTICLE HYDRODYNAMICS SIMULATIONS OF DWARF GALAXIES

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

Williamson, David; Martel, Hugo; Kawata, Daisuke, E-mail: david-john.williamson.1@ulaval.ca

2016-05-10

We perform a series of smoothed particle hydrodynamics simulations of isolated dwarf galaxies to compare different metal mixing models. In particular, we examine the role of diffusion in the production of enriched outflows and in determining the metallicity distributions of gas and stars. We investigate different diffusion strengths by changing the pre-factor of the diffusion coefficient, by varying how the diffusion coefficient is calculated from the local velocity distribution, and by varying whether the speed of sound is included as a velocity term. Stronger diffusion produces a tighter [O/Fe]–[Fe/H] distribution in the gas and cuts off the gas metallicity distributionmore » function at lower metallicities. Diffusion suppresses the formation of low-metallicity stars, even with weak diffusion, and also strips metals from enriched outflows. This produces a remarkably tight correlation between “metal mass-loading” (mean metal outflow rate divided by mean metal production rate) and the strength of diffusion, even when the diffusion coefficient is calculated in different ways. The effectiveness of outflows at removing metals from dwarf galaxies and the metal distribution of the gas is thus dependent on the strength of diffusion. By contrast, we show that the metallicities of stars are not strongly dependent on the strength of diffusion, provided that some diffusion is present.« less

Molecular modeling of polymers 16. Gaseous diffusion in polymers: a quantitative structure-property relationship (QSPR) analysis.

PubMed

Patel, H C; Tokarski, J S; Hopfinger, A J

1997-10-01

The purpose of this study was to identify the key physicochemical molecular properties of polymeric materials responsible for gaseous diffusion in the polymers. Quantitative structure-property relationships, QSPRs were constructed using a genetic algorithm on a training set of 16 polymers for which CO2, N2, O2 diffusion constants were measured. Nine physicochemical properties of each of the polymers were used in the trial basis set for QSPR model construction. The linear cross-correlation matrices were constructed and investigated for colinearity among the members of the training sets. Common water diffusion measures for a limited training set of six polymers was used to construct a "semi-QSPR" model. The bulk modulus of the polymer was overwhelmingly found to be the dominant physicochemical polymer property that governs CO2, N2 and O2 diffusion. Some secondary physicochemical properties controlling diffusion, including conformational entropy, were also identified as correlation descriptors. Very significant QSPR diffusion models were constructed for all three gases. Cohesive energy was identified as the main correlation physicochemical property with aqueous diffusion measures. The dominant role of polymer bulk modulus on gaseous diffusion makes it difficult to develop criteria for selective transport of gases through polymers. Moreover, high bulk moduli are predicted to be necessary for effective gas barrier materials. This property requirement may limit the processing and packaging features of the material. Aqueous diffusion in polymers may occur by a different mechanism than gaseous diffusion since bulk modulus does not correlate with aqueous diffusion, but rather cohesive energy of the polymer.

Relative Roles of Gap Junction Channels and Cytoplasm in Cell-to-Cell Diffusion of Fluorescent Tracers

NASA Astrophysics Data System (ADS)

Safranyos, Richard G. A.; Caveney, Stanley; Miller, James G.; Petersen, Nils O.

1987-04-01

Intercellular (tissue) diffusion of molecules requires cytoplasmic diffusion and diffusion through gap junctional (or cell-to-cell) channels. The rates of tissue and cytoplasmic diffusion of fluorescent tracers, expressed as an effective diffusion coefficient, De, and a cytoplasmic diffusion coefficient, Dcyt, have been measured among the developing epidermal cells of a larval beetle, Tenebrio molitor L., to determine the contribution of the junctional channels to intercellular diffusion. Tracer diffusion was measured by injecting fluorescent tracers into cells and quantitating the rate of subsequent spread into adjacent cells. Cytoplasmic diffusion was determined by fluorescence photobleaching. These experiments show that gap junctional channels constitute approximately 70-80% of the total cell-to-cell resistance to the diffusion of organic tracers at high concentrations in this tissue. At low concentrations, however, the binding of tracer to cytoplasm slows down the cytoplasmic diffusion, which may limit intercellular diffusion.

Development of a High-Performance Wind Turbine Equipped with a Brimmed Diffuser Shroud

NASA Astrophysics Data System (ADS)

Ohya, Yuji; Karasudani, Takashi; Sakurai, Akira; Inoue, Masahiro

We have developed a new wind turbine system that consists of a diffuser shroud with a broad-ring brim at the exit periphery and a wind turbine inside it. The brimmed-diffuser shroud plays the role of a device for collecting and accelerating the approaching wind. Emphasis is placed on positioning the brim at the exit of the diffuser shroud. Namely, the brim generates a very low-pressure region in the exit neighborhood of the diffuser by strong vortex formation and draws more mass flow to the wind turbine inside the diffuser shroud. To obtain a higher power output of the shrouded wind turbine, we have examined the optimal form for the brimmed diffuser, such as the diffuser open angle, brim height, hub ratio, centerbody length, inlet shroud shape and so on. As a result, a shrouded wind turbine equipped with a brimmed diffuser has been developed, and demonstrated power augmentation for a given turbine diameter and wind speed by a factor of about five compared to a standard (bare) wind turbine.

Heat transport in electrically aligned multiwalled carbon nanotubes dispersed in water

NASA Astrophysics Data System (ADS)

Cervantes-Alvarez, F.; Macias, J. D.; Alvarado-Gil, J. J.

2018-02-01

A modified Ångström method was used to determine the thermal diffusivity and thermal conductivity of aqueous dispersions of multiwalled carbon nanotubes as a function of their weight fraction concentration and in the presence of an externally applied electric field. Measurements were performed in planar samples, with a fixed thickness of 3.18 mm applying an AC voltage in the range from 0 to 70~V_RMS and for concentrations of carbon nanotubes from 0 to 2 wf%. It is shown that this field induces the formation of clusters followed by their alignment along the electric field, which can favor heat transfer in that direction. Heat transfer measurements show two regimes, in the first one under 0.5 wf%, voltages lower than 30~V_RMS are not strong enough to induce the adequate order of the carbon nanostructures, and as a consequence, thermal diffusivity of the dispersion remains close to the thermal diffusivity of water. In contrast for higher concentrations (above 1.5 wf%), 10~V_RMS are enough to get a good alignment. Above such thresholds of concentrations and voltages, thermal diffusivity and conductivity increase, when the electric field is increased, in such a way that for an applied voltage of 20~V_RMS and for a concentration of 1.5 wf%, an increase of 49% of the thermal conductivity was obtained. It is also shown that this approach exhibits limits, due to the fact that the electric-field induced structure, can act as a heating element at high electric field intensities and carbon nanotubes concentrations, which can induce convection and evaporation of the liquid matrix.

Anisotropic scene geometry resampling with occlusion filling for 3DTV applications

NASA Astrophysics Data System (ADS)

Kim, Jangheon; Sikora, Thomas

2006-02-01

Image and video-based rendering technologies are receiving growing attention due to their photo-realistic rendering capability in free-viewpoint. However, two major limitations are ghosting and blurring due to their sampling-based mechanism. The scene geometry which supports to select accurate sampling positions is proposed using global method (i.e. approximate depth plane) and local method (i.e. disparity estimation). This paper focuses on the local method since it can yield more accurate rendering quality without large number of cameras. The local scene geometry has two difficulties which are the geometrical density and the uncovered area including hidden information. They are the serious drawback to reconstruct an arbitrary viewpoint without aliasing artifacts. To solve the problems, we propose anisotropic diffusive resampling method based on tensor theory. Isotropic low-pass filtering accomplishes anti-aliasing in scene geometry and anisotropic diffusion prevents filtering from blurring the visual structures. Apertures in coarse samples are estimated following diffusion on the pre-filtered space, the nonlinear weighting of gradient directions suppresses the amount of diffusion. Aliasing artifacts from low density are efficiently removed by isotropic filtering and the edge blurring can be solved by the anisotropic method at one process. Due to difference size of sampling gap, the resampling condition is defined considering causality between filter-scale and edge. Using partial differential equation (PDE) employing Gaussian scale-space, we iteratively achieve the coarse-to-fine resampling. In a large scale, apertures and uncovered holes can be overcoming because only strong and meaningful boundaries are selected on the resolution. The coarse-level resampling with a large scale is iteratively refined to get detail scene structure. Simulation results show the marked improvements of rendering quality.

Soot Formation in Laminar Acetylene/Air Diffusion Flames at Atmospheric Pressure. Appendix C

NASA Technical Reports Server (NTRS)

Xu, F.; Faeth, G. M.; Urban, D. L. (Technical Monitor); Yuan, Z.-G. (Technical Monitor)

2000-01-01

The flame structure and soot-formation (soot nucleation and growth) properties of axisymmetric laminar coflowing jet diffusion flames were studied experimentally. Test conditions involved acetylene-nitrogen jets burning in coflowing air at atmospheric pressure. Measurements were limited to the axes of the flames and included soot concentrations, soot temperatures, soot structure, major gas species concentrations, radical species (H, OH, and O) concentrations, and gas velocities. The results show that as distance increases along the axes of the flames, detectable soot formation begins when significant H concentrations are present, and ends when acetylene concentrations become small. Species potentially associated with soot oxidation-O2, CO2, H2O, O, and OH-are present throughout the soot-formation region so that soot formation and oxidation proceed at the same time. Strong rates of soot growth compared to soot nucleation early in the soot-formation process, combined with increased rates of soot nucleation and oxidation as soot formation proceeds, causes primary soot particle diameters to reach a maximum relatively early in the soot-formation process. Aggregation of primary soot particles proceeds, however, until the final stages of soot oxidation. Present measurements of soot growth (corrected for soot oxidation) in laminar diffusion flames were consistent with earlier measurements of soot growth in laminar premixed flames and exhibited encouraging agreement with existing hydrogen-abstraction/carbon-addition (HACA) soot growth mechanisms in the literature that were developed based on measurements within laminar premixed flames. Measured primary soot particle nucleation rates in the present laminar diffusion flames also were consistent with corresponding rates measured in laminar premixed flames and yielded a crude correlation in terms of acetylene and H concentrations and the temperature.

Soot Formation in Laminar Acetylene/Air Diffusion Flames at Atmospheric Pressure. Appendix H

NASA Technical Reports Server (NTRS)

Xu, F.; Faeth, G. M.; Yuan, Z.-G. (Technical Monitor); Urban, D. L. (Technical Monitor); Yuan, Z.-G. (Technical Monitor)

2001-01-01

The flame structure and soot-formation (soot nucleation and growth) properties of axisymmetric laminar coflowing jet diffusion flames were studied experimentally. Test conditions involved acetylene-nitrogen jets burning in coflowing air at atmospheric pressure. Measurements were limited to the axes of the flames and included soot concentrations, soot temperatures, soot structure, major gas species concentrations, radical species (H, OH, and O) concentrations, and gas velocities. The results show that as distance increases along the axes of the flames, detectable soot formation begins when significant H concentrations are present, and ends when acetylene concentrations become small. Species potentially associated with soot oxidation-O2, CO2, H2O, O, and OH-are present throughout the soot-formation region so that soot formation and oxidation proceed at the same time. Strong rates of soot growth compared to soot nucleation early in the soot-formation process, combined with increased rates of soot nucleation and oxidation as soot formation proceeds, causes primary soot particle diameters to reach a maximum relatively early in the soot-formation process. Aggregation of primary soot particles proceeds, however, until the final stages of soot oxidation. Present measurements of soot growth (corrected for soot oxidation) in laminar diffusion flames were consistent with earlier measurements of soot growth in laminar premixed flames and exhibited encouraging agreement with existing hydrogen-abstraction/carbon-addition (HACA) soot growth mechanisms in the literature that were developed based on measurements within laminar premixed flames. Measured primary soot particle nucleation rates in the present laminar diffusion flames also were consistent with corresponding rates measured in laminar premixed flames and yielded a crude correlation in terms of acetylene and H concentrations and the temperature.

Soot Formation in Laminar Acetylene/Air Diffusion Flames at Atmospheric Pressure. Appendix J

NASA Technical Reports Server (NTRS)

Xu, F.; Faeth, G. M.; Urban, D. L. (Technical Monitor); Yuan, Z.-G. (Technical Monitor)

2001-01-01

The flame structure and soot-formation (soot nucleation and growth) properties of axisymmetric laminar coflowing jet diffusion flames were studied experimentally. Test conditions involved acetylene-nitrogen jets burning in coflowing air at atmospheric pressure. Measurements were limited to the axes of the flames and included soot concentrations, soot temperatures, soot structure, major gas species concentrations, radical species (H, OH, and O) concentrations, and gas velocities. The results show that as distance increases along the axes of the flames, detectable soot formation begins when significant H concentrations are present, and ends when acetylene concentrations become small. Species potentially associated with soot oxidation--O2, CO2, H2O, O, and OH-are present throughout the soot-formation region so that soot formation and oxidation proceed at the same time. Strong rates of soot growth compared to soot nucleation early in the soot-formation process, combined with increased rates of soot nucleation and oxidation as soot formation proceeds, causes primary soot particle diameters to reach a maximum relatively early in the soot-formation process. Aggregation of primary soot particles proceeds, however, until the final stages of soot oxidation. Present measurements of soot growth (corrected for soot oxidation) in laminar diffusion flames were consistent with earlier measurements of soot growth in laminar premixed flames and exhibited encouraging agreement with existing hydrogen-abstraction/carbon-addition (HACA) soot growth mechanisms in the literature that were developed based on measurements within laminar premixed flames. Measured primary soot particle nucleation rates in the present laminar diffusion flames also were consistent with corresponding rates measured in laminar premixed flames and yielded a crude correlation in terms of acetylene and H concentrations and the temperature.

Inter-diffusion analysis of joint interface of tungsten-rhenium couple

NASA Astrophysics Data System (ADS)

Hua, Y. F.; Li, Z. X.; Zhang, X.; Du, J. H.; Huang, C. L.; Du, M. H.

2011-09-01

The tungsten-rhenium couple was prepared by using glow plasma physical vapor deposition (PVD) on the isotropic fine grained graphite (IG) substrates. Diffusion anneals of the tungsten-rhenium couple were conducted at the temperature from 1100 °C to 1400 °C to investigate the inter-diffusion behaviors. The results showed that the thickness of the inter-diffusion zone increased with increasing annealing temperature. The relationship between the inter-diffusion coefficient and the annealing temperature accorded with the Arrhenius manner. The value of inter-diffusion activation energies was 189 kJ/mole (1.96 eV). The service time of tungsten-rhenium multilayer diffusion barrier was limited by the inter-diffusion for rhenium and tungsten rather than the diffusion of carbon in rhenium.

Low temperature delays timing and enhances the cost of nitrogen fixation in the unicellular cyanobacterium Cyanothece

PubMed Central

Brauer, Verena S; Stomp, Maayke; Rosso, Camillo; van Beusekom, Sebastiaan AM; Emmerich, Barbara; Stal, Lucas J; Huisman, Jef

2013-01-01

Marine nitrogen-fixing cyanobacteria are largely confined to the tropical and subtropical ocean. It has been argued that their global biogeographical distribution reflects the physiologically feasible temperature range at which they can perform nitrogen fixation. In this study we refine this line of argumentation for the globally important group of unicellular diazotrophic cyanobacteria, and pose the following two hypotheses: (i) nitrogen fixation is limited by nitrogenase activity at low temperature and by oxygen diffusion at high temperature, which is manifested by a shift from strong to weak temperature dependence of nitrogenase activity, and (ii) high respiration rates are required to maintain very low levels of oxygen for nitrogenase, which results in enhanced respiratory cost per molecule of fixed nitrogen at low temperature. We tested these hypotheses in laboratory experiments with the unicellular cyanobacterium Cyanothece sp. BG043511. In line with the first hypothesis, the specific growth rate increased strongly with temperature from 18 to 30 °C, but leveled off at higher temperature under nitrogen-fixing conditions. As predicted by the second hypothesis, the respiratory cost of nitrogen fixation and also the cellular C:N ratio rose sharply at temperatures below 21 °C. In addition, we found that low temperature caused a strong delay in the onset of the nocturnal nitrogenase activity, which shortened the remaining nighttime available for nitrogen fixation. Together, these results point at a lower temperature limit for unicellular nitrogen-fixing cyanobacteria, which offers an explanation for their (sub)tropical distribution and suggests expansion of their biogeographical range by global warming. PMID:23823493

Nonlinear ionic transport through microstructured solid electrolytes: homogenization estimates

NASA Astrophysics Data System (ADS)

Curto Sillamoni, Ignacio J.; Idiart, Martín I.

2016-10-01

We consider the transport of multiple ionic species by diffusion and migration through microstructured solid electrolytes in the presence of strong electric fields. The assumed constitutive relations for the constituent phases follow from convex energy and dissipation potentials which guarantee thermodynamic consistency. The effective response is heuristically deduced from a multi-scale convergence analysis of the relevant field equations. The resulting homogenized response involves an effective dissipation potential per species. Each potential is mathematically akin to that of a standard nonlinear heterogeneous conductor. A ‘linear-comparison’ homogenization technique is then used to generate estimates for these nonlinear potentials in terms of available estimates for corresponding linear conductors. By way of example, use is made of the Maxwell-Garnett and effective-medium linear approximations to generate estimates for two-phase systems with power-law dissipation. Explicit formulas are given for some limiting cases. In the case of threshold-type behavior, the estimates exhibit non-analytical dilute limits and seem to be consistent with fields localized in low energy paths.

Direct detection constraints on dark photon dark matter

NASA Astrophysics Data System (ADS)

An, Haipeng; Pospelov, Maxim; Pradler, Josef; Ritz, Adam

2015-07-01

Dark matter detectors built primarily to probe elastic scattering of WIMPs on nuclei are also precise probes of light, weakly coupled, particles that may be absorbed by the detector material. In this paper, we derive constraints on the minimal model of dark matter comprised of long-lived vector states V (dark photons) in the 0.01- 100 keV mass range. The absence of an ionization signal in direct detection experiments such as XENON10 and XENON100 places a very strong constraint on the dark photon mixing angle, down to O (10-15), assuming that dark photons comprise the dominant fraction of dark matter. This sensitivity to dark photon dark matter exceeds the indirect bounds derived from stellar energy loss considerations over a significant fraction of the available mass range. We also revisit indirect constraints from V → 3 γ decay and show that limits from modifications to the cosmological ionization history are comparable to the updated limits from the diffuse γ-ray flux.

Explaining the apparent impenetrable barrier to ultra-relativistic electrons in the outer Van Allen belt.

PubMed

Ozeke, Louis G; Mann, Ian R; Murphy, Kyle R; Degeling, Alex W; Claudepierre, Seth G; Spence, Harlan E

2018-05-10

Recent observations have shown the existence of an apparent impenetrable barrier at the inner edge of the ultra-relativistic outer electron radiation belt. This apparent impenetrable barrier has not been explained. However, recent studies have suggested that fast loss, such as associated with scattering into the atmosphere from man-made very-low frequency transmissions, is required to limit the Earthward extent of the belt. Here we show that the steep flux gradient at the implied barrier location is instead explained as a natural consequence of ultra-low frequency wave radial diffusion. Contrary to earlier claims, sharp boundaries in fast loss processes at the barrier are not needed. Moreover, we show that penetration to the barrier can occur on the timescale of days rather than years as previously reported, with the Earthward extent of the belt being limited by the finite duration of strong solar wind driving, which can encompass only a single geomagnetic storm.

Decoupled Method for Reconstruction of Surface Conditions From Internal Temperatures On Ablative Materials With Uncertain Recession Model

NASA Technical Reports Server (NTRS)

Oliver, A. Brandon

2017-01-01

Obtaining measurements of flight environments on ablative heat shields is both critical for spacecraft development and extremely challenging due to the harsh heating environment and surface recession. Thermocouples installed several millimeters below the surface are commonly used to measure the heat shield temperature response, but an ill-posed inverse heat conduction problem must be solved to reconstruct the surface heating environment from these measurements. Ablation can contribute substantially to the measurement response making solutions to the inverse problem strongly dependent on the recession model, which is often poorly characterized. To enable efficient surface reconstruction for recession model sensitivity analysis, a method for decoupling the surface recession evaluation from the inverse heat conduction problem is presented. The decoupled method is shown to provide reconstructions of equivalent accuracy to the traditional coupled method but with substantially reduced computational effort. These methods are applied to reconstruct the environments on the Mars Science Laboratory heat shield using diffusion limit and kinetically limited recession models.

Direct detection constraints on dark photon dark matter

DOE PAGES

An, Haipeng; Pospelov, Maxim; Pradler, Josef; ...

2015-06-11

Dark matter detectors built primarily to probe elastic scattering of WIMPs on nuclei are also precise probes of light, weakly coupled, particles that may be absorbed by the detector material. In this paper, we derive constraints on the minimal model of dark matter comprised of long-lived vector states V (dark photons) in the 0.01–100KeV mass range. The absence of an ionization signal in direct detection experiments such as XENON10 and XENON100 places a very strong constraint on the dark photon mixing angle, down to Ο(10 –15), assuming that dark photons comprise the dominant fraction of dark matter. This sensitivity tomore » dark photon dark matter exceeds the indirect bounds derived from stellar energy loss considerations over a significant fraction of the available mass range. As a result, we also revisit indirect constraints from V → 3γ decay and show that limits from modifications to the cosmological ionization history are comparable to the updated limits from the diffuse γ-ray flux.« less

"We noticed that suddenly the country has become full of MRI". Policy makers' views on diffusion and use of health technologies in Iran

PubMed Central

2010-01-01

Objective Uncontrolled proliferation of health technologies (HT) is one contributor to the increasing pressure on health systems to adopt new technologies. With limited resources, policy-makers encounter difficulties in fulfilling their responsibility to meet the healthcare needs of the population. The aim of this study is to explore how policy-makers' reason about the diffusion and utilization of health technologies in Iran using magnetic resonance imaging (MRI) and interferon beta as tracers. Method This qualitative exploration complements quantitative data generated in a research project investigating the diffusion and utilization of MRI and interferon beta in Iran. Qualitative semi-structured interviews were conducted with 13 informants in different positions and levels of authority in the Ministry of Health (MOH), University of Medical Sciences, Health Insurance Organizations, and Parliament. The data was analysed using the framework approach. Findings Although policy-makers appeared to be positive to health technology assessment (HTA), the processes of policy-making described by the interviewees did not seem to be based on a full understanding of this (discipline). Several obstacles to applying knowledge about HT and HTA were described. The current official plan for MRI adoption and diffusion in the country was said not to be followed, and no such plan was described for interferon beta. Instead, market forces such as advertising, and physician and consumer demand, appear to have strong influence on HT diffusion and use. Dual practice may have increased the induced demand and also reduced the supervision of the private sector by the MOH. Conclusion Management instability and lack of coordination in the MOH were found to be important obstacles to accumulation of knowledge and experience which, in turn, could have led to suboptimal managerial and policy-making processes. Furthermore marketing should be controlled in order to avoid creating unnecessary patient demands and negative influences on physicians' behavior. PMID:20370906

Discontinuous Galerkin method for multicomponent chemically reacting flows and combustion

NASA Astrophysics Data System (ADS)

Lv, Yu; Ihme, Matthias

2014-08-01

This paper presents the development of a discontinuous Galerkin (DG) method for application to chemically reacting flows in subsonic and supersonic regimes under the consideration of variable thermo-viscous-diffusive transport properties, detailed and stiff reaction chemistry, and shock capturing. A hybrid-flux formulation is developed for treatment of the convective fluxes, combining a conservative Riemann-solver and an extended double-flux scheme. A computationally efficient splitting scheme is proposed, in which advection and diffusion operators are solved in the weak form, and the chemically stiff substep is advanced in the strong form using a time-implicit scheme. The discretization of the viscous-diffusive transport terms follows the second form of Bassi and Rebay, and the WENO-based limiter due to Zhong and Shu is extended to multicomponent systems. Boundary conditions are developed for subsonic and supersonic flow conditions, and the algorithm is coupled to thermochemical libraries to account for detailed reaction chemistry and complex transport. The resulting DG method is applied to a series of test cases of increasing physico-chemical complexity. Beginning with one- and two-dimensional multispecies advection and shock-fluid interaction problems, computational efficiency, convergence, and conservation properties are demonstrated. This study is followed by considering a series of detonation and supersonic combustion problems to investigate the convergence-rate and the shock-capturing capability in the presence of one- and multistep reaction chemistry. The DG algorithm is then applied to diffusion-controlled deflagration problems. By examining convergence properties for polynomial order and spatial resolution, and comparing these with second-order finite-volume solutions, it is shown that optimal convergence is achieved and that polynomial refinement provides advantages in better resolving the localized flame structure and complex flow-field features associated with multidimensional and hydrodynamic/thermo-diffusive instabilities in deflagration and detonation systems. Comparisons with standard third- and fifth-order WENO schemes are presented to illustrate the benefit of the DG scheme for application to detonation and multispecies flow/shock-interaction problems.

"We noticed that suddenly the country has become full of MRI". Policy makers' views on diffusion and use of health technologies in Iran.

PubMed

Palesh, Mohammad; Tishelman, Carol; Fredrikson, Sten; Jamshidi, Hamidreza; Tomson, Göran; Emami, Azita

2010-04-06

Uncontrolled proliferation of health technologies (HT) is one contributor to the increasing pressure on health systems to adopt new technologies. With limited resources, policy-makers encounter difficulties in fulfilling their responsibility to meet the healthcare needs of the population. The aim of this study is to explore how policy-makers' reason about the diffusion and utilization of health technologies in Iran using magnetic resonance imaging (MRI) and interferon beta as tracers. This qualitative exploration complements quantitative data generated in a research project investigating the diffusion and utilization of MRI and interferon beta in Iran. Qualitative semi-structured interviews were conducted with 13 informants in different positions and levels of authority in the Ministry of Health (MOH), University of Medical Sciences, Health Insurance Organizations, and Parliament. The data was analysed using the framework approach. Although policy-makers appeared to be positive to health technology assessment (HTA), the processes of policy-making described by the interviewees did not seem to be based on a full understanding of this (discipline). Several obstacles to applying knowledge about HT and HTA were described. The current official plan for MRI adoption and diffusion in the country was said not to be followed, and no such plan was described for interferon beta. Instead, market forces such as advertising, and physician and consumer demand, appear to have strong influence on HT diffusion and use. Dual practice may have increased the induced demand and also reduced the supervision of the private sector by the MOH. Management instability and lack of coordination in the MOH were found to be important obstacles to accumulation of knowledge and experience which, in turn, could have led to suboptimal managerial and policy-making processes. Furthermore marketing should be controlled in order to avoid creating unnecessary patient demands and negative influences on physicians' behavior.

Diffusive wave in the low Mach limit for non-viscous and heat-conductive gas

NASA Astrophysics Data System (ADS)

Liu, Yechi

2018-06-01

The low Mach number limit for one-dimensional non-isentropic compressible Navier-Stokes system without viscosity is investigated, where the density and temperature have different asymptotic states at far fields. It is proved that the solution of the system converges to a nonlinear diffusion wave globally in time as Mach number goes to zero. It is remarked that the velocity of diffusion wave is proportional with the variation of temperature. Furthermore, it is shown that the solution of compressible Navier-Stokes system also has the same phenomenon when Mach number is suitably small.

Thermal Skin fabrication technology

NASA Technical Reports Server (NTRS)

Milam, T. B.

1972-01-01

Advanced fabrication techniques applicable to Thermal Skin structures were investigated, including: (1) chemical machining; (2) braze bonding; (3) diffusion bonding; and (4) electron beam welding. Materials investigated were nickel and nickel alloys. Sample Thermal Skin panels were manufactured using the advanced fabrication techniques studied and were structurally tested. Results of the program included: (1) development of improved chemical machining processes for nickel and several nickel alloys; (2) identification of design geometry limits; (3) identification of diffusion bonding requirements; (4) development of a unique diffusion bonding tool; (5) identification of electron beam welding limits; and (6) identification of structural properties of Thermal Skin material.

New product forecasting with limited or no data

NASA Astrophysics Data System (ADS)

Ismai, Zuhaimy; Abu, Noratikah; Sufahani, Suliadi

2016-10-01

In the real world, forecasts would always be based on historical data with the assumption that the behaviour be the same for the future. But how do we forecast when there is no such data available? New product or new technologies normally has limited amount of data available. Knowing that forecasting is valuable for decision making, this paper presents forecasting of new product or new technologies using aggregate diffusion models and modified Bass Model. A newly launched Proton car and its penetration was chosen to demonstrate the possibility of forecasting sales demand where there is limited or no data available. The model was developed to forecast diffusion of new vehicle or an innovation in the Malaysian society. It is to represent the level of spread on the new vehicle among a given set of the society in terms of a simple mathematical function that elapsed since the introduction of the new product. This model will forecast the car sales volume. A procedure of the proposed diffusion model was designed and the parameters were estimated. Results obtained by applying the proposed diffusion model and numerical calculation shows that the model is robust and effective for forecasting demand of the new vehicle. The results reveal that newly developed modified Bass diffusion of demand function has significantly contributed for forecasting the diffusion of new Proton car or new product.

Breakdown of the reaction-diffusion master equation with nonelementary rates

NASA Astrophysics Data System (ADS)

Smith, Stephen; Grima, Ramon

2016-05-01

The chemical master equation (CME) is the exact mathematical formulation of chemical reactions occurring in a dilute and well-mixed volume. The reaction-diffusion master equation (RDME) is a stochastic description of reaction-diffusion processes on a spatial lattice, assuming well mixing only on the length scale of the lattice. It is clear that, for the sake of consistency, the solution of the RDME of a chemical system should converge to the solution of the CME of the same system in the limit of fast diffusion: Indeed, this has been tacitly assumed in most literature concerning the RDME. We show that, in the limit of fast diffusion, the RDME indeed converges to a master equation but not necessarily the CME. We introduce a class of propensity functions, such that if the RDME has propensities exclusively of this class, then the RDME converges to the CME of the same system, whereas if the RDME has propensities not in this class, then convergence is not guaranteed. These are revealed to be elementary and nonelementary propensities, respectively. We also show that independent of the type of propensity, the RDME converges to the CME in the simultaneous limit of fast diffusion and large volumes. We illustrate our results with some simple example systems and argue that the RDME cannot generally be an accurate description of systems with nonelementary rates.

Viscous organic aerosol particles in the upper troposphere: diffusivity-controlled water uptake and ice nucleation?

DOE PAGES

Lienhard, D. M.; Huisman, A. J.; Krieger, U. K.; ...

2015-01-01

New measurements of water diffusion in secondary organic aerosol (SOA) material produced by oxidation of α-pinene and in a number of organic/inorganic model mixtures (3-methylbutane-1,2,3-tricarboxylic acid (3-MBTCA), levoglucosan, levoglucosan/NH 4HSO 4, raffinose) are presented. These indicate that water diffusion coefficients are determined by several properties of the aerosol substance and cannot be inferred from the glass transition temperature or bouncing properties. Our results suggest that water diffusion in SOA particles is faster than often assumed and imposes no significant kinetic limitation on water uptake and release at temperatures above 220 K. The fast diffusion of water suggests that heterogeneous icemore » nucleation on a glassy core is very unlikely in these systems. At temperatures below 220 K, model simulations of SOA particles suggest that heterogeneous ice nucleation may occur in the immersion mode on glassy cores which remain embedded in a liquid shell when experiencing fast updraft velocities. The particles absorb significant quantities of water during these updrafts which plasticize their outer layers such that these layers equilibrate readily with the gas phase humidity before the homogeneous ice nucleation threshold is reached. Glass formation is thus unlikely to restrict homogeneous ice nucleation. Only under most extreme conditions near the very high tropical tropopause may the homogeneous ice nucleation rate coefficient be reduced as a consequence of slow condensed-phase water diffusion. Since the differences between the behavior limited or non limited by diffusion are small even at the very high tropical tropopause, condensed-phase water diffusivity is unlikely to have significant consequences on the direct climatic effects of SOA particles under tropospheric conditions.« less

Sub-Fickean Diffusion in a One-Dimensional Plasma Ring

NASA Astrophysics Data System (ADS)

Theisen, W. L.

2013-12-01

A one-dimensional dusty plasma ring is formed in a strongly-coupled complex plasma. The dust particles in the ring can be characterized as a one-dimensional system where the particles cannot pass each other. The particles perform random walks due to thermal motions. This single-file self diffusion is characterized by the mean-squared displacement (msd) of the individual particles which increases with time t. Diffusive processes that follow Ficks law predict that the msd increases as t, however, single-file diffusion is sub-Fickean meaning that the msd is predicted to increase as t^(1/2). Particle position data from the dusty plasma ring is analyzed to determine the scaling of the msd with time. Results are compared with predictions of single-file diffusion theory.

Deterministic diffusion in flower-shaped billiards.

PubMed

Harayama, Takahisa; Klages, Rainer; Gaspard, Pierre

2002-08-01

We propose a flower-shaped billiard in order to study the irregular parameter dependence of chaotic normal diffusion. Our model is an open system consisting of periodically distributed obstacles in the shape of a flower, and it is strongly chaotic for almost all parameter values. We compute the parameter dependent diffusion coefficient of this model from computer simulations and analyze its functional form using different schemes, all generalizing the simple random walk approximation of Machta and Zwanzig. The improved methods we use are based either on heuristic higher-order corrections to the simple random walk model, on lattice gas simulation methods, or they start from a suitable Green-Kubo formula for diffusion. We show that dynamical correlations, or memory effects, are of crucial importance in reproducing the precise parameter dependence of the diffusion coefficent.

Diffusion in Coulomb crystals.

PubMed

Hughto, J; Schneider, A S; Horowitz, C J; Berry, D K

2011-07-01

Diffusion in Coulomb crystals can be important for the structure of neutron star crusts. We determine diffusion constants D from molecular dynamics simulations. We find that D for Coulomb crystals with relatively soft-core 1/r interactions may be larger than D for Lennard-Jones or other solids with harder-core interactions. Diffusion, for simulations of nearly perfect body-centered-cubic lattices, involves the exchange of ions in ringlike configurations. Here ions "hop" in unison without the formation of long lived vacancies. Diffusion, for imperfect crystals, involves the motion of defects. Finally, we find that diffusion, for an amorphous system rapidly quenched from Coulomb parameter Γ=175 to Coulomb parameters up to Γ=1750, is fast enough that the system starts to crystalize during long simulation runs. These results strongly suggest that Coulomb solids in cold white dwarf stars, and the crust of neutron stars, will be crystalline and not amorphous.

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

Weiser, Philip; Qin, Ying; Yin, Weikai

Uniaxial stress experiments performed for the 3306 cm -1 vibrational line assigned to the interstitial-hydrogen, shallow-donor center in In 2O 3 reveal its symmetry and transition- moment direction. The defect alignment that can be produced by a [001] stress applied at 165 K is due to a process that is also a hydrogen- diffusion jump, providing a microscopic determination of the diffusion constant for H in In 2O 3 and its mechanism. Lastly, our experimental results strongly complement theoretical predictions for the structure and diffusion of the interstitial hydrogen donor center in In 2O 3.

Influence of coupling on thermal forces and dynamic friction in plasmas with multiple ion species

NASA Astrophysics Data System (ADS)

Kagan, Grigory; Baalrud, Scott D.; Daligault, Jérôme

2017-07-01

The recently proposed effective potential theory [Phys. Rev. Lett. 110, 235001 (2013)] is used to investigate the influence of coupling on inter-ion-species diffusion and momentum exchange in multi-component plasmas. Thermo-diffusion and the thermal force are found to diminish rapidly as strong coupling onsets. For the same coupling parameters, the dynamic friction coefficient is found to tend to unity. These results provide an impetus for addressing the role of coupling on diffusive processes in inertial confinement fusion experiments.

Influence of coupling on thermal forces and dynamic friction in plasmas with multiple ion species

DOE PAGES

Kagan, Grigory; Baalrud, Scott D.; Daligault, Jérôme

2017-07-05

The recently proposed effective potential theory [Phys. Rev. Lett. 110, 235001 (2013)] is used to investigate the influence of coupling on inter-ion-species diffusion and momentum exchange in multi-component plasmas. Thermo-diffusion and the thermal force are found to diminish rapidly as strong coupling onsets. We found that for the same coupling parameters, the dynamic friction coefficient there tends to be unity. Our results provide an impetus for addressing the role of coupling on diffusive processes in inertial confinement fusion experiments.

Vibronic Origin for the Diffuse Band Spectrum

NASA Astrophysics Data System (ADS)

Duley, W. W.

1983-09-01

The two arguments outlined by Nuth and Donn (1983) against an interpretation of the diffuse band spectrum between 677 and 536 nm as vibronic systems associated with forbidden origins at 14321, 15153, and 15343 cm-1 (Duley, 1982) are controverted. It is concluded that the vibronic analysis presented by Duley, 1983 for the diffuse band spectrum is in keeping with current spectroscopic practice. The identification of a forbidden origin for 19 of these bands at 14321 cm-1 strongly suggests the involvement of Cr3+ ions in MgO solids in the production of these features.

Numerical investigations of hybrid rocket engines

NASA Astrophysics Data System (ADS)

Betelin, V. B.; Kushnirenko, A. G.; Smirnov, N. N.; Nikitin, V. F.; Tyurenkova, V. V.; Stamov, L. I.

2018-03-01

Paper presents the results of numerical studies of hybrid rocket engines operating cycle including unsteady-state transition stage. A mathematical model is developed accounting for the peculiarities of diffusion combustion of fuel in the flow of oxidant, which is composed of oxygen-nitrogen mixture. Three dimensional unsteady-state simulations of chemically reacting gas mixture above thermochemically destructing surface are performed. The results show that the diffusion combustion brings to strongly non-uniform fuel mass regression rate in the flow direction. Diffusive deceleration of chemical reaction brings to the decrease of fuel regression rate in the longitudinal direction.

Proteins as micro viscosimeters: Brownian motion revisited.

PubMed

Lavalette, Daniel; Hink, Mark A; Tourbez, Martine; Tétreau, Catherine; Visser, Antonie J

2006-08-01

Translational and rotational diffusion coefficients of proteins in solution strongly deviate from the Stokes-Einstein laws when the ambient viscosity is induced by macromolecular co-solutes rather than by a solvent of negligible size as was assumed by A. Einstein one century ago for deriving the laws of Brownian motion and diffusion. Rotational and translational motions experience different micro viscosities and both become a function of the size ratio of protein and macromolecular co-solute. Possible consequences upon fluorescence spectroscopy observations of diffusing proteins within living cells are discussed.

Influence of coupling on thermal forces and dynamic friction in plasmas with multiple ion species

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

Kagan, Grigory; Baalrud, Scott D.; Daligault, Jérôme

The recently proposed effective potential theory [Phys. Rev. Lett. 110, 235001 (2013)] is used to investigate the influence of coupling on inter-ion-species diffusion and momentum exchange in multi-component plasmas. Thermo-diffusion and the thermal force are found to diminish rapidly as strong coupling onsets. We found that for the same coupling parameters, the dynamic friction coefficient there tends to be unity. Our results provide an impetus for addressing the role of coupling on diffusive processes in inertial confinement fusion experiments.

Modeling Vertical Structure and Heat Transport within the Oceans of Ice-covered Worlds (Invited)

NASA Astrophysics Data System (ADS)

Goodman, J. C.

2010-12-01

Indirect observational evidence provides a strong case for liquid oceans beneath the icy crust of Europa and several other frozen moons in the outer solar system. However, little is known about the fluid circulation within these exotic oceans. As a first step toward understanding circulations driven by buoyancy (rather than mechanical forcing from tides), one must understand the typical vertical structure of temperature, salinity, and thus density within the ocean. Following a common approach from terrestrial oceanography, I have built a "single column convection model" for icy world oceans, which describes the density structure of the ocean as a function of depth only: horizontal variations are ignored. On Earth, this approach is of limited utility, because of the strong influence of horizontal wind-driven currents and sea-surface temperature gradients set in concert with the overlying atmosphere. Neither of these confounding issues is present in an icy world's ocean. In the model, mixing of fluid properties via overturning convection is modeled as a strong diffusive process which only acts when the ocean is vertically unstable. "Double diffusive" processes (salt fingering and diffusive layering) are included: these are mixing processes resulting from the unequal molecular diffusivities of heat and salt. Other important processes, such as heating on adiabatic compression, and freshwater fluxes from melting overlying ice, are also included. As a simple test case, I considered an ocean of Europa-like depth (~100 km) and gravity, heated from the seafloor. To simplify matters, I specified an equation of state appropriate to terrestrial seawater, and a simple isothermal ocean as an initial condition. As expected, convection gradually penetrates upward, warming the ocean to an adiabatic, unstratified equilibrium density profile on a timescale of 50 kyr if 4.5 TW of heat are emitted by the silicate interior; the same result is achieved in proportionally more/less time for weaker/stronger internal heating. Unlike Earth's oceans, I predict that since icy worlds' oceans are heated from below, they will generally be unstratified, with constant potential density from top to bottom. There will be no pycnocline as on Earth, so global ocean currents supported by large-scale density gradients seem unlikely. However, icy world oceans may be "weird" in ways which are unheard-of in terrestrial oceanography The density of sulfate brine has a very different equation of state than chloride brines: does this affect the vertical structure? If the ocean water is very pure, cold water can be less dense than warm. Can this lead to periodic catastrophic overturning, as proposed by other authors? These and other questions are currently being investigated using the single-column convection model as a primary tool.

The Role of Dopant Ions on Charge Injection and Transport in Electrochemically Doped Quantum Dot Films.

PubMed

Gudjonsdottir, Solrun; van der Stam, Ward; Kirkwood, Nicholas; Evers, Wiel H; Houtepen, Arjan J

2018-05-16

Control over the charge density is very important for implementation of colloidal semiconductor nanocrystals into various optoelectronic applications. A promising approach to dope nanocrystal assemblies is charge injection by electrochemistry, in which the charge compensating electrolyte ions can be regarded as external dopant ions. To gain insight into the doping mechanism and the role of the external dopant ions, we investigate charge injection in ZnO nanocrystal assemblies for a large series of charge compensating electrolyte ions with spectroelectrochemical and electrochemical transistor measurements. We show that charge injection is limited by the diffusion of cations in the nanocrystal films as their diffusion coefficient are found to be ∼7 orders of magnitude lower than those of electrons. We further show that the rate of charge injection depends strongly on the cation size and cation concentration. Strikingly, the onset of electron injection varies up to 0.4 V, depending on the size of the electrolyte cation. For the small ions Li + and Na + the onset is at significantly less negative potentials. For larger ions (K + , quaternary ammonium ions) the onset is always at the same, more negative potential, suggesting that intercalation may take place for Li + and Na + . Finally, we show that the nature of the charge compensating cation does not affect the source-drain electronic conductivity and mobility, indicating that shallow donor levels from intercalating ions fully hybridize with the quantum confined energy levels and that the reorganization energy due to intercalating ions does not strongly affect electron transport in these nanocrystal assemblies.

The Role of Dopant Ions on Charge Injection and Transport in Electrochemically Doped Quantum Dot Films

PubMed Central

2018-01-01

Control over the charge density is very important for implementation of colloidal semiconductor nanocrystals into various optoelectronic applications. A promising approach to dope nanocrystal assemblies is charge injection by electrochemistry, in which the charge compensating electrolyte ions can be regarded as external dopant ions. To gain insight into the doping mechanism and the role of the external dopant ions, we investigate charge injection in ZnO nanocrystal assemblies for a large series of charge compensating electrolyte ions with spectroelectrochemical and electrochemical transistor measurements. We show that charge injection is limited by the diffusion of cations in the nanocrystal films as their diffusion coefficient are found to be ∼7 orders of magnitude lower than those of electrons. We further show that the rate of charge injection depends strongly on the cation size and cation concentration. Strikingly, the onset of electron injection varies up to 0.4 V, depending on the size of the electrolyte cation. For the small ions Li+ and Na+ the onset is at significantly less negative potentials. For larger ions (K+, quaternary ammonium ions) the onset is always at the same, more negative potential, suggesting that intercalation may take place for Li+ and Na+. Finally, we show that the nature of the charge compensating cation does not affect the source-drain electronic conductivity and mobility, indicating that shallow donor levels from intercalating ions fully hybridize with the quantum confined energy levels and that the reorganization energy due to intercalating ions does not strongly affect electron transport in these nanocrystal assemblies. PMID:29718666

Heavy quark diffusion in strong magnetic fields at weak coupling and implications for elliptic flow

DOE PAGES

Fukushima, Kenji; Hattori, Koichi; Yee, Ho -Ung; ...

2016-04-20

In this paper, we compute the momentum diffusion coefficients of heavy quarks, κ ∥ and κ ⊥, in a strong magnetic field B along the directions parallel and perpendicular to B, respectively, at the leading order in QCD coupling constant α s. We consider a regime relevant for the relativistic heavy ion collisions, α seB << T 2 << eB, so that thermal excitations of light quarks are restricted to the lowest Landau level (LLL) states. In the vanishing light-quark mass limit, we find κ LO ⊥ ∝ α 2 sTeB in the leading order that arises from screened Coulombmore » scatterings with (1+1)-dimensional LLL quarks, while κ ∥ gets no contribution from the scatterings with LLL quarks due to kinematic restrictions. We show that the first nonzero leading order contributions to κ LO ∥ come from the two separate effects: 1) the screened Coulomb scatterings with thermal gluons, and 2) a finite light-quark mass m q. The former leads to κ LO,gluon ∥ ∝ α 2 sT 3 and the latter to κ LO,massive ∥ ∝ α s(α seB) 1/2m 2 q. Based on our results, we propose a new scenario for the large value of heavy-quark elliptic flow observed in RHIC and LHC. Namely, when κ ⊥ >> κ ∥, an anisotropy in drag forces gives rise to a sizable amount of the heavy-quark elliptic flow even if heavy quarks do not fully belong to an ellipsoidally expanding background fluid.« less

Heavy quark diffusion in strong magnetic fields at weak coupling and implications for elliptic flow

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

Fukushima, Kenji; Hattori, Koichi; Yee, Ho -Ung

In this paper, we compute the momentum diffusion coefficients of heavy quarks, κ ∥ and κ ⊥, in a strong magnetic field B along the directions parallel and perpendicular to B, respectively, at the leading order in QCD coupling constant α s. We consider a regime relevant for the relativistic heavy ion collisions, α seB << T 2 << eB, so that thermal excitations of light quarks are restricted to the lowest Landau level (LLL) states. In the vanishing light-quark mass limit, we find κ LO ⊥ ∝ α 2 sTeB in the leading order that arises from screened Coulombmore » scatterings with (1+1)-dimensional LLL quarks, while κ ∥ gets no contribution from the scatterings with LLL quarks due to kinematic restrictions. We show that the first nonzero leading order contributions to κ LO ∥ come from the two separate effects: 1) the screened Coulomb scatterings with thermal gluons, and 2) a finite light-quark mass m q. The former leads to κ LO,gluon ∥ ∝ α 2 sT 3 and the latter to κ LO,massive ∥ ∝ α s(α seB) 1/2m 2 q. Based on our results, we propose a new scenario for the large value of heavy-quark elliptic flow observed in RHIC and LHC. Namely, when κ ⊥ >> κ ∥, an anisotropy in drag forces gives rise to a sizable amount of the heavy-quark elliptic flow even if heavy quarks do not fully belong to an ellipsoidally expanding background fluid.« less

Carrier diffusion as a measure of carrier/exciton transfer rate in InAs/InGaAsP/InP hybrid quantum dot-quantum well structures emitting at telecom spectral range

NASA Astrophysics Data System (ADS)

Rudno-Rudziński, W.; Biegańska, D.; Misiewicz, J.; Lelarge, F.; Rousseau, B.; Sek, G.

2018-01-01

We investigate the diffusion of photo-generated carriers (excitons) in hybrid two dimensional-zero dimensional tunnel injection structures, based on strongly elongated InAs quantum dots (called quantum dashes, QDashes) of various heights, designed for emission at around 1.5 μm, separated by a 3.5 nm wide barrier from an 8 nm wide In0.64Ga0.36As0.78P0.22 quantum well (QW). By measuring the spectrally filtered real space images of the photoluminescence patterns with high resolution, we probe the spatial extent of the emission from QDashes. Deconvolution with the exciting light spot shape allows us to extract the carrier/exciton diffusion lengths. For the non-resonant excitation case, the diffusion length depends strongly on excitation power, pointing at carrier interactions and phonons as its main driving mechanisms. For the case of excitation resonant with absorption in the adjacent QW, the diffusion length does not depend on excitation power for low excitation levels since the generated carriers do not have sufficient excess kinetic energy. It is also found that the diffusion length depends on the quantum-mechanical coupling strength between QW and QDashes, controlled by changing the dash size. It influences the energy difference between the QDash ground state of the system and the quantum well levels, which affects the tunneling rates. When that QW-QDash level separation decreases, the probability of capturing excitons generated in the QW by QDashes increases, which is reflected by the decreased diffusion length from approx. 5 down to 3 μm.

Some notes on hydrogen-related point defects and their role in the isotope exchange and electrical conductivity in olivine

NASA Astrophysics Data System (ADS)

Karato, Shun-ichiro

2015-11-01

Nominally anhydrous minerals such as olivine dissolve hydrogen in a variety of forms including free (or interstitial) proton (Hrad) and two protons trapped at the M-site ((2 H)M×). The strength of chemical bonding between protons and the surrounding atoms are different among different species, and consequently protons belonging to different species likely have different mobility (diffusion coefficients). I discuss the role of diffusion of protons in different species in the isotope exchange and hydrogen-assisted electrical conductivity adding a few notes to the previous work by Karato (2013) including a new way to test the model. I conclude that in the case of isotope exchange, the interaction among these species is strong because diffusion is heterogeneous, whereas there is no strong interaction among different species in electrical conduction where diffusion is homogeneous (in an infinite crystal). Consequently, the slowest diffusing species controls the rate of isotope exchange, whereas the fastest diffusing species controls electrical conductivity leading to a different temperature dependence of activation energy and anisotropy. This model explains the differences in the activation energy and anisotropy between isotope diffusion and electrical conductivity, and predicts that the mechanism of electrical conductivity changes with temperature providing an explanation for most of the discrepancies among different experimental observations at different temperatures except for those by Poe et al. (2010) who reported anomalously high water content dependence and highly anisotropic activation energy. When the results obtained at high temperatures are used, most of the geophysically observed high and highly anisotropic electrical conductivity in the asthenosphere can be explained without invoking partial melting.

MAGNETIC FIELD LINE RANDOM WALK FOR DISTURBED FLUX SURFACES: TRAPPING EFFECTS AND MULTIPLE ROUTES TO BOHM DIFFUSION

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

Ghilea, M. C.; Ruffolo, D.; Sonsrettee, W.

2011-11-01

The magnetic field line random walk (FLRW) is important for the transport of energetic particles in many astrophysical situations. While all authors agree on the quasilinear diffusion of field lines for fluctuations that mainly vary parallel to a large-scale field, for the opposite case of fluctuations that mainly vary in the perpendicular directions, there has been an apparent conflict between concepts of Bohm diffusion and percolation/trapping effects. Here computer simulation and non-perturbative analytic techniques are used to re-examine the FLRW in magnetic turbulence with slab and two-dimensional (2D) components, in which 2D flux surfaces are disturbed by the slab fluctuations.more » Previous non-perturbative theories for D{sub perpendicular}, based on Corrsin's hypothesis, have identified a slab contribution with quasilinear behavior and a 2D contribution due to Bohm diffusion with diffusive decorrelation (DD), combined in a quadratic formula. Here we present analytic theories for other routes to Bohm diffusion, with random ballistic decorrelation (RBD) either due to the 2D component itself (for a weak slab contribution) or the total fluctuation field (for a strong slab contribution), combined in a direct sum with the slab contribution. Computer simulations confirm the applicability of RBD routes for weak or strong slab contributions, while the DD route applies for a moderate slab contribution. For a very low slab contribution, interesting trapping effects are found, including a depressed diffusion coefficient and subdiffusive behavior. Thus quasilinear, Bohm, and trapping behaviors are all found in the same system, together with an overall viewpoint to explain these behaviors.« less

Interferometric measurements of a dendritic growth front solutal diffusion layer

NASA Technical Reports Server (NTRS)

Hopkins, John A.; Mccay, T. D.; Mccay, Mary H.

1991-01-01

An experimental study was undertaken to measure solutal distributions in the diffusion layer produced during the vertical directional solidification (VDS) of an ammonium chloride - water (NH4Cl-H2O) solution. Interferometry was used to obtain concentration measurements in the 1-2 millimeter region defining the diffusion layer. These measurements were fitted to an exponential form to extract the characteristic diffusion parameter for various times after the start of solidification. The diffusion parameters are within the limits predicted by steady state theory and suggest that the effective solutal diffusivity is increasing as solidification progresses.

Determination of the diffusion coefficient and phase-transfer rate parameter in LaNi{sub 5} and MmNi{sub 3.6}Co{sub 0.8}Mn{sub 0.4}Al{sub 0.3} using microelectrodes

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

Lundqvist, A.; Lindbergh, G.

1998-11-01

A potential-step method for determining the diffusion coefficient and phase-transfer parameter in metal hydrides by using microelectrodes was investigated. It was shown that a large potential step is not enough to ensure a completely diffusion-limited mass transfer if a surface-phase transfer reaction takes place at a finite rate. It was shown, using a kinetic expression for the surface phase-transfer reaction, that the slope of the logarithm of the current vs. time curve will be constant both in the case of the mass-transfer limited by diffusion or by diffusion and a surface-phase transfer. The diffusion coefficient and phase-transfer rate parameter weremore » accurately determined for MmNi{sub 3.6}Co{sub 0.8}Mn{sub 0.4}Al{sub 0.3} using a fit to the whole transient. The diffusion coefficient was found to be (1.3 {+-} 0.3) {times} 10{sup {minus}13} m{sup 2}/s. The fit was good and showed that a pure diffusion model was not enough to explain the observed transient. The diffusion coefficient and phase-transfer rate parameter were also estimated for pure LaNi{sub 5}. A fit of the whole curve showed that neither a pure diffusion model nor a model including phase transfer could explain the whole transient.« less

On the use of flux limiters in the discrete ordinates method for 3D radiation calculations in absorbing and scattering media

NASA Astrophysics Data System (ADS)

Godoy, William F.; DesJardin, Paul E.

2010-05-01

The application of flux limiters to the discrete ordinates method (DOM), SN, for radiative transfer calculations is discussed and analyzed for 3D enclosures for cases in which the intensities are strongly coupled to each other such as: radiative equilibrium and scattering media. A Newton-Krylov iterative method (GMRES) solves the final systems of linear equations along with a domain decomposition strategy for parallel computation using message passing libraries in a distributed memory system. Ray effects due to angular discretization and errors due to domain decomposition are minimized until small variations are introduced by these effects in order to focus on the influence of flux limiters on errors due to spatial discretization, known as numerical diffusion, smearing or false scattering. Results are presented for the DOM-integrated quantities such as heat flux, irradiation and emission. A variety of flux limiters are compared to "exact" solutions available in the literature, such as the integral solution of the RTE for pure absorbing-emitting media and isotropic scattering cases and a Monte Carlo solution for a forward scattering case. Additionally, a non-homogeneous 3D enclosure is included to extend the use of flux limiters to more practical cases. The overall balance of convergence, accuracy, speed and stability using flux limiters is shown to be superior compared to step schemes for any test case.

Diffusion profiling of tumor volumes using a histogram approach can predict proliferation and further microarchitectural features in medulloblastoma.

PubMed

Schob, Stefan; Beeskow, Anne; Dieckow, Julia; Meyer, Hans-Jonas; Krause, Matthias; Frydrychowicz, Clara; Hirsch, Franz-Wolfgang; Surov, Alexey

2018-05-31

Medulloblastomas are the most common central nervous system tumors in childhood. Treatment and prognosis strongly depend on histology and transcriptomic profiling. However, the proliferative potential also has prognostical value. Our study aimed to investigate correlations between histogram profiling of diffusion-weighted images and further microarchitectural features. Seven patients (age median 14.6 years, minimum 2 years, maximum 20 years; 5 male, 2 female) were included in this retrospective study. Using a Matlab-based analysis tool, histogram analysis of whole apparent diffusion coefficient (ADC) volumes was performed. ADC entropy revealed a strong inverse correlation with the expression of the proliferation marker Ki67 (r = - 0.962, p = 0.009) and with total nuclear area (r = - 0.888, p = 0.044). Furthermore, ADC percentiles, most of all ADCp90, showed significant correlations with Ki67 expression (r = 0.902, p = 0.036). Diffusion histogram profiling of medulloblastomas provides valuable in vivo information which potentially can be used for risk stratification and prognostication. First of all, entropy revealed to be the most promising imaging biomarker. However, further studies are warranted.

Mapping the exciton diffusion in semiconductor nanocrystal solids.

PubMed

Kholmicheva, Natalia; Moroz, Pavel; Bastola, Ebin; Razgoniaeva, Natalia; Bocanegra, Jesus; Shaughnessy, Martin; Porach, Zack; Khon, Dmitriy; Zamkov, Mikhail

2015-03-24

Colloidal nanocrystal solids represent an emerging class of functional materials that hold strong promise for device applications. The macroscopic properties of these disordered assemblies are determined by complex trajectories of exciton diffusion processes, which are still poorly understood. Owing to the lack of theoretical insight, experimental strategies for probing the exciton dynamics in quantum dot solids are in great demand. Here, we develop an experimental technique for mapping the motion of excitons in semiconductor nanocrystal films with a subdiffraction spatial sensitivity and a picosecond temporal resolution. This was accomplished by doping PbS nanocrystal solids with metal nanoparticles that force the exciton dissociation at known distances from their birth. The optical signature of the exciton motion was then inferred from the changes in the emission lifetime, which was mapped to the location of exciton quenching sites. By correlating the metal-metal interparticle distance in the film with corresponding changes in the emission lifetime, we could obtain important transport characteristics, including the exciton diffusion length, the number of predissociation hops, the rate of interparticle energy transfer, and the exciton diffusivity. The benefits of this approach to device applications were demonstrated through the use of two representative film morphologies featuring weak and strong interparticle coupling.

Classification Order of Surface-Confined Intermixing at Epitaxial Interface

NASA Astrophysics Data System (ADS)

Michailov, M.

The self-organization phenomena at epitaxial interface hold special attention in contemporary material science. Being relevant to the fundamental physical problem of competing, long-range and short-range atomic interactions in systems with reduced dimensionality, these phenomena have found exacting academic interest. They are also of great technological importance for their ability to bring spontaneous formation of regular nanoscale surface patterns and superlattices with exotic properties. The basic phenomenon involved in this process is surface diffusion. That is the motivation behind the present study which deals with important details of diffusion scenarios that control the fine atomic structure of epitaxial interface. Consisting surface imperfections (terraces, steps, kinks, and vacancies), the interface offers variety of barriers for surface diffusion. Therefore, the adatoms and clusters need a certain critical energy to overcome the corresponding diffusion barriers. In the most general case the critical energies can be attained by variation of the system temperature. Hence, their values define temperature limits of system energy gaps associated with different diffusion scenarios. This systematization imply classification order of surface alloying: blocked, incomplete, and complete. On that background, two diffusion problems, related to the atomic-scale surface morphology, will be discussed. The first problem deals with diffusion of atomic clusters on atomically smooth interface. On flat domains, far from terraces and steps, we analyzed the impact of size, shape, and cluster/substrate lattice misfit on the diffusion behavior of atomic clusters (islands). We found that the lattice constant of small clusters depends on the number N of building atoms at 1 < N ≤ 10. In heteroepitaxy, this effect of variable lattice constant originates from the enhanced charge transfer and the strong influence of the surface potential on cluster atomic arrangement. At constant temperature, the variation of the lattice constant leads to variable misfit which affects the island migration. The cluster/substrate commensurability influences the oscillation behavior of the diffusion coefficient caused by variation in the cluster shape. We discuss the results in a physical model that implies cluster diffusion with size-dependent cluster/substrate misfit. The second problem is devoted to diffusion phenomena in the vicinity of atomic terraces on stepped or vicinal surfaces. Here, we develop a computational model that refines important details of diffusion behavior of adatoms accounting for the energy barriers at specific atomic sites (smooth domains, terraces, and steps) located on the crystal surface. The dynamic competition between energy gained by mixing and substrate strain energy results in diffusion scenario where adatoms form alloyed islands and alloyed stripes in the vicinity of terrace edges. Being in agreement with recent experimental findings, the observed effect of stripe and island alloy formation opens up a way regular surface patterns to be configured at different atomic levels on the crystal surface. The complete surface alloying of the entire interface layer is also briefly discussed with critical analysis and classification of experimental findings and simulation data.

Continuum modelling of silicon diffusion in indium gallium arsenide

NASA Astrophysics Data System (ADS)

Aldridge, Henry Lee, Jr.

A possible method to overcome the physical limitations experienced by continued transistor scaling and continue improvements in performance and power consumption is integration of III-V semiconductors as alternative channel materials for logic devices. Indium Gallium Arsenide (InGaAs) is such a material from the III-V semiconductor family, which exhibit superior electron mobilities and injection velocities than that of silicon. In order for InGaAs integration to be realized, contact resistances must be minimized through maximizing activation of dopants in this material. Additionally, redistribution of dopants during processing must be clearly understood and ultimately controlled at the nanometer-scale. In this work, the activation and diffusion behavior of silicon, a prominent n-type dopant in InGaAs, has been characterized and subsequently modelled using the Florida Object Oriented Process and Device Simulator (FLOOPS). In contrast to previous reports, silicon exhibits non-negligible diffusion in InGaAs, even for smaller thermal budget rapid thermal anneals (RTAs). Its diffusion is heavily concentration-dependent, with broadening "shoulder-like" profiles when doping levels exceed 1-3x1019cm -3, for both ion-implanted and Molecular Beam Epitaxy (MBE)-grown cases. Likewise a max net-activation value of ˜1.7x1019cm -3 is consistently reached with enough thermal processing, regardless of doping method. In line with experimental results and several ab-initio calculation results, rapid concentration-dependent diffusion of Si in InGaAs and the upper limits of its activation is believed to be governed by cation vacancies that serve as compensating defects in heavily n-type regions of InGaAs. These results are ultimately in line with an amphoteric defect model, where the activation limits of dopants are an intrinsic limitation of the material, rather than governed by individual dopant species or their methods of incorporation. As a result a Fermi level dependent point defect diffusion model and activation limit model were subsequently developed in FLOOPS with outputs in good agreement with experimental results.

Some Problems in Using Diffusion Models for New Products.

ERIC Educational Resources Information Center

Bernhardt, Irwin; Mackenzie, Kenneth D.

This paper analyzes some of the problems of using diffusion models to formulate marketing strategies for new products. Though future work in this area appears justified, many unresolved problems limit its application. There is no theory for adoption and diffusion processes; such a theory is outlined in this paper. The present models are too…