Thermodynamic stability of nanosized multicomponent bubbles/droplets: The square gradient theory and the capillary approach

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

Wilhelmsen, Øivind, E-mail: oivind.wilhelmsen@ntnu.no; Bedeaux, Dick; Kjelstrup, Signe

Formation of nanosized droplets/bubbles from a metastable bulk phase is connected to many unresolved scientific questions. We analyze the properties and stability of multicomponent droplets and bubbles in the canonical ensemble, and compare with single-component systems. The bubbles/droplets are described on the mesoscopic level by square gradient theory. Furthermore, we compare the results to a capillary model which gives a macroscopic description. Remarkably, the solutions of the square gradient model, representing bubbles and droplets, are accurately reproduced by the capillary model except in the vicinity of the spinodals. The solutions of the square gradient model form closed loops, which showsmore » the inherent symmetry and connected nature of bubbles and droplets. A thermodynamic stability analysis is carried out, where the second variation of the square gradient description is compared to the eigenvalues of the Hessian matrix in the capillary description. The analysis shows that it is impossible to stabilize arbitrarily small bubbles or droplets in closed systems and gives insight into metastable regions close to the minimum bubble/droplet radii. Despite the large difference in complexity, the square gradient and the capillary model predict the same finite threshold sizes and very similar stability limits for bubbles and droplets, both for single-component and two-component systems.« less

Investigation of the charging characteristics of micrometer sized droplets based on parallel plate capacitor model.

PubMed

Zhang, Yanzhen; Liu, Yonghong; Wang, Xiaolong; Shen, Yang; Ji, Renjie; Cai, Baoping

2013-02-05

The charging characteristics of micrometer sized aqueous droplets have attracted more and more attentions due to the development of the microfluidics technology since the electrophoretic motion of a charged droplet can be used as the droplet actuation method. This work proposed a novel method of investigating the charging characteristics of micrometer sized aqueous droplets based on parallel plate capacitor model. With this method, the effects of the electric field strength, electrolyte concentration, and ion species on the charging characteristics of the aqueous droplets was investigated. Experimental results showed that the charging characteristics of micrometer sized droplets can be investigated by this method.

Droplet size and velocity distributions for spray modelling

NASA Astrophysics Data System (ADS)

Jones, D. P.; Watkins, A. P.

2012-01-01

Methods for constructing droplet size distributions and droplet velocity profiles are examined as a basis for the Eulerian spray model proposed in Beck and Watkins (2002,2003) [5,6]. Within the spray model, both distributions must be calculated at every control volume at every time-step where the spray is present and valid distributions must be guaranteed. Results show that the Maximum Entropy formalism combined with the Gamma distribution satisfy these conditions for the droplet size distributions. Approximating the droplet velocity profile is shown to be considerably more difficult due to the fact that it does not have compact support. An exponential model with a constrained exponent offers plausible profiles.

Alcohol cold starting - A theoretical study

NASA Technical Reports Server (NTRS)

Browning, L. H.

1983-01-01

Two theoretical computer models have been developed to study cold starting problems with alcohol fuels. The first model, a droplet fall-out and sling-out model, shows that droplets must be smaller than 50 microns to enter the cylinder under cranking conditions without being slung-out in the intake manifold. The second model, which examines the fate of droplets during the compression process, shows that the heat of compression can be used to vaporize small droplets (less than 50 microns) producing flammable mixtures below freezing ambient temperatures. While droplet size has the greater effect on startability, a very high compression ratio can also aid cold starting.

Modeling Droplet Heat and Mass Transfer during Spray Bar Pressure Control of the Multipurpose Hydrogen Test Bed (MHTB) Tank in Normal Gravity

NASA Technical Reports Server (NTRS)

Kartuzova, O.; Kassemi, M.

2016-01-01

A CFD model for simulating pressure control in cryogenic storage tanks through the injection of a subcooled liquid into the ullage is presented and applied to the 1g MHTB spray bar cooling experiments. An Eulerian-Lagrangian approach is utilized to track the spray droplets and capture the interaction between the discrete droplets and the continuous ullage phase. The spray model is coupled with the VOF model by performing particle tracking in the ullage, removing particles from the ullage when they reach the interface, and then adding their contributions to the liquid. A new model for calculating the droplet-ullage heat and mass transfer is developed. In this model, a droplet is allowed to warm up to the saturation temperature corresponding to the ullage vapor pressure, after which it evaporates while remaining at the saturation temperature. The droplet model is validated against the results of the MHTB spray-bar cooling experiments with 50% and 90% tank fill ratios. The predictions of the present T-sat based model are compared with those of a previously developed kinetic-based droplet mass transfer model. The predictions of the two models regarding the evolving tank pressure and temperature distributions, as well as the droplets' trajectories and temperatures, are examined and compared in detail. Finally, the ullage pressure and local vapor and liquid temperature evolutions are validated against the corresponding data provided by the MHTB spray bar mixing experiment.

Modeling of Rapid Solidification with Undercooling Effect During Droplet Flattening on a Substrate in Coating Formation

NASA Astrophysics Data System (ADS)

Shukla, Rajesh Kumar; Patel, Virendra; Kumar, Arvind

2018-02-01

The coating deposit on the substrate in thermal spray coating process develops by solidification of individual molten particle which impacts, flattens and solidifies on the surface of the substrate. Droplet flattening and solidification typically involves rapid cooling. In this paper, a model for non-equilibrium rapid solidification of a molten droplet spreading onto a substrate is presented. Transient flow during droplet impact and its subsequent spreading is considered using the volume of fluid surface tracking method which was fully coupled with the rapid solidification model. The rapid solidification model includes undercooling, nucleation, interface tracking, non-equilibrium solidification kinetics and combined heat transfer and fluid flow as required to treat a non-stagnant splat formed from droplet flattening. The model is validated with the literature results on stagnant splats. Subsequently, using the model the characteristics of the rapidly solidifying interface for non-stagnant splat, such as interface velocity and interface temperature, are described and the effect of undercooling and interfacial heat transfer coefficient are highlighted. In contrast to the stagnant splat, the non-stagnant splat considered in this study displays interesting features in the rapidly solidifying interface. These are attributed to droplet thinning and droplet recoiling that occur during the droplet spreading process.

Derivation and calibration of a gas metal arc welding (GMAW) dynamic droplet model

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

Reutzel, E.W.; Einerson, C.J.; Johnson, J.A.

1996-12-31

A rudimentary, existing dynamic model for droplet growth and detachment in gas metal arc welding (GMAW) was improved and calibrated to match experimental data. The model simulates droplets growing at the end of an imaginary spring. Mass is added to the drop as the electrode melts, the droplet grows, and the spring is displaced. Detachment occurs when one of two criteria is met, and the amount of mass that is detached is a function of the droplet velocity at the time of detachment. Improvements to the model include the addition of a second criterion for drop detachment, a more sophisticatedmore » model of the power supply and secondary electric circuit, and the incorporation of a variable electrode resistance. Relevant physical parameters in the model were adjusted during model calibration. The average current, droplet frequency, and parameter-space location of globular-to-streaming mode transition were used as criteria for tuning the model. The average current predicted by the calibrated model matched the experimental average current to within 5% over a wide range of operating conditions.« less

Prediction of the structure of fuel sprays in gas turbine combustors

NASA Technical Reports Server (NTRS)

Shuen, J. S.

1985-01-01

The structure of fuel sprays in a combustion chamber is theoretically investigated using computer models of current interest. Three representative spray models are considered: (1) a locally homogeneous flow (LHF) model, which assumes infinitely fast interphase transport rates; (2) a deterministic separated flow (DSF) model, which considers finite rates of interphase transport but ignores effects of droplet/turbulence interactions; and (3) a stochastic separated flow (SSF) model, which considers droplet/turbulence interactions using random sampling for turbulence properties in conjunction with random-walk computations for droplet motion and transport. Two flow conditions are studied to investigate the influence of swirl on droplet life histories and the effects of droplet/turbulence interactions on flow properties. Comparison of computed results with the experimental data show that general features of the flow structure can be predicted with reasonable accuracy using the two separated flow models. In contrast, the LHF model overpredicts the rate of development of the flow. While the SSF model provides better agreement with measurements than the DSF model, definitive evaluation of the significance of droplet/turbulence interaction is not achieved due to uncertainties in the spray initial conditions.

Multiphase Modeling of Secondary Atomization in a Shock Environment

NASA Astrophysics Data System (ADS)

St. Clair, Jeffrey; McGrath, Thomas; Balachandar, Sivaramakrishnan

2017-06-01

Understanding and developing accurate modeling strategies for shock-particulate interaction remains a challenging and important topic, with application to energetic materials development, volcanic eruptions, and safety/risk assessment. This work presents computational modeling of compressible multiphase flows with shock-induced droplet atomization. Droplet size has a strong influence on the interphase momentum and heat transfer. A test case is presented that is sensitive to this, requiring the dynamic modeling of the secondary atomization process occurring when the shock impacts the droplets. An Eulerian-Eulerian computational model that treats all phases as compressible, is hyperbolic and satisfies the 2nd Law of Thermodynamics is applied. Four different breakup models are applied to the test case in which a planar shock wave encounters a cloud of water droplets. The numerical results are compared with both experimental and previously-generated modeling results. The effect of the drag relation used is also investigated. The computed results indicate the necessity of using a droplet breakup model for this application, and the relative accuracy of results obtained with the different droplet breakup and drag models is discussed.

Emulsion droplet interactions: a front-tracking treatment

NASA Astrophysics Data System (ADS)

Mason, Lachlan; Juric, Damir; Chergui, Jalel; Shin, Seungwon; Craster, Richard V.; Matar, Omar K.

2017-11-01

Emulsion coalescence influences a multitude of industrial applications including solvent extraction, oil recovery and the manufacture of fast-moving consumer goods. Droplet interaction models are vital for the design and scale-up of processing systems, however predictive modelling at the droplet-scale remains a research challenge. This study simulates industrially relevant moderate-inertia collisions for which a high degree of droplet deformation occurs. A hybrid front-tracking/level-set approach is used to automatically account for interface merging without the need for `bookkeeping' of interface connectivity. The model is implemented in Code BLUE using a parallel multi-grid solver, allowing both film and droplet-scale dynamics to be resolved efficiently. Droplet interaction simulations are validated using experimental sequences from the literature in the presence and absence of background turbulence. The framework is readily extensible for modelling the influence of surfactants and non-Newtonian fluids on droplet interaction processes. EPSRC, UK, MEMPHIS program Grant (EP/K003976/1), RAEng Research Chair (OKM), PETRONAS.

Wetting of crystalline polymer surfaces: A molecular dynamics simulation

NASA Astrophysics Data System (ADS)

Fan, Cun Feng; Caǧin, Tahir

1995-11-01

Molecular dynamics has been used to study the wetting of model polymer surfaces, the crystal surfaces of polyethylene (PE), poly(tetrafluoroethylene) (PTFE), and poly(ethylene terephthalate) (PET) by water and methylene iodide. In the simulation a liquid droplet is placed on a model surface and constant temperature, rigid body molecular dynamics is carried out while the model surface is kept fixed. A generally defined microscopic contact angle between a liquid droplet and a solid surface is quantitatively calculated from the volume of the droplet and the interfacial area between the droplet and the surface. The simulation results agree with the trend in experimental data for both water and methylene iodide. The shape of the droplets on the surface is analyzed and no obvious anisotropy of the droplets is seen in the surface plane, even though the crystal surfaces are highly oriented. The surface free energies of the model polymer surfaces are estimated from their contact angles with the two different liquid droplets.

Numerical simulation of the gas-liquid interaction of a liquid jet in supersonic crossflow

NASA Astrophysics Data System (ADS)

Li, Peibo; Wang, Zhenguo; Sun, Mingbo; Wang, Hongbo

2017-05-01

The gas-liquid interaction process of a liquid jet in supersonic crossflow with a Mach number of 1.94 was investigated numerically using the Eulerian-Lagrangian method. The KH (Kelvin-Helmholtz) breakup model was used to calculate the droplet stripping process, and the secondary breakup process was simulated by the competition of RT (Rayleigh-Taylor) breakup model and TAB (Taylor Analogy Breakup) model. A correction of drag coefficient was proposed by considering the compressible effects and the deformation of droplets. The location and velocity models of child droplets after breakup were improved according to droplet deformation. It was found that the calculated spray features, including spray penetration, droplet size distribution and droplet velocity profile agree reasonably well with the experiment. Numerical results revealed that the streamlines of air flow could intersect with the trajectory of droplets and are deflected towards the near-wall region after they enter into spray zone around the central plane. The analysis of gas-liquid relative velocity and droplet deformation suggested that the breakup of droplets mainly occurs around the front region of the spray where gathered a large number of droplets with different sizes. The liquid trailing phenomenon of jet spray which has been discovered by the previous experiment was successfully captured, and a reasonable explanation was given based on the analysis of gas-liquid interaction process.

Transient Droplet Behavior and Droplet Breakup during Bulk and Confined Shear Flow in Blends with One Viscoelastic Component: Experiments, Modelling and Simulations

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

Cardinaels, Ruth; Verhulst, Kristof; Moldenaers, Paula

2008-07-07

The transient droplet deformation and droplet orientation after inception of shear, the shape relaxation after cessation of shear and droplet breakup during shear, are microscopically studied, both under bulk and confined conditions. The studied blends contain one viscoelastic Boger fluid phase. A counter rotating setup, based on a Paar Physica MCR300, is used for the droplet visualisation. For bulk shear flow, it is shown that the droplet deformation during startup of shear flow and the shape relaxation after cessation of shear flow are hardly influenced by droplet viscoelasticity, even at moderate to high capillary and Deborah numbers. The effects ofmore » droplet viscoelasticity only become visible close to the critical conditions and a novel break-up mechanism is observed. Matrix viscoelasticity has a more pronounced effect, causing overshoots in the deformation and significantly inhibiting relaxation. However, different applied capillary numbers prior to cessation of shear flow, with the Deborah number fixed, still result in a single master curve for shape retraction, as in fully Newtonian systems. The long tail in the droplet relaxation can be qualitatively described with a phenomenological model for droplet deformation, when using a 5-mode Giesekus model for the fluid rheology. It is found that the shear flow history significantly affects the droplet shape evolution and the breakup process in blends with one viscoelastic component. Confining a droplet between two plates accelerates the droplet deformation kinetics, similar to fully Newtonian systems. However, the increased droplet deformation, due to wall effects, causes the steady state to be reached at a later instant in time. Droplet relaxation is less sensitive to confinement, leading to slower relaxation kinetics only for highly confined droplets. For the blend with a viscoelastic droplet, a non-monotonous trend is found for the critical capillary number as a function of the confinement ratio. Finally, experimental data are compared with 3D simulations, performed with a volume-of-fluid algorithm.« less

Mechanics of a gaseous film barrier to lubricant wetting of elastohydrodynamically lubricated conjunctions

NASA Technical Reports Server (NTRS)

Prahl, J. M.; Hamrock, B. J.

1985-01-01

Two analytical models, one based on simple hydrodynamic lubrication and the other on soft elastohydrodynamic lubrication, are presented and compared to delineate the dominant physical parameters that govern the mechanics of a gaseous film between a small droplet of lubricant and the outer race of a ball bearing. Both models are based on the balance of gravity forces, air drag forces, and air film lubrication forces and incorporate a drag coefficient C sub D and a lubrication coefficient C sub L to be determined from experiment. The soft elastohydrodynamic lubrication (EHL) model considers the effects of droplet deformation and solid-surface geometry; the simpler hydrodynamic lubrication (HL) model assumes that the droplet remains essentially spherical. The droplet's angular position depended primarily on the ratio of gas inertia to droplet gravity forces and on the gas Reynolds number and weakly on the ratio of droplet gravity forces to surface tension forces (Bond number) and geometric ratios for the soft EHL. An experimental configuration in which an oil droplet is supported by an air film on the rotating outer race of a ball bearing within a pressure-controlled chamber produced measurements of droplet angular position as a function of outer-race velocity droplet size and type, and chamber pressure.

Large Eddy Simulation including population dynamics model for polydisperse droplet evolution

NASA Astrophysics Data System (ADS)

Aiyer, Aditya; Yang, Di; Chamecki, Marcelo; Meneveau, Charles

2017-11-01

Previous studies have shown that dispersion patterns of oil droplets in the ocean following a deep sea oil spill depend critically on droplet diameter. Hence predicting the evolution of the droplet size distribution is of critical importance for predicting macroscopic features of dispersion in the ocean. We adopt a population dynamics model of polydisperse droplet distributions for use in LES. We generalize a breakup model from Reynolds averaging approaches to LES in which the breakup is modeled as due to bombardment of droplets by turbulent eddies of various sizes. The breakage rate is expressed as an integral of a collision frequency times a breakage efficiency over all eddy sizes. An empirical fit to the integral is proposed in order to avoid having to recalculate the integral at every LES grid point and time step. The fit is tested by comparison with various stirred tank experiments. As a flow application for LES we consider a jet of bubbles and large droplets injected at the bottom of the tank. The advected velocity and concentration fields of the drops are described using an Eulerian approach. We study the change of the oil droplet distribution due to breakup caused by interaction of turbulence with the oil droplets. This research was made possible by a Grant from the Gulf of Mexico Research Initiative.

One-to-one encapsulation based on alternating droplet generation

NASA Astrophysics Data System (ADS)

Hirama, Hirotada; Torii, Toru

2015-10-01

This paper reports the preparation of encapsulated particles as models of cells using an alternating droplet generation encapsulation method in which the number of particles in a droplet is controlled by a microchannel to achieve one-to-one encapsulation. Using a microchannel in which wettability is treated locally, the fluorescent particles used as models of cells were successfully encapsulated in uniform water-in-oil-in-water (W/O/W) emulsion droplets. Furthermore, 20% of the particle-containing droplets contained one particle. Additionally, when a surfactant with the appropriate properties was used, the fluorescent particles within each inner aqueous droplet were enclosed in the merged droplet by spontaneous droplet coalescence. This one-to-one encapsulation method based on alternating droplet generation could be used for a variety of applications, such as high-throughput single-cell assays, gene transfection into cells or one-to-one cell fusion.

One-to-one encapsulation based on alternating droplet generation.

PubMed

Hirama, Hirotada; Torii, Toru

2015-10-21

This paper reports the preparation of encapsulated particles as models of cells using an alternating droplet generation encapsulation method in which the number of particles in a droplet is controlled by a microchannel to achieve one-to-one encapsulation. Using a microchannel in which wettability is treated locally, the fluorescent particles used as models of cells were successfully encapsulated in uniform water-in-oil-in-water (W/O/W) emulsion droplets. Furthermore, 20% of the particle-containing droplets contained one particle. Additionally, when a surfactant with the appropriate properties was used, the fluorescent particles within each inner aqueous droplet were enclosed in the merged droplet by spontaneous droplet coalescence. This one-to-one encapsulation method based on alternating droplet generation could be used for a variety of applications, such as high-throughput single-cell assays, gene transfection into cells or one-to-one cell fusion.

Modelling Oil Droplet Breakup in a Turbulent Jet

NASA Astrophysics Data System (ADS)

Philip, Rachel; Hewitt, Ian; Howell, Peter

2017-11-01

In a deep-sea oil spill, a broken pipe near the seabed can result in the release of a turbulent oil jet into the surrounding ocean. The jet's shearing motion will typically cause the oil to break up into smaller droplets, which are then more readily dispersed and decomposed by sea microbes. In order to understand this natural clean-up process, we develop analytical and numerical models for the drop size distribution at different locations in the jet. This involves examining and unifying disparate scales, from the macroscopic jet to the microscopic droplets. We first examine the turbulent jet and we can use its self-similarity to simplify our models. We then turn to the droplet scale, considering the rate at which drops are deformed and broken up. Droplet deformation is precipitated by the jet's turbulent mixing and shearing and thus depends on the macroscopic jet models. We combine these large and small scale models to determine the droplet size distribution, as it varies with jet location. By varying the initial conditions and parameters in these models, we obtain insights into the factors affecting this droplet breakup process and how it may be optimised.

Simulating oil droplet dispersal from the Deepwater Horizon spill with a Lagrangian approach

USGS Publications Warehouse

North, Elizabeth W.; Schlag, Zachary; Adams, E. Eric; Sherwood, Christopher R.; He, Ruoying; Hyun, Hoon; Socolofsky, Scott A.

2011-01-01

An analytical multiphase plume model, combined with time-varying flow and hydrographic fields generated by the 3-D South Atlantic Bight and Gulf of Mexico model (SABGOM) hydrodynamic model, were used as input to a Lagrangian transport model (LTRANS), to simulate transport of oil droplets dispersed at depth from the recent Deepwater Horizon MC 252 oil spill. The plume model predicts a stratification-dominated near field, in which small oil droplets detrain from the central plume containing faster rising large oil droplets and gas bubbles and become trapped by density stratification. Simulated intrusion (trap) heights of ∼ 310–370 m agree well with the midrange of conductivity-temperature-depth observations, though the simulated variation in trap height was lower than observed, presumably in part due to unresolved variability in source composition (percentage oil versus gas) and location (multiple leaks during first half of spill). Simulated droplet trajectories by the SABGOM-LTRANS modeling system showed that droplets with diameters between 10 and 50 μm formed a distinct subsurface plume, which was transported horizontally and remained in the subsurface for >1 month. In contrast, droplets with diameters ≥90 μm rose rapidly to the surface. Simulated trajectories of droplets ≤50 μm in diameter were found to be consistent with field observations of a southwest-tending subsurface plume in late June 2010 reported by Camilli et al. [2010]. Model results suggest that the subsurface plume looped around to the east, with potential subsurface oil transport to the northeast and southeast. Ongoing work is focusing on adding degradation processes to the model to constrain droplet dispersal.

Theoretical Basis for Estimated Test Times and Conditions for Drop Tower and Space-Based Droplet Burning Experiments With Methanol and N-Heptane

NASA Technical Reports Server (NTRS)

Marchese, Anthony J.; Dryer, Fredrick L.; Choi, Mun Y.

1994-01-01

In order to develop an extensive envelope of test conditions for NASA's space-based Droplet Combustion Experiment (DCE) as well those droplet experiments which can be performed using a drop tower, the transient vaporization and combustion of methanol and n-heptane droplets were simulated using a recently developed fully time-dependent, spherically symmetric droplet combustion model. The transient vaporization of methanol and n-heptane was modeled to characterize the instantaneous gas phase composition surrounding the droplet prior to the introduction of an ignition source. The results for methanol/air showed that the entire gas phase surrounding a 2 mm methanol droplet deployed in zero-g .quickly falls outside the lean flammability limit. The gas phase surrounding an identically-sized n-heptane droplet, on the other hand, remains flammable. The combustion of methanol was then modeled considering a detailed gas phase chemical kinetic mechanism (168 steps, 26 species) and the effect of the dissolution of flame-generated water into the liquid droplet. These results were used to determine the critical ignition diameter required to achieve quasi-steady droplet combustion in a given oxidizing environment. For droplet diameters greater than the critical ignition diameter, the model predicted a finite diameter at which the flame would extinguish. These extinction diameters were found to vary significantly with initial droplet diameter. This phenomenon appears to be unique to the transient heat transfer, mass transfer and chemical kinetics of the system and thus has not been reported elsewhere to date. The extinction diameter was also shown to vary significantly with the liquid phase Lewis number since the amount of water present in the droplet at extinction is largely governed by the rate at which water is transported into the droplet via mass diffusion. Finally, the numerical results for n-heptane combustion were obtained using both 2 step and 96 step semi-emperical chemical kinetic mechanisms. Neither mechanism exhibited the variation of extinction diameter with initial diameter.

A Lagrangian stochastic model for aerial spray transport above an oak forest

USGS Publications Warehouse

Wang, Yansen; Miller, David R.; Anderson, Dean E.; McManus, Michael L.

1995-01-01

An aerial spray droplets' transport model has been developed by applying recent advances in Lagrangian stochastic simulation of heavy particles. A two-dimensional Lagrangian stochastic model was adopted to simulate the spray droplet dispersion in atmospheric turbulence by adjusting the Lagrangian integral time scale along the drop trajectory. The other major physical processes affecting the transport of spray droplets above a forest canopy, the aircraft wingtip vortices and the droplet evaporation, were also included in each time step of the droplets' transport.The model was evaluated using data from an aerial spray field experiment. In generally neutral stability conditions, the accuracy of the model predictions varied from run-to-run as expected. The average root-mean-square error was 24.61 IU cm−2, and the average relative error was 15%. The model prediction was adequate in two-dimensional steady wind conditions, but was less accurate in variable wind condition. The results indicated that the model can simulate successfully the ensemble; average transport of aerial spray droplets under neutral, steady atmospheric wind conditions.

A Model for Particle Microphysics,Turbulent Mixing, and Radiative Transfer in the Stratocumulus-Topped Marine Boundary Layer and Comparisons with Measurements

NASA Technical Reports Server (NTRS)

Ackerman, Andrew S.; Toon, Owen B.; Hobbs, Peter V.

1995-01-01

A detailed 1D model of the stratocumulus-topped marine boundary layer is described. The model has three coupled components: a microphysics module that resolves the size distributions of aerosols and cloud droplets, a turbulence module that treats vertical mixing between layers, and a multiple wavelength radiative transfer module that calculates radiative heating rates and cloud optical properties. The results of a 12-h model simulation reproduce reasonably well the bulk thermodynamics, microphysical properties, and radiative fluxes measured in an approx. 500-m thick, summertime marine stratocumulus cloud layer by Nicholls. However, in this case, the model predictions of turbulent fluxes between the cloud and subcloud layers exceed the measurements. Results of model simulations are also compared to measurements of a marine stratus layer made under gate conditions and with measurements of a high, thin marine stratocumulus layer. The variations in cloud properties are generally reproduced by the model, although it underpredicts the entrainment of overlying air at cloud top under gale conditions. Sensitivities of the model results are explored. The vertical profile of cloud droplet concentration is sensitive to the lower size cutoff of the droplet size distribution due to the presence of unactivated haze particles in the lower region of the modeled cloud. Increases in total droplet concentrations do not always produce less drizzle and more cloud water in the model. The radius of the mean droplet volume does not correlate consistently with drizzle, but the effective droplet radius does. The greatest impacts on cloud properties predicted by the model are produced by halving the width of the size distribution of input condensation nuclei and by omitting the effect of cloud-top radiative cooling on the condensational growth of cloud droplets. The omission of infrared scattering produces noticeable changes in cloud properties. The collection efficiencies for droplets less than 30-micron radius, and the value of the accommodation coefficient for condensational droplet growth, have noticeable effects on cloud properties. The divergence of the horizontal wind also has a significant effect on a 12-h model simulation of cloud structure. Conclusions drawn from the model are tentative because of the limitations of the 1D model framework. A principal simplification is that the model assumes horizontal homogeneity, and, therefore, does not resolve updrafts and downdrafts. Likely consequences of this simplification include overprediction of the growth of droplets by condensation in the upper region of the cloud, underprediction of droplet condensational growth in the lower region of the cloud, and under-prediction of peak supersaturations.

A Model for Particle Microphysics, Turbulent Mixing, and Radiative Transfer in the Stratocumulus-Topped Marine Boundary Layer and Comparisons with Measurements

NASA Technical Reports Server (NTRS)

Ackerman, Andrew S.; Toon, Owen B.; Hobbs, Peter V.

1995-01-01

A detailed 1D model of the stratocumulus-topped marine boundary layer is described. The model has three coupled components: a microphysics module that resolves the size distributions of aerosols and cloud droplets, a turbulence module that treats vertical mixing between layers, and a multiple wavelength radiative transfer module that calculates radiative heating rates and cloud optical properties. The results of a 12-h model simulation reproduce reasonably well the bulk thermodynamics, microphysical properties, and radiative fluxes measured in an approx. 500-m thick, summertime marine stratocumulus cloud layer by Nicholls. However, in this case, the model predictions of turbulent fluxes between the cloud and subcloud layers exceed the measurements. Results of model simulations are also compared to measurements of a marine stratus layer made under gale conditions and with measurements of a high, thin marine stratocumulus layer. The variations in cloud properties are generally reproduced by the model, although it underpredicts the entrainment of overlying air at cloud top under gale conditions. Sensitivities of the model results are explored. The vertical profile of cloud droplet concentration is sensitive to the lower size cutoff of the droplet size distribution due to the presence of unactivated haze particles in the lower region of the modeled cloud. Increases in total droplet concentrations do not always produce less drizzle and more cloud water in the model. The radius of the mean droplet volume does not correlate consistently with drizzle, but the effective droplet radius does. The greatest impacts on cloud properties predicted by the model are produced by halving the width of the size distribution of input condensation nuclei and by omitting the effect of cloud-top radiative cooling on the condensational growth of cloud droplets. The omission of infrared scattering produces noticeable changes in cloud properties. The collection efficiencies for droplets less than 30-micrometers radius, and the value of the accommodation coefficient for condensational droplet growth, have noticeable effects on cloud properties. The divergence of the horizontal wind also has a significant effect on a 12-h model simulation of cloud structure. Conclusions drawn from the model are tentative because of the limitations of the 1D model framework. A principal simplification is that the model assumes horizontal homogeneity, and, therefore, does not resolve updrafts and downdrafts. Likely consequences of this simplification include overprediction of the growth of droplets by condensation in the upper region of the cloud, underprediction of droplet condensational growth in the lower region of the cloud, and underprediction of peak supersaturations.

Microphysical response of cloud droplets in a fluctuating updraft. Ph.D. Thesis

NASA Technical Reports Server (NTRS)

Harding, D. D.

1977-01-01

The effect of a fluctuating updraft upon a distribution of cloud droplets is examined. Computations are performed for fourteen vertical velocity patterns; each allows a closed parcel of cloud air to undergo downward as well as upward motion. Droplet solution and curvature effects are included. The classical equations for the growth rate of an individual droplet by vapor condensation relies on simplifying assumptions. Those assumptions are isolated and examined. A unique approach is presented in which all energy sources and sinks of a droplet may be considered and is termed the explicit model. It is speculated that the explicit model may enhance the growth of large droplets at greater heights. Such a model is beneficial to the studies of pollution scavenging and acid rain.

On-Line, Real-Time Diagnostics of a Single Fluid Atomization System

NASA Technical Reports Server (NTRS)

DelshadKhatibi, P.; Ilbagi, A.; Henein, H.

2012-01-01

A drop tube-Impulse Atomization technique was used to produce copper droplets. In this method, energy is transferred to a liquid by plunger movement resulting in spherical droplets emanating from orifices. A mathematical model of the evolution of droplet velocity and temperature at various heights for different sized droplets was developed. A two-color pyrometer, DPV-2000, and a shadowgraph were used to measure droplets radiant energy, diameter and velocity. The temperature values from the model were used to assess the two color pyrometer assumption over the temperature range of measurement. The DVP 2000 measurements were found to be dependent of droplet size wavelength and position of droplets below the atomizing nozzle. By calibrating the instrument for effective emissivity over the range of measurements, the thermal history of droplets may be recorded using a single color pyrometer approach.

Combustion of Interacting Droplet Arrays in a Microgravity Environment

NASA Technical Reports Server (NTRS)

Dietrich, D. L.; Struk, P. M.; Ikegami, M.; Nagaishi, H.; Honma, S.; Ikeda, K.

2001-01-01

Investigations into droplet interactions date back to Rex et al. Annamalai and Ryan and Annamalai published extensive reviews of droplet array and cloud combustion studies. In the majority of the reviewed studies, the authors examined the change in the burning rate constant, k, (relative to that of the single droplet) that results from interactions. More recently, Niioka and co-workers have examined ignition and flame propagation along arrays of interacting droplets with the goal of relating these phenomena in this simplified geometry to the more practical spray configuration. Our work has focussed on droplet interactions under conditions where flame extinction occurs at a finite droplet diameter. In our previous work, we reported that in normal gravity, reduced pressure conditions, droplet interactions improved flame stability and extended flammability limits (by inference). In our recent work, we examine droplet interactions under conditions where the flame extinguishes at a finite droplet diameter in microgravity. The microgravity experiments were in the NASA GRC 2.2 and 5.2 second drop towers, and the JAMIC (Japan Microgravity Center) 10 second drop tower. We also present progress on a numerical model of single droplet combustion that is in the process of being extended to model a binary droplet array.

Development of an Evaporation Sub-model and Simulation of Multiple Droplet Impingement in Volume of Fluid Method

NASA Astrophysics Data System (ADS)

Potham, Sathya Prasad

Droplet collision and impingement on a substrate are widely observed phenomenon in many applications like spray injection of Internal Combustion Engines, spray cooling, spray painting and atomizers used in propulsion applications. Existing Lagrangian models do not provide a comprehensive picture of the outcome of these events and may involve model constants requiring experimental data for validation. Physics based models like Volume of Fluid (VOF) method involve no parametric tuning and are more accurate. The aim of this thesis is to extend the basic VOF method with an evaporation sub-model and implement in an open source Computational Fluid Dynamics (CFD) software, OpenFOAM. The new model is applied to numerically study the evaporation of spherical n-heptane droplets impinging on a hot wall at atmospheric pressure and a temperature above the Leidenfrost temperature. An additional vapor phase is introduced apart from the liquid and gas phases to understand the mixing and diffusion of vapor and gas phases. The evaporation model is validated quantitatively and qualitatively with fundamental problems having analytical solutions and published results. The effect of droplet number and arrangement on evaporation is studied by three cases with one (Case 1), two (Case 2) and four (Case 3) droplets impinging on hot wall in film boiling regime at a fixed temperature of wall and a constant non-dimensional distance between droplets. Droplet lift and spread, surface temperature, heat transfer, and evaporation rate are examined. It was observed that more liquid mass evaporated in Case 1 compared to the other cases. Droplet levitation begins early in Case 1 and very high levitation observed was partially due to contraction of its shape from elongated to a more circular form. Average surface temperature was also considerably reduced in Case 1 due to high droplet levitation.

Coalescence-induced jumping of micro-droplets on heterogeneous superhydrophobic surfaces

NASA Astrophysics Data System (ADS)

Attarzadeh, Reza; Dolatabadi, Ali

2017-01-01

The phenomenon of droplets coalescence-induced self-propelled jumping on homogeneous and heterogeneous superhydrophobic surfaces was numerically modeled using the volume of fluid method coupled with a dynamic contact angle model. The heterogeneity of the surface was directly modeled as a series of micro-patterned pillars. To resolve the influence of air around a droplet and between the pillars, extensive simulations were performed for different droplet sizes on a textured surface. Parallel computations with the OpenMP algorithm were used to accelerate computation speed to meet the convergence criteria. The composition of the air-solid surface underneath the droplet facilitated capturing the transition from a no-slip/no-penetration to a partial-slip with penetration as the contact line at triple point started moving to the air pockets. The wettability effect from the nanoscopic roughness and the coating was included in the model by using the intrinsic contact angle obtained from a previously published study. As the coalescence started, the radial velocity of the coalescing liquid bridge was partially reverted to the upward direction due to the counter-action of the surface. However, we found that the velocity varied with the size of the droplets. A part of the droplet kinetic energy was dissipated as the merged droplet started penetrating into the cavities. This was due to a different area in contact between the liquid and solid and, consequently, a higher viscous dissipation rate in the system. We showed that the effect of surface roughness is strongly significant when the size of the micro-droplet is comparable with the size of the roughness features. In addition, the relevance of droplet size to surface roughness (critical relative roughness) was numerically quantified. We also found that regardless of the viscous cutoff radius, as the relative roughness approached the value of 44, the direct inclusion of surface topography was crucial in the modeling of the droplet-surface interaction. Finally, we validated our model against existing experimental data in the literature, verifying the effect of relative roughness on the jumping velocity of a merged droplet.

Fiber Supported Droplet Combustion-2 (FSDC-2)

NASA Technical Reports Server (NTRS)

Colantonio, Renato; Dietrich, Daniel; Haggard, John B., Jr.; Nayagan, Vedha; Dryer, Frederick L.; Shaw, Benjamin D.; Williams, Forman A.

1998-01-01

Experimental results for the burning characteristics of fiber supported, liquid droplets in ambient Shuttle cabin air (21% oxygen, 1 bar pressure) were obtained from the Glove Box Facility aboard the STS-94/MSL-1 mission using the Fiber Supported Droplet Combustion - 2 (FSDC-2) apparatus. The combustion of individual droplets of methanol/water mixtures, ethanol, ethanol/water azeotrope, n-heptane, n-decane, and n-heptane/n-hexadecane mixtures were studied in quiescent air. The effects of low velocity, laminar gas phase forced convection on the combustion of individual droplets of n-heptane and n-decane were investigated and interactions of two droplet-arrays of n-heptane and n-decane droplets were also studied with and without gas phase convective flow. Initial diameters ranging from about 2mm to over 6mm were burned on 80-100 micron silicon fibers. In addition to phenomenological observations, quantitative data were obtained in the form of backlit images of the burning droplets, overall flame images, and radiometric combustion emission measurements as a function of the burning time in each experiment. In all, 124 of the 129 attempted experiments (or about twice the number of experiments originally planned for the STS-94/MSL-1 mission) were conducted successfully. The experimental results contribute new observations on the combustion properties of pure alkanes, binary alkane mixtures, and simple alcohols for droplet sizes not studied previously, including measurements on individual droplets and two-droplet arrays, inclusive of the effects of forced gas phase convection. New phenomena characterized experimentally for the first time include radiative extinction of droplet burning for alkanes and the "twin effect" which occurs as a result of interactions during the combustion of two-droplet arrays. Numerical modeling of isolated droplet combustion phenomenon has been conducted for methanol/water mixtures, n-heptane, and n-heptane/n-hexadecane mixtures, and results compare quantitatively with those found experimentally for methanol/water mixtures. Initial computational results qualitatively predict experimental results obtained for isolated n-heptane and n-heptane/n-hexadecane droplet combustion, although the effects of sooting are not yet included in the modeling work. Numerical modeling of ethanol and ethanol/water droplet burning is under development. Considerable data remain to be fully analyzed and will provide a large database for comparisons with further numerical and analytical modeling and development of future free droplet experiments aboard space platforms.

Experimental study on the droplet formation around pins of different geometry for the design of a compact falling-droplet absorber

NASA Astrophysics Data System (ADS)

Cola, Fabrizio; Romagnoli, Alessandro; Hey, Jonathan

2018-05-01

Absorber downsizing for the development of compact absorption chillers is a known challenge of this type of refrigerator. Past studies have revealed how a droplet flow regime can increase the interface area and enhance absorption rates, especially during the droplet formation. This study proposes a space-efficient design for an adiabatic absorber based on a bank of solid pins coupled with a droplet flow regime. Manufacturing through 3D printing technique is used to study the effect of different fin shapes during droplet formation. Droplet behavior is firstly studied analytically through a variational approach. Experiments on pure water are then carried out to validate the model and produce design guidelines for a H2O-LiBr absorber. Results show that the analytical model is more accurate in the regions close to the droplet bottom. The rhomboidal geometry with 120° returned the smallest droplet volume without allowing coalescence of more droplets, ensuring the maintenance of droplet flow and a high surface area for mass transfer. Disturbances in the droplet profiles were observed, caused by the pin-droplet interaction. A map has been then created to allow a quick sizing of the absorber and find its main geometrical and operational features.

Reaction Force of Micro-scale Liquid Droplets Constrained Between Parallel Plates through CFD

NASA Astrophysics Data System (ADS)

Free, Robert; Hekiri, Haider; Hawa, Takumi

2012-02-01

Micro-scale liquid droplets responding to depression between parallel plates are investigated analytically and numerically. The functional dependence of the reaction force accrued in such droplets on droplet size, surface tension, depression amount, and contact angle is explored. For both the 2D and 3D case, an analytical model is developed based on first principles. Computational fluid dynamics is then utilized to evaluate the validity of these models. The reaction force is highly nonlinear, initially increasing very slowly with increasing depression of the droplet, but eventually moving asymptotically to infinity. The force scales linearly with both the droplet free radius and surface tension of the liquid, but has a much more complicated dependence on the contact angle and depression. Explicit expressions for the reaction force have been determined, showing these dependencies. The 3D model has been largely supported by the CFD results. It very accurately predicts the reaction force on the upper plate as the droplet is crushed, accounting for the effect of contact angle, surface tension, and droplet size.

Mechanism and simulation of droplet coalescence in molten steel

NASA Astrophysics Data System (ADS)

Ni, Bing; Zhang, Tao; Ni, Hai-qi; Luo, Zhi-guo

2017-11-01

Droplet coalescence in liquid steel was carefully investigated through observations of the distribution pattern of inclusions in solidified steel samples. The process of droplet coalescence was slow, and the critical Weber number ( We) was used to evaluate the coalescence or separation of droplets. The relationship between the collision parameter and the critical We indicated whether slow coalescence or bouncing of droplets occurred. The critical We was 5.5, which means that the droplets gradually coalesce when We ≤ 5.5, whereas they bounce when We > 5.5. For the carbonate wire feeding into liquid steel, a mathematical model implementing a combined computational fluid dynamics (CFD)-discrete element method (DEM) approach was developed to simulate the movement and coalescence of variably sized droplets in a bottom-argon-blowing ladle. In the CFD model, the flow field was solved on the premise that the fluid was a continuous medium. Meanwhile, the droplets were dispersed in the DEM model, and the coalescence criterion of the particles was added to simulate the collision- coalescence process of the particles. The numerical simulation results and observations of inclusion coalescence in steel samples are consistent.

Distillation-based Droplet Modeling of Non-Ideal Oxygenated Gasoline Blends: Investigating the Role of Droplet Evaporation on PM Emissions

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

Burke, Stephen C.; Ratcliff, Matthew; McCormick, Robert

In some studies, a relationship has been observed between increasing ethanol content in gasoline and increased particulate matter (PM) emissions from vehicles equipped with spark ignition engines. The fundamental cause of the PM increase seen for moderate ethanol concentrations is not well understood. Ethanol features a greater heat of vaporization (HOV) than gasoline and also influences vaporization by altering the liquid and vapor composition throughout the distillation process. A droplet vaporization model was developed to explore ethanol's effect on the evaporation of aromatic compounds known to be PM precursors. The evolving droplet composition is modeled as a distillation process, withmore » non-ideal interactions between oxygenates and hydrocarbons accounted for using UNIFAC group contribution theory. Predicted composition and distillation curves were validated by experiments. Detailed hydrocarbon analysis was applied to fuel samples and to distillate fractions, and used as input for the initial droplet composition. With composition calculated throughout the distillation, the changing HOV and other physical properties can be found using reference data. The droplet can thus be modeled in terms of energy transfer, which in turn provides the transient mass transfer, droplet temperature, and droplet diameter. Model predictions suggest that non-ideal vapor-liquid equilibrium along with an increase in HOV can alter the droplet composition evolution. Results predict that the presence of ethanol causes enrichment of the higher boiling fractions (T90+) in the aromatic components as well as lengthens the droplet lifetime. A simulation of the evaporation process in a transient environment as experienced within an engine cylinder predicts a decrease in mixing time of the heaviest fractions of the fuel prior to spark initiation, possibly explaining observations linking ethanol to PM.« less

Distillation-based Droplet Modeling of Non-Ideal Oxygenated Gasoline Blends: Investigating the Role of Droplet Evaporation on PM Emissions

DOE PAGES

Burke, Stephen C.; Ratcliff, Matthew; McCormick, Robert; ...

2017-03-28

In some studies, a relationship has been observed between increasing ethanol content in gasoline and increased particulate matter (PM) emissions from vehicles equipped with spark ignition engines. The fundamental cause of the PM increase seen for moderate ethanol concentrations is not well understood. Ethanol features a greater heat of vaporization (HOV) than gasoline and also influences vaporization by altering the liquid and vapor composition throughout the distillation process. A droplet vaporization model was developed to explore ethanol's effect on the evaporation of aromatic compounds known to be PM precursors. The evolving droplet composition is modeled as a distillation process, withmore » non-ideal interactions between oxygenates and hydrocarbons accounted for using UNIFAC group contribution theory. Predicted composition and distillation curves were validated by experiments. Detailed hydrocarbon analysis was applied to fuel samples and to distillate fractions, and used as input for the initial droplet composition. With composition calculated throughout the distillation, the changing HOV and other physical properties can be found using reference data. The droplet can thus be modeled in terms of energy transfer, which in turn provides the transient mass transfer, droplet temperature, and droplet diameter. Model predictions suggest that non-ideal vapor-liquid equilibrium along with an increase in HOV can alter the droplet composition evolution. Results predict that the presence of ethanol causes enrichment of the higher boiling fractions (T90+) in the aromatic components as well as lengthens the droplet lifetime. A simulation of the evaporation process in a transient environment as experienced within an engine cylinder predicts a decrease in mixing time of the heaviest fractions of the fuel prior to spark initiation, possibly explaining observations linking ethanol to PM.« less

Spontaneous emulsification and self-propulsion of oil droplets induced by the synthesis of amino acid-based surfactants.

PubMed

Nagasaka, Yuriko; Tanaka, Shinpei; Nehira, Tatsuo; Amimoto, Tomoko

2017-09-27

It is well known that oil droplets in or on water exhibit spontaneous movement induced by surfactants, and this self-propulsion is regarded as an important factor in droplet-based models for a living cell. We report here an oil-droplet system spontaneously producing amino acid-based surfactants, which are then utilized for the droplets' self-propulsion. Thus this system is an active system capable of producing the fuel for the propulsion by itself, which can be used as a conceptual model for cell metabolism.

Droplet size in flow: Theoretical model and application to polymer blends

NASA Astrophysics Data System (ADS)

Fortelný, Ivan; Jůza, Josef

2017-05-01

The paper is focused on prediction of the average droplet radius, R, in flowing polymer blends where the droplet size is determined by dynamic equilibrium between the droplet breakup and coalescence. Expressions for the droplet breakup frequency in systems with low and high contents of the dispersed phase are derived using available theoretical and experimental results for model blends. Dependences of the coalescence probability, Pc, on system parameters, following from recent theories, is considered and approximate equation for Pc in a system with a low polydispersity in the droplet size is proposed. Equations for R in systems with low and high contents of the dispersed phase are derived. Combination of these equations predicts realistic dependence of R on the volume fraction of dispersed droplets, φ. Theoretical prediction of the ratio of R to the critical droplet radius at breakup agrees fairly well with experimental values for steadily mixed polymer blends.

Theoretical model of droplet wettability on a low-surface-energy solid under the influence of gravity.

PubMed

Yonemoto, Yukihiro; Kunugi, Tomoaki

2014-01-01

The wettability of droplets on a low surface energy solid is evaluated experimentally and theoretically. Water-ethanol binary mixture drops of several volumes are used. In the experiment, the droplet radius, height, and contact angle are measured. Analytical equations are derived that incorporate the effect of gravity for the relationships between the droplet radius and height, radius and contact angle, and radius and liquid surface energy. All the analytical equations display good agreement with the experimental data. It is found that the fundamental wetting behavior of the droplet on the low surface energy solid can be predicted by our model which gives geometrical information of the droplet such as the contact angle, droplet radius, and height from physical values of liquid and solid.

Energy exchange analysis in droplet dynamics via the Navier-Stokes-Cahn-Hilliard model

NASA Astrophysics Data System (ADS)

Espath, L. F. R.; Sarmiento, A. F.; Vignal, P.; Varga, B. O. N.; Cortes, A. M. A.; Dalcin, L.; Calo, V. M.

2016-06-01

We develop the energy budget equation of the coupled Navier-Stokes-Cahn-Hilliard (NSCH) system. We use the NSCH equations to model the dynamics of liquid droplets in a liquid continuum. Buoyancy effects are accounted for through the Boussinesq assumption. We physically interpret each quantity involved in the energy exchange to further insight into the model. Highly resolved simulations involving density-driven flows and merging of droplets allow us to analyze these energy budgets. In particular, we focus on the energy exchanges when droplets merge, and describe flow features relevant to this phenomenon. By comparing our numerical simulations to analytical predictions and experimental results available in the literature, we conclude that modeling droplet dynamics within the framework of NSCH equations is a sensible approach worth further research.

Transport characteristics of expiratory droplets and droplet nuclei in indoor environments with different ventilation airflow patterns.

PubMed

Wan, M P; Chao, C Y H

2007-06-01

Expiratory droplets and droplet nuclei can be pathogen carriers for airborne diseases. Their transport characteristics were studied in detail in two idealized floor-supply-type ventilation flow patterns: Unidirectional-upward and single-side-floor, using a multiphase numerical model. The model was validated by running interferometric Mie imaging experiments using test droplets with nonvolatile content, which formed droplet nuclei, ultimately, in a class-100 clean-room chamber. By comparing the droplet dispersion and removal characteristics with data of two other ceiling-supply ventilation systems collected from a previous work, deviations from the perfectly mixed ventilation condition were found to exist in various cases to different extent. The unidirectional-upward system was found to be more efficient in removing the smallest droplet nuclei (formed from 1.5 mum droplets) by air extraction, but it became less effective for larger droplets and droplet nuclei. Instead, the single-side-floor system was shown to be more favorable in removing these large droplets and droplet nuclei. In the single-side-floor system, the lateral overall dispersion coefficients for the small droplets and nuclei (initial size

Fiber-Supported Droplet Combustion Experiment-2

NASA Technical Reports Server (NTRS)

Colantonio, Renato O.

1998-01-01

A major portion of the energy produced in the world today comes from the burning of liquid hydrocarbon fuels in the form of droplets. Understanding the fundamental physical processes involved in droplet combustion is not only important in energy production but also in propulsion, in the mitigation of combustion-generated pollution, and in the control of the fire hazards associated with handling liquid combustibles. Microgravity makes spherically symmetric combustion possible, allowing investigators to easily validate their droplet models without the complicating effects of gravity. The Fiber-Supported Droplet Combustion (FSDC-2) investigation was conducted in the Microgravity Glovebox facility of the shuttles' Spacelab during the reflight of the Microgravity Science Laboratory (MSL- 1R) on STS-94 in July 1997. FSDC-2 studied fundamental phenomena related to liquid fuel droplet combustion in air. Pure fuels and mixtures of fuels were burned as isolated single and duo droplets with and without forced air convection. FSDC-2 is sponsored by the NASA Lewis Research Center, whose researchers are working in cooperation with several investigators from industry and academia. The rate at which a droplet burns is important in many commercial applications. The classical theory of droplet burning assumes that, for an isolated, spherically symmetric, single-fuel droplet, the gas-phase combustion processes are much faster than the droplet surface regression rate and that the liquid phase is at a uniform temperature equal to the boiling point. Recent, more advanced models predict that both the liquid and gas phases are unsteady during a substantial portion of the droplet's burning history, thus affecting the instantaneous and average burning rates, and that flame radiation is a dominant mechanism that can extinguish flames in a microgravity environment. FSDC-2 has provided well-defined, symmetric droplet burning data including radiative emissions to validate these theoretical models for heptane, decane, ethanol, and methanol fuels. Since most commercial combustion systems burn droplets in a convective environment, data were obtained without and with convective flow over the burning droplet (see the following photos).

Investigation of water droplet trajectories within the NASA icing research tunnel

NASA Technical Reports Server (NTRS)

Reehorst, Andrew; Ibrahim, Mounir

1995-01-01

Water droplet trajectories within the NASA Lewis Research Center's Icing Research Tunnel (IRT) were studied through computer analysis. Of interest was the influence of the wind tunnel contraction and wind tunnel model blockage on the water droplet trajectories. The computer analysis was carried out with a program package consisting of a three-dimensional potential panel code and a three-dimensional droplet trajectory code. The wind tunnel contraction was found to influence the droplet size distribution and liquid water content distribution across the test section from that at the inlet. The wind tunnel walls were found to have negligible influence upon the impingement of water droplets upon a wing model.

Photoacoustic spectral characterization of perfluorocarbon droplets

NASA Astrophysics Data System (ADS)

Strohm, Eric; Gorelikov, Ivan; Matsuura, Naomi; Kolios, Michael

2012-02-01

Perfluorocarbon droplets containing optical absorbing nanoparticles have been developed for use as theranostic agents (for both imaging and therapy) and as dual-mode contrast agents. Droplets can be used as photoacoustic contrast agents, vaporized via optical irradiation, then the resulting bubbles can be used as ultrasound imaging and therapeutic agents. The photoacoustic signals from micron-sized droplets containing silica coated gold nanospheres were measured using ultra-high frequencies (100-1000 MHz). The spectra of droplets embedded in a gelatin phantom were compared to a theoretical model which calculates the pressure wave from a spherical homogenous liquid undergoing thermoelastic expansion resulting from laser absorption. The location of the spectral features of the theoretical model and experimental spectra were in agreement after accounting for increases in the droplet sound speed with frequency. The agreement between experiment and model indicate that droplets (which have negligible optical absorption in the visible and infrared spectra by themselves) emitted pressure waves related to the droplet composition and size, and was independent of the physical characteristics of the optical absorbing nanoparticles. The diameter of individual droplets was calculated using three independent methods: the time domain photoacoustic signal, the time domain pulse echo ultrasound signal, and a fit to the photoacoustic model, then compared to the diameter as measured by optical microscopy. It was found the photoacoustic and ultrasound methods calculated diameters an average of 2.6% of each other, and 8.8% lower than that measured using optical microscopy. The discrepancy between the calculated diameters and the optical measurements may be due to the difficulty in resolving the droplet edges after being embedded in the translucent gelatin medium.

Experimental and Modeling Study of the Burning of an Ethanol Droplet in Microgravity

NASA Technical Reports Server (NTRS)

Kazakov, Andrei; Conley, Jordan; Dryer, Frederick L.; Ferkul, Paul (Technical Monitor)

2000-01-01

The microgravity ethanol droplet combustion experiments were performed aboard the STS-94/MSL-1 Shuttle mission within the Fiber-Supported Droplet Combustion-2 (FSDC-2) program. The burning histories and flame standoffs for pure ethanol and ethanol/water droplets were obtained from the images recorded with two 8 mm videocameras. The obtained results show that average gasification rate is related to the initial droplet size in a manner similar to n-alkanes and methanol and consistent with the results of Hara and Kumagai and the data taken recently in the NASA-Lewis 2.2 s droptower. A transient, moving finite-element chemically reacting flow model applied previously to sphero-symmetric combustion of methanol, methanol/water, n-alkane, and n-alkane binary mixture droplets was adopted for the problem of ethanol droplet combustion. The model includes detailed description of gas-phase reaction chemistry and transport, a simplified description of liquid phase transport, and non-luminous radiative heat transfer. Gas-phase chemistry was described with the detailed reaction mechanism of Norton and Dryer, which consists of 142 reversible elementary reactions of 33 species. Another recently published reaction mechanism of high-temperature ethanol oxidation was also considered. The model predictions were found to compare favorably with the experimental data. The model analysis also indicates that water condensation in the case of ethanol has smaller effect on average droplet gasification rate as compared with previously studied methanol cases. This effect is explained by non-ideal (azeotropic) behavior of binary ethanol-water mixtures. Further analysis of computational results and ethanol droplet radiative extinction behavior will be discussed.

Three-dimensional lattice Boltzmann simulations of microdroplets including contact angle hysteresis on topologically structured surfaces

DOE PAGES

Ba, Yan; Kang, Qinjun; Liu, Haihu; ...

2016-04-14

In this study, the dynamical behavior of a droplet on topologically structured surface is investigated by using a three-dimensional color-gradient lattice Boltzmann model. A wetting boundary condition is proposed to model fluid-surface interactions, which is advantageous to improve the accuracy of the simulation and suppress spurious velocities at the contact line. The model is validated by the droplet partial wetting test and reproduction of the Cassie and Wenzel states. A series of simulations are conducted to investigate the behavior of a droplet when subjected to a shear flow. It is found that in Cassie state, the droplet undergoes a transitionmore » from stationary, to slipping and finally to detachment states as the capillary number increases, while in Wenzel state, the last state changes to the breakup state. The critical capillary number, above which the droplet slipping occurs, is small for the Cassie droplet, but is significantly enhanced for the Wenzel droplet due to the increased contact angle hysteresis. In Cassie state, the receding contact angle nearly equals the prediction by the Cassie relation, and the advancing contact angle is close to 180°, leading to a small contact angle hysteresis. In Wenzel state, however, the contact angle hysteresis is extremely large (around 100°). Finally, high droplet mobility can be easily achieved for Cassie droplets, whereas in Wenzel state, extremely low droplet mobility is identified.« less

A Numerical Analysis of Droplet Breakup in Asymmetric T-Junctions with Different Outlet Pressure Gradients

NASA Astrophysics Data System (ADS)

Cheng, Way Lee; Han, Arum; Sadr, Reza

2016-11-01

Droplet splitting is the breakup of a parent droplet into two or more daughter droplets of desired sizes. It is done to improve production efficiency and investigational capacity in microfluidic devices. Passive splitting is the breakup of droplets into precise volume ratios at predetermined locations without external power sources. In this study, a 3-D simulation was conducted using the Volume-of-Fluid method to analysis the breakup process of a droplet in asymmetric T-junctions with different outlet arm lengths. The arrangement allows a droplet to be split into two smaller droplets of different sizes, where the volumetric ratio of the daughter droplets depends on the length ratios of the outlet arms. The study identified different breakup regimes such as primary, transition, bubble and non-breakup under different flow conditions and channel configurations. Furthermore, a close analysis to the primary breakup regimes were done to determine the breakup mechanisms at various flow conditions. The analysis show that the breakup mechanisms in asymmetric T-junctions is different than a regular split. A pseudo-phenomenological model for the breakup criteria was presented at the end. The model was an expanded version to a theoretically derived model for the symmetric droplet breakup. The Qatar National Research Fund (a member of the Qatar Founda- tion), under Grant NPRP 5-671-2-278, supported this work.

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

Ba, Yan; Kang, Qinjun; Liu, Haihu

In this study, the dynamical behavior of a droplet on topologically structured surface is investigated by using a three-dimensional color-gradient lattice Boltzmann model. A wetting boundary condition is proposed to model fluid-surface interactions, which is advantageous to improve the accuracy of the simulation and suppress spurious velocities at the contact line. The model is validated by the droplet partial wetting test and reproduction of the Cassie and Wenzel states. A series of simulations are conducted to investigate the behavior of a droplet when subjected to a shear flow. It is found that in Cassie state, the droplet undergoes a transitionmore » from stationary, to slipping and finally to detachment states as the capillary number increases, while in Wenzel state, the last state changes to the breakup state. The critical capillary number, above which the droplet slipping occurs, is small for the Cassie droplet, but is significantly enhanced for the Wenzel droplet due to the increased contact angle hysteresis. In Cassie state, the receding contact angle nearly equals the prediction by the Cassie relation, and the advancing contact angle is close to 180°, leading to a small contact angle hysteresis. In Wenzel state, however, the contact angle hysteresis is extremely large (around 100°). Finally, high droplet mobility can be easily achieved for Cassie droplets, whereas in Wenzel state, extremely low droplet mobility is identified.« less

A Priori Subgrid Scale Modeling for a Droplet Laden Temporal Mixing Layer

NASA Technical Reports Server (NTRS)

Okongo, Nora; Bellan, Josette

2000-01-01

Subgrid analysis of a transitional temporal mixing layer with evaporating droplets has been performed using a direct numerical simulation (DNS) database. The DNS is for a Reynolds number (based on initial vorticity thickness) of 600, with droplet mass loading of 0.2. The gas phase is computed using a Eulerian formulation, with Lagrangian droplet tracking. Since Large Eddy Simulation (LES) of this flow requires the computation of unfiltered gas-phase variables at droplet locations from filtered gas-phase variables at the grid points, it is proposed to model these by assuming the gas-phase variables to be given by the filtered variables plus a correction based on the filtered standard deviation, which can be computed from the sub-grid scale (SGS) standard deviation. This model predicts unfiltered variables at droplet locations better than simply interpolating the filtered variables. Three methods are investigated for modeling the SGS standard deviation: Smagorinsky, gradient and scale-similarity. When properly calibrated, the gradient and scale-similarity methods give results in excellent agreement with the DNS.

Modelling and investigation of partial wetting surfaces for drop dynamics using lattice Boltzmann method

NASA Astrophysics Data System (ADS)

Pravinraj, T.; Patrikar, Rajendra

2017-07-01

Partial wetting surfaces and its influence on the droplet movement of micro and nano scale being contemplated for many useful applications. The dynamics of the droplet usually analyzed with a multiphase lattice Boltzmann method (LBM). In this paper, the influence of partial wetting surface on the dynamics of droplet is systematically analyzed for various cases. Splitting of droplets due to chemical gradient of the surface is studied and analyses of splitting time for various widths of the strips for different Weber numbers are computed. With the proposed model one can tune the splitting volume and time by carefully choosing a strip width and droplet position. The droplet spreading on chemically heterogeneous surfaces shows that the spreading can be controlled not only by parameters of Weber number but also by tuning strip width ratio. The transportation of the droplet from hydrophobic surface to hydrophilic surface due to chemical gradient is simulated and analyzed using our hybrid thermodynamic-image processing technique. The results prove that with the progress of time the surface free energy decreases with increase in spreading area. Finally, the transportation of a droplet on microstructure gradient is demonstrated. The model explains the temporal behaviour of droplet during the spreading, recoiling and translation along with tracking of contact angle hysteresis phenomenon.

Detailed finite element method modeling of evaporating multi-component droplets

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

Diddens, Christian, E-mail: C.Diddens@tue.nl

The evaporation of sessile multi-component droplets is modeled with an axisymmetic finite element method. The model comprises the coupled processes of mixture evaporation, multi-component flow with composition-dependent fluid properties and thermal effects. Based on representative examples of water–glycerol and water–ethanol droplets, regular and chaotic examples of solutal Marangoni flows are discussed. Furthermore, the relevance of the substrate thickness for the evaporative cooling of volatile binary mixture droplets is pointed out. It is shown how the evaporation of the more volatile component can drastically decrease the interface temperature, so that ambient vapor of the less volatile component condenses on the droplet.more » Finally, results of this model are compared with corresponding results of a lubrication theory model, showing that the application of lubrication theory can cause considerable errors even for moderate contact angles of 40°. - Graphical abstract:.« less

Development of a unified oil droplet size distribution model with application to surface breaking waves and subsea blowout releases considering dispersant effects.

PubMed

Li, Zhengkai; Spaulding, Malcolm; French McCay, Deborah; Crowley, Deborah; Payne, James R

2017-01-15

An oil droplet size model was developed for a variety of turbulent conditions based on non-dimensional analysis of disruptive and restorative forces, which is applicable to oil droplet formation under both surface breaking-wave and subsurface-blowout conditions, with or without dispersant application. This new model was calibrated and successfully validated with droplet size data obtained from controlled laboratory studies of dispersant-treated and non-treated oil in subsea dispersant tank tests and field surveys, including the Deep Spill experimental release and the Deepwater Horizon blowout oil spill. This model is an advancement over prior models, as it explicitly addresses the effects of the dispersed phase viscosity, resulting from dispersant application and constrains the maximum stable droplet size based on Rayleigh-Taylor instability that is invoked for a release from a large aperture. Copyright © 2016 Elsevier Ltd. All rights reserved.

Theoretical study of liquid droplet dispersion in a venturi scrubber.

PubMed

Fathikalajahi, J; Talaie, M R; Taheri, M

1995-03-01

The droplet concentration distribution in an atomizing scrubber was calculated based on droplet eddy diffusion by a three-dimensional dispersion model. This model is also capable of predicting the liquid flowing on the wall. The theoretical distribution of droplet concentration agrees well with experimental data given by Viswanathan et al. for droplet concentration distribution in a venturi-type scrubber. The results obtained by the model show a non-uniform distribution of drops over the cross section of the scrubber, as noted by the experimental data. While the maximum of droplet concentration distribution may depend on many operating parameters of the scrubber, the results of this study show that the highest uniformity of drop distribution will be reached when penetration length is approximately equal to one-fourth of the depth of the scrubber. The results of this study can be applied to evaluate the removal efficiency of a venturi scrubber.

Ground deposition of liquid droplets released from a point source in the atmospheric surface layer

NASA Astrophysics Data System (ADS)

Panneton, Bernard

1989-01-01

A series of field experiments is presented in which the ground deposition of liquid droplets, 120 and 150 microns in diameter, released from a point source at 7 m above ground level, was measured. A detailed description of the experimental technique is provided, and the results are presented and compared to the predictions of a few models. A new rotating droplet generator is described. Droplets are produced by the forced breakup of capillary liquid jets and droplet coalescence is inhibited by the rotational motion of the spray head. The two dimensional deposition patterns are presented in the form of plots of contours of constant density, normalized arcwise distributions and crosswind integrated distributions. The arcwise distributions follow a Gaussian distribution whose standard deviation is evaluated using a modified Pasquill's technique. Models of the crosswind integrated deposit from Godson, Csanady, Walker, Bache and Sayer, and Wilson et al are evaluated. The results indicate that the Wilson et al random walk model is adequate for predicting the ground deposition of the 150 micron droplets. In one case, where the ratio of the droplet settling velocity to the mean wind speed was largest, Walker's model proved to be adequate. Otherwise, none of the models were acceptable in light of the experimental data.

On the Development of Spray Submodels Based on Droplet Size Moments

NASA Astrophysics Data System (ADS)

Beck, J. C.; Watkins, A. P.

2002-11-01

Hitherto, all polydisperse spray models have been based on discretising the liquid flow field into groups of equally sized droplets. The authors have recently developed a spray model that captures the full polydisperse nature of the spray flow without using droplet size classes (Beck, 2000, Ph.D thesis, UMIST; Beck and Watkins, 2001, Proc. R. Soc. London A). The parameters used to describe the distribution of droplet sizes are the moments of the droplet size distribution function. Transport equations are written for the two moments which represent the liquid mass and surface area, and two more moments representing the sum of drop radii and droplet number are approximated via use of a presumed distribution function, which is allowed to vary in space and time. The velocities to be used in the two transport equations are obtained by defining moment-average quantities and constructing further transport equations for the relevant moment-average velocities. An equation for the energy of the liquid phase and standard gas phase equations, including a k-ɛ turbulence model, are also solved. All the equations are solved in an Eulerian framework using the finite-volume approach, and the phases are coupled through source terms. Effects such as interphase drag, droplet breakup, and droplet-droplet collisions are also captured through the use of source terms. The development of the submodels to describe these effects is the subject of this paper. All the source terms for the hydrodynamics of the spray are derived in this paper in terms of the four moments of the droplet size distribution in order to find the net effect on the whole spray flow field. The development of similar submodels to describe heat and mass transfer effects between the phases is the subject of a further paper (Beck and Watkins, 2001, J. Heat Fluid Flow). The model has been applied to a wide variety of different sprays, including high-pressure diesel sprays, wide-angle solid-cone water sprays, hollow-cone spray s, and evaporating sprays. The comparisons of the results with experimental data show that the model performs well. The interphase drag model, along with the model for the turbulent dispersion of the liquid, produces excellent agreement in the spray penetration results, and the moment-average velocity approach gives good radial distributions of droplet size, showing the capability of the model to predict polydisperse behaviour. Good submodel performance results in droplet breakup, collisions, and evaporation effects (see (Beck and Watkins, 2001, J. Heat Fluid Flow)) also being captured successfully.

Droplet Breakup in Asymmetric T-Junctions at Intermediate to Large Capillary Numbers

NASA Astrophysics Data System (ADS)

Sadr, Reza; Cheng, Way Lee

2017-11-01

Splitting of a parent droplet into multiple daughter droplets of desired sizes is usually desired to enhance production and investigational efficiency in microfluidic devices. This can be done in an active or passive mode depending on whether an external power sources is used or not. In this study, three-dimensional simulations were done using the Volume-of-Fluid (VOF) method to analyze droplet splitting in asymmetric T-junctions with different outlet lengths. The parent droplet is divided into two uneven portions the volumetric ratio of the daughter droplets, in theory, depends on the length ratios of the outlet branches. The study identified various breakup modes such as primary, transition, bubble and non-breakup under various flow conditions and the configuration of the T-junctions. In addition, an analysis with the primary breakup regimes were conducted to study the breakup mechanisms. The results show that the way the droplet splits in an asymmetric T-junction is different than the process in a symmetric T-junction. A model for the asymmetric breakup criteria at intermediate or large Capillary number is presented. The proposed model is an expanded version to a theoretically derived model for the symmetric droplet breakup under similar flow conditions.

Modeling waves forced by a drop bouncing on a vibrating bath

NASA Astrophysics Data System (ADS)

Turton, Sam; Rosales, Ruben; Bush, John

2017-11-01

We study the wavefield generated by a droplet bouncing on a bath of silicon oil undergoing vertical oscillations. Such droplets may bounce indefinitely below the Faraday threshold, and in certain parameter regimes destabilize into a walking state in which they are propelled by their own wavefield. While previous theoretical models have rationalize the behavior of single droplets, difficulties have arisen in rationalizing the behavior of multi-droplet systems. We here present a refined wave model that allows us to do so. In particular, we give a detailed account of the spatio-temporal decay of the waves, in addition to the couping between the wave amplitude and modulations in the droplet's vertical dynamics. Our analytic model is compared with the results of direct numerical simulations and experiments. We gratefully acknowledge the financial support of the NSF.

Modeling the Transport Phenomena in the Solution Precursor Plasma Spraying

NASA Astrophysics Data System (ADS)

Shan, Yanguang

2008-10-01

Solution precursor plasma spraying has been used to produce finely structured ceramic coatings with nano- and sub-micrometric features. This process involves the injection of a solution spray of ceramic salts into a DC plasma jet under atmospheric condition. During the process, the solvent vaporizes as the droplet travel downstream. Solid particles are finally formed due to the precipitation of the solute, and the particle are heated up and accelerated to the substrate to generate the coating. This work describes a 3D model to simulate the transport phenomena and the trajectory and heating of the solution spray in the process. The jet-spray two-way interactions are considered. A simplified model is employed to simulate the evolution process and the formation of the solid particle from the solution droplet in the plasma jet. O'Rourke's droplet collision model is used to take into account of the influence of droplet collision. The influence of droplet breakup is also considered by implementing TAB droplet breakup models into the plasma jet model. The temperature and velocity fields of the jet are obtained and validated. The particle size, velocity, temperature and position distribution on the substrate are predicted.

Mathematical Modeling of an Oscillating Droplet

NASA Technical Reports Server (NTRS)

Berry, S.; Hyers, R. W.; Racz, L. M.; Abedian, B.; Rose, M. Franklin (Technical Monitor)

2000-01-01

Oscillating droplets are of interest in a number of disciplines. A practical application is the oscillating drop method, which is a technique for measuring surface tension and viscosity of liquid metals. It is especially suited to undercooled and highly reactive metals, because it is performed by electromagnetic levitation. The natural oscillation frequency of the droplets is related to the surface tension of the material, and the decay of oscillations is related to its viscosity. The fluid flow inside the droplet must be laminar in order for this technique to yield good results. Because no experimental method has yet been developed to visualize flow in electromagnetically-levitated oscillating metal droplets, mathematical modeling is required to determine whether or not turbulence occurs. Three mathematical models of the flow: (1) assuming laminar conditions, (2) using the k-epsilon turbulence model, and (3) using the RNG turbulence model, respectively, are compared and contrasted to determine the physical characteristics of the flow. It is concluded that the RNG model is the best suited for describing this problem. The goal of the presented work was to characterize internal flow in an oscillating droplet of liquid metal, and to verify the accuracy of the characterization by comparing calculated surface tension and viscosity.

Large Diameter, Radiative Extinction Experiments with Decane Droplets in Microgravity

NASA Technical Reports Server (NTRS)

Easton, John; Tien, James; Dietrich, Daniel

1999-01-01

The extinction of a diffusion flame is of fundamental interest in combustion science. Linan, Law, and Chung and Law analytically and experimentally determined an extinction boundary in terms of droplet diameter and pressure for a single droplet due to Damkohler, or blowoff, extinction. More recently, other researchers demonstrated extinction due to finite rate kinetics in reduced gravity for free droplets of heptane. Chao modeled the effect of radiative heat loss on a quasi-steady spherically symmetric single droplet burning in the absence of buoyancy. They determined that for increasing droplet diameter, a second limit can be reached such that combustion is no longer possible. This second, larger droplet diameter limit arises due to radiative heat loss, which increases with increasing droplet and flame diameter. This increase in radiative heat loss arises due to an increase in the surface area of the flame. Recently, Marchese modeled fuel droplets with detailed chemistry and radiative effects, and compared the results to other work. The modeling also showed the importance of radiative loss and radiative extinction Experiments examined the behavior of a large droplet of decane burning in reduced gravity onboard the NASA Lewis DC-9 aircraft, but did not show a radiative extinction boundary due to g-jitter (Variations in gravitational level and direction) effects. Dietrich conducted experiments in the reduced gravity environment of the Space Shuttle. This work showed that the extinction diameter of methanol droplets increased when the initial diameter of the droplets was large (in this case, approximately 5 mm). Theoretical results agreed with these experimental results only when the theory included radiative effects . Radiative extinction was experimentally verified by Nayagam in a later Shuttle mission. The following work focuses on the combustion and extinction of a single fuel droplet. The goal is to experimentally determine a large droplet diameter limit that arises due to radiative heat loss from the flame to the surroundings.

Analytical Model for Diffusive Evaporation of Sessile Droplets Coupled with Interfacial Cooling Effect.

PubMed

Nguyen, Tuan A H; Biggs, Simon R; Nguyen, Anh V

2018-05-30

Current analytical models for sessile droplet evaporation do not consider the nonuniform temperature field within the droplet and can overpredict the evaporation by 20%. This deviation can be attributed to a significant temperature drop due to the release of the latent heat of evaporation along the air-liquid interface. We report, for the first time, an analytical solution of the sessile droplet evaporation coupled with this interfacial cooling effect. The two-way coupling model of the quasi-steady thermal diffusion within the droplet and the quasi-steady diffusion-controlled droplet evaporation is conveniently solved in the toroidal coordinate system by applying the method of separation of variables. Our new analytical model for the coupled vapor concentration and temperature fields is in the closed form and is applicable for a full range of spherical-cap shape droplets of different contact angles and types of fluids. Our analytical results are uniquely quantified by a dimensionless evaporative cooling number E o whose magnitude is determined only by the thermophysical properties of the liquid and the atmosphere. Accordingly, the larger the magnitude of E o , the more significant the effect of the evaporative cooling, which results in stronger suppression on the evaporation rate. The classical isothermal model is recovered if the temperature gradient along the air-liquid interface is negligible ( E o = 0). For substrates with very high thermal conductivities (isothermal substrates), our analytical model predicts a reversal of temperature gradient along the droplet-free surface at a contact angle of 119°. Our findings pose interesting challenges but also guidance for experimental investigations.

Design of Experiments to Study the Impact of Process Parameters on Droplet Size and Development of Non-Invasive Imaging Techniques in Tablet Coating

PubMed Central

Dennison, Thomas J.; Smith, Julian; Hofmann, Michael P.; Bland, Charlotte E.; Badhan, Raj K.; Al-Khattawi, Ali; Mohammed, Afzal R.

2016-01-01

Atomisation of an aqueous solution for tablet film coating is a complex process with multiple factors determining droplet formation and properties. The importance of droplet size for an efficient process and a high quality final product has been noted in the literature, with smaller droplets reported to produce smoother, more homogenous coatings whilst simultaneously avoiding the risk of damage through over-wetting of the tablet core. In this work the effect of droplet size on tablet film coat characteristics was investigated using X-ray microcomputed tomography (XμCT) and confocal laser scanning microscopy (CLSM). A quality by design approach utilising design of experiments (DOE) was used to optimise the conditions necessary for production of droplets at a small (20 μm) and large (70 μm) droplet size. Droplet size distribution was measured using real-time laser diffraction and the volume median diameter taken as a response. DOE yielded information on the relationship three critical process parameters: pump rate, atomisation pressure and coating-polymer concentration, had upon droplet size. The model generated was robust, scoring highly for model fit (R2 = 0.977), predictability (Q2 = 0.837), validity and reproducibility. Modelling confirmed that all parameters had either a linear or quadratic effect on droplet size and revealed an interaction between pump rate and atomisation pressure. Fluidised bed coating of tablet cores was performed with either small or large droplets followed by CLSM and XμCT imaging. Addition of commonly used contrast materials to the coating solution improved visualisation of the coating by XμCT, showing the coat as a discrete section of the overall tablet. Imaging provided qualitative and quantitative evidence revealing that smaller droplets formed thinner, more uniform and less porous film coats. PMID:27548263

Estimation and control of droplet size and frequency in projected spray mode of a gas metal arc welding (GMAW) process.

PubMed

Anzehaee, Mohammad Mousavi; Haeri, Mohammad

2011-07-01

New estimators are designed based on the modified force balance model to estimate the detaching droplet size, detached droplet size, and mean value of droplet detachment frequency in a gas metal arc welding process. The proper droplet size for the process to be in the projected spray transfer mode is determined based on the modified force balance model and the designed estimators. Finally, the droplet size and the melting rate are controlled using two proportional-integral (PI) controllers to achieve high weld quality by retaining the transfer mode and generating appropriate signals as inputs of the weld geometry control loop. Copyright © 2011 ISA. Published by Elsevier Ltd. All rights reserved.

Modeling for Standoff Surface Detection

DTIC Science & Technology

2013-11-01

Height and Volume of a Droplet ..............................................37 9. INKJET PRINTING OF DROPLET DISTRIBUTION...38 9.1 Inkjet Printing of SF96-5 on Teflon Material ..................................................38 9.2 VLSTRACK Witness Card...modeled droplet distribution on relevant surfaces with actual chemicals. This unique printer is a flatbed inkjet printer that can be used to deposit

A new model for the biodegradation kinetics of oil droplets: application to the Deepwater Horizon oil spill in the Gulf of Mexico

PubMed Central

2013-01-01

Oil biodegradation by native bacteria is one of the most important natural processes that can attenuate the environmental impacts of marine oil spills. Existing models for oil biodegradation kinetics are mostly for dissolved oil. This work developed a new mathematical model for the biodegradation of oil droplets and applied the model to estimate the time scale for oil biodegradation under conditions relevant to the Deepwater Horizon oil spill in the Gulf of Mexico. In the model, oil is composed of droplets of various sizes following the gamma function distribution. Each oil droplet shrinks during the microbe-mediated degradation at the oil-water interface. Using our developed model, we find that the degradation of oil droplets typically goes through two stages. The first stage is characterized by microbial activity unlimited by oil-water interface with higher biodegradation rates than that of the dissolved oil. The second stage is governed by the availability of the oil-water interface, which results in much slower rates than that of soluble oil. As a result, compared to that of the dissolved oil, the degradation of oil droplets typically starts faster and then quickly slows down, ultimately reaching a smaller percentage of degraded oil in longer time. The availability of the water-oil interface plays a key role in determining the rates and extent of degradation. We find that several parameters control biodegradation rates, including size distribution of oil droplets, initial microbial concentrations, initial oil concentration and composition. Under conditions relevant to the Deepwater Horizon spill, we find that the size distribution of oil droplets (mean and coefficient of variance) is the most important parameter because it determines the availability of the oil-water interface. Smaller oil droplets with larger variance leads to faster and larger extent of degradation. The developed model will be useful for evaluating transport and fate of spilled oil, different remediation strategies, and risk assessment. PMID:24138161

Droplet-turbulence interactions in subcritical and supercritical evaporating sprays

NASA Technical Reports Server (NTRS)

Santavicca, Domenic A.; Coy, Edward; Greenfield, Stuart; Song, Young-Hoon

1991-01-01

The objective of this research is to obtain an improved understanding of droplet turbulence interactions in vaporizing liquid sprays under conditions typical of those encountered in liquid fueled rocket engines. The interaction between liquid droplets and the surrounding turbulent gas flow affects droplet dispersion, droplet collisions, droplet vaporization and gas-phase, fuel-oxidant mixing, and therefore has a significant effect on the engine's combustion characteristics. An example of this is the role which droplet-turbulence interactions are believed to play in combustion instabilities. Despite their importance, droplet-turbulence interactions and their effect on liquid fueled rocket engine performance are not well understood. This is particularly true under supercritical conditions, where many conventional concepts, such as surface tension, no longer apply. Our limited understanding of droplet-turbulence interactions, under both subcritical conditions, represents a major limitation in our ability to design improved liquid previously unavailable information and valuable new insights which will directly impact the design of future liquid fueled rocket engines, as well as, allow for the development of significantly improved spray combustion models, making such models useful design tools.

The effects of van der Waals attractions on cloud droplet growth by coalescence

NASA Technical Reports Server (NTRS)

Rogers, Jan R.; Davis, Robert H.

1990-01-01

The inclusion of van der Waals attractions in the interaction between cloud droplets has been recently shown to significantly increase the collision efficiencies of the smaller droplets. In the current work, these larger values for the collision efficiencies are used in a population dynamics model of the droplet size distribution evolution with time, in hopes of at least partially resolving the long-standing paradox in cloud microphysics that predicted rates of the onset of precipitation are generally much lower than those which are observed. Evolutions of several initial cloud droplet spectra have been tracked in time. Size evolutions are compared as predicted from the use of collision efficiencies computed using two different models to allow for droplet-droplet contact: one which considers slip flow effects only, and one which considers the combined effects of van der Waals forces and slip flow. The rate at which the droplet mass density function shifts to larger droplet sizes is increased by typically 20-25 percent, when collision efficiencies which include van der Waals forces are used.

The clouds of Venus. II - An investigation of the influence of coagulation on the observed droplet size distribution

NASA Technical Reports Server (NTRS)

Rossow, W. B.

1977-01-01

An approximate numerical technique is used to investigate the influence of coagulation, sedimentation and turbulent motions on the observed droplet size distribution in the upper layers of the Venus clouds. If the cloud mass mixing ratio is less than 0.000001 at 250 K or the eddy diffusivity throughout the cloud is greater than 1,000,000 sq cm per sec, then coagulation is unimportant. In this case, the observed droplet size distribution is the initial size distribution produced by the condensation of the droplets. It is found that all cloud models with droplet formation near the cloud top (e.g., a photochemical model) must produce the observed droplet size distribution by condensation without subsequent modification by coagulation. However, neither meteoritic or surface dust can supply sufficient nucleating particles to account for the observed droplet number density. If the cloud droplets are formed near the cloud bottom, the observed droplet size distribution can be produced solely by the interaction of coagulation and dynamics; all information about the initial size distribution is lost. If droplet formation occurs near the cloud bottom, the lower atmosphere of Venus is oxidizing rather than reducing.

Modeling of spray droplets deformation and breakup

NASA Technical Reports Server (NTRS)

Ibrahim, E. A.; Yang, H. Q.; Przekwas, A. J.

1993-01-01

A droplet deformation and breakup (DDB) model is proposed to study shear-type mechanism of spray droplets in pure extentional flows. A numerical solution of the DDB model equation is obtained using a fourth-order Runge-Kutta initial-value solver. The predictions of the DDB model as well as semianalytical and the Taylor analogy models are compared with the experimental data (Krzeczkowski, 1980) for shear breakup, which depict the dimensionless deformation of the drop vs dimensionless time.

Modeling the spray casting process

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

El-Haggar, S.M.; Muoio, N.; Crowe, C.T.

1995-12-31

Spray forming is a process in which a liquid metal is atomized into very small droplets and deposited on a substrate. These small droplets cool very rapidly in a high velocity gas jet, giving rise to smaller grain structure and improved mechanical properties. This paper presents a numerical model, based on the trajectory approach, for the velocity and thermal properties of the droplets in the jet and predicts the deposition pattern and the state of the droplets upon contact with the substrate.

Evaluation of evaporation coefficient for micro-droplets exposed to low pressure: A semi-analytical approach

NASA Astrophysics Data System (ADS)

Chakraborty, Prodyut R.; Hiremath, Kirankumar R.; Sharma, Manvendra

2017-02-01

Evaporation rate of water is strongly influenced by energy barrier due to molecular collision and heat transfer limitations. The evaporation coefficient, defined as the ratio of experimentally measured evaporation rate to that maximum possible theoretical limit, varies over a conflicting three orders of magnitude. In the present work, a semi-analytical transient heat diffusion model of droplet evaporation is developed considering the effect of change in droplet size due to evaporation from its surface, when the droplet is injected into vacuum. Negligible effect of droplet size reduction due to evaporation on cooling rate is found to be true. However, the evaporation coefficient is found to approach theoretical limit of unity, when the droplet radius is less than that of mean free path of vapor molecules on droplet surface contrary to the reported theoretical predictions. Evaporation coefficient was found to reduce rapidly when the droplet under consideration has a radius larger than the mean free path of evaporating molecules, confirming the molecular collision barrier to evaporation rate. The trend of change in evaporation coefficient with increasing droplet size predicted by the proposed model will facilitate obtaining functional relation of evaporation coefficient with droplet size, and can be used for benchmarking the interaction between multiple droplets during evaporation in vacuum.

Hydrodynamic resistance and mobility of deformable objects in microfluidic channels

PubMed Central

Sajeesh, P.; Doble, M.; Sen, A. K.

2014-01-01

This work reports experimental and theoretical studies of hydrodynamic behaviour of deformable objects such as droplets and cells in a microchannel. Effects of mechanical properties including size and viscosity of these objects on their deformability, mobility, and induced hydrodynamic resistance are investigated. The experimental results revealed that the deformability of droplets, which is quantified in terms of deformability index (D.I.), depends on the droplet-to-channel size ratio ρ and droplet-to-medium viscosity ratio λ. Using a large set of experimental data, for the first time, we provide a mathematical formula that correlates induced hydrodynamic resistance of a single droplet ΔRd with the droplet size ρ and viscosity λ. A simple theoretical model is developed to obtain closed form expressions for droplet mobility ϕ and ΔRd. The predictions of the theoretical model successfully confront the experimental results in terms of the droplet mobility ϕ and induced hydrodynamic resistance ΔRd. Numerical simulations are carried out using volume-of-fluid model to predict droplet generation and deformation of droplets of different size ratio ρ and viscosity ratio λ, which compare well with that obtained from the experiments. In a novel effort, we performed experiments to measure the bulk induced hydrodynamic resistance ΔR of different biological cells (yeast, L6, and HEK 293). The results reveal that the bulk induced hydrodynamic resistance ΔR is related to the cell concentration and apparent viscosity of the cells. PMID:25538806

Combustion of interacting droplet arrays in a microgravity environment

NASA Technical Reports Server (NTRS)

Dietrich, Daniel L.

1995-01-01

This research program involves the study of one and two dimensional arrays of droplets in a buoyant-free environment. The purpose of the work is to extend the database and theories that exist for single droplets into the regime where droplet interactions are important. The eventual goal being to use the results of this work as inputs to models on spray combustion where droplets seldom burn individually; instead the combustion history of a droplet is strongly influenced by the presence of the neighboring droplets. Throughout the course of the work, a number of related aspects of isolated droplet combustion have also been investigated. This paper will review our progress in microgravity droplet array combustion, advanced diagnostics (specifically L2) applied to isolated droplet combustion, and radiative extinction large droplet flames. A small-scale droplet combustion experiment being developed for the Space Shuttle will also be described.

Heat transfer studies on the liquid droplet radiator

NASA Technical Reports Server (NTRS)

Mattick, A. T.; Nelson, M.

1987-01-01

This paper examines radiation transfer in the droplet sheet of a liquid droplet radiator including non-isotropic scattering by the droplets. Non-isotropic scattering becomes significant for small droplets (diameter less than 0.1 mm) and for low emissivity liquids. For droplets with an emittance of 0.1 and for a droplet sheet optical depth or 5, the radiated power varies by about 12 percent, depending on whether scattering is predominantly forward or backward. An experimental measurement of the power emitted by a cylindrical cloud of heated droplets of silicone fluid is also reported. The measured cloud emissivity correlates, within experimental error, with the analytical model.

Optics of retinal oil droplets: a model of light collection and polarization detection in the avian retina.

PubMed

Young, S R; Martin, G R

1984-01-01

A wave optical model was used to analyse the scattering properties of avian retinal oil droplets. Computations for the near field region showed that oil droplets perform significant light collection in cone photoreceptors and so enhance outer segment photon capture rates. Scattering by the oil droplet of the principal cone of a double cone pair, combined with accessory cone dichroic absorption under conditions of transverse illumination, may mediate avian polarization sensitivity.

Motion behavior of water droplets driven by triboelectric nanogenerator

NASA Astrophysics Data System (ADS)

Nie, Jinhui; Jiang, Tao; Shao, Jiajia; Ren, Zewei; Bai, Yu; Iwamoto, Mitsumasa; Chen, Xiangyu; Wang, Zhong Lin

2018-04-01

By integrating a triboelectric nanogenerator (TENG) and a simple circuit board, the motion of water droplets can be controlled by the output of the TENG, which demonstrates a self-powered microfluidic system toward various practical applications in the fields of microfluidic system and soft robotics. This paper describes a method to construct a physical model for this self-powered system on the basis of electrostatic induction theory. The model can precisely simulate the detailed motion behavior of the droplet under driving of TENG, and it can also reveal the influences of surface hydrophobicity on the motion of the droplet, which can help us to better understand the key parameters that decide the performance of the system. The experimental observation of the dynamic performance of the droplet has also been done with a high speed camera system. A comparison between simulation results and real measurements confirms that the proposed model can predict the velocity and position of the water droplet driven by high voltage source as well as TENG. Hence, the proposed model in this work could serve as a guidance for optimizing the self-powered systems in future studies.

Theoretical model of chirality-induced helical self-propulsion

NASA Astrophysics Data System (ADS)

Yamamoto, Takaki; Sano, Masaki

2018-01-01

We recently reported the experimental realization of a chiral artificial microswimmer exhibiting helical self-propulsion [T. Yamamoto and M. Sano, Soft Matter 13, 3328 (2017), 10.1039/C7SM00337D]. In the experiment, cholesteric liquid crystal (CLC) droplets dispersed in surfactant solutions swam spontaneously, driven by the Marangoni flow, in helical paths whose handedness is determined by the chirality of the component molecules of CLC. To study the mechanism of the emergence of the helical self-propelled motion, we propose a phenomenological model of the self-propelled helical motion of the CLC droplets. Our model is constructed by symmetry argument in chiral systems, and it describes the dynamics of CLC droplets with coupled time-evolution equations in terms of a velocity, an angular velocity, and a tensor variable representing the symmetry of the helical director field of the droplet. We found that helical motions as well as other chiral motions appear in our model. By investigating bifurcation behaviors between each chiral motion, we found that the chiral coupling terms between the velocity and the angular velocity, the structural anisotropy of the CLC droplet, and the nonlinearity of model equations play a crucial role in the emergence of the helical motion of the CLC droplet.

Repulsion-based model for contact angle saturation in electrowetting

PubMed Central

2015-01-01

We introduce a new model for contact angle saturation phenomenon in electrowetting on dielectric systems. This new model attributes contact angle saturation to repulsion between trapped charges on the cap and base surfaces of the droplet in the vicinity of the three-phase contact line, which prevents these surfaces from converging during contact angle reduction. This repulsion-based saturation is similar to repulsion between charges accumulated on the surfaces of conducting droplets which causes the well known Coulombic fission and Taylor cone formation phenomena. In our model, both the droplet and dielectric coating were treated as lossy dielectric media (i.e., having finite electrical conductivities and permittivities) contrary to the more common assumption of a perfectly conducting droplet and perfectly insulating dielectric. We used theoretical analysis and numerical simulations to find actual charge distribution on droplet surface, calculate repulsion energy, and minimize energy of the total system as a function of droplet contact angle. Resulting saturation curves were in good agreement with previously reported experimental results. We used this proposed model to predict effect of changing liquid properties, such as electrical conductivity, and system parameters, such as thickness of the dielectric layer, on the saturation angle, which also matched experimental results. PMID:25759748

Repulsion-based model for contact angle saturation in electrowetting.

PubMed

Ali, Hassan Abdelmoumen Abdellah; Mohamed, Hany Ahmed; Abdelgawad, Mohamed

2015-01-01

We introduce a new model for contact angle saturation phenomenon in electrowetting on dielectric systems. This new model attributes contact angle saturation to repulsion between trapped charges on the cap and base surfaces of the droplet in the vicinity of the three-phase contact line, which prevents these surfaces from converging during contact angle reduction. This repulsion-based saturation is similar to repulsion between charges accumulated on the surfaces of conducting droplets which causes the well known Coulombic fission and Taylor cone formation phenomena. In our model, both the droplet and dielectric coating were treated as lossy dielectric media (i.e., having finite electrical conductivities and permittivities) contrary to the more common assumption of a perfectly conducting droplet and perfectly insulating dielectric. We used theoretical analysis and numerical simulations to find actual charge distribution on droplet surface, calculate repulsion energy, and minimize energy of the total system as a function of droplet contact angle. Resulting saturation curves were in good agreement with previously reported experimental results. We used this proposed model to predict effect of changing liquid properties, such as electrical conductivity, and system parameters, such as thickness of the dielectric layer, on the saturation angle, which also matched experimental results.

Diffusive growth of a single droplet with three different boundary conditions

NASA Astrophysics Data System (ADS)

Tavassoli, Z.; Rodgers, G. J.

2000-02-01

We study a single, motionless three-dimensional droplet growing by adsorption of diffusing monomers on a 2D substrate. The diffusing monomers are adsorbed at the aggregate perimeter of the droplet with different boundary conditions. Models with both an adsorption boundary condition and a radiation boundary condition, as well as a phenomenological model, are considered and solved in a quasistatic approximation. The latter two models allow particle detachment. In the short time limit, the droplet radius grows as a power of the time with exponents of 1/4, 1/2 and 3/4 for the models with adsorption, radiation and phenomenological boundary conditions, respectively. In the long time limit a universal growth rate as $[t/\\ln(t)]^{1/3}$ is observed for the radius of the droplet for all models independent of the boundary conditions. This asymptotic behaviour was obtained by Krapivsky \\cite{krapquasi} where a similarity variable approach was used to treat the growth of a droplet with an adsorption boundary condition based on a quasistatic approximation. Another boundary condition with a constant flux of monomers at the aggregate perimeter is also examined. The results exhibit a power law growth rate with an exponent of 1/3 for all times.

Experimental and Numerical Study of Spacecraft Contamination Problems Associated With Gas and Gas-Droplet Thruster Plume Flows

DTIC Science & Technology

2006-04-17

of the droplet phase are then used for validation of theoretical models of the gas-droplet plume flow. Based on experimental and numerical results...with the continuous model adequately reproduces the Arrhenius rate at high temperatures but significantly underpredicts the theoretical rate at low...continuous model and discrete model of real gas effects, and the results on the shock -wave stand-off distance were compared with the experimental data of

Performance estimation of a Venturi scrubber using a computational model for capturing dust particles with liquid spray.

PubMed

Pak, S I; Chang, K S

2006-12-01

A Venturi scrubber has dispersed three-phase flow of gas, dust, and liquid. Atomization of a liquid jet and interaction between the phases has a large effect on the performance of Venturi scrubbers. In this study, a computational model for the interactive three-phase flow in a Venturi scrubber has been developed to estimate pressure drop and collection efficiency. The Eulerian-Lagrangian method is used to solve the model numerically. Gas flow is solved using the Eulerian approach by using the Navier-Stokes equations, and the motion of dust and liquid droplets, described by the Basset-Boussinesq-Oseen (B-B-O) equation, is solved using the Lagrangian approach. This model includes interaction between gas and droplets, atomization of a liquid jet, droplet deformation, breakup and collision of droplets, and capture of dust by droplets. A circular Pease-Anthony Venturi scrubber was simulated numerically with this new model. The numerical results were compared with earlier experimental data for pressure drop and collection efficiency, and gave good agreements.

Dynamics of skirting droplets

NASA Astrophysics Data System (ADS)

Akers, Caleb; Hale, Jacob

2014-11-01

It has been observed that non-coalescence between a droplet and pool of like fluid can be prolonged or inhibited by sustained relative motion between the two fluids. In this study, we quantitatively describe the motion of freely moving droplets that skirt across the surface of a still pool of like fluid. Droplets of different sizes and small Weber number were directed horizontally onto the pool surface. After stabilization of the droplet shape after impact, the droplets smoothly moved across the surface, slowing until coalescence. Using high-speed imaging, we recorded the droplet's trajectory from a top-down view as well as side views both slightly above and below the fluid surface. The droplets' speed is observed to decrease exponentially, with the smaller droplets slowing down at a greater rate. Droplets infused with neutral density micro beads showed that the droplet rolls along the surface of the pool. A qualitative model of this motion is presented.

Insights into Evaporative Droplet Dynamics in the High-Wind Atmospheric Boundary Layer

NASA Astrophysics Data System (ADS)

Peng, T.; Richter, D. H.

2017-12-01

Sea-spray droplets ejected into the air-sea boundary layer take part in a series of complex transport processes. To model the air-sea exchange of heat and moisture under high-wind conditions, it is important yet challenging to understand influences of evaporative droplets in the atmospheric boundary layer. We implement a high-resolution Eulerian-Lagrangian algorithm with droplets laden in a turbulent open-channel flow to reveal the dynamic and thermodynamic characteristics of evaporating sea spray. Our past numerical simulations demonstrated an overall weak modification to the total heat flux by evaporative droplets. This is due to redistributed sensible and latent heat fluxes from relatively small droplets that respond rapidly to the ambient environment or the limited residence time of larger droplets. However, droplets with a slower thermodynamic response to the environment indicate a potential to enhance the total heat flux, but this is dependent on concentration and suspension time. In the current study, we focus on correlations between the residence time and thermodynamic statistics of droplets in order to better understand how best to parameterize in large-scale models. In addition, we focus in detail on the different scales of turbulence to further characterize the range of influence that evaporating droplets have on the surrounding fluid.

Ignition and combustion characteristics of metallized propellants

NASA Technical Reports Server (NTRS)

Turns, Stephen R.; Mueller, D. C.

1993-01-01

Experimental and analytical investigations focusing on secondary atomization and ignition characteristics of aluminum/liquid hydrocarbon slurry propellants were conducted. Experimental efforts included the application of a laser-based, two-color, forward-scatter technique to simultaneously measure free-flying slurry droplet diameters and velocities for droplet diameters in the range of 10-200 microns. A multi-diffusion flame burner was used to create a high-temperature environment into which a dilute stream of slurry droplets could be introduced. Narrowband measurements of radiant emission were used to determine if ignition of the aluminum in the slurry droplet had occurred. Models of slurry droplet shell formation were applied to aluminum/liquid hydrocarbon propellants and used to ascertain the effects of solids loading and ultimate particle size on the minimum droplet diameter that will permit secondary atomization. For a 60 weight-percent Al slurry, the limiting critical diameter was predicted to be 34.7 microns which is somewhat greater than the 20-25 micron limiting diameters determined in the experiments. A previously developed model of aluminum ignition in a slurry droplet was applied to the present experiments and found to predict ignition times in reasonable agreement with experimental measurements. A model was also developed that predicts the mechanical stress in the droplet shell and a parametric study was conducted. A one-dimensional model of a slurry-fueled rocket combustion chamber was developed. This model includes the processes of liquid hydrocarbon burnout, secondary atomization, aluminum ignition, and aluminum combustion. Also included is a model for radiant heat transfer from the hot aluminum oxide particles to the chamber walls. Exercising this model shows that only a modest amount of secondary atomization is required to reduce residence times for aluminum burnout, and thereby maintain relatively short chamber lengths. The model also predicts radiant heat transfer losses to the walls to be only approximately 3 percent of the fuel energy supplied. Additional work is required to determine the effects of secondary atomization on two-phase losses in the nozzle.

Effects of Deformation on Drag and Lift Forces Acting on a Droplet in a Shear Flow

NASA Astrophysics Data System (ADS)

Suh, Youngho; Lee, Changhoon

2010-11-01

The droplet behavior in a linear shear flow is studied numerically to investigate the effect of deformation on the drag and lift acting on droplet. The droplet shape is calculated by a level set method which is improved by incorporating a sharp-interface modeling technique for accurately enforcing the matching conditions at the liquid- gas interface. By adopting the feedback forces which can maintain the droplet at a fixed position, we determine the acting force on a droplet in shear flow field with efficient handling of deformation. Based on the numerical results, drag and lift forces acting on a droplet are observed to depend strongly on the deformation. Droplet shapes are observed to be spherical, deformed, and oscillating depending on the Reynolds number. Also, the present method is proven to be applicable to a three- dimensional deformation of droplet in the shear flow, which cannot be properly analyzed by the previous studies. Comparisons of the calculated results by the current method with those obtained from body-fitted methods [Dandy and Leal, J. Fluid Mech. 208, 161 (1989)] and empirical models [Feng and Beard, J. Atmos. Sci. 48, 1856 (1991)] show good agreement.

Microfluidic generation of aqueous two-phase system (ATPS) droplets by controlled pulsating inlet pressures.

PubMed

Moon, Byeong-Ui; Jones, Steven G; Hwang, Dae Kun; Tsai, Scott S H

2015-06-07

We present a technique that generates droplets using ultralow interfacial tension aqueous two-phase systems (ATPS). Our method combines a classical microfluidic flow focusing geometry with precisely controlled pulsating inlet pressure, to form monodisperse ATPS droplets. The dextran (DEX) disperse phase enters through the central inlet with variable on-off pressure cycles controlled by a pneumatic solenoid valve. The continuous phase polyethylene glycol (PEG) solution enters the flow focusing junction through the cross channels at a fixed flow rate. The on-off cycles of the applied pressure, combined with the fixed flow rate cross flow, make it possible for the ATPS jet to break up into droplets. We observe different droplet formation regimes with changes in the applied pressure magnitude and timing, and the continuous phase flow rate. We also develop a scaling model to predict the size of the generated droplets, and the experimental results show a good quantitative agreement with our scaling model. Additionally, we demonstrate the potential for scaling-up of the droplet production rate, with a simultaneous two-droplet generating geometry. We anticipate that this simple and precise approach to making ATPS droplets will find utility in biological applications where the all-biocompatibility of ATPS is desirable.

Acoustic and photoacoustic characterization of micron-sized perfluorocarbon emulsions

NASA Astrophysics Data System (ADS)

Strohm, Eric M.; Gorelikov, Ivan; Matsuura, Naomi; Kolios, Michael C.

2012-09-01

Perfluorocarbon droplets containing nanoparticles (NPs) have recently been investigated as theranostic and dual-mode contrast agents. These droplets can be vaporized via laser irradiation or used as photoacoustic contrast agents below the vaporization threshold. This study investigates the photoacoustic mechanism of NP-loaded droplets using photoacoustic frequencies between 100 and 1000 MHz, where distinct spectral features are observed that are related to the droplet composition. The measured photoacoustic spectrum from NP-loaded perfluorocarbon droplets was compared to a theoretical model that assumes a homogenous liquid. Good agreement in the location of the spectral features was observed, which suggests the NPs act primarily as optical absorbers to induce thermal expansion of the droplet as a single homogenous object. The NP size and composition do not affect the photoacoustic spectrum; therefore, the photoacoustic signal can be maximized by optimizing the NP optical absorbing properties. To confirm the theoretical parameters in the model, photoacoustic, ultrasonic, and optical methods were used to estimate the droplet diameter. Photoacoustic and ultrasonic methods agreed to within 1.4%, while the optical measurement was 8.5% higher; this difference decreased with increasing droplet size. The small discrepancy may be attributed to the difficulty in observing the small droplets through the partially translucent phantom.

Acoustic and photoacoustic characterization of micron-sized perfluorocarbon emulsions.

PubMed

Strohm, Eric M; Gorelikov, Ivan; Matsuura, Naomi; Kolios, Michael C

2012-09-01

Perfluorocarbon droplets containing nanoparticles (NPs) have recently been investigated as theranostic and dual-mode contrast agents. These droplets can be vaporized via laser irradiation or used as photoacoustic contrast agents below the vaporization threshold. This study investigates the photoacoustic mechanism of NP-loaded droplets using photoacoustic frequencies between 100 and 1000 MHz, where distinct spectral features are observed that are related to the droplet composition. The measured photoacoustic spectrum from NP-loaded perfluorocarbon droplets was compared to a theoretical model that assumes a homogenous liquid. Good agreement in the location of the spectral features was observed, which suggests the NPs act primarily as optical absorbers to induce thermal expansion of the droplet as a single homogenous object. The NP size and composition do not affect the photoacoustic spectrum; therefore, the photoacoustic signal can be maximized by optimizing the NP optical absorbing properties. To confirm the theoretical parameters in the model, photoacoustic, ultrasonic, and optical methods were used to estimate the droplet diameter. Photoacoustic and ultrasonic methods agreed to within 1.4%, while the optical measurement was 8.5% higher; this difference decreased with increasing droplet size. The small discrepancy may be attributed to the difficulty in observing the small droplets through the partially translucent phantom.

Effects of current on droplet generation and arc plasma in gas metal arc welding

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

Hu, J.; Tsai, H. L.

2006-09-01

In gas metal arc welding (GMAW), a technology using pulsed currents has been employed to achieve the one-droplet-per-pulse (ODPP) metal transfer mode with the advantages of low average currents, a stable and controllable droplet generation, and reduced spatter. In this paper, a comprehensive model was developed to study the effects of different current profiles on the droplet formation, plasma generation, metal transfer, and weld pool dynamics in GMAW. Five types of welding currents were studied, including two constant currents and three wave form currents. In each type, the transient temperature and velocity distributions of the arc plasma and the moltenmore » metal, and the shapes of the droplet and the weld pool were calculated. The results showed that a higher current generates smaller droplets, higher droplet frequency, and higher electromagnetic force that becomes the dominant factor detaching the droplet from the electrode tip. The model has demonstrated that a stable ODPP metal transfer mode can be achieved by choosing a current with proper wave form for given welding conditions.« less

Droplets size evolution of dispersion in a stirred tank

NASA Astrophysics Data System (ADS)

Kysela, Bohus; Konfrst, Jiri; Chara, Zdenek; Sulc, Radek; Jasikova, Darina

2018-06-01

Dispersion of two immiscible liquids is commonly used in chemical industry as wall as in metallurgical industry e. g. extraction process. The governing property is droplet size distribution. The droplet sizes are given by the physical properties of both liquids and flow properties inside a stirred tank. The first investigation stage is focused on in-situ droplet size measurement using image analysis and optimizing of the evaluation method to achieve maximal result reproducibility. The obtained experimental results are compared with multiphase flow simulation based on Euler-Euler approach combined with PBM (Population Balance Modelling). The population balance model was, in that specific case, simplified with assumption of pure breakage of droplets.

Direct numerical simulation of a combusting droplet with convection

NASA Technical Reports Server (NTRS)

Liang, Pak-Yan

1992-01-01

The evaporation and combustion of a single droplet under forced and natural convection was studied numerically from first principles using a numerical scheme that solves the time-dependent multiphase and multispecies Navier-Stokes equations and tracks the sharp gas-liquid interface cutting across an arbitrary Eulerian grid. The flow fields both inside and outside of the droplet are resolved in a unified fashion. Additional governing equations model the interphase mass, energy, and momentum exchange. Test cases involving iso-octane, n-hexane, and n-propanol droplets show reasonable comparison rate, and flame stand-off distance. The partially validated code is, thus, readied to be applied to more demanding droplet combustion situations where substantial drop deformation render classical models inadequate.

Heat Transfer through a Condensate Droplet on Hydrophobic and Nanostructured Superhydrophobic Surfaces.

PubMed

Chavan, Shreyas; Cha, Hyeongyun; Orejon, Daniel; Nawaz, Kashif; Singla, Nitish; Yeung, Yip Fun; Park, Deokgeun; Kang, Dong Hoon; Chang, Yujin; Takata, Yasuyuki; Miljkovic, Nenad

2016-08-09

Understanding the fundamental mechanisms governing vapor condensation on nonwetting surfaces is crucial to a wide range of energy and water applications. In this paper, we reconcile classical droplet growth modeling barriers by utilizing two-dimensional axisymmetric numerical simulations to study individual droplet heat transfer on nonwetting surfaces (90° < θa < 170°). Incorporation of an appropriate convective boundary condition at the liquid-vapor interface reveals that the majority of heat transfer occurs at the three phase contact line, where the local heat flux can be up to 4 orders of magnitude higher than at the droplet top. Droplet distribution theory is incorporated to show that previous modeling approaches underpredict the overall heat transfer by as much as 300% for dropwise and jumping-droplet condensation. To verify our simulation results, we study condensed water droplet growth using optical and environmental scanning electron microscopy on biphilic samples consisting of hydrophobic and nanostructured superhydrophobic regions, showing excellent agreement with the simulations for both constant base area and constant contact angle growth regimes. Our results demonstrate the importance of resolving local heat transfer effects for the fundamental understanding and high fidelity modeling of phase change heat transfer on nonwetting surfaces.

An interfacial mechanism for cloud droplet formation on organic aerosols

DOE PAGES

Ruehl, C. R.; Davies, J. F.; Wilson, K. R.

2016-03-25

Accurate predictions of aerosol/cloud interactions require simple, physically accurate parameterizations of the cloud condensation nuclei (CCN) activity of aerosols. Current models assume that organic aerosol species contribute to CCN activity by lowering water activity. We measured droplet diameters at the point of CCN activation for particles composed of dicarboxylic acids or secondary organic aerosol and ammonium sulfate. Droplet activation diameters were 40 to 60% larger than predicted if the organic was assumed to be dissolved within the bulk droplet, suggesting that a new mechanism is needed to explain cloud droplet formation. A compressed film model explains how surface tension depressionmore » by interfacial organic molecules can alter the relationship between water vapor supersaturation and droplet size (i.e., the Köhler curve), leading to the larger diameters observed at activation.« less

An interfacial mechanism for cloud droplet formation on organic aerosols

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

Ruehl, C. R.; Davies, J. F.; Wilson, K. R.

Accurate predictions of aerosol/cloud interactions require simple, physically accurate parameterizations of the cloud condensation nuclei (CCN) activity of aerosols. Current models assume that organic aerosol species contribute to CCN activity by lowering water activity. We measured droplet diameters at the point of CCN activation for particles composed of dicarboxylic acids or secondary organic aerosol and ammonium sulfate. Droplet activation diameters were 40 to 60% larger than predicted if the organic was assumed to be dissolved within the bulk droplet, suggesting that a new mechanism is needed to explain cloud droplet formation. A compressed film model explains how surface tension depressionmore » by interfacial organic molecules can alter the relationship between water vapor supersaturation and droplet size (i.e., the Köhler curve), leading to the larger diameters observed at activation.« less

An interfacial mechanism for cloud droplet formation on organic aerosols.

PubMed

Ruehl, Christopher R; Davies, James F; Wilson, Kevin R

2016-03-25

Accurate predictions of aerosol/cloud interactions require simple, physically accurate parameterizations of the cloud condensation nuclei (CCN) activity of aerosols. Current models assume that organic aerosol species contribute to CCN activity by lowering water activity. We measured droplet diameters at the point of CCN activation for particles composed of dicarboxylic acids or secondary organic aerosol and ammonium sulfate. Droplet activation diameters were 40 to 60% larger than predicted if the organic was assumed to be dissolved within the bulk droplet, suggesting that a new mechanism is needed to explain cloud droplet formation. A compressed film model explains how surface tension depression by interfacial organic molecules can alter the relationship between water vapor supersaturation and droplet size (i.e., the Köhler curve), leading to the larger diameters observed at activation. Copyright © 2016, American Association for the Advancement of Science.

Droplet Growth Kinetics in Various Environments

NASA Astrophysics Data System (ADS)

Raatikainen, T. E.; Lathem, T. L.; Moore, R.; Lin, J. J.; Cerully, K. M.; Padro, L.; Lance, S.; Cozic, J.; Anderson, B. E.; Nenes, A.

2012-12-01

The largest uncertainties in the effects of atmospherics aerosols on the global radiation budget are related to their indirect effects on cloud properties (IPCC, the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007). Cloud formation is a kinetic process where the resulting cloud properties depend on aerosol properties and meteorological parameters such as updraft velocity (e.g. McFiggans et al., Atmos. Chem. Phys., 6, 2593-2649, 2006). Droplet growth rates are limited by the water vapor diffusion, but additional kinetic limitations, e.g., due to organic surface films, slow solute dissociation or highly viscous or glassy aerosol states have been hypothesized. Significant additional kinetic limitations can lead to increased cloud droplet number concentration, thus the effect is similar to those of increased aerosol number concentration or changes in vertical velocity (e.g. Nenes et al., Geophys. Res. Lett., 29, 1848, 2002). There are a few studies where slow droplet growth has been observed (e.g. Ruehl et al., Geophys. Res. Lett., 36, L15814, 2009), however, little is currently known about their global occurrence and magnitude. Cloud micro-physics models often describe kinetic limitations by an effective water vapor uptake coefficient or similar parameter. Typically, determining aerosol water vapor uptake coefficients requires experimental observations of droplet growth which are interpreted by a numerical droplet growth model where the uptake coefficient is an adjustable parameter (e.g. Kolb et al., Atmos. Chem. Phys., 10, 10561-10605, 2010). Such methods have not been practical for high time-resolution or long term field measurements, until a model was recently developed for analyzing Droplet Measurement Technologies (DMT) cloud condensation nuclei (CCN) counter data (Raatikainen et al., Atmos. Chem. Phys., 12, 4227-4243, 2012). Model verification experiments showed that the calibration aerosol droplet size can be predicted accurately for various instrument settings and also in the case of high CCN concentrations when water vapor depletion decreases supersaturation and droplet size (Lathem and Nenes, Aerosol Sci. Tech., 45, 604-615, 2011). The model also accounts for aerosol hygroscopicity and size distribution variations, which can have significant effects on the droplet size. We have examined cloud droplet activation and growth kinetics by analyzing several DMT CCN counter data sets collected from various environments including boreal forests, arctic areas, fresh and aged biomass burning plumes, and polluted and biogenically influenced urban areas (Raatikainen et al., In preparation, 2012). Model simulations show that the variations in observed droplet size are caused by water vapor depletion effects, changes in dry particle size distributions and hygroscopicity, and changes in instrument supersaturation profiles. This means that fast droplet growth kinetics with water uptake coefficient close to 0.2 is prevalent at least for the studied environments.

Coalescence-Induced Jumping of Multiple Condensate Droplets on Hierarchical Superhydrophobic Surfaces

PubMed Central

Chen, Xuemei; Patel, Ravi S.; Weibel, Justin A.; Garimella, Suresh V.

2016-01-01

Coalescence-induced jumping of condensate droplets from a superhydrophobic surface with hierarchical micro/nanoscale roughness is quantitatively characterized. Experimental observations show that the condensate droplet jumping is induced by coalescence of multiple droplets of different sizes, and that the coalesced droplet trajectories typically deviate from the surface normal. A depth-from-defocus image processing technique is developed to track the out-of-plane displacement of the jumping droplets, so as to accurately measure the droplet size and velocity. The results demonstrate that the highest jumping velocity is achieved when two droplets coalesce. The jumping velocity decreases gradually with an increase in the number of coalescing droplets, despite the greater potential surface energy released upon coalescence. A general theoretical model that accounts for viscous dissipation, surface adhesion, line tension, the initial droplet wetting states, and the number and sizes of the coalescing droplets is developed to explain the trends of droplet jumping velocity observed in the experiments. PMID:26725512

Cloud droplet size distribution broadening during diffusional growth: ripening amplified by deactivation and reactivation

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

Yang, Fan; Kollias, Pavlos; Shaw, Raymond A.

Cloud droplet size distributions (CDSDs), which are related to cloud albedo and lifetime, are usually broader in warm clouds than predicted from adiabatic parcel calculations. We investigate a mechanism for the CDSD broadening using a Lagrangian bin-microphysics cloud parcel model that considers the condensational growth of cloud droplets formed on polydisperse, sub-micrometer aerosols in an adiabatic cloud parcel that undergoes vertical oscillations, such as those due to cloud circulations or turbulence. Results show that the CDSD can be broadened during condensational growth as a result of Ostwald ripening amplified by droplet deactivation and reactivation, which is consistent with Korolev (1995).more » The relative roles of the solute effect, curvature effect, deactivation and reactivation on CDSD broadening are investigated. Deactivation of smaller cloud droplets, which is due to the combination of curvature and solute effects in the downdraft region, enhances the growth of larger cloud droplets and thus contributes particles to the larger size end of the CDSD. Droplet reactivation, which occurs in the updraft region, contributes particles to the smaller size end of the CDSD. In addition, we find that growth of the largest cloud droplets strongly depends on the residence time of cloud droplet in the cloud rather than the magnitude of local variability in the supersaturation fluctuation. This is because the environmental saturation ratio is strongly buffered by smaller cloud droplets. Two necessary conditions for this CDSD broadening, which generally occur in the atmosphere, are: (1) droplets form on polydisperse aerosols of varying hygroscopicity and (2) the cloud parcel experiences upwards and downwards motions. Therefore we expect that this mechanism for CDSD broadening is possible in real clouds. Our results also suggest it is important to consider both curvature and solute effects before and after cloud droplet activation in a cloud model. The importance of this mechanism compared with other mechanisms on cloud properties should be investigated through in-situ measurements and 3-D dynamic models.« less

Cloud droplet size distribution broadening during diffusional growth: ripening amplified by deactivation and reactivation

DOE PAGES

Yang, Fan; Kollias, Pavlos; Shaw, Raymond A.; ...

2017-12-06

Cloud droplet size distributions (CDSDs), which are related to cloud albedo and lifetime, are usually broader in warm clouds than predicted from adiabatic parcel calculations. We investigate a mechanism for the CDSD broadening using a Lagrangian bin-microphysics cloud parcel model that considers the condensational growth of cloud droplets formed on polydisperse, sub-micrometer aerosols in an adiabatic cloud parcel that undergoes vertical oscillations, such as those due to cloud circulations or turbulence. Results show that the CDSD can be broadened during condensational growth as a result of Ostwald ripening amplified by droplet deactivation and reactivation, which is consistent with Korolev (1995).more » The relative roles of the solute effect, curvature effect, deactivation and reactivation on CDSD broadening are investigated. Deactivation of smaller cloud droplets, which is due to the combination of curvature and solute effects in the downdraft region, enhances the growth of larger cloud droplets and thus contributes particles to the larger size end of the CDSD. Droplet reactivation, which occurs in the updraft region, contributes particles to the smaller size end of the CDSD. In addition, we find that growth of the largest cloud droplets strongly depends on the residence time of cloud droplet in the cloud rather than the magnitude of local variability in the supersaturation fluctuation. This is because the environmental saturation ratio is strongly buffered by smaller cloud droplets. Two necessary conditions for this CDSD broadening, which generally occur in the atmosphere, are: (1) droplets form on polydisperse aerosols of varying hygroscopicity and (2) the cloud parcel experiences upwards and downwards motions. Therefore we expect that this mechanism for CDSD broadening is possible in real clouds. Our results also suggest it is important to consider both curvature and solute effects before and after cloud droplet activation in a cloud model. The importance of this mechanism compared with other mechanisms on cloud properties should be investigated through in-situ measurements and 3-D dynamic models.« less

Cloud droplet size distribution broadening during diffusional growth: ripening amplified by deactivation and reactivation

NASA Astrophysics Data System (ADS)

Yang, Fan; Kollias, Pavlos; Shaw, Raymond A.; Vogelmann, Andrew M.

2018-05-01

Cloud droplet size distributions (CDSDs), which are related to cloud albedo and rain formation, are usually broader in warm clouds than predicted from adiabatic parcel calculations. We investigate a mechanism for the CDSD broadening using a moving-size-grid cloud parcel model that considers the condensational growth of cloud droplets formed on polydisperse, submicrometer aerosols in an adiabatic cloud parcel that undergoes vertical oscillations, such as those due to cloud circulations or turbulence. Results show that the CDSD can be broadened during condensational growth as a result of Ostwald ripening amplified by droplet deactivation and reactivation, which is consistent with early work. The relative roles of the solute effect, curvature effect, deactivation and reactivation on CDSD broadening are investigated. Deactivation of smaller cloud droplets, which is due to the combination of curvature and solute effects in the downdraft region, enhances the growth of larger cloud droplets and thus contributes particles to the larger size end of the CDSD. Droplet reactivation, which occurs in the updraft region, contributes particles to the smaller size end of the CDSD. In addition, we find that growth of the largest cloud droplets strongly depends on the residence time of cloud droplet in the cloud rather than the magnitude of local variability in the supersaturation fluctuation. This is because the environmental saturation ratio is strongly buffered by numerous smaller cloud droplets. Two necessary conditions for this CDSD broadening, which generally occur in the atmosphere, are as follows: (1) droplets form on aerosols of different sizes, and (2) the cloud parcel experiences upwards and downwards motions. Therefore we expect that this mechanism for CDSD broadening is possible in real clouds. Our results also suggest it is important to consider both curvature and solute effects before and after cloud droplet activation in a cloud model. The importance of this mechanism compared with other mechanisms on cloud properties should be investigated through in situ measurements and 3-D dynamic models.

Modeling Oblique Impact Dynamics of Particle-Laden Nanodroplets

NASA Astrophysics Data System (ADS)

Yong, Xin; Qin, Shiyi

2016-11-01

A fundamental understanding of the impact dynamics of nanoscopic droplets laden with nanoparticles has important implications for materials printing and thin film processing. Using many-body dissipative particle dynamics (MDPD), we model nanometer sized suspension droplets imping on dry solid substrate with oblique angles, and compare their behavior with pure liquid droplets. Equilibrated floating droplets containing two types of nanoparticles, namely fully-wetted hydrophilic particles and surface-active Janus particles, impact onto the solid surface with varying initial velocities and impact angles. The velocity components in the normal and tangential directions to the substrate defines normal and tangential Reynolds and Weber numbers, which are used to classify impact regimes. Droplets with nanoparticles dispersed in the bulk and covering the droplet surface (resembling liquid marbles) exhibit quite different behavior in the course of impact. We also reveal the influences of substrate wettability and its interaction with nanoparticles on the impact dynamics. In addition, the vapor film beneath an impinging droplet shows no significant effect on the impact dynamics in our MDPD simulations.

Multi-scale simulations of droplets in generic time-dependent flows

NASA Astrophysics Data System (ADS)

Milan, Felix; Biferale, Luca; Sbragaglia, Mauro; Toschi, Federico

2017-11-01

We study the deformation and dynamics of droplets in time-dependent flows using a diffuse interface model for two immiscible fluids. The numerical simulations are at first benchmarked against analytical results of steady droplet deformation, and further extended to the more interesting case of time-dependent flows. The results of these time-dependent numerical simulations are compared against analytical models available in the literature, which assume the droplet shape to be an ellipsoid at all times, with time-dependent major and minor axis. In particular we investigate the time-dependent deformation of a confined droplet in an oscillating Couette flow for the entire capillary range until droplet break-up. In this way these multi component simulations prove to be a useful tool to establish from ``first principles'' the dynamics of droplets in complex flows involving multiple scales. European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Grant Agreement No 642069. & European Research Council under the European Community's Seventh Framework Program, ERC Grant Agreement No 339032.

Uptake and withdrawal of droplets from carbon nanotubes.

PubMed

Schebarchov, D; Hendy, S C

2011-01-01

We give an account of recent studies of droplet uptake and withdrawal from carbon nanotubes using simple theoretical arguments and molecular dynamics simulations. Firstly, the thermodynamics of droplet uptake and release is considered and tested via simulation. We show that the Laplace pressure acting on a droplet assists capillary uptake, allowing sufficiently small non-wetting droplets to be absorbed. We then demonstrate how the uptake and release of droplets of non-wetting fluids can be exploited for the use of carbon nanotubes as nanopipettes. Finally, we extend the Lucas-Washburn model to deal with the dynamics of droplet capillary uptake, and again test this by comparison with molecular dynamics simulations.

Uptake and withdrawal of droplets from carbon nanotubes

NASA Astrophysics Data System (ADS)

Schebarchov, D.; Hendy, S. C.

2011-01-01

We give an account of recent studies of droplet uptake and withdrawal from carbon nanotubes using simple theoretical arguments and molecular dynamics simulations. Firstly, the thermodynamics of droplet uptake and release is considered and tested via simulation. We show that the Laplace pressure acting on a droplet assists capillary uptake, allowing sufficiently small non-wetting droplets to be absorbed. We then demonstrate how the uptake and release of droplets of non-wetting fluids can be exploited for the use of carbon nanotubes as nanopipettes. Finally, we extend the Lucas-Washburn model to deal with the dynamics of droplet capillary uptake, and again test this by comparison with molecular dynamics simulations.

RAIN-Droplet: A Novel 3-D in vitro Angiogenesis Model

PubMed Central

Zeitlin, Benjamin D.; Dong, Zhihong; Nör, Jacques E.

2012-01-01

Angiogenesis is fundamentally required for the initialization, development and metastatic spread of cancer. A rapidly expanding number of new experimental, chemical modulators of endothelial cell function have been described for the therapeutic inhibition of angiogenesis in cancer. Despite this expansion there has been very limited parallel growth of in vitro angiogenesis models or experimental tools. Here we present the Responsive Angiogenic Implanted Network (RAIN)-Droplet model and novel angiogenesis assay using an endothelial cell culture model of microvascular endothelial cells encapsulated in a spontaneously self-assembling, toroidal hydrogel droplet uniquely yielding discrete, pre-formed, angiogenic networks that may be embedded in 3-D matrices. On embedding, radial growth of capillary-like sprouts and cell invasion was observed. The sprouts formed as both outgrowths from endothelial cells on the surface of the droplets but also, uniquely, from the pre-formed network structures within the droplet. We demonstrate proof-of-principle for the utility of the model showing significant inhibition of sprout formation (p<0.001) in the presence of bevacizumab, an anti-angiogenic antibody. Using the RAIN-Droplet assay we also demonstrate a novel dose dependent pro-angiogenic function for the characteristically anti-angiogenic multi-kinase inhibitor sorafenib. Exposure of endothelial cells in 3-D culture to low, non-lethal doses (<1 μM) of sorafenib after initiation of sprouting resulted in the formation of significantly (p<0.05) more endothelial sprouts compared to controls over a 48-hour period. Higher doses of sorafenib (5 μM) resulted in a significant (p<0.05) reduction of sprouting over the same time period. The RAIN-Droplet model is a highly versatile and simply constructed 3-D focal sprouting approach well suited for the study of vascular morphogenesis and for preclinical testing of drugs. Furthermore, the RAIN-Droplet model has facilitated the discovery of a novel pro-angiogenic capacity for sorafenib which may impact the clinical application and dosing regimen of that drug. PMID:22565576

Modeling micro-droplet formation in near-field electrohydrodynamic jet printing

NASA Astrophysics Data System (ADS)

Popell, George Colin

Near-field electrohydrodynamic jet (E-jet) printing has recently gained significant interest within the manufacturing research community because of its ability to produce micro/sub-micron-scale droplets using a wide variety of inks and substrates. However, the process currently operates in open-loop and as a result suffers from unpredictable printing quality. The use of physics-based, control-oriented process models is expected to enable closed-loop control of this printing technique. The objective of this research is to perform a fundamental study of the substrate-side droplet shape-evolution in near-field E-jet printing and to develop a physics-based model of the same that links input parameters such as voltage magnitude and ink properties to the height and diameter of the printed droplet. In order to achieve this objective, a synchronized high-speed imaging and substrate-side current-detection system was used implemented to enable a correlation between the droplet shape parameters and the measured current signal. The experimental data reveals characteristic process signatures and droplet spreading regimes. The results of these studies are then used as the basis for a model that predicts the droplet diameter and height using the measured current signal as the input. A unique scaling factor based on the measured current signal is used in this model instead of relying on empirical scaling laws found in literature. For each of the three inks tested in this study, the average absolute error in the model predictions is under 4.6% for diameter predictions and under 10.6% for height predictions of the steady-state droplet. While printing under non-conducive ambient conditions of low humidity and high temperatures, the use of the environmental correction factor in the model is seen to result in average absolute errors of 10.35% and 12.5% for diameter and height predictions.

Entropic multiple-relaxation-time multirange pseudopotential lattice Boltzmann model for two-phase flow

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

Qin, Feifei; Mazloomi Moqaddam, Ali; Kang, Qinjun

Here, an entropic multiple-relaxation-time lattice Boltzmann approach is coupled to a multirange Shan-Chen pseudopotential model to study the two-phase flow. Compared with previous multiple-relaxation-time multiphase models, this model is stable and accurate for the simulation of a two-phase flow in a much wider range of viscosity and surface tension at a high liquid-vapor density ratio. A stationary droplet surrounded by equilibrium vapor is first simulated to validate this model using the coexistence curve and Laplace’s law. Then, two series of droplet impact behavior, on a liquid film and a flat surface, are simulated in comparison with theoretical or experimental results.more » Droplet impact on a liquid film is simulated for different Reynolds numbers at high Weber numbers. With the increase of the Sommerfeld parameter, onset of splashing is observed and multiple secondary droplets occur. The droplet spreading ratio agrees well with the square root of time law and is found to be independent of Reynolds number. Moreover, shapes of simulated droplets impacting hydrophilic and superhydrophobic flat surfaces show good agreement with experimental observations through the entire dynamic process. The maximum spreading ratio of a droplet impacting the superhydrophobic flat surface is studied for a large range of Weber numbers. Results show that the rescaled maximum spreading ratios are in good agreement with a universal scaling law. This series of simulations demonstrates that the proposed model accurately captures the complex fluid-fluid and fluid-solid interfacial physical processes for a wide range of Reynolds and Weber numbers at high density ratios.« less

Entropic multiple-relaxation-time multirange pseudopotential lattice Boltzmann model for two-phase flow

NASA Astrophysics Data System (ADS)

Qin, Feifei; Mazloomi Moqaddam, Ali; Kang, Qinjun; Derome, Dominique; Carmeliet, Jan

2018-03-01

An entropic multiple-relaxation-time lattice Boltzmann approach is coupled to a multirange Shan-Chen pseudopotential model to study the two-phase flow. Compared with previous multiple-relaxation-time multiphase models, this model is stable and accurate for the simulation of a two-phase flow in a much wider range of viscosity and surface tension at a high liquid-vapor density ratio. A stationary droplet surrounded by equilibrium vapor is first simulated to validate this model using the coexistence curve and Laplace's law. Then, two series of droplet impact behavior, on a liquid film and a flat surface, are simulated in comparison with theoretical or experimental results. Droplet impact on a liquid film is simulated for different Reynolds numbers at high Weber numbers. With the increase of the Sommerfeld parameter, onset of splashing is observed and multiple secondary droplets occur. The droplet spreading ratio agrees well with the square root of time law and is found to be independent of Reynolds number. Moreover, shapes of simulated droplets impacting hydrophilic and superhydrophobic flat surfaces show good agreement with experimental observations through the entire dynamic process. The maximum spreading ratio of a droplet impacting the superhydrophobic flat surface is studied for a large range of Weber numbers. Results show that the rescaled maximum spreading ratios are in good agreement with a universal scaling law. This series of simulations demonstrates that the proposed model accurately captures the complex fluid-fluid and fluid-solid interfacial physical processes for a wide range of Reynolds and Weber numbers at high density ratios.

Entropic multiple-relaxation-time multirange pseudopotential lattice Boltzmann model for two-phase flow

DOE PAGES

Qin, Feifei; Mazloomi Moqaddam, Ali; Kang, Qinjun; ...

2018-03-22

Here, an entropic multiple-relaxation-time lattice Boltzmann approach is coupled to a multirange Shan-Chen pseudopotential model to study the two-phase flow. Compared with previous multiple-relaxation-time multiphase models, this model is stable and accurate for the simulation of a two-phase flow in a much wider range of viscosity and surface tension at a high liquid-vapor density ratio. A stationary droplet surrounded by equilibrium vapor is first simulated to validate this model using the coexistence curve and Laplace’s law. Then, two series of droplet impact behavior, on a liquid film and a flat surface, are simulated in comparison with theoretical or experimental results.more » Droplet impact on a liquid film is simulated for different Reynolds numbers at high Weber numbers. With the increase of the Sommerfeld parameter, onset of splashing is observed and multiple secondary droplets occur. The droplet spreading ratio agrees well with the square root of time law and is found to be independent of Reynolds number. Moreover, shapes of simulated droplets impacting hydrophilic and superhydrophobic flat surfaces show good agreement with experimental observations through the entire dynamic process. The maximum spreading ratio of a droplet impacting the superhydrophobic flat surface is studied for a large range of Weber numbers. Results show that the rescaled maximum spreading ratios are in good agreement with a universal scaling law. This series of simulations demonstrates that the proposed model accurately captures the complex fluid-fluid and fluid-solid interfacial physical processes for a wide range of Reynolds and Weber numbers at high density ratios.« less

Application of Compressible Volume of Fluid Model in Simulating the Impact and Solidification of Hollow Spherical ZrO2 Droplet on a Surface

NASA Astrophysics Data System (ADS)

Safaei, Hadi; Emami, Mohsen Davazdah; Jazi, Hamidreza Salimi; Mostaghimi, Javad

2017-12-01

Applications of hollow spherical particles in thermal spraying process have been developed in recent years, accompanied by attempts in the form of experimental and numerical studies to better understand the process of impact of a hollow droplet on a surface. During such process, volume and density of the trapped gas inside droplet change. The numerical models should be able to simulate such changes and their consequent effects. The aim of this study is to numerically simulate the impact of a hollow ZrO2 droplet on a flat surface using the volume of fluid technique for compressible flows. An open-source, finite-volume-based CFD code was used to perform the simulations, where appropriate subprograms were added to handle the studied cases. Simulation results were compared with the available experimental data. Results showed that at high impact velocities ( U 0 > 100 m/s), the compression of trapped gas inside droplet played a significant role in the impact dynamics. In such velocities, the droplet splashed explosively. Compressibility effects result in a more porous splat, compared to the corresponding incompressible model. Moreover, the compressible model predicted a higher spread factor than the incompressible model, due to planetary structure of the splat.

Weaving colloidal webs around droplets: spontaneous assembly of extended colloidal networks encasing microfluidic droplet ensembles.

PubMed

Zheng, Lu; Ho, Leon Yoon; Khan, Saif A

2016-10-26

The ability to form transient, self-assembling solid networks that 'cocoon' emulsion droplets on-demand allows new possibilities in the rapidly expanding area of microfluidic droplet-based materials science. In this communication, we demonstrate the spontaneous formation of extended colloidal networks that encase large microfluidic droplet ensembles, thus completely arresting droplet motion and effectively isolating each droplet from others in the ensemble. To do this, we employ molecular inclusion complexes of β-cyclodextrin, which spontaneously form and assemble into colloidal solids at the droplet interface and beyond, via the outward diffusion of a guest molecule (dichloromethane) from the droplets. We illustrate the advantage of such transient network-based droplet stabilization in the area of pharmaceutical crystallization, where we are able to fabricate monodisperse spherical crystalline microgranules of 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile (ROY), a model hydrophobic drug, with a dramatic enhancement of particle properties compared to conventional methods.

Dancing droplets: Contact angle, drag, and confinement

NASA Astrophysics Data System (ADS)

Benusiglio, Adrien; Cira, Nate; Prakash, Manu

2015-11-01

When deposited on a clean glass slide, a mixture of water and propylene glycol forms a droplet of given contact angle, when both pure liquids spread. (Cira, Benusiglio, Prakash: Nature, 2015). The droplet is stabilized by a gradient of surface tension due to evaporation that induces a Marangoni flow from the border to the apex of the droplets. The apparent contact angle of the droplets depends on both their composition and the external humidity as captured by simple models. These droplets present remarkable properties such as lack of a large pinning force. We discuss the drag on these droplets as a function of various parameters. We show theoretical and experimental results of how various confinement geometries change the vapor gradient and the dynamics of droplet attraction.

Acoustic droplet vaporization of vascular droplets in gas embolotherapy

NASA Astrophysics Data System (ADS)

Bull, Joseph

2016-11-01

This work is primarily motivated by a developmental gas embolotherapy technique for cancer treatment. In this methodology, infarction of tumors is induced by selectively formed vascular gas bubbles that arise from the acoustic vaporization of vascular droplets. Additionally, micro- or nano-droplets may be used as vehicles for localized drug delivery, with or without flow occlusion. In this talk, we examine the dynamics of acoustic droplet vaporization through experiments and theoretical/computational fluid mechanics models, and investigate the bioeffects of acoustic droplet vaporization on endothelial cells and in vivo. Functionalized droplets that are targeted to tumor vasculature are examined. The influence of fluid mechanical and acoustic parameters, as well as droplet functionalization, is explored. This work was supported by NIH Grant R01EB006476.

Lagrangian condensation microphysics with Twomey CCN activation

NASA Astrophysics Data System (ADS)

Grabowski, Wojciech W.; Dziekan, Piotr; Pawlowska, Hanna

2018-01-01

We report the development of a novel Lagrangian microphysics methodology for simulations of warm ice-free clouds. The approach applies the traditional Eulerian method for the momentum and continuous thermodynamic fields such as the temperature and water vapor mixing ratio, and uses Lagrangian super-droplets to represent condensed phase such as cloud droplets and drizzle or rain drops. In other applications of the Lagrangian warm-rain microphysics, the super-droplets outside clouds represent unactivated cloud condensation nuclei (CCN) that become activated upon entering a cloud and can further grow through diffusional and collisional processes. The original methodology allows for the detailed study of not only effects of CCN on cloud microphysics and dynamics, but also CCN processing by a cloud. However, when cloud processing is not of interest, a simpler and computationally more efficient approach can be used with super-droplets forming only when CCN is activated and no super-droplet existing outside a cloud. This is possible by applying the Twomey activation scheme where the local supersaturation dictates the concentration of cloud droplets that need to be present inside a cloudy volume, as typically used in Eulerian bin microphysics schemes. Since a cloud volume is a small fraction of the computational domain volume, the Twomey super-droplets provide significant computational advantage when compared to the original super-droplet methodology. Additional advantage comes from significantly longer time steps that can be used when modeling of CCN deliquescence is avoided. Moreover, other formulation of the droplet activation can be applied in case of low vertical resolution of the host model, for instance, linking the concentration of activated cloud droplets to the local updraft speed. This paper discusses the development and testing of the Twomey super-droplet methodology, focusing on the activation and diffusional growth. Details of the activation implementation, transport of super-droplets in the physical space, and the coupling between super-droplets and the Eulerian temperature and water vapor field are discussed in detail. Some of these are relevant to the original super-droplet methodology as well and to the ice phase modeling using the Lagrangian approach. As a computational example, the scheme is applied to an idealized moist thermal rising in a stratified environment, with the original super-droplet methodology providing a benchmark to which the new scheme is compared.

Modelling the Evolution of Sea Spray Droplets on a Global Scale

NASA Astrophysics Data System (ADS)

Staniec, A.; Vlahos, P.; Monahan, E. C.

2017-12-01

Sea spray droplets are an important mechanism for the transport of moisture, heat, and organic material between the ocean and the atmosphere. Spume droplets are the largest of the size spectrum and as such have the potential to transport significant amounts of energy and gases despite their generally short residence time in the atmosphere. A model is developed based on the physical parameterizations from Andreas et al. (1995, 2005)and a range of spume generation functions, coupled with a biogeochemical exchange model for gases developed here to examine the equilibrium temperature and gas exchange of spume droplets under representative open ocean conditions. The modelling approach uses micro-physics to simulate the expected changes to the droplet as it equilibrates with the atmospheric temperature and relative humidity. The effect of temperature differentials and relative humidity variations is explored. A global approach is simulated by using average summer and winter values for SST, salinity, and air temperature throughout the various ocean basins.

Modeling and Validation of a Three-Stage Solidification Model for Sprays

NASA Astrophysics Data System (ADS)

Tanner, Franz X.; Feigl, Kathleen; Windhab, Erich J.

2010-09-01

A three-stage freezing model and its validation are presented. In the first stage, the cooling of the droplet down to the freezing temperature is described as a convective heat transfer process in turbulent flow. In the second stage, when the droplet has reached the freezing temperature, the solidification process is initiated via nucleation and crystal growth. The latent heat release is related to the amount of heat convected away from the droplet and the rate of solidification is expressed with a freezing progress variable. After completion of the solidification process, in stage three, the cooling of the solidified droplet (particle) is described again by a convective heat transfer process until the particle approaches the temperature of the gaseous environment. The model has been validated by experimental data of a single cocoa butter droplet suspended in air. The subsequent spray validations have been performed with data obtained from a cocoa butter melt in an experimental spray tower using the open-source computational fluid dynamics code KIVA-3.

Experimental Investigation of Lagrangian Statistics of Motion of Diesel Oil Droplets and Fluid Particles in Isotropic Turbulence

NASA Astrophysics Data System (ADS)

Gopalan, Balaji; Malkiel, Edwin; Katz, Joseph

2007-11-01

Lagrangian motion in isotropic turbulence of slightly buoyant diesel oil droplets (specific gravity 0.85 and size 0.6-1.1 mm) and almost neutrally buoyant, 50 μm tracer particles are studied using high speed, in-line digital holographic cinematography. Droplets and particles are injected into a 50x50x70 mm^3 sample volume located at the center of a nearly isotropic turbulence facility, and data are obtained for Reλ of 190, 195 and 214. The turbulence is characterized by 2D PIV measurements at different planes. An automated tracking program has been used for measuring velocity time history of more than 22000 droplet tracks and 15000 particle tracks. Analysis compares probability density functions (PDF) of Lagrangian velocity and acceleration, spectra, as well as velocity and acceleration autocorrelation functions of droplets with those of particles. For most of the present conditions, rms values of horizontal droplet velocity exceed those of the fluid. The rms values of droplet vertical velocity are higher than those of the fluid only for the highest turbulence level. PDFs of droplet velocity have nearly Gaussian distributions, justifying use of Taylor's (1921) model to calculate diffusion parameters. The fluid particle diffusion coefficient exceeds that of the droplet primarily because the fluid diffusion timescale is higher than that of the droplet. For all droplet sizes and Reynolds numbers, the diffusion coefficient, calculated using Taylor's model, scaled by quiescent rise velocity and turbulence integral length scale, is a monotonically increasing function of the turbulence level normalized by droplet quiescent rise velocity.

Active fluids at circular boundaries: swim pressure and anomalous droplet ripening.

PubMed

Jamali, Tayeb; Naji, Ali

2018-06-13

We investigate the swim pressure exerted by non-chiral and chiral active particles on convex or concave circular boundaries. Active particles are modeled as non-interacting and non-aligning self-propelled Brownian particles. The convex and concave circular boundaries are used to model a fixed inclusion immersed in an active bath and a cavity (or container) enclosing the active particles, respectively. We first present a detailed analysis of the role of convex versus concave boundary curvature and of the chirality of active particles in their spatial distribution, chirality-induced currents, and the swim pressure they exert on the bounding surfaces. The results will then be used to predict the mechanical equilibria of suspended fluid enclosures (generically referred to as 'droplets') in a bulk with active particles being present either inside the bulk fluid or within the suspended droplets. We show that, while droplets containing active particles behave in accordance with standard capillary paradigms when suspended in a normal bulk, those containing a normal fluid exhibit anomalous behaviors when suspended in an active bulk. In the latter case, the excess swim pressure results in non-monotonic dependence of the inside droplet pressure on the droplet radius; hence, revealing an anomalous regime of behavior beyond a threshold radius, in which the inside droplet pressure increases upon increasing the droplet size. Furthermore, for two interconnected droplets, mechanical equilibrium can occur also when the droplets have different sizes. We thus identify a regime of anomalous droplet ripening, where two unequal-sized droplets can reach a final state of equal size upon interconnection, in stark contrast with the standard Ostwald ripening phenomenon, implying shrinkage of the smaller droplet in favor of the larger one.

Droplet Traffic at a Simple Junction at Low Capillary Numbers

NASA Astrophysics Data System (ADS)

Engl, Wilfried; Roche, Matthieu; Colin, Annie; Panizza, Pascal; Ajdari, Armand

2005-11-01

We report that, when a train of confined droplets flowing through a channel reaches a junction, the droplets either are alternately distributed between the different outlets or all collect into the shortest one. We argue that this behavior is due to the hydrodynamic feedback of droplets in the different outlets on the selection process occurring at the junction. A “mean field” model, yielding semiquantitative results, offers a first guide to predict droplet traffic in branched networks.

Modeling metal droplet sprays in spray forming

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

Muoio, N.G.; Crowe, C.T.; Fritsching, U.

1995-12-31

Spray casting is a process whereby a molten metal stream is atomized and deposited on a substrate. The rapid solidification of the metal droplets gives rise to a fine grain structure and improved material properties. This paper presents a simulation for the fluid and thermal interaction of the fluid and droplets in the spray and the effect on the droplet spray pattern. Good agreement is obtained between the measured and predicted droplet mass flux distribution in the spray.

Fractal Analyses of High-Resolution Cloud Droplet Measurements.

NASA Astrophysics Data System (ADS)

Malinowski, Szymon P.; Leclerc, Monique Y.; Baumgardner, Darrel G.

1994-02-01

Fractal analyses of individual cloud droplet distributions using aircraft measurements along one-dimensional horizontal cross sections through clouds are performed. Box counting and cluster analyses are used to determine spatial scales of inhomogeneity of cloud droplet spacing. These analyses reveal that droplet spatial distributions do not exhibit a fractal behavior. A high variability in local droplet concentration in cloud volumes undergoing mixing was found. In these regions, thin filaments of cloudy air with droplet concentration close to those observed in cloud cores were found. Results suggest that these filaments may be anisotropic. Additional box counting analyses performed for various classes of cloud droplet diameters indicate that large and small droplets are similarly distributed, except for the larger characteristic spacing of large droplets.A cloud-clear air interface defined by a certain threshold of total droplet count (TDC) was investigated. There are indications that this interface is a convoluted surface of a fractal nature, at least in actively developing cumuliform clouds. In contrast, TDC in the cloud interior does not have fractal or multifractal properties. Finally a random Cantor set (RCS) was introduced as a model of a fractal process with an ill-defined internal scale. A uniform measure associated with the RCS after several generations was introduced to simulate the TDC records. Comparison of the model with real TDC records indicates similar properties of both types of data series.

The Phospholipid:Diacylglycerol Acyltransferase Lro1 Is Responsible for Hepatitis C Virus Core-Induced Lipid Droplet Formation in a Yeast Model System

PubMed Central

Wang, Chao-Wen; Cheng, Yun-Hsin; Irokawa, Hayato; Hwang, Gi-Wook; Naganuma, Akira; Kuge, Shusuke

2016-01-01

Chronic infection with the hepatitis C virus frequently induces steatosis, which is a significant risk factor for liver pathogenesis. Steatosis is characterized by the accumulation of lipid droplets in hepatocytes. The structural protein core of the virus induces lipid droplet formation and localizes on the surface of the lipid droplets. However, the precise molecular mechanisms for the core-induced formation of lipid droplets remain elusive. Recently, we showed that the expression of the core protein in yeast as a model system could induce lipid droplet formation. In this study, we probed the cellular factors responsible for the formation of core-induced lipid-droplets in yeast cells. We demonstrated that one of the enzymes responsible for triglyceride synthesis, a phospholipid:diacylglycerol acyltransferase (Lro1), is required for the core-induced lipid droplet formation. While core proteins inhibit Lro1 degradation and alter Lro1 localization, the characteristic localization of Lro1 adjacent to the lipid droplets appeared to be responsible for the core-induced lipid droplet formation. RNA virus genomes have evolved using high mutation rates to maintain their ability to replicate. Our observations suggest a functional relationship between the core protein with hepatocytes and yeast cells. The possible interactions between core proteins and the endoplasmic reticulum membrane affect the mobilization of specific proteins. PMID:27459103

Dynamics of Droplet Extinction in Slow Convective Flows

NASA Technical Reports Server (NTRS)

Nayagam, V.; Haggard, J. B., Jr.; Williams, F. A.

1999-01-01

The classical model for droplet combustion predicts that the square of the droplet diameter decreases linearly with time. It also predicts that a droplet of any size will burn to completion over a period of time. However, it has been known for some time that under certain conditions flames surrounding a droplet, in a quiescent environment, could extinguish because of insufficient residence time for the chemistry to proceed to completion. This type of extinction that occurs for smaller droplets has been studied extensively in the past. Large droplets, on the other hand, exhibit a different type of extinction where excessive radiative heat loss from the flame zone leads to extinction. This mode of "radiative extinction" was theoretically predicted for droplet burning by Chao et al. and was observed in recent space experiments in a quiescent environment. Thus far, the fundamental flammability limit prescribed by radiative extinction of liquid droplets has been measured only under quiescent environmental conditions. In many space platforms, however, ventilation systems produce small convective flows and understanding of the influences of this convection on the extinction process will help better define the radiative extinction flammability boundaries. Boundaries defined by experiments and captured using theoretical models could provide enhanced fire safety margin in space explor1999063d investigation of convective effects will help in interpretations of burning-rate data obtained during free-floated droplet combustion experiments with small residual velocities.

Modeling of surface tension effects in venturi scrubbing

NASA Astrophysics Data System (ADS)

Ott, Robert M.; Wu, Tatsu K. L.; Crowder, Jerry W.

A modified model of venturi scrubber performance has been developed that addresses two effects of liquid surface tension: its effect on droplet size and its effect on particle penetration into the droplet. The predictions of the model indicate that, in general, collection efficiency increases with a decrease in liquid surface tension, but the range over which this increase is significant depends on the particle size and on the scrubber operating parameters. The predictions further indicate that the increases in collection efficiency are almost totally due to the effect of liquid surface tension on the mean droplet size, and that the collection efficiency is not significantly affected by the ability of the particle to penetrate the droplet.

An infrared scattering by evaporating droplets at the initial stage of a pool fire suppression by water sprays

NASA Astrophysics Data System (ADS)

Dombrovsky, Leonid A.; Dembele, Siaka; Wen, Jennifer X.

2018-06-01

The computational analysis of downward motion and evaporation of water droplets used to suppress a typical transient pool fire shows local regions of a high volume fraction of relatively small droplets. These droplets are comparable in size with the infrared wavelength in the range of intense flame radiation. The estimated scattering of the radiation by these droplets is considerable throughout the entire spectrum except for a narrow region in the vicinity of the main absorption peak of water where the anomalous refraction takes place. The calculations of infrared radiation field in the model pool fire indicate the strong effect of scattering which can be observed experimentally to validate the fire computational model.

Modeling of turbulence effects on the heat and mass transfer of evaporating sprays

NASA Astrophysics Data System (ADS)

Madhanabharatam, Balasubramanyam

A large diversity of two-phase gas-liquid flows of both scientific and practical interest involves the evaporation of near spherical liquid droplets in high temperature turbulent environments. Current numerical modeling approaches are predominantly focused towards the effects of continuous phase (gas phase) turbulence on the evaporation rates of liquid fuel sprays during the evaporation process, failing to account for the inherent turbulence present in the dispersed phase (liquid phase), due to the injection of sprays at high velocities. Existing models accounting for internal turbulence effects use Direct Numerical Simulations and Large Eddy Simulations that are computationally intensive. This research provides an alternative phenomenological approach of modeling droplet internal turbulence effects through the mass and heat transfer between the droplet surface and the external gas phase within a thin film inside the droplet. This finite conductivity (F-C) model was based on the two-temperature film theory, where the turbulence characteristics of the droplet are used to estimate the effective thermal diffusivity (alphaeff) within the droplet phase. The alphaeff is estimated from the physical properties of the flow within the droplet rather than from a 'curve-fit' as done conventionally. The results of the one-way coupled study indicated that the equilibrium drop temperature predictions were higher than calculations by the infinite conductivity (I-C) model. The liquid internal turbulence has a considerable effect on the diffusivity in the primary atomization regime. The thermal boundary layer was found to be substantially thick initially, decreasing quickly to a small value, exhibiting a reasonable physical trend. The two-way coupled studies (CFD) indicated that the F-C model, slowed down the evaporation process, produced larger droplets and longer tip penetration lengths during the initial stages of injection. For a jet in a supersonic cross-flow, results indicated that jet penetration increased rapidly in the vicinity of the injector exit and then gradually increased due to increase in the drag of the air stream. A modified drag coefficient was incorporated to improve model accuracy in predictions. Overall the results obtained from the numerical calculations during this study were reasonably comparable to measured data and showed more accurate comparisons to that of the I-C model.

A multiphase ion-transport analysis of the electrostatic disjoining pressure: implications for binary droplet coalescence

NASA Astrophysics Data System (ADS)

Mason, Lachlan; Gebauer, Felix; Bart, Hans-Jörg; Stevens, Geoffrey; Harvie, Dalton

2016-11-01

Understanding the physics of emulsion coalescence is critical for the robust simulation of industrial solvent extraction processes, in which loaded organic and raffinate phases are separated via the coalescence of dispersed droplets. At the droplet scale, predictive collision-outcome models require an accurate description of the repulsive surface forces arising from electrical-double-layer interactions. The conventional disjoining-pressure treatment of double-layer forces, however, relies on assumptions which do not hold generally for deformable droplet collisions: namely, low interfacial curvature and negligible advection of ion species. This study investigates the validity bounds of the disjoining pressure approximation for low-inertia droplet interactions. A multiphase ion-transport model, based on a coupling of droplet-scale Nernst-Planck and Navier-Stokes equations, predicts ion-concentration fields that are consistent with the equilibrium Boltzmann distribution; indicating that the disjoining-pressure approach is valid for both static and dynamic interactions in low-Reynolds-number settings. The present findings support the development of coalescence kernels for application in macro-scale population balance modelling.

Statistical steady states in turbulent droplet condensation

NASA Astrophysics Data System (ADS)

Bec, Jeremie; Krstulovic, Giorgio; Siewert, Christoph

2017-11-01

We investigate the general problem of turbulent condensation. Using direct numerical simulations we show that the fluctuations of the supersaturation field offer different conditions for the growth of droplets which evolve in time due to turbulent transport and mixing. This leads to propose a Lagrangian stochastic model consisting of a set of integro-differential equations for the joint evolution of the squared radius and the supersaturation along droplet trajectories. The model has two parameters fixed by the total amount of water and the thermodynamic properties, as well as the Lagrangian integral timescale of the turbulent supersaturation. The model reproduces very well the droplet size distributions obtained from direct numerical simulations and their time evolution. A noticeable result is that, after a stage where the squared radius simply diffuses, the system converges exponentially fast to a statistical steady state independent of the initial conditions. The main mechanism involved in this convergence is a loss of memory induced by a significant number of droplets undergoing a complete evaporation before growing again. The statistical steady state is characterised by an exponential tail in the droplet mass distribution.

Coalescence driven self-organization of growing nanodroplets around a microcap

NASA Astrophysics Data System (ADS)

Dyett, Brendan; Hao, Hao; Lohse, Detlef; Zhang, Xuehua

The coalescence between growing droplets is important for the surface coverage and spatial arrangements of droplets on surfaces. In this work, total internal reflection fluorescence (TIRF) microscopy is utilized to in-situ investigate the formation of nanodroplets around the rim of a polymer microcap, with sub-micron spatial and millisecond temporal resolution. We observe that the coalescence among droplets occurs frequently during their growth by solvent exchange. Our experimental results show that the position of the droplet from two merged droplets is related to the size of the parent droplets. The position of the coalesced droplet and the ratio of parent droplet sizes obey a scaling law, reflecting a coalescence preference based on the size inequality. As a result of droplet coalescence, the angles between the centroids of two neighbouring droplets increase with time, obeying a nearly symmetrical arrangement of droplets at various time intervals. The evolution of the position and number from coalescence of growing droplets is modelled. The mechanism for coalescence driven self-organization of growing droplets is general, applicable to microcaps of different sizes and droplets of different liquids. The understanding from this work may be valuable for positioning nanodroplets by nucleation and growth without using templates.

Coalescence driven self-organization of growing nanodroplets around a microcap.

PubMed

Dyett, Brendan; Hao, Hao; Lohse, Detlef; Zhang, Xuehua

2018-04-04

The coalescence between growing droplets is important for the surface coverage and spatial arrangements of droplets on surfaces. In this work, total internal reflection fluorescence (TIRF) microscopy is utilized to in situ investigate the formation of nanodroplets around the rim of a polymer microcap, with sub-micron spatial and millisecond temporal resolution. We observe that the coalescence among droplets occurs frequently during their growth by solvent exchange. Our experimental results show that the position of the droplet from two merged droplets is related to the size of the parent droplets. The position of the coalesced droplet and the ratio of parent droplet sizes obey a scaling law, reflecting a coalescence preference based on the size inequality. As a result of droplet coalescence, the angles between the centroids of two neighbouring droplets increase with time, obeying a nearly symmetrical arrangement of droplets at various time intervals. The evolution of the position and number from coalescence of growing droplets is modelled. The mechanism for coalescence driven self-organization of growing droplets is general, applicable to microcaps of different sizes and droplets of different liquids. The understanding from this work may be valuable for positioning nanodroplets by nucleation and growth without using templates.

Electric field makes Leidenfrost droplets take a leap.

PubMed

Wildeman, Sander; Sun, Chao

2016-12-06

Leidenfrost droplets, i.e. droplets whose mobility is ensured by a thin vapor film between the droplet and a hot plate, are exposed to an external electric field. We find that in a strong vertical electric field the droplet can start to bounce progressively higher, defying gravitational attraction. From the droplet's trajectory we infer the temporal evolution of the amount of charge on the droplet. This reveals that the charge starts high and then decreases in steps as the droplet slowly evaporates. After each discharge event the charge is in a fixed proportion to the droplet's surface area. We show that this behavior can be accurately modeled by treating the droplet as a conducting sphere that occasionally makes electrical contact with the hot plate, at intervals dictated by an electro-capillary instability in the vapor film. An analysis of the kinetic and potential energies of the bouncing droplet reveals that, while the overall motion is damped, the droplet occasionally experiences a sudden boost, keeping its energy close to the value for which the free fall trajectory and droplet oscillation are in sync. This helps the droplet to escape from the hot surface when finally the electrical surface forces overtake gravity.

Analysis of isothermal and cooling rate dependent immersion freezing by a unifying stochastic ice nucleation model

NASA Astrophysics Data System (ADS)

Alpert, P. A.; Knopf, D. A.

2015-05-01

Immersion freezing is an important ice nucleation pathway involved in the formation of cirrus and mixed-phase clouds. Laboratory immersion freezing experiments are necessary to determine the range in temperature (T) and relative humidity (RH) at which ice nucleation occurs and to quantify the associated nucleation kinetics. Typically, isothermal (applying a constant temperature) and cooling rate dependent immersion freezing experiments are conducted. In these experiments it is usually assumed that the droplets containing ice nuclei (IN) all have the same IN surface area (ISA), however the validity of this assumption or the impact it may have on analysis and interpretation of the experimental data is rarely questioned. A stochastic immersion freezing model based on first principles of statistics is presented, which accounts for variable ISA per droplet and uses physically observable parameters including the total number of droplets (Ntot) and the heterogeneous ice nucleation rate coefficient, Jhet(T). This model is applied to address if (i) a time and ISA dependent stochastic immersion freezing process can explain laboratory immersion freezing data for different experimental methods and (ii) the assumption that all droplets contain identical ISA is a valid conjecture with subsequent consequences for analysis and interpretation of immersion freezing. The simple stochastic model can reproduce the observed time and surface area dependence in immersion freezing experiments for a variety of methods such as: droplets on a cold-stage exposed to air or surrounded by an oil matrix, wind and acoustically levitated droplets, droplets in a continuous flow diffusion chamber (CFDC), the Leipzig aerosol cloud interaction simulator (LACIS), and the aerosol interaction and dynamics in the atmosphere (AIDA) cloud chamber. Observed time dependent isothermal frozen fractions exhibiting non-exponential behavior with time can be readily explained by this model considering varying ISA. An apparent cooling rate dependence ofJhet is explained by assuming identical ISA in each droplet. When accounting for ISA variability, the cooling rate dependence of ice nucleation kinetics vanishes as expected from classical nucleation theory. The model simulations allow for a quantitative experimental uncertainty analysis for parameters Ntot, T, RH, and the ISA variability. In an idealized cloud parcel model applying variability in ISAs for each droplet, the model predicts enhanced immersion freezing temperatures and greater ice crystal production compared to a case when ISAs are uniform in each droplet. The implications of our results for experimental analysis and interpretation of the immersion freezing process are discussed.

Rebounding droplet-droplet collisions on superhydrophobic surfaces: from the phenomenon to droplet logic.

PubMed

Mertaniemi, Henrikki; Forchheimer, Robert; Ikkala, Olli; Ras, Robin H A

2012-11-08

When water droplets impact each other while traveling on a superhydrophobic surface, we demonstrate that they are able to rebound like billiard balls. We present elementary Boolean logic operations and a flip-flop memory based on these rebounding water droplet collisions. Furthermore, bouncing or coalescence can be easily controlled by process parameters. Thus by the controlled coalescence of reactive droplets, here using the quenching of fluorescent metal nanoclusters as a model reaction, we also demonstrate an elementary operation for programmable chemistry. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Experimental Study of Unsupported Nonane fuel Droplet Combustion in Microgravity

NASA Technical Reports Server (NTRS)

Callahan, B. J.; Avedisian, C. T.; Hertzog, D. E.; Berkery, J. W.

1999-01-01

Soot formation in droplet flames is the basic component of the particulate emission process that occurs in spray combustion. The complexity of soot formation motivates a one-dimensional transport condition which has obvious advantages in modeling. Recent models of spherically symmetric droplet combustion have made this assumption when incorporating such aspects as detailed chemistry and radiation. Interestingly, spherical symmetry does not necessarily restrict the results because it has been observed that the properties of carbon formed in flames are not strongly affected by the nature of the fuel or flaming configuration. What is affected, however, are the forces acting on the soot aggregates and where they are trapped by a balance of drag and thermophoretic forces. The distribution of these forces depends on the transport conditions of the flame. Prior studies of spherical droplet flames have examined the droplet burning history of alkanes, alcohols and aromatics. Data are typically the evolution of droplet, flame, extinction, and soot shell diameters. These data are only now just beginning to find their way into comprehensive numerical models of droplet combustion to test proposed oxidation schemes for fuels such as methanol and heptane. In the present study, we report new measurements on the burning history of unsupported nonane droplets in a convection-free environment to promote spherical symmetry. The far-field gas is atmospheric pressure air at room temperature. The evolution of droplet diameter was measured using high speed cine photography of a spark-ignited, droplet within a confined volume in a drop tower. The initial droplet diameters varied between 0.5 mm and 0.6 mm. The challenge of unsupported droplets is to form, deploy and ignite them with minimal disturbance, and then to keep them in the camera field of view. Because of the difficulty of this undertaking, more sophisticated diagnostics for studying soot than photographic were not used. Supporting the test droplet by a fiber fixes the droplet position but the fiber can perturb the burning process especially for a sooting fuel. Prior studies on heptane showed little evidence for soot formation due to g-droplets of similar size the relationship between sooting and droplet diameter. For nonane droplets we expect increased sooting due to the greater number of carbon atoms. As a sooting droplet burns and its diameter decreases, proportionally less soot should form. This reduced soot, as well as the influence of soot formed earlier in the burning process which collects in a 'shell', on heat transport to the flame offers the potential for a time-varying burning rate. Such an effect was investigated and revealed in results reported here. Speculation is offered for the cause of this effect and its possible relation to soot formation.

Interaction mechanisms between ceramic particles and atomized metallic droplets

NASA Astrophysics Data System (ADS)

Wu, Yue; Lavernia, Enrique J.

1992-10-01

The present study was undertaken to provide insight into the dynamic interactions that occur when ceramic particles are placed in intimate contact with a metallic matrix undergoing a phase change. To that effect, Al-4 wt pct Si/SiCp composite droplets were synthesized using a spray atomization and coinjection approach, and their solidification microstructures were studied both qualitatively and quantitatively. The present results show that SiC particles (SiCp) were incor- porated into the matrix and that the extent of incorporation depends on the solidification con- dition of the droplets at the moment of SiC particle injection. Two factors were found to affect the distribution and volume fraction of SiC particles in droplets: the penetration of particles into droplets and the entrapment and/or rejection of particles by the solidification front. First, during coinjection, particles collide with the atomized droplets with three possible results: they may penetrate the droplets, adhere to the droplet surface, or bounce back after impact. The extent of penetration of SiC particles into droplets was noted to depend on the kinetic energy of the particles and the magnitude of the surface energy change in the droplets that occurs upon impact. In liquid droplets, the extent of penetration of SiC particles was shown to depend on the changes in surface energy, ΔEs, experienced by the droplets. Accordingly, large SiC particles encoun- tered more resistance to penetration relative to small ones. In solid droplets, the penetration of SiC particles was correlated with the dynamic pressure exerted by the SiC particles on the droplets during impact and the depth of the ensuing crater. The results showed that no pene- tration was possible in such droplets. Second, once SiC particles have penetrated droplets, their final location in the microstructure is governed by their interactions with the solidification front. As a result of these interactions, both entrapment and rejection of SiC particles occurred during droplet solidification. A comparison of the present results to those anticipated from well-established kinetic and thermodynamic models led to some interesting findings. First, the models proposed by Boiling and Cisse[24] and Chernov et al.[58] predict relative low critical interface velocities necessary for entrapment, inconsistent with the present experimental findings. Second, although the observed correlation between the critical front velocity and droplet diameter was generally consistent with that predicted by Stefanescu et a/.’s model,[27] the dependence on the size of SiC particles was not. In view of this discrepancy, three possible mechanisms were proposed to account for the experimental findings: nucleation of α-Al on SiC particles, entrapment of SiC particles between primary dendrite arms, and entrapment of SiC particles between secondary dendrite arms.

Metal droplet erosion and shielding plasma layer under plasma flows typical of transient processes in tokamaks

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

Martynenko, Yu. V., E-mail: Martynenko-YV@nrcki.ru

It is shown that the shielding plasma layer and metal droplet erosion in tokamaks are closely interrelated, because shielding plasma forms from the evaporated metal droplets, while droplet erosion is caused by the shielding plasma flow over the melted metal surface. Analysis of experimental data and theoretical models of these processes is presented.

Evaporation-induced patterns from droplets containing motile and nonmotile bacteria.

PubMed

Nellimoottil, Tittu Thomas; Rao, Pinjala Nagaraju; Ghosh, Siddhartha Sankar; Chattopadhyay, Arun

2007-08-14

In this letter, we report the observations of specific pattern formation from the evaporation of aqueous droplets containing motile and nonmotile bacteria. We found that when motile bacteria were present the droplet evaporated into disclike patterned deposits of bacteria. However, when the bacteria were made nonmotile by treatment with liquid nitrogen, the droplet evaporated into ringlike deposits. We also observed that bacteria with higher motility produced more uniformly deposited disclike patterns. Furthermore, we propose a model with numerical simulations to explain the mechanism of formation of these patterns. The model is based on the advective fluid flow from the center of the droplet toward the edge due to enhanced evaporation from the edge of the pinned droplet in comparison to that from the free surface. For the case of motile bacteria, we have added another velocity parameter toward the axis of the droplet and directed against the fluid flow in order to account for the disclike pattern formation. The numerical simulations match the experimental observations well. The present work, by qualitative and quantitative understanding of the evaporation of bacteria droplets, demonstrates that the inherent bacterial motility is primarily responsible for the formation of these differential patterns.

Evaporation kinetics of surfactant solution droplets on rice (Oryza sativa) leaves

PubMed Central

Cao, Li-Dong; Zheng, Li; Xu, Jun; Li, Feng-Min; Huang, Qi-Liang

2017-01-01

The dynamics of evaporating sessile droplets on hydrophilic or hydrophobic surfaces is widely studied, and many models for these processes have been developed based on experimental evidence. However, few research has been explored on the evaporation of sessile droplets of surfactant or pesticide solutions on target crop leaves. Thus, in this paper the impact of surfactant concentrations on contact angle, contact diameter, droplet height, and evolution of the droplets’ evaporative volume on rice leaf surfaces have been investigated. The results indicate that the evaporation kinetics of surfactant droplets on rice leaves were influenced by both the surfactant concentrations and the hydrophobicity of rice leaf surfaces. When the surfactant concentration is lower than the surfactant CMC (critical micelle concentration), the droplet evaporation time is much longer than that of the high surfactant concentration. This is due to the longer existence time of a narrow wedge region under the lower surfactant concentration, and such narrow wedge region further restricts the droplet evaporation. Besides, our experimental data are shown to roughly collapse onto theoretical curves based on the model presented by Popov. This study could supply theoretical data on the evaporation of the adjuvant or pesticide droplets for practical applications in agriculture. PMID:28472108

Magnetic water-in-water droplet microfluidics

NASA Astrophysics Data System (ADS)

Navi, Maryam; Abbasi, Niki; Tsai, Scott

2017-11-01

Aqueous two-phase systems (ATPS) have shown to be ideal candidates for replacing the conventional water-oil systems used in droplet microfluidics. We use an ATPS of Polyethylene Glycol (PEG) and Dextran (DEX) for microfluidic generation of magnetic water-in-water droplets. As ferrofluid partitions to DEX phase, there is no significant diffusion of ferrofluid at the interface of the droplets, rendering generation of magnetic DEX droplets in a non-magnetic continuous phase of PEG possible. In this system, both phases are water-based and highly biocompatible. We microfluidically generate magnetic DEX droplets at a flow-focusing junction in a jetting regime. We sort the droplets based on their size by placing a permanent magnet downstream of the droplet generation region, and show that the deflection of droplets is in good agreement with a mathematical model. We also show that the magnetic DEX droplets can be stabilized by lysozyme and be used for separation of single cell containing water-in-water droplets. This system of magnetic water-in-water droplet manipulation may find biomedical applications such as single-cell studies and drug delivery.

Meteorological and Aerosol effects on Marine Cloud Microphysical Properties

NASA Astrophysics Data System (ADS)

Sanchez, K. J.; Russell, L. M.; Modini, R. L.; Frossard, A. A.; Ahlm, L.; Roberts, G.; Hawkins, L. N.; Schroder, J. C.; Wang, Z.; Lee, A.; Abbatt, J.; Lin, J.; Nenes, A.; Wonaschuetz, A.; Sorooshian, A.; Noone, K.; Jonsson, H.; Albrecht, B. A.; Desiree, T. S.; Macdonald, A. M.; Seinfeld, J.; Zhao, R.

2015-12-01

Both meteorology and microphysics affect cloud formation and consequently their droplet distributions and shortwave reflectance. The Eastern Pacific Emitted Aerosol Cloud Experiment (EPEACE) and the Stratocumulus Observations of Los-Angeles Emissions Derived Aerosol-Droplets (SOLEDAD) studies provide detailed measurements in 6 case studies of both cloud thermodynamic properties and initial particle number distribution and composition, as well as the resulting cloud drop distribution and composition. This study uses simulations of a detailed chemical and microphysical aerosol-cloud parcel (ACP) model with explicit kinetic drop activation to reproduce the observed cloud droplet distribution and composition. Four of the cases examined had a sub-adiabatic lapse rate, which was shown to have fewer droplets due to decreased maximum supersaturation, lower LWC and higher cloud base height, consistent with previous findings. These detailed case studies provided measured thermodynamics and microphysics that constrained the simulated droplet size distribution sufficiently to match the droplet number within 6% and the size within 19% for 4 of the 6 cases, demonstrating "closure" or consistency of the measured composition with the measured CCN spectra and the inferred and modeled supersaturation. The contribution of organic components to droplet formation shows small effects on the droplet number and size in the 4 marine cases that had background aerosol conditions with varying amounts of coastal, ship or other non-biogenic sources. In contrast, the organic fraction and hygroscopicity increased the droplet number and size in the cases with generated smoke and cargo ship plumes that were freshly emitted and not yet internally mixed with the background particles. The simulation results show organic hygroscopicity causes small effects on cloud reflectivity (<0.7%) with the exception of the cargo ship plume and smoke plume which increased absolute cloud reflectivity fraction by 0.02 and 0.20 respectively. In addition, the ACP model simulations are compared to those from a numerical parameterization of cloud droplet activation that is suitable for GCMs and show droplet concentrations are comparable between the two methods.

A New Finite-Conductivity Droplet Evaporation Model Including Liquid Turbulence Effect

NASA Technical Reports Server (NTRS)

Balasubramanyam, M. S.; Chen, C. P.; Trinh, H. P.

2006-01-01

A new approach to account for finite thermal conductivity and turbulence effects within atomizing droplets of an evaporating spray is presented in this paper. The model is an extension of the T-blob and T-TAB atomization/spray model of Trinh and Chen [9]. This finite conductivity model is based on the two-temperature film theory in which the turbulence characteristics of the droplet are used to estimate the effective thermal diffusivity for the liquid-side film thickness. Both one-way and two-way coupled calculations were performed to investigate the performance cf this model against the published experimental data.

Sectorial oscillation of acoustically levitated nanoparticle-coated droplet

NASA Astrophysics Data System (ADS)

Zang, Duyang; Chen, Zhen; Geng, Xingguo

2016-01-01

We have investigated the dynamics of a third mode sectorial oscillation of nanoparticle-coated droplets using acoustic levitation in combination with active modulation. The presence of nanoparticles at the droplet surface changes its oscillation amplitude and frequency. A model linking the interfacial rheology and oscillation dynamics has been proposed in which the compression modulus ɛ of the particle layer is introduced into the analysis. The ɛ obtained with the model is in good agreement with that obtained by the Wilhelmy plate approach, highlighting the important role of interfacial rheological properties in the sectorial oscillation of droplets.

Droplet motion in microfluidic networks: Hydrodynamic interactions and pressure-drop measurements

NASA Astrophysics Data System (ADS)

Sessoms, D. A.; Belloul, M.; Engl, W.; Roche, M.; Courbin, L.; Panizza, P.

2009-07-01

We present experimental, numerical, and theoretical studies of droplet flows in hydrodynamic networks. Using both millifluidic and microfluidic devices, we study the partitioning of monodisperse droplets in an asymmetric loop. In both cases, we show that droplet traffic results from the hydrodynamic feedback due to the presence of droplets in the outlet channels. We develop a recently-introduced phenomenological model [W. Engl , Phys. Rev. Lett. 95, 208304 (2005)] and successfully confront its predictions to our experimental results. This approach offers a simple way to measure the excess hydrodynamic resistance of a channel filled with droplets. We discuss the traffic behavior and the variations in the corresponding hydrodynamic resistance length Ld and of the droplet mobility β , as a function of droplet interdistance and confinement for channels having circular or rectangular cross sections.

Numerical Modeling of Turbulence Effects within an Evaporating Droplet in Atomizing Sprays

NASA Technical Reports Server (NTRS)

Balasubramanyam, M. S.; Chen, C. P.; Trinh, H. P.

2006-01-01

A new approach to account for finite thermal conductivity and turbulence effects within atomizing liquid sprays is presented in this paper. The model is an extension of the T-blob and T-TAB atomization/spray model of Trinh and Chen (2005). This finite conductivity model is based on the two-temperature film theory, where the turbulence characteristics of the droplet are used to estimate the effective thermal diffhsivity within the droplet phase. Both one-way and two-way coupled calculations were performed to investigate the performance of this model. The current evaporation model is incorporated into the T-blob atomization model of Trinh and Chen (2005) and implemented in an existing CFD Eulerian-Lagrangian two-way coupling numerical scheme. Validation studies were carried out by comparing with available evaporating atomization spray experimental data in terms of jet penetration, temperature field, and droplet SMD distribution within the spray. Validation results indicate the superiority of the finite-conductivity model in low speed parallel flow evaporating spray.

Application of welding science to welding engineering: A lumped parameter gas metal arc welding dynamic process model

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

Murray, P.E.; Smartt, H.B.; Johnson, J.A.

1997-12-31

We develop a model of the depth of penetration of the weld pool in gas metal arc welding (GMAW) which demonstrates interaction between the arc, filler wire and weld pool. This model is motivated by the observations of Essers and Walter which suggest a relationship between droplet momentum and penetration depth. A model of gas metal arc welding was augmented to include an improved model of mass transfer and a simple model of accelerating droplets in a plasma jet to obtain the mass and momentum of impinging droplets. The force of the droplets and depth of penetration is correlated bymore » a dimensionless linear relation used to predict weld pool depth for a range of values of arc power and contact tip to workpiece distance. Model accuracy is examined by comparing theoretical predictions and experimental measurements of the pool depth obtained from bead on plate welds of carbon steel in an argon rich shielding gas. Moreover, theoretical predictions of pool depth are compared to the results obtained from the heat conduction model due to Christensen et al. which suggest that in some cases the momentum of impinging droplets is a better indicator of the depth of the weld pool and the presence of a deep, narrow penetration.« less

A theoretical model of the influence of spray on the exchange of momentum, with storm and hurricane winds

NASA Astrophysics Data System (ADS)

Troitskaya, Yuliya; Ezhova, Ekaterina; Soustova, Irina

2013-04-01

A stochastic model of the "life cycle" of a droplet, the torn off the crest of a steep surface wave and then falling down to the water is constructed. The model includes the following constituents: i) a model of motion of a heavy particle in the forcing air flow (equation of motion), ii) a model of the wind flow (wind velocity, wave-induced disturbances, turbulent fluctuations), iii) a model of spray injection, iiii) the droplet statistics (size distribution, wind-speed dependence) The interaction of water droplets in the atmospheric boundary layer with turbulent fluctuations is described in terms of the Markovian chain. The mean wind field in the marine atmospheric boundary layer is determined by the momentum exchange associated with the turbulent and wave momentum transfer and by sprays. The wave and turbulent momentum exchange is parameterized by the Charnok expression for the roughness parameter. Wave disturbances induced in the air flow by waves at the surface, were calculated within the model of the marine atmospheric boundary suggested in [1]. The greatest uncertainty in this model is the mechanism of droplets injection. We consider two models for the injection of droplets in the air flow. In the first model the droplets formed by the development of the Kelvin-Helmholtz instability, are entered in the flow with the orbital velocity of the wave (Koga's model [2]), The second mechanism, investigated in many papers, considers droplets from the breakdown of a jet which rises at high speeds from the bottom of the collapsing air bubble cavity [3]. To determine the number of drops injected to the atmospheric boundary layer from the sea surface, the Spray generation function proposed in [4] was in use. Within the model the momentum acquired by every droplet in the interaction with the air flow was calculated. Depending on the particular field of air velocity, wave parameters and the radius of the droplet, it can both get and deliver momentum give impetus to the air flow during the life cycle from taking them off the water to fall into the water. Contribution of droplets to the momentum balance of air flow is determined by the total momentum balance of sea sprays. The calculations in the model showed that the momentum exchange with the spray can lead to either a weak (less than 10%) increase of the aerodynamic surface drag or to a weak reduction (within Koga's model [2]). Recommendations for the experiment on investigation of the "life cycle" of spray in the air flow are suggested. This work was supported by RFBR (project 11-05-12047-ofi-m, 13-05-00865-a, 12-05-33070 mol-a-ved, 12-05-31435 mol-a, 12-05-01064_A). References 1. Troitskaya, Y. I., D. A. Sergeev, A. A. Kandaurov, G. A. Baidakov, M. A. Vdovin, and V. I. Kazakov Laboratory and theoretical modeling of air-sea momentum transfer under severe wind conditions J.Geophys. Res., 117, C00J21, doi:10.1029/2011JC007778. 2. Koga M. Direct production of droplets from breaking wind-waves - its observation by a multi-colored overlapping exposure photographing technique // Tellus. 1981. V.33. Issue 6. P. 552-563. 3. Spiel D.E. On the birth of jet drops from bubbles bursting on water surfaces // J. Geophys. Res. 1995. V.100. P. 4995-5006. 4. Andreas E. L., 1998: A new sea spray generation function for wind speeds up to 32 m s21. J. Phys. Oceanogr., 28, 2175-2184.

Droplets on bent fibers

NASA Astrophysics Data System (ADS)

Weyer, Floriane; Pan, Zhao; Pitt, William; Truscott, Tadd; Vandewalle, Nicolas

Droplets on fibers are part of our everyday lives. Many phenomena involve drops and fibers such as the formation of dew droplets on a spiderweb, the trapping of water droplets on cactus spines or the motion of droplets on wetted moss hairs. These topics have been widely studied. In particular, Lorenceau et al. determined the critical volume of a water droplet hanging on a horizontal fiber. Here, we address a similar question : we try to find out the maximum droplet size on bent fibers, which are able to hold significantly more water than horizontal fibers. Indeed, we noticed that, in nature, some specific plants can hold large rain droplets thanks to their Y-shaped leaves. We try to mimic these structures with nylon fibers, of different diameters, bent with various angles. For each set-up, the critical water volume is determined. Finally, we propose models of the physics involved in determining droplet size that could be implemented in future fiber-based microfluidic devices.

Charging and Release Mechanisms of Flexible Macromolecules in Droplets

NASA Astrophysics Data System (ADS)

Oh, Myong In; Consta, Styliani

2017-08-01

We study systematically the charging and release mechanisms of a flexible macromolecule, modeled by poly(ethylene glycol) (PEG), in a droplet by using molecular dynamics simulations. We compare how PEG is solvated and charged by sodium Na+ ions in a droplet of water (H2O), acetonitrile (MeCN), and their mixtures. Initially, we examine the location and the conformation of the macromolecule in a droplet bearing no net charge. It is revealed that the presence of charge carriers do not affect the location of PEG in aqueous and MeCN droplets compared with that in the neutral droplets, but the location of the macromolecule and the droplet size do affect the PEG conformation. PEG is charged on the surface of a sodiated aqueous droplet that is found close to the Rayleigh limit. Its charging is coupled to the extrusion mechanism, where PEG segments leave the droplet once they coordinate a Na+ ion or in a correlated motion with Na+ ions. In contrast, as PEG resides in the interior of a MeCN droplet, it is sodiated inside the droplet. The compact macro-ion transitions through partially unwound states to an extended conformation, a process occurring during the final stage of desolvation and in the presence of only a handful of MeCN molecules. For charged H2O/MeCN droplets, the sodiation of PEG is determined by the H2O component, reflecting its slower evaporation and preference over MeCN for solvating Na+ ions. We use the simulation data to construct an analytical model that suggests that the droplet surface electric field may play a role in the macro-ion-droplet interactions that lead to the extrusion of the macro-ion. This study provides the first evidence of the effect of the surface electric field by using atomistic simulations. [Figure not available: see fulltext.

Impinging Water Droplets on Inclined Glass Surfaces

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

Armijo, Kenneth Miguel; Lance, Blake; Ho, Clifford K.

Multiphase computational models and tests of falling water droplets on inclined glass surfaces were developed to investigate the physics of impingement and potential of these droplets to self-clean glass surfaces for photovoltaic modules and heliostats. A multiphase volume-of-fluid model was developed in ANSYS Fluent to simulate the impinging droplets. The simulations considered different droplet sizes (1 mm and 3 mm), tilt angles (0°, 10°, and 45°), droplet velocities (1 m/s and 3 m/s), and wetting characteristics (wetting=47° contact angle and non-wetting = 93° contact angle). Results showed that the spread factor (maximum droplet diameter during impact divided by the initialmore » droplet diameter) decreased with increasing inclination angle due to the reduced normal force on the surface. The hydrophilic surface yielded greater spread factors than the hydrophobic surface in all cases. With regard to impact forces, the greater surface tilt angles yielded lower normal forces, but higher shear forces. Experiments showed that the experimentally observed spread factor (maximum droplet diameter during impact divided by the initial droplet diameter) was significantly larger than the simulated spread factor. Observed spread factors were on the order of 5 - 6 for droplet velocities of ~3 m/s, whereas the simulated spread factors were on the order of 2. Droplets were observed to be mobile following impact only for the cases with 45° tilt angle, which matched the simulations. An interesting phenomenon that was observed was that shortly after being released from the nozzle, the water droplet oscillated (like a trampoline) due to the "snapback" caused by the surface tension of the water droplet being released from the nozzle. This oscillation impacted the velocity immediately after the release. Future work should evaluate the impact of parameters such as tilt angle and surface wettability on the impact of particle/soiling uptake and removal to investigate ways that photovoltaic modules and heliostats can be designed to maximize self-cleaning.« less

Ignition of an organic water-coal fuel droplet floating in a heated-air flow

NASA Astrophysics Data System (ADS)

Valiullin, T. R.; Strizhak, P. A.; Shevyrev, S. A.; Bogomolov, A. R.

2017-01-01

Ignition of an organic water-coal fuel (CWSP) droplet floating in a heated-air flow has been studied experimentally. Rank B2 brown-coal particles with a size of 100 μm, used crankcase Total oil, water, and a plasticizer were used as the main CWSP components. A dedicated quartz-glass chamber has been designed with inlet and outlet elements made as truncated cones connected via a cylindrical ring. The cones were used to shape an oxidizer flow with a temperature of 500-830 K and a flow velocity of 0.5-5.0 m/s. A technique that uses a coordinate-positioning gear, a nichrome thread, and a cutter element has been developed for discharging CWSP droplets into the working zone of the chamber. Droplets with an initial size of 0.4 to 2.0 mm were used. Conditions have been determined for a droplet to float in the oxidizer flow long enough for the sustainable droplet burning to be initiated. Typical stages and integral ignition characteristics have been established. The integral parameters (ignition-delay times) of the examined processes have been compared to the results of experiments with CWSP droplets suspended on the junction of a quick-response thermocouple. It has been shown that floating fuel droplets ignite much quicker than the ones that sit still on the thermocouple due to rotation of an CWSP droplet in the oxidizer flow, more uniform heating of the droplet, and lack of heat drainage towards the droplet center. High-speed video recording of the peculiarities of floatation of a burning fuel droplet makes it possible to complement the existing models of water-coal fuel burning. The results can be used for a more substantiated modeling of furnace CWSP burning with the ANSYS, Fluent, and Sigma-Flow software packages.

Numerical study of liquid-hydrogen droplet generation from a vibrating orifice

NASA Astrophysics Data System (ADS)

Xu, J.; Celik, D.; Hussaini, M. Y.; Van Sciver, S. W.

2005-08-01

Atomic hydrogen propellant feed systems for far-future spacecraft may utilize solid-hydrogen particle carriers for atomic species that undergo recombination to create hot rocket exhaust. Such technology will require the development of particle generation techniques. One such technique could involve the production of hydrogen droplets from a vibrating orifice that would then freeze in cryogenic helium vapor. Among other quantities, the shape and size of the droplet are of particular interest. The present paper addresses this problem within the framework of the incompressible Navier-Stokes equations for multiphase flows, in order to unravel the basic mechanisms of droplet formation with a view to control them. Surface tension, one of the most important mechanisms to determine droplet shape, is modeled as the source term in the momentum equation. Droplet shape is tracked using a volume-of-fluid approach. A dynamic meshing technique is employed to accommodate the vibration of the generator orifice. Numerically predicted droplet shapes show satisfactory agreement with photographs of droplets generated in experiments. A parametric study is carried out to understand the influence of injection velocity, nozzle vibrational frequency, and amplitude on the droplet shape and size. The computational model provides a definitive qualitative picture of the evolution of droplet shape as a function of the operating parameters. It is observed that, primarily, the orifice vibrational frequency affects the shape, the vibrational amplitude affects the time until droplet detachment from the orifice, and the injection velocity affects the size. However, it does not mean that, for example, there is no secondary effect of amplitude on shape or size.

On the transient flow inside and around a deforming millimetre class oil droplet falling under the action of gravity in stagnant air

NASA Astrophysics Data System (ADS)

Bergeles, K.; Hardalupas, Y.; Taylor, A. M. K. P.

2018-01-01

The liquid flow inside, and the induced air flow around, a falling droplet in stagnant air was numerically investigated using the volume of fluid method to describe the droplet interface. The droplet consisted of oil with the same surface tension and with viscosity as parameter. It was injected into stagnant air with an initial velocity of 1 m/s; therefore, the initial Weber (We = 0.14), Reynolds (Re = 141), and Bond (Bo = 2.4) numbers remained constant during the parametric study whilst the initial Capillary (Ca) and Ohnesorge (Oh) numbers varied by an order of magnitude from 0.46 to 4.6 and from 0.044 to 0.44, respectively. We examined the effect of viscosity on the flow inside, and around, the droplet as well as on the droplet deformation and its natural frequency. This investigation showed a strong dependence of the deformation with liquid viscosity. Specifically, the droplets achieved their final deformation in under-damped, for low viscosity, and in over-damped, for high viscosity, oscillation modes. After a critical time tcrit (or Recrit), the instantaneous air flow symmetry was disturbed, initially in the wake and soon after in the interior of the droplet and in the vortex shedding downstream of the droplet. The air flow in the wake region detached from the droplet surface and resulted in a wake which was approximately 1.5 times longer and wider than the wake behind a solid sphere at the same Re number at steady state conditions. A roller-vortex structure (called rollex) was established upon injection in the immediate wake of the droplet, forming the necessary kinematic link between the directions of the internal circulation in the droplet (Hill vortex) and of the external recirculating air flow in the droplet's wake. The droplet drag coefficients were compared with corresponding values used in droplet breakup models: although, ultimately, the droplet drag coefficient converged to the values given by the models, the initial magnitudes after injection were incorrect.

A Priori Subgrid Analysis of Temporal Mixing Layers with Evaporating Droplets

NASA Technical Reports Server (NTRS)

Okongo, Nora; Bellan, Josette

1999-01-01

Subgrid analysis of a transitional temporal mixing layer with evaporating droplets has been performed using three sets of results from a Direct Numerical Simulation (DNS) database, with Reynolds numbers (based on initial vorticity thickness) as large as 600 and with droplet mass loadings as large as 0.5. In the DNS, the gas phase is computed using a Eulerian formulation, with Lagrangian droplet tracking. The Large Eddy Simulation (LES) equations corresponding to the DNS are first derived, and key assumptions in deriving them are first confirmed by computing the terms using the DNS database. Since LES of this flow requires the computation of unfiltered gas-phase variables at droplet locations from filtered gas-phase variables at the grid points, it is proposed to model these by assuming the gas-phase variables to be the sum of the filtered variables and a correction based on the filtered standard deviation; this correction is then computed from the Subgrid Scale (SGS) standard deviation. This model predicts the unfiltered variables at droplet locations considerably better than simply interpolating the filtered variables. Three methods are investigated for modeling the SGS standard deviation: the Smagorinsky approach, the Gradient model and the Scale-Similarity formulation. When the proportionality constant inherent in the SGS models is properly calculated, the Gradient and Scale-Similarity methods give results in excellent agreement with the DNS.

Characteristics of sprinklers and water spray mists for fire safety

NASA Astrophysics Data System (ADS)

Jackman, Louise A.; Lavelle, Stephen P.; Nolan, P. F.

1991-04-01

In order to predict the type of sprinkler or spray head required for fire safety in buildings and transport systems (e.g. aircraft) it is necessary to model the interaction of water droplets with the thermally buoyant fire gases. Such modelling requires a detailed knowledge of the mean droplet size, the droplet size distribution, droplet velocity and trajectory. Many existing systems for the characterisation of droplets are indirect in that an optical property is measured and the results are subject to "black box" data processing. A direct method can be developed using a synchronised metal vapour laser and high speed cine camera with appropriate optics. Results on both sprinkler and spray mist will be presented and a basis for the choice of active fire protection systems will be outlined.

Effect of indium droplets on growth of InGaN film by molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Zheng, Xiantong; Liang, Hongwei; Wang, Ping; Sun, Xiaoxiao; Chen, Zhaoying; Wang, Tao; Sheng, Bowen; Wang, Yixin; Chen, Ling; Wang, Ding; Rong, Xin; Li, Mo; Zhang, Jian; Wang, Xinqiang

2018-01-01

Effect of indium (In) droplets on InGaN thin films grown by molecular beam epitaxy (MBE) has been investigated. The surface of InGaN covered by In droplets shows a smoother topography than that without droplets, indicating that the presence of In droplets is beneficial to the two dimensional growth. Beneath the In droplets, many ring-like structures are observed. The arrangement of these "ring" shows the movement of the In droplets during the InGaN growth. A qualitative growth model is proposed to explain the evolution of the InGaN surface morphology in In-droplet-induced-epitaxy process, giving an explanation that a local vapor-liquid-solid (VLS) system is preferentially formed at the edge of the droplets, leading to a high growth rate. Furthermore, the energy dispersive X-ray spectroscopy results reveal that the relatively higher In/Ga flux ratio in the region covered by the In droplet results in a locally higher In content.

Recent results and persisting problems in modeling flow induced coalescence

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

Fortelný, I., E-mail: fortelny@imc.cas.cz, E-mail: juza@imc.cas.cz; Jza, J., E-mail: fortelny@imc.cas.cz, E-mail: juza@imc.cas.cz

2014-05-15

The contribution summarizes recent results of description of the flow induced coalescence in immiscible polymer blends and addresses problems that call for which solving. The theory of coalescence based on the switch between equations for matrix drainage between spherical or deformed droplets provides a good agreement with more complicated modeling and available experimental data for probability, P{sub c}, that the collision of droplets will be followed by their fusion. A new equation for description of the matrix drainage between deformed droplets, applicable to the whole range of viscosity ratios, p, of the droplets and matrixes, is proposed. The theory facilitatesmore » to consider the effect of the matrix elasticity on coalescence. P{sub c} decreases with the matrix relaxation time but this decrease is not pronounced for relaxation times typical of most commercial polymers. Modeling of the flow induced coalescence in concentrated systems is needed for prediction of the dependence of coalescence rate on volume fraction of droplets. The effect of the droplet anisometry on P{sub c} should be studied for better understanding the coalescence in flow field with high and moderate deformation rates. A reliable description of coalescence in mixing and processing devices requires proper modeling of complex flow fields.« less

A lattice Boltzmann model for substrates with regularly structured surface roughness

NASA Astrophysics Data System (ADS)

Yagub, A.; Farhat, H.; Kondaraju, S.; Singh, T.

2015-11-01

Superhydrophobic surface characteristics are important in many industrial applications, ranging from the textile to the military. It was observed that surfaces fabricated with nano/micro roughness can manipulate the droplet contact angle, thus providing an opportunity to control the droplet wetting characteristics. The Shan and Chen (SC) lattice Boltzmann model (LBM) is a good numerical tool, which holds strong potentials to qualify for simulating droplets wettability. This is due to its realistic nature of droplet contact angle (CA) prediction on flat smooth surfaces. But SC-LBM was not able to replicate the CA on rough surfaces because it lacks a real representation of the physics at work under these conditions. By using a correction factor to influence the interfacial tension within the asperities, the physical forces acting on the droplet at its contact lines were mimicked. This approach allowed the model to replicate some experimentally confirmed Wenzel and Cassie wetting cases. Regular roughness structures with different spacing were used to validate the study using the classical Wenzel and Cassie equations. The present work highlights the strength and weakness of the SC model and attempts to qualitatively conform it to the fundamental physics, which causes a change in the droplet apparent contact angle, when placed on nano/micro structured surfaces.

Compositional evolution of particle-phase reaction products and water in the heterogeneous OH oxidation of model aqueous organic aerosols

NASA Astrophysics Data System (ADS)

Chim, Man Mei; Cheng, Chiu Tung; Davies, James F.; Berkemeier, Thomas; Shiraiwa, Manabu; Zuend, Andreas; Nin Chan, Man

2017-12-01

Organic compounds present at or near the surface of aqueous droplets can be efficiently oxidized by gas-phase OH radicals, which alter the molecular distribution of the reaction products within the droplet. A change in aerosol composition affects the hygroscopicity and leads to a concomitant response in the equilibrium amount of particle-phase water. The variation in the aerosol water content affects the aerosol size and physicochemical properties, which in turn governs the oxidation kinetics and chemistry. To attain better knowledge of the compositional evolution of aqueous organic droplets during oxidation, this work investigates the heterogeneous OH-radical-initiated oxidation of aqueous methylsuccinic acid (C5H8O4) droplets, a model compound for small branched dicarboxylic acids found in atmospheric aerosols, at a high relative humidity of 85 % through experimental and modeling approaches. Aerosol mass spectra measured by a soft atmospheric pressure ionization source (Direct Analysis in Real Time, DART) coupled with a high-resolution mass spectrometer reveal two major products: a five carbon atom (C5) hydroxyl functionalization product (C5H8O5) and a C4 fragmentation product (C4H6O3). These two products likely originate from the formation and subsequent reactions (intermolecular hydrogen abstraction and carbon-carbon bond scission) of tertiary alkoxy radicals resulting from the OH abstraction occurring at the methyl-substituted carbon site. Based on the identification of the reaction products, a kinetic model of oxidation (a two-product model) coupled with the Aerosol Inorganic-Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model is built to simulate the size and compositional changes of aqueous methylsuccinic acid droplets during oxidation. Model results show that at the maximum OH exposure, the droplets become slightly more hygroscopic after oxidation, as the mass fraction of water is predicted to increase from 0.362 to 0.424; however, the diameter of the droplets decreases by 6.1 %. This can be attributed to the formation of volatile fragmentation products that partition to the gas phase, leading to a net loss of organic species and associated particle-phase water, and thus a smaller droplet size. Overall, fragmentation and volatilization processes play a larger role than the functionalization process in determining the evolution of aerosol water content and droplet size at high-oxidation stages.

Interface-Resolving Simulation of Collision Efficiency of Cloud Droplets

NASA Astrophysics Data System (ADS)

Wang, Lian-Ping; Peng, Cheng; Rosa, Bodgan; Onishi, Ryo

2017-11-01

Small-scale air turbulence could enhance the geometric collision rate of cloud droplets while large-scale air turbulence could augment the diffusional growth of cloud droplets. Air turbulence could also enhance the collision efficiency of cloud droplets. Accurate simulation of collision efficiency, however, requires capture of the multi-scale droplet-turbulence and droplet-droplet interactions, which has only been partially achieved in the recent past using the hybrid direct numerical simulation (HDNS) approach. % where Stokes disturbance flow is assumed. The HDNS approach has two major drawbacks: (1) the short-range droplet-droplet interaction is not treated rigorously; (2) the finite-Reynolds number correction to the collision efficiency is not included. In this talk, using two independent numerical methods, we will develop an interface-resolved simulation approach in which the disturbance flows are directly resolved numerically, combined with a rigorous lubrication correction model for near-field droplet-droplet interaction. This multi-scale approach is first used to study the effect of finite flow Reynolds numbers on the droplet collision efficiency in still air. Our simulation results show a significant finite-Re effect on collision efficiency when the droplets are of similar sizes. Preliminary results on integrating this approach in a turbulent flow laden with droplets will also be presented. This work is partially supported by the National Science Foundation.

Splashing Droplets

NASA Technical Reports Server (NTRS)

VanderWal, Randall L.; Kizito, John Patrick; Berger, Gordon M.; Iwan, J.; Alexander, D.; Tryggvason, Gretar

2002-01-01

Current data on droplet breakup is scarce for the sizes and velocities typical of practical applications such as in spray combustion processes and coating processes. While much more representative of practical applications, the small spatial scales and rapid time-scales prevent detailed measurement of the internal fluid dynamics and liquid property gradients produced by impinging upon surfaces. Realized through the extended spatial and temporal scales afforded by a microgravity environment, an improved understanding of drop breakup dynamics is sought to understand and ultimately control the impingement dynamics of droplets upon surfaces in practical situations. The primary objective of this research will be to mark the onset of different 'splashing modes' and to determine their temperature, pressure and angle dependence for impinging droplets representative of practical fluids. In addition, we are modeling the evolution of droplets that do not initially splash but rather undergo a 'fingering' evolution observed on the spreading fluid front and the transformation of these fingers into splashed products. An example of our experimental data is presented below. These images are of Isopar V impacting a mirror-polished surface. They were acquired using a high-speed camera at 1000 frames per second. They show the spreading of a single droplet after impact and ensuing finger instabilities. Normal gravity experimental data such as this will guide low gravity measurements in the 2.2 second drop tower and KC-135 aircraft as available. Presently we are in the process of comparing the experimental data of droplet shape evolution to numerical models, which can also capture the internal fluid dynamics and liquid property gradients such as produced by impingement upon a heated surface. To-date isothermal numerical data has been modeled using direct numerical simulations of representative splashing droplets. The data obtained so far indicates that the present model describes well the droplet wall interactions to a point in time just before splash. Additional information is included in the original extended abstract.

Capillary droplet propulsion on a fibre.

PubMed

Haefner, Sabrina; Bäumchen, Oliver; Jacobs, Karin

2015-09-21

A viscous liquid film coating a fibre becomes unstable and decays into droplets due to the Rayleigh-Plateau instability (RPI). Here, we report on the generation of uniform droplets on a hydrophobized fibre by taking advantage of this effect. In the late stages of liquid column breakup, a three-phase contact line can be formed at one side of the droplet by spontaneous rupture of the thinning film. The resulting capillary imbalance leads to droplet propulsion along the fibre. We study the dynamics and the dewetting speed of the droplet as a function of molecular weight as well as temperature and compare to a force balance model based on purely viscous dissipation.

Dynamics of water droplets detached from porous surfaces of relevance to PEM fuel cells.

PubMed

Theodorakakos, A; Ous, T; Gavaises, M; Nouri, J M; Nikolopoulos, N; Yanagihara, H

2006-08-15

The detachment of liquid droplets from porous material surfaces used with proton exchange membrane (PEM) fuel cells under the influence of a cross-flowing air is investigated computationally and experimentally. CCD images taken on a purpose-built transparent fuel cell have revealed that the water produced within the PEM is forming droplets on the surface of the gas-diffusion layer. These droplets are swept away if the velocity of the flowing air is above a critical value for a given droplet size. Static and dynamic contact angle measurements for three different carbon gas-diffusion layer materials obtained inside a transparent air-channel test model have been used as input to the numerical model; the latter is based on a Navier-Stokes equations flow solver incorporating the volume of fluid (VOF) two-phase flow methodology. Variable contact angle values around the gas-liquid-solid contact-line as well as their dynamic change during the droplet shape deformation process, have allowed estimation of the adhesion force between the liquid droplet and the solid surface and successful prediction of the separation line at which droplets loose their contact from the solid surface under the influence of the air stream flowing around them. Parametric studies highlight the relevant importance of various factors affecting the detachment of the liquid droplets from the solid surface.

Electrostatic field and charge distribution in small charged dielectric droplets

NASA Astrophysics Data System (ADS)

Storozhev, V. B.

2004-08-01

The charge distribution in small dielectric droplets is calculated on the basis of continuum medium approximation. There are considered charged liquid spherical droplets of methanol in the range of nanometer sizes. The problem is solved by the following way. We find the free energy of some ion in dielectric droplet, which is a function of distribution of other ions in the droplet. The probability of location of the ion in some element of volume in the droplet is a function of its free energy in this element of volume. The same approach can be applied to other ions in the droplet. The obtained charge distribution differs considerably from the surface distribution. The curve of the charge distribution in the droplet as a function of radius has maximum near the surface. Relative concentration of charges in the vicinity of the center of the droplet does not equal to zero, and it is the higher, the less is the total charge of the droplet. According to the estimates the model is applicable if the droplet radius is larger than 10 nm.

Effect of surface roughness on contact line dynamics of a thin droplet

NASA Astrophysics Data System (ADS)

Bhattacharjee, Debanik; Soltannia, Babak; Nazaripoor, Hadi; Sadrzadeh, Mohtada

2017-11-01

Any surface possesses inherent roughness. Droplet spreading on a surface is an example of a contact line problem. The tri-phase contact line is prone to stress singularity which can be relieved by using precursor film assumption and disjoining pressure. In this study, an axisymmetric, incompressible, Newtonian droplet spreading on a surface was investigated. An evolution equation which tracks the droplet height over time was obtained considering the lubrication approximation. The nonlinear PDE of evolution equation was solved using finite difference scheme. A simplified Gaussian model was used as a starting point to assess the role of roughness in the dynamics of contact line. The preliminary results revealed that, for both impermeable and permeable surfaces, the apparent contact angle increased in the presence of defects whereas the equilibrium stage remained unaffected. The apparent contact angle, however, was more strongly dependent on the nature and density of defects for impermeable surfaces due to the longer droplet lifetime. Furthermore, random self-affine and non-Gaussian models are employed. The mathematical model results are finally compared with theoretical models like the Cassie-Baxter, Wenzel, and Penetration modes. NSERC.

Axisymmetric Lattice Boltzmann Model of Droplet Impact on Solid Surfaces

NASA Astrophysics Data System (ADS)

Dalgamoni, Hussein; Yong, Xin

2017-11-01

Droplet impact is a ubiquitous fluid phenomena encountered in scientific and engineering applications such as ink-jet printing, coating, electronics manufacturing, and many others. It is of great technological importance to understand the detailed dynamics of drop impact on various surfaces. The lattice Boltzmann method (LBM) emerges as an efficient method for modeling complex fluid systems involving rapidly evolving fluid-fluid and fluid-solid interfaces with complex geometries. In this work, we model droplet impact on flat solid substrates with well-defined wetting behavior using a two-phase axisymmetric LBM with high density and viscosity contrasts. We extend the two-dimensional Lee and Liu model to capture axisymmetric effect in the normal impact. First we compare the 2D axisymmetric results with the 2D and 3D results reported by Lee and Liu to probe the effect of axisymmetric terms. Then, we explore the effects of Weber number, Ohnesorge number, and droplet-surface equilibrium contact angle on the impact. The dynamic contact angle and spreading factor of the droplet during impact are investigated to qualitatively characterize the impact dynamics.

Acid rain: Microphysical model

NASA Technical Reports Server (NTRS)

Dingle, A. N.

1980-01-01

A microphysical model was used to simulate the case of a ground cloud without dilution by entrainment and without precipitation. The numerical integration techniques of the model are presented. The droplet size spectra versus time and the droplet molalities for each value of time are discussed.

Impact of a compound droplet on a flat surface: A model for single cell epitaxy.

PubMed

Tasoglu, Savas; Kaynak, Gozde; Szeri, Andrew J; Demirci, Utkan; Muradoglu, Metin

2010-08-01

The impact and spreading of a compound viscous droplet on a flat surface are studied computationally using a front-tracking method as a model for the single cell epitaxy. This is a technology developed to create two-dimensional and three-dimensional tissue constructs cell by cell by printing cell-encapsulating droplets precisely on a substrate using an existing ink-jet printing method. The success of cell printing mainly depends on the cell viability during the printing process, which requires a deeper understanding of the impact dynamics of encapsulated cells onto a solid surface. The present study is a first step in developing a model for deposition of cell-encapsulating droplets. The inner droplet representing the cell, the encapsulating droplet, and the ambient fluid are all assumed to be Newtonian. Simulations are performed for a range of dimensionless parameters to probe the deformation and rate of deformation of the encapsulated cell, which are both hypothesized to be related to cell damage. The deformation of the inner droplet consistently increases: as the Reynolds number increases; as the diameter ratio of the encapsulating droplet to the cell decreases; as the ratio of surface tensions of the air-solution interface to the solution-cell interface increases; as the viscosity ratio of the cell to encapsulating droplet decreases; or as the equilibrium contact angle decreases. It is observed that maximum deformation for a range of Weber numbers has (at least) one local minimum at We=2. Thereafter, the effects of cell deformation on viability are estimated by employing a correlation based on the experimental data of compression of cells between parallel plates. These results provide insight into achieving optimal parameter ranges for maximal cell viability during cell printing.

Brief Communication: A Simplified Approach to Transient Convective Droplet Evaporation and Burning

NASA Technical Reports Server (NTRS)

Madooglu, K.; Karagozian, A. R.

1994-01-01

Empirical correlations for evaporation rates from single fuel droplets have existed since the 1930s. These correlations, which will be referred to in this article as Froessling/Ranz-Marshall types of correlations, are appropriate to the special cases of steady-state evaporation in the absence of chemical reaction. In a previous article by the authors, the quasi-steady evaporation and burning processes associated with a fuel drop in a convective environment are examined through a droplet model based on the boundary layer approach. For droplet Reynolds numbers of practical interest, this model produces very reasonable steady state as well as quasi-time-dependent droplet simulations, requiring relatively short computational times and yielding good agreement with the above-mentioned empirical correlations. The steady-state case, however, is usually relevant to practical combustor situations only when the drop has reached a nearly uniform temperature since the heating process of the drop cannot be considered to be quasi-steady. In the present study, the transient heating process of the droplet interior during evaporation and/or burning is taken into account, and thus calculations pertaining to the entire life-time of the droplet are carried out. It is of particular interest here to obtain simplified correlations to describe the transient behavior of evaporating and burning droplets; these may be incorporated with greater ease into spray calculations. Accordingly, we have chosen to use stagnation conditions in the present model in a modification of the Froessling/Ranz-Marshall correlations. These modified correlations, incorporating an effective transfer number, produce a fairly accurate representation of droplet evaporation and burning, while requiring only one tenth the computational effort used in a full boundary layer solution.

Experiments and Model Development for the Investigation of Sooting and Radiation Effects in Microgravity Droplet Combustion

NASA Technical Reports Server (NTRS)

Choi, Mun Young; Yozgatligil, Ahmet; Dryer, Frederick L.; Kazakov, Andrei; Dobashi, Ritsu

2001-01-01

Today, despite efforts to develop and utilize natural gas and renewable energy sources, nearly 97% of the energy used for transportation is derived from combustion of liquid fuels, principally derived from petroleum. While society continues to rely on liquid petroleum-based fuels as a major energy source in spite of their finite supply, it is of paramount importance to maximize the efficiency and minimize the environmental impact of the devices that burn these fuels. The development of improved energy conversion systems, having higher efficiencies and lower emissions, is central to meeting both local and regional air quality standards. This development requires improvements in computational design tools for applied energy conversion systems, which in turn requires more robust sub-model components for combustion chemistry, transport, energy transport (including radiation), and pollutant emissions (soot formation and burnout). The study of isolated droplet burning as a unidimensional, time dependent model diffusion flame system facilitates extensions of these mechanisms to include fuel molecular sizes and pollutants typical of conventional and alternative liquid fuels used in the transportation sector. Because of the simplified geometry, sub-model components from the most detailed to those reduced to sizes compatible for use in multi-dimensional, time dependent applied models can be developed, compared and validated against experimental diffusion flame processes, and tested against one another. Based on observations in microgravity experiments on droplet combustion, it appears that the formation and lingering presence of soot within the fuel-rich region of isolated droplets can modify the burning rate, flame structure and extinction, soot aerosol properties, and the effective thermophysical properties. These observations led to the belief that perhaps one of the most important outstanding contributions of microgravity droplet combustion is the observation that in the absence of asymmetrical forced and natural convection, a soot shell is formed between the droplet surface and the flame, exerting an influence on the droplet combustion response far greater than previously recognized. The effects of soot on droplet burning parameters, including burning rate, soot shell dynamics, flame structure, and extinction phenomena provide significant testing parameters for studying the structure and coupling of soot models with other sub-model components.

Simulating the Effects of Semivolatile Compounds on Cloud Processing of Aerosol

NASA Astrophysics Data System (ADS)

Kokkola, H.; Kudzotsa, I.; Tonttila, J.; Raatikainen, T.; Romakkaniemi, S.

2017-12-01

Aerosol removal processes largely dictate how well aerosol is transported in the atmosphere and thus the aerosol load over remote regions depends on how effectively aerosol is removed during its transport from the source regions. This means that in order to model the global distribution aerosol, both in vertical and horizontal, wet deposition processes have to be properly modelled. However, in large scale models, the description of wet removal and the vertical redistribution of aerosol by cloud processes is often extremely simplified.Here we present a novel aerosol-cloud model SALSA, where the aerosol properties are tracked through different cloud processes. These processes include: cloud droplet activation, precipitation formation, ice nucleation, melting, and evaporation. It is a sectional model that includes separate size sections for non-activated aerosol, cloud droplets, precipitation droplets, and ice crystals. The aerosol-cloud model was coupled to a large eddy model UCLALES which simulates the boundary-layer dynamics. In this study, the model has been applied in studying the wet removal as well as interactions between aerosol, clouds, and semi-volatile compounds, ammonia and nitric acid. These semi-volative compounds are special in the sense that they co-condense together with water during cloud activation and have been suggested to form droplets that can be considered cloud-droplet-like already in subsaturated conditions. In our model, we calculate the kinetic partitioning of ammonia and sulfate thus explicitly taking into account the effect of ammonia and nitric acid in the cloud formation. Our simulations indicate that especially in polluted conditions, these compounds significantly affect the properties of cloud droplets thus significantly affecting the lifecycle of different aerosol compounds.

Broadening of cloud droplet spectra through turbulent entrainment and eddy hopping

NASA Astrophysics Data System (ADS)

Abade, Gustavo; Grabowski, Wojciech; Pawlowska, Hanna

2017-11-01

This work discusses the effect of cloud turbulence and turbulent entrainment on the evolution of the cloud droplet-size spectrum. We simulate an ensemble of idealized turbulent cloud parcels that are subject to entrainment events, modeled as a random Poisson process. Entrainment events, subsequent turbulent mixing inside the parcel, supersaturation fluctuations, and the resulting stochastic droplet growth by condensation are simulated using a Monte Carlo scheme. Quantities characterizing the turbulence intensity, entrainment rate and the mean fraction of environmental air entrained in an event are specified as external parameters. Cloud microphysics is described by applying Lagrangian particles, the so-called superdroplets. They are either unactivated cloud condensation nuclei (CCN) or cloud droplets that form from activated CCN. The model accounts for the transport of environmental CCN into the cloud by the entraining eddies at the cloud edge. Turbulent mixing of the entrained dry air with cloudy air is described using a linear model. We show that turbulence plays an important role in aiding entrained CCN to activate, providing a source of small cloud droplets and thus broadening the droplet size distribution. Further simulation results will be reported at the meeting.

An economical model for simulating droplet spectrum evolution in turbulent cloud chambers and wind tunnels

NASA Astrophysics Data System (ADS)

Krueger, Steven; Cantrell, W.; Niedermeier, D.; Shaw, R.; Stratmann, F.

2017-11-01

Although airborne instruments provide detailed information about the microphysical structure of clouds, the measurements provide only a few snapshots of each cloud. Deducing the droplet spectrum evolution from such measurements is next to impossible. We are using two alternative approaches: laboratory studies and numerical simulations. The former relies on a new turbulent cloud chamber (the Pi Chamber) at Michigan Technical University, as well as the first humid turbulent wind tunnel (LACIS-T) at the Leibniz Institute for Tropospheric Research. Both produce conditions for droplet growth (i.e., supersaturation) by mixing saturated vapor at different temperatures. The Pi Chamber produces turbulence by inducing Rayleigh-Bénard convection, while the wind tunnel generates turbulence with a grid. We are using the Explicit Mixing Parcel Model (EMPM) to numerically simulate droplet spectrum evolution in these flows. The EMPM explicitly links turbulent mixing and droplet spectrum evolution by representing a turbulent flow in a 1D domain with the linear eddy model. The EMPM can economically span scales from those of the smallest turbulent eddies to those of the largest. The EMPM grows or evaporates thousands of individual cloud droplets according to their local environments.

Ground Deposition of Liquid Droplets Released from a Point Source in the Atmospheric Surface Layer.

NASA Astrophysics Data System (ADS)

Panneton, Bernard

1989-09-01

A series of field experiments is presented in which the ground deposition of liquid droplets, 120 and 150 μm in diameter, released from a point source at 7 meters above the ground level, was measured. A detailed description of the experimental technique is provided, and the results are presented and compared to the predictions of a few models. A new rotating droplet generator is described. Droplets are produced by the forced breakup of capillary liquid jets and droplet coalescence is inhibited by the rotational motion of the spray head. A system for analyzing spray samples has been developed. This is a specialized image analysis system based on an electronic digitizing camera which measures the area and perimeter of stains left by dyed droplets collected on Kromekote^{rm TM } cards. A complete set of meteorological data supports the ground-deposition data. The turbulent air velocities at two levels above the ground and the temperature of the air at one level were measured with one sonic anemometer and a sonic anemometer-thermometer. The vertical heat and momentum fluxes were estimated using the eddy-correlation technique. The two-dimensional deposition patterns are presented in the form of plots of contours of constant density, normalized arcwise distributions and crosswind integrated distributions. The arcwise distributions follow a Gaussian distribution whose standard deviation is evaluated using a modified Pasquill's beta technique. Models of the crosswind integrated deposit from Godson, Csanady, Walker, Bache and Sayer, and Wilson et al are evaluated. The results indicate that the Wilson et al random walk model is adequate for predicting the ground deposition of the 150 μm droplets. In one case, where the ratio of the droplet settling velocity to the mean wind speed was largest, Walker's model proved to be adequate. Otherwise, none of the models were acceptable in light of our experimental data.

Detailed assessment of diesel spray atomization models using visible and X-ray extinction measurements

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

Magnotti, G. M.; Genzale, C. L.

The physical mechanisms characterizing the breakup of a diesel spray into droplets are still unknown. This gap in knowledge has largely been due to the challenges of directly imaging this process or quantitatively measuring the outcomes of spray breakup, such as droplet size. Recent x-ray measurements by Argonne National Laboratory, utilized in this work, provide needed information about the spatial evolution of droplet sizes in selected regions of the spray under a range of injection pressures (50–150 MPa) and ambient densities (7.6–22.8 kg/m3) relevant for diesel operating conditions. Ultra-small angle x-ray scattering (USAXS) measurements performed at the Advanced Photon Sourcemore » are presented, which quantify Sauter mean diameters (SMD) within optically thick regions of the spray that are inaccessible by conventional droplet sizing measurement techniques, namely in the near-nozzle region, along the spray centerline, and within the core of the spray. To quantify droplet sizes along the periphery of the spray, a complementary technique is proposed and introduced, which leverages the ratio of path-integrated x-ray and visible laser extinction (SAMR) measurements to quantify SMD. The SAMR and USAXS measurements are then utilized to evaluate current spray models used for engine computational fluid dynamic (CFD) simulations. We explore the ability of a carefully calibrated spray model, premised on aerodynamic wave growth theory, to capture the experimentally observed trends of SMD throughout the spray. The spray structure is best predicted with an aerodynamic primary and secondary breakup process that is represented with a slower time constant and larger formed droplet size than conventionally recommended for diesel spray models. Additionally, spray model predictions suggest that droplet collisions may not influence the resultant droplet size distribution along the spray centerline in downstream regions of the spray.« less

Modelling the impact, spreading and freezing of a water droplet on horizontal and inclined superhydrophobic cooled surfaces

NASA Astrophysics Data System (ADS)

Yao, Yina; Li, Cong; Zhang, Hui; Yang, Rui

2017-10-01

It is quite important to clearly understand the dynamic and freezing process of water droplets impacting a cold substrate for the prevention of ice accretion. In this study, a three-dimensional model including an extended phase change method was developed on OpenFOAM platform to simulate the impact, spreading and freezing of a water droplet on a cooled solid substrate. Both normal and oblique impact conditions were studied numerically. The evolution of the droplet shape and dynamic characteristics such as area ratio and spread factor were compared between numerical and experimental results. Good agreements were obtained. The effects of Weber number and Ohnersorge number on the oblique impact and freezing process were investigated. A regime map which depicts the different responses of droplets as a function of normal Weber number and Ohnesorge number was obtained. Moreover, the impact, spreading and freezing behaviour of water droplets were analyzed in detail from the numerical results.

Assessing the size distribution of droplets in a cloud chamber from light extinction data during a transient regime

NASA Astrophysics Data System (ADS)

Vâjâiac, Sorin Nicolae; Filip, Valeriu; Štefan, Sabina; Boscornea, Andreea

2014-03-01

The paper describes a method of assessing the size distribution of fog droplets in a cloud chamber, based on measuring the time variation of the transmission of a light beam during the gravitational settling of droplets. Using a model of light extinction by floating spherical particles, the size distribution of droplets is retrieved, along with characteristic structural parameters of the fog (total droplet concentration, liquid water content and effective radius). Moreover, the time variation of the effective radius can be readily extracted from the model. The errors of the method are also estimated and fall within acceptable limits. The method proves sensitive enough to resolve various modes in the droplet distribution and to point out changes in the distribution due to diverse types of aerosol present in the chamber or to the thermal condition of the fog. It is speculated that the method can be further simplified to reach an in-situ version for real-time field measurements.

Binding Affinity Effects on Physical Characteristics of a Model Phase-Separated Protein Droplet

NASA Astrophysics Data System (ADS)

Chuang, Sara; Banani, Salman; Rosen, Michael; Brangwynne, Clifford

2015-03-01

Non-membrane bound organelles are associated with a range of biological functions. Several of these structures exhibit liquid-like properties, and may represent droplets of phase-separated RNA and/or proteins. These structures are often enriched in multi-valent molecules, however little is known about the interactions driving the assembly, properties, and function. Here, we address this question using a model multi-valent protein system consisting of repeats of Small Ubiquitin-like Modifier (SUMO) protein and a SUMO-interacting motif (SIM). These proteins undergo phase separation into liquid-like droplets. We combine microrheology and quantitative microscopy to determine affect of binding affinity on the viscosity, density and surface tension of these droplets. We also use fluorescence recovery after photobleaching (FRAP), fluorescence correlation spectroscopy (FCS) and partitioning experiments to probe the structure and dynamics within these droplets. Our results shed light on how inter-molecular interactions manifests in droplet properties, and lay the groundwork for a comprehensive biophysical picture of intracellular RNA/protein organelles.

Dynamics and Energetics of Deformable Evaporating Droplets at Intermediate Reynolds Numbers.

NASA Astrophysics Data System (ADS)

Haywood, Ross Jeffrey

The behaviour of vaporizing droplets, representative of droplets present in hydrocarbon fuel sprays, has been investigated. A finite volume numerical model using a non-orthogonal, adaptive grid has been developed to examine both steady deformed and transient deforming droplet behaviour. Computations are made of the shapes of, and the velocity, pressure, temperature and concentration fields around and within n-heptane droplets evaporating in high temperature air environments at intermediate Reynolds and Weber numbers (10 <= Re <= 100, We <= 10). The numerical model has been rigorously tested by comparison with existing theoretical and numerical solutions and experimental data for problems of intermediate Reynolds number flows over spheroids, inviscid deforming droplets, viscous oscillating droplets, and transient deforming liquid droplets subjected to electrostatic fields. Computations show steady deformed droplets assuming oblate shapes with major axes perpendicular to the mean flow direction. When based on volume equivalent diameters, existing quasi-steady correlations of Nusselt and Sherwood numbers (Renksizbulut and Yuen (1983), Haywood et al. (1989), and Renksizbulut et al. (1991)) for spherical droplets are in good agreement with the numerical results. Providing they are based on actual frontal area, the computed drag coefficients are also reasonably well predicted by the existing quasi-steady drag correlation (Haywood et al. (1989), Renksizbulut and Yuen (1983)). A new correlation is developed for the total drag coefficient of quasi-steady deformed vaporizing droplets. The computed transient histories of droplets injected with an initial Reynolds number of 100 into 1000 K air at 1 and 10 atmospheres ambient pressure show strongly damped initial oscillations at frequencies within 25 percent of the theoretical natural frequency of Lamb (1932). Gas phase shear induced circulation within the droplets is responsible for the observed strong damping and promotes the formation of prolate shapes. The computed rates of heat and mass transfer of transient deforming drops are well predicted by the quasi-steady correlations indicated above.

Dynamic Chemically Driven Dewetting, Spreading, and Self-Running of Sessile Droplets on Crystalline Silicon.

PubMed

Arscott, Steve

2016-12-06

A chemically driven dewetting effect is demonstrated using sessile droplets of dilute hydrofluoric acid on chemically oxidized silicon wafers. The dewetting occurs as the thin oxide is slowly etched by the droplet and replaced by a hydrogen-terminated surface; the result of this is a gradual increase in the contact angle of the droplet with time. The time-varying work of adhesion is calculated from the time-varying contact angle; this corresponds to the changing chemical nature of the surface during dewetting and can be modeled by the well-known logistic (sigmoid) function often used for the modeling of restricted growth, in this case, the transition from an oxidized surface to a hydrogen-terminated silicon surface. The observation of the time-varying contact angle allows one to both measure the etch rate of the silicon oxide and estimate the hydrogenation rate as a function of HF concentration and wafer type. In addition to this, at a certain HF concentration, a self-running droplet effect is observed. In contrast, on hydrogen-terminated silicon wafers, a chemically induced spreading effect is observed using sessile droplets of nitric acid. The droplet spreading can also be modeled using a logistical function, where the restricted growth is the transition from hydrogen-terminated to a chemically induced oxidized silicon surface. The chemically driven dewetting and spreading observed here add to the methods available to study dynamic wetting (e.g., the moving three-phase contact line) of sessile droplets on surfaces. By slowing down chemical kinetics of the wetting, one is able to record the changing profile of the sessile droplet with time and gather information concerning the time-varying surface chemistry. The data also indicates a chemical interface hysteresis (CIH) that is compared to contact angle hysteresis (CAH). The approach can also be used to study the chemical etching and deposition behavior of thin films using liquids by monitoring the macroscopic droplet profile and relating this to the time-varying physical and chemical interface phenomena.

Methodology for calculating the volume of condensate droplets on topographically modified, microgrooved surfaces.

PubMed

Sommers, A D

2011-05-03

Liquid droplets on micropatterned surfaces consisting of parallel grooves tens of micrometers in width and depth are considered, and a method for calculating the droplet volume on these surfaces is presented. This model, which utilizes the elongated and parallel-sided nature of droplets condensed on these microgrooved surfaces, requires inputs from two droplet images at ϕ = 0° and ϕ = 90°--namely, the droplet major axis, minor axis, height, and two contact angles. In this method, a circular cross-sectional area is extruded the length of the droplet where the chord of the extruded circle is fixed by the width of the droplet. The maximum apparent contact angle is assumed to occur along the side of the droplet because of the surface energy barrier to wetting imposed by the grooves--a behavior that was observed experimentally. When applied to water droplets condensed onto a microgrooved aluminum surface, this method was shown to calculate the actual droplet volume to within 10% for 88% of the droplets analyzed. This method is useful for estimating the volume of retained droplets on topographically modified, anisotropic surfaces where both heat and mass transfer occur and the surface microchannels are aligned parallel to gravity to assist in condensate drainage.

Drag Coefficient of Water Droplets Approaching the Leading Edge of an Airfoil

NASA Technical Reports Server (NTRS)

Vargas, Mario; Sor, Suthyvann; Magarino, Adelaida Garcia

2013-01-01

This work presents results of an experimental study on droplet deformation and breakup near the leading edge of an airfoil. The experiment was conducted in the rotating rig test cell at the Instituto Nacional de Tecnica Aeroespacial (INTA) in Madrid, Spain. An airfoil model was placed at the end of the rotating arm and a monosize droplet generator produced droplets that fell from above, perpendicular to the path of the airfoil. The interaction between the droplets and the airfoil was captured with high speed imaging and allowed observation of droplet deformation and breakup as the droplet approached the airfoil near the stagnation line. Image processing software was used to measure the position of the droplet centroid, equivalent diameter, perimeter, area, and the major and minor axes of an ellipse superimposed over the deforming droplet. The horizontal and vertical displacement of each droplet against time was also measured, and the velocity, acceleration, Weber number, Bond number, Reynolds number, and the drag coefficients were calculated along the path of the droplet to the beginning of breakup. Results are presented and discussed for drag coefficients of droplets with diameters in the range of 300 to 1800 micrometers, and airfoil velocities of 50, 70 and 90 meters/second. The effect of droplet oscillation on the drag coefficient is discussed.

Predictive modeling of low solubility semiconductor alloys

NASA Astrophysics Data System (ADS)

Rodriguez, Garrett V.; Millunchick, Joanna M.

2016-09-01

GaAsBi is of great interest for applications in high efficiency optoelectronic devices due to its highly tunable bandgap. However, the experimental growth of high Bi content films has proven difficult. Here, we model GaAsBi film growth using a kinetic Monte Carlo simulation that explicitly takes cation and anion reactions into account. The unique behavior of Bi droplets is explored, and a sharp decrease in Bi content upon Bi droplet formation is demonstrated. The high mobility of simulated Bi droplets on GaAsBi surfaces is shown to produce phase separated Ga-Bi droplets as well as depressions on the film surface. A phase diagram for a range of growth rates that predicts both Bi content and droplet formation is presented to guide the experimental growth of high Bi content GaAsBi films.

Multiphase chemistry in a microphysical radiation fog model—A numerical study

NASA Astrophysics Data System (ADS)

Bott, Andreas; Carmichael, Gregory R.

A microphysical radiation fog model is coupled with a detailed chemistry module to simulate chemical reactions in the gas phase and in fog water during a radiation fog event. In the chemical part of the model the microphysical particle spectrum is subdivided into three size classes corresponding to non-activated aerosol particles, small and large fog droplets. Chemical reactions in the liquid phase are separately calculated in the small and in the large droplet size class. The impact of the chemical constitution of activated aerosols on fogwater chemistry is considered in the model simulations. The mass transfer of chemical species between the gas phase and the two liquid phases is treated in detail by solving the corresponding coupled differential equation system. The model also accounts for concentration changes of gas-phase and aqueous-phase chemical species which are induced by turbulence, gravitational settling and by evaporation/condensation processes. Numerical results demonstrate that fogwater chemistry is strongly controlled by dynamic processes, i.e. the vertical growth of the fog, turbulent mixing processes and the gravitational settling of the particles. The concentrations of aqueous-phase chemical species are different in the two droplet size classes. Reactands with lower water solubility are mainly found in the large droplet size class because the characteristic time for their mass transfer from the gas phase into the liquid phase is essentially longer than the characteristic time for the formation of large fog droplets. Species with high water solubility are rapidly transferred into the small fog droplets and are then washed out by wet deposition before these particles grow further to form large droplets. Thus, the concentrations of the major ions (NO 3-, NH 4+) are much higher in small than in large droplets, yielding distinctly lower pH values of the small particles. In the present study the reaction of sulfur with H 2O 2 and the Fe(III)-catalysed autoxidation of S(IV) are the major S(VI) producing mechanisms in fog water. Most of the time the sulfur oxidation rates are higher in the large than in the small droplets. Fogwater deposition by gravitational settling occurs mainly in the large droplet size class. However, since in the small droplets the concentrations of chemical species with very good water solubility are relatively high, in both droplet size classes the total wet deposition of these reactands is of the same order of magnitude.

An experimental study on the coalescence process of binary droplets in oil under ultrasonic standing waves.

PubMed

Luo, Xiaoming; Cao, Juhang; He, Limin; Wang, Hongping; Yan, Haipeng; Qin, Yahua

2017-01-01

The coalescence process of binary droplets in oil under ultrasonic standing waves was investigated with high-speed photography. Three motion models of binary droplets in coalescence process were illustrated: (1) slight translational oscillation; (2) sinusoidal translational oscillation; (3) migration along with acoustic streaming. To reveal the droplets coalescence mechanisms, the influence of main factors (such as acoustic intensity, droplet size, viscosity and interfacial tension, etc) on the motion and coalescence of binary droplets was studied under ultrasonic standing waves. Results indicate that the shortest coalescence time is achieved when binary droplets show sinusoidal translational oscillation. The corresponding acoustic intensity in this case is the optimum acoustic intensity. Under the optimum acoustic intensity, drop size decrease will bring about coalescence time decrease by enhancing the binary droplets oscillation. Moreover, there is an optimum interfacial tension to achieve the shortest coalescence time. Copyright © 2016 Elsevier B.V. All rights reserved.

A test of current models for the mechanism of milk-lipid droplet secretion

PubMed Central

Jeong, Jaekwang; Lisinski, Ivonne; Kadegowda, Anil K.G.; Shin, Hyunsu; Wooding, F.B. Peter; Daniels, Brian R.; Schaack, Jerome; Mather, Ian H.

2013-01-01

Milk lipid is secreted by a unique process, during which triacylglycerol droplets bud from mammary cells coated with an outer bilayer of apical membrane. In all current schemes, the integral protein butyrophilin 1A1 (BTN) is postulated to serve as a transmembrane scaffold, which interacts, either with itself, or with the peripheral proteins, xanthine oxidoreductase (XOR) and possibly perilipin-2 (PLIN2), to form an immobile bridging complex between the droplet and apical surface. In one such scheme, BTN on the surface of cytoplasmic lipid droplets interacts directly with BTN in the apical membrane without binding to either XOR or PLIN2. We tested these models using both biochemical and morphological approaches. BTN was concentrated in the apical membrane in all species examined and contained mature N-linked glycans. We found no evidence for the association of unprocessed BTN with intracellular lipid droplets. BTN-enhanced-green-fluorescent-protein was highly mobile in areas of mouse milk-lipid droplets that had not undergone post-secretion changes, and endogenous mouse BTN comprised only 0.5–0.7%, (w/w) of the total protein, i.e., over fifty-fold less than in the milk-lipid droplets of cow and other species. These data are incompatible with models of milk-lipid secretion in which BTN is the major component of an immobile global adhesive complex and suggest that interactions between BTN and other proteins at the time of secretion are more transient than previously predicted. The high mobility of BTN in lipid droplets, mark it as a potential mobile signaling molecule in milk. PMID:23738536

Combustion properties of Kraft Black Liquors

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

Frederick, W.J. Jr.; Hupa, M.

1993-04-01

In a previous study of the phenomena involved in the combustion of black liquor droplets a numerical model was developed. The model required certain black liquor specific combustion information which was then not currently available, and additional data were needed for evaluating the model. The overall objectives of the project reported here was to provide experimental data on key aspects of black liquor combustion, to interpret the data, and to put it into a form which would be useful for computational models for recovery boilers. The specific topics to be investigated were the volatiles and char carbon yields from pyrolysismore » of single black liquor droplets; a criterion for the onset of devolatilization and the accompanying rapid swelling; and the surface temperature of black liquor droplets during pyrolysis, combustion, and gasification. Additional information on the swelling characteristics of black liquor droplets was also obtained as part of the experiments conducted.« less

Cough aerosol in healthy participants: fundamental knowledge to optimize droplet-spread infectious respiratory disease management

PubMed Central

2012-01-01

Background The Influenza A H1N1 virus can be transmitted via direct, indirect, and airborne route to non-infected subjects when an infected patient coughs, which expels a number of different sized droplets to the surrounding environment as an aerosol. The objective of the current study was to characterize the human cough aerosol pattern with the aim of developing a standard human cough bioaerosol model for Influenza Pandemic control. Method 45 healthy non-smokers participated in the open bench study by giving their best effort cough. A laser diffraction system was used to obtain accurate, time-dependent, quantitative measurements of the size and number of droplets expelled by the cough aerosol. Results Voluntary coughs generated droplets ranging from 0.1 - 900 microns in size. Droplets of less than one-micron size represent 97% of the total number of measured droplets contained in the cough aerosol. Age, sex, weight, height and corporal mass have no statistically significant effect on the aerosol composition in terms of size and number of droplets. Conclusions We have developed a standard human cough aerosol model. We have quantitatively characterized the pattern, size, and number of droplets present in the most important mode of person-to-person transmission of IRD: the cough bioaerosol. Small size droplets (< 1 μm) predominated the total number of droplets expelled when coughing. The cough aerosol is the single source of direct, indirect and/or airborne transmission of respiratory infections like the Influenza A H1N1 virus. Study design Open bench, Observational, Cough, Aerosol study PMID:22436202

Cough aerosol in healthy participants: fundamental knowledge to optimize droplet-spread infectious respiratory disease management.

PubMed

Zayas, Gustavo; Chiang, Ming C; Wong, Eric; MacDonald, Fred; Lange, Carlos F; Senthilselvan, Ambikaipakan; King, Malcolm

2012-03-21

The Influenza A H1N1 virus can be transmitted via direct, indirect, and airborne route to non-infected subjects when an infected patient coughs, which expels a number of different sized droplets to the surrounding environment as an aerosol. The objective of the current study was to characterize the human cough aerosol pattern with the aim of developing a standard human cough bioaerosol model for Influenza Pandemic control. 45 healthy non-smokers participated in the open bench study by giving their best effort cough. A laser diffraction system was used to obtain accurate, time-dependent, quantitative measurements of the size and number of droplets expelled by the cough aerosol. Voluntary coughs generated droplets ranging from 0.1 - 900 microns in size. Droplets of less than one-micron size represent 97% of the total number of measured droplets contained in the cough aerosol. Age, sex, weight, height and corporal mass have no statistically significant effect on the aerosol composition in terms of size and number of droplets. We have developed a standard human cough aerosol model. We have quantitatively characterized the pattern, size, and number of droplets present in the most important mode of person-to-person transmission of IRD: the cough bioaerosol. Small size droplets (< 1 μm) predominated the total number of droplets expelled when coughing. The cough aerosol is the single source of direct, indirect and/or airborne transmission of respiratory infections like the Influenza A H1N1 virus. Open bench, Observational, Cough, Aerosol study. © 2012 Zayas et al; licensee BioMed Central Ltd.

Physics of puffing and microexplosion of emulsion fuel droplets

NASA Astrophysics Data System (ADS)

Shinjo, J.; Xia, J.; Ganippa, L. C.; Megaritis, A.

2014-10-01

The physics of water-in-oil emulsion droplet microexplosion/puffing has been investigated using high-fidelity interface-capturing simulation. Varying the dispersed-phase (water) sub-droplet size/location and the initiation location of explosive boiling (bubble formation), the droplet breakup processes have been well revealed. The bubble growth leads to local and partial breakup of the parent oil droplet, i.e., puffing. The water sub-droplet size and location determine the after-puffing dynamics. The boiling surface of the water sub-droplet is unstable and evolves further. Finally, the sub-droplet is wrapped by boiled water vapor and detaches itself from the parent oil droplet. When the water sub-droplet is small, the detachment is quick, and the oil droplet breakup is limited. When it is large and initially located toward the parent droplet center, the droplet breakup is more extensive. For microexplosion triggered by the simultaneous growth of multiple separate bubbles, each explosion is local and independent initially, but their mutual interactions occur at a later stage. The degree of breakup can be larger due to interactions among multiple explosions. These findings suggest that controlling microexplosion/puffing is possible in a fuel spray, if the emulsion-fuel blend and the ambient flow conditions such as heating are properly designed. The current study also gives us an insight into modeling the puffing and microexplosion of emulsion droplets and sprays.

Numerical modelling of microdroplet self-propelled jumping on micro-textured surface

NASA Astrophysics Data System (ADS)

Attarzadeh, S. M. Reza; Dolatabadi, Ali; Chun Kim, Kyung

2015-11-01

Understanding various stages of single and multiple droplet impact on a super-hydrophobic surface is of interest for many industrial applications such as aerospace industry. In this study, the phenomenon of coalescence induced droplets self-propelled jumping on a micro-textured super-hydrophobic surface is numerically simulated using Volume of Fluid (VOF) method. This model mimics the scenario of coalescing cloud-sized particles over the surface structure of an aircraft. The VOF coupled with a dynamic contact angle model is used to simulate the coalescence of two equal size droplets, that are initially placed very closed to each other with their interface overlapping with each other's which triggers the incipience of their coalescence. The textured surface is modeled as a series of equally spaced squared pillars, with 111° as the intrinsic contact angle all over the solid contact area. It is shown that the radial velocity of coalescing liquid bridge is reverted to upward direction due to the counter action of the surface to the basal area of droplet in contact. The presence of air beneath the droplet inside micro grooves which aimed at repelling water droplet is also captured in this model. The simulated results are found in good agreement with experimental observations. The authors gratefully acknowledge the financial support from Natural Sciences and Engineering Research Council of Canada (NSERC), Consortium de Recherche et d'innovation en Aerospatiale au Quebec (CRIAQ), Bombardier Aerospace, Pratt Whitney Canada.

Fundamentals of magnet-actuated droplet manipulation on an open hydrophobic surface.

PubMed

Long, Zhicheng; Shetty, Abhishek M; Solomon, Michael J; Larson, Ronald G

2009-06-07

We systematically investigate droplet movement, coalescence, and splitting on an open hydrophobic surface. These processes are actuated by magnetic beads internalized in an oil-coated aqueous droplet using an external magnet. Results are organized into an 'operating diagram' that describes regions of droplet stable motion, breakage, and release from the magnet. The results are explained theoretically with a simple model that balances magnetic, friction, and capillary-induced drag forces and includes the effects of particle type, droplet size, surrounding oil layer, surface tension, and viscosity. Finally, we discuss the implications of the results for the design of magnet-actuated droplet systems for applications such as nucleic acid purification, immunoassay and drug delivery.

Dynamic Morphologies and Stability of Droplet Interface Bilayers

NASA Astrophysics Data System (ADS)

Guiselin, Benjamin; Law, Jack O.; Chakrabarti, Buddhapriya; Kusumaatmaja, Halim

2018-06-01

We develop a theoretical framework for understanding dynamic morphologies and stability of droplet interface bilayers (DIBs), accounting for lipid kinetics in the monolayers and bilayer, and droplet evaporation due to imbalance between osmotic and Laplace pressures. Our theory quantitatively describes distinct pathways observed in experiments when DIBs become unstable. We find that when the timescale for lipid desorption is slow compared to droplet evaporation, the lipid bilayer will grow and the droplets approach a hemispherical shape. In contrast, when lipid desorption is fast, the bilayer area will shrink and the droplets eventually detach. Our model also suggests there is a critical size below which DIBs can become unstable, which may explain experimental difficulties in miniaturizing the DIB platform.

Size-isolation of ultrasound-mediated phase change perfluorocarbon droplets using differential centrifugation.

PubMed

Mercado, Karla P; Radhakrishnan, Kirthi; Stewart, Kyle; Snider, Lindsay; Ryan, Devin; Haworth, Kevin J

2016-05-01

Perfluorocarbon droplets that are capable of an ultrasound-mediated phase transition have applications in diagnostic and therapeutic ultrasound. Techniques to modify the droplet size distribution are of interest because of the size-dependent acoustic response of the droplets. Differential centrifugation has been used to isolate specific sizes of microbubbles. In this work, differential centrifugation was employed to isolate droplets with diameters between 1 and 3 μm and 2 and 5 μm from an initially polydisperse distribution. Further, an empirical model was developed for predicting the droplet size distribution following differential centrifugation and to facilitate the selection of centrifugation parameters for obtaining desired size distributions.

Size-isolation of ultrasound-mediated phase change perfluorocarbon droplets using differential centrifugation

PubMed Central

Mercado, Karla P.; Radhakrishnan, Kirthi; Stewart, Kyle; Snider, Lindsay; Ryan, Devin; Haworth, Kevin J.

2016-01-01

Perfluorocarbon droplets that are capable of an ultrasound-mediated phase transition have applications in diagnostic and therapeutic ultrasound. Techniques to modify the droplet size distribution are of interest because of the size-dependent acoustic response of the droplets. Differential centrifugation has been used to isolate specific sizes of microbubbles. In this work, differential centrifugation was employed to isolate droplets with diameters between 1 and 3 μm and 2 and 5 μm from an initially polydisperse distribution. Further, an empirical model was developed for predicting the droplet size distribution following differential centrifugation and to facilitate the selection of centrifugation parameters for obtaining desired size distributions. PMID:27250199

Fundamentals of magnet-actuated droplet manipulation on an open hydrophobic surface†

PubMed Central

Long, Zhicheng; Shetty, Abhishek M.; Solomon, Michael J.; Larson, Ronald G.

2010-01-01

We systematically investigate droplet movement, coalescence, and splitting on an open hydrophobic surface. These processes are actuated by magnetic beads internalized in an oil-coated aqueous droplet using an external magnet. Results are organized into an ‘operating diagram’ that describes regions of droplet stable motion, breakage, and release from the magnet. The results are explained theoretically with a simple model that balances magnetic, friction, and capillary-induced drag forces and includes the effects of particle type, droplet size, surrounding oil layer, surface tension, and viscosity. Finally, we discuss the implications of the results for the design of magnet-actuated droplet systems for applications such as nucleic acid purification, immunoassay and drug delivery. PMID:19458864

A lattice Boltzmann simulation of coalescence-induced droplet jumping on superhydrophobic surfaces with randomly distributed structures

NASA Astrophysics Data System (ADS)

Zhang, Li-Zhi; Yuan, Wu-Zhi

2018-04-01

The motion of coalescence-induced condensate droplets on superhydrophobic surface (SHS) has attracted increasing attention in energy-related applications. Previous researches were focused on regularly rough surfaces. Here a new approach, a mesoscale lattice Boltzmann method (LBM), is proposed and used to model the dynamic behavior of coalescence-induced droplet jumping on SHS with randomly distributed rough structures. A Fast Fourier Transformation (FFT) method is used to generate non-Gaussian randomly distributed rough surfaces with the skewness (Sk), kurtosis (K) and root mean square (Rq) obtained from real surfaces. Three typical spreading states of coalesced droplets are observed through LBM modeling on various rough surfaces, which are found to significantly influence the jumping ability of coalesced droplet. The coalesced droplets spreading in Cassie state or in composite state will jump off the rough surfaces, while the ones spreading in Wenzel state would eventually remain on the rough surfaces. It is demonstrated that the rough surfaces with smaller Sks, larger Rqs and a K at 3.0 are beneficial to coalescence-induced droplet jumping. The new approach gives more detailed insights into the design of SHS.

Fast Evaporation of Spreading Droplets of Colloidal Suspensions

NASA Astrophysics Data System (ADS)

Maki, Kara; Kumar, Satish

2011-11-01

When a coffee droplet dries on a countertop, a dark ring of coffee solute is left behind, a phenomenon often referred to as ``the coffee-ring effect.'' A closely related yet less-well-explored phenomenon is the formation of a layer of particles, or skin, at the surface of the droplet. In this work, we explore the behavior of a mathematical model that can qualitatively describe both phenomena. We consider a thin axisymmetric droplet of a colloidal suspension on a horizontal substrate undergoing spreading and rapid evaporation. The lubrication approximation is applied to simplify the mass and momentum conservation equations, and the colloidal particles are allowed to influence droplet rheology through their effect on the viscosity. By describing the transport of the colloidal particles with the full convection-diffusion equation, we are able to capture depthwise gradients in particle concentration and thus describe skin formation, a feature neglected in prior models of droplet evaporation. Whereas capillarity creates a flow that drives particles to the contact line to produce a coffee-ring, Marangoni flows can compete with this and promote skin formation. Increases in viscosity due to particle concentration slow down droplet dynamics, and can lead to a significant reduction in the spreading rate.

Hydrodynamics and PIV study in the impingement zone formed by a droplet train

NASA Astrophysics Data System (ADS)

Kanjirakat, Anoop; Sadr, Reza; Zhang, Taolue; Muthusamy, Jayaveera; Alvarado, Jorge; Texas A; M University at Qatar Collaboration; Texas A; M University College Station Collaboration

2016-11-01

Droplet impingement is encountered in numerous technical applications, such as ink jet printing, spray cooling, and fuel injection in internal combustion engines. Even though many studies in droplet impingement were conducted in past, not many have measured the near-wall velocities in the droplet impingement zone. With the goal of gaining a better understanding of the hydrodynamics in the impingement zone, well-controlled experiments are performed in combination with micro-PIV measurements and numerical simulations. Hydrodynamics of HFE-7100 droplets generated using a piezoelectric droplet generator, impinging on a pre-wetted surface is investigated. Micro-PIV studies in the high-velocity impingement zone are performed using one-micron meter fluorescent particles dispersed in HFE-7100 along with the double exposed images. Three-dimensional and 2D-axisymmetric numerical modeling for a transient droplet crown development is performed. The interface between the gas and the liquid is modeled using a Volume of Fluid (VOF) method. Numerical simulation results obtained are observed to be in good agreement with that of the experimental observations. Supported by National Priority Research Program (NPRP) of Qatar National Research Fund (QNRF), Grant No.: NPRP 6-1304-2-525.

Application of rainbow refractometry for measurement of droplets with solid inclusions

NASA Astrophysics Data System (ADS)

Li, Can; Wu, Xue-cheng; Cao, Jian-zheng; Chen, Ling-hong; Gréhan, Gerard; Cen, Ke-fa

2018-01-01

Characterization of droplets with solid inclusions is of great research interest and has wide industrial applications. Reported here is a theoretical and experimental investigation of the measurement of droplets with solid inclusions using rainbow refractometry. A rainbow extinction model of a droplet with solid inclusions was deduced based on Beer-Lambert's Law. It takes into account the volume concentration, relative size, scattering efficiency of the solid inclusion, and liquid refractive index. An acoustic levitation system for a single droplet and a global rainbow instrumentation system for spray were integrated to study the effect of the H2O-CaCO3 suspension droplets on the rainbow signal and the measured parameters. The results showed that the rainbow encountered unusual disturbances, introduced by the solid inclusions, but its overall structure was not destroyed. Discoveries also included that for volume concentrations of 2.5% or less the CaCO3 particles with diameters below 4 μm had little effect on the measured parameters of the host droplet. The extinction characteristic was also analyzed. The rainbow extinction model failed to quantity the volume concentration of CaCO3, but succeeded in its qualitative analysis.

Spontaneous Droplet Motion on a Periodically Compliant Substrate.

PubMed

Liu, Tianshu; Nadermann, Nichole; He, Zhenping; Strogatz, Steven H; Hui, Chung-Yuen; Jagota, Anand

2017-05-23

Droplet motion arises in many natural phenomena, ranging from the familiar gravity-driven slip and arrest of raindrops on windows to the directed transport of droplets for water harvesting by plants and animals under dry conditions. Deliberate transportation and manipulation of droplets are also important in many technological applications, including droplet-based microfluidic chemical reactors and for thermal management. Droplet motion usually requires gradients of surface energy or temperature or external vibration to overcome contact angle hysteresis. Here, we report a new phenomenon in which a drying droplet placed on a periodically compliant surface undergoes spontaneous, erratic motion in the absence of surface energy gradients and external stimuli such as vibration. By modeling the droplet as a mass-spring system on a substrate with periodically varying compliance, we show that the stability of equilibrium depends on the size of the droplet. Specifically, if the center of mass of the drop lies at a stable equilibrium point of the system, it will stay there until evaporation reduces its size and this fixed point becomes unstable; with any small perturbation, the droplet then moves to one of its neighboring fixed points.

Droplet size effects on film drainage between droplet and substrate.

PubMed

Steinhaus, Benjamin; Spicer, Patrick T; Shen, Amy Q

2006-06-06

When a droplet approaches a solid surface, the thin liquid film between the droplet and the surface drains until an instability forms and then ruptures. In this study, we utilize microfluidics to investigate the effects of film thickness on the time to film rupture for water droplets in a flowing continuous phase of silicone oil deposited on solid poly(dimethylsiloxane) (PDMS) surfaces. The water droplets ranged in size from millimeters to micrometers, resulting in estimated values of the film thickness at rupture ranging from 600 nm down to 6 nm. The Stefan-Reynolds equation is used to model film drainage beneath both millimeter- and micrometer-scale droplets. For millimeter-scale droplets, the experimental and analytical film rupture times agree well, whereas large differences are observed for micrometer-scale droplets. We speculate that the differences in the micrometer-scale data result from the increases in the local thin film viscosity due to confinement-induced molecular structure changes in the silicone oil. A modified Stefan-Reynolds equation is used to account for the increased thin film viscosity of the micrometer-scale droplet drainage case.

Self-Pressurization and Spray Cooling Simulations of the Multipurpose Hydrogen Test Bed (MHTB) Ground-Based Experiment

NASA Technical Reports Server (NTRS)

Kartuzova, O.; Kassemi, M.; Agui, J.; Moder, J.

2014-01-01

This paper presents a CFD (computational fluid dynamics) model for simulating the self-pressurization of a large scale liquid hydrogen storage tank. In this model, the kinetics-based Schrage equation is used to account for the evaporative and condensing interfacial mass flows. Laminar and turbulent approaches to modeling natural convection in the tank and heat and mass transfer at the interface are compared. The flow, temperature, and interfacial mass fluxes predicted by these two approaches during tank self-pressurization are compared against each other. The ullage pressure and vapor temperature evolutions are also compared against experimental data obtained from the MHTB (Multipuprpose Hydrogen Test Bed) self-pressurization experiment. A CFD model for cooling cryogenic storage tanks by spraying cold liquid in the ullage is also presented. The Euler- Lagrange approach is utilized for tracking the spray droplets and for modeling interaction between the droplets and the continuous phase (ullage). The spray model is coupled with the VOF (volume of fluid) model by performing particle tracking in the ullage, removing particles from the ullage when they reach the interface, and then adding their contributions to the liquid. Droplet ullage heat and mass transfer are modeled. The flow, temperature, and interfacial mass flux predicted by the model are presented. The ullage pressure is compared with experimental data obtained from the MHTB spray bar mixing experiment. The results of the models with only droplet/ullage heat transfer and with heat and mass transfer between the droplets and ullage are compared.

Numerical investigation of interfacial transport resistance due to water droplets in proton exchange membrane fuel cell air channels

NASA Astrophysics Data System (ADS)

Koz, Mustafa; Kandlikar, Satish G.

2013-12-01

Oxygen transport resistance at the air flow channel and gas diffusion layer (GDL) interface is needed in modelling the performance of a proton exchange membrane fuel cell (PEMFC). This resistance is expressed through the non-dimensional Sherwood number (Sh). The effect of the presence of a droplet on Sh is studied numerically in an isolated air flow channel using a commercially available package, COMSOL Multiphysics®. A droplet is represented as a solid obstruction placed on the GDL-channel interface and centred along the channel width. The effect of a single droplet is first studied for a range of superficial mean air velocities and droplet sizes. Secondly, the effect of droplet spacing on Sh is studied through simulations of two consecutive droplets. Lastly, multiple droplets in a row are studied as a more representative case of a PEMFC air flow channel. The results show that the droplets significantly increase Sh above the fully developed value in the wake region. This enhancement increases with the number of droplets, droplet size, and superficial mean air velocity. Moreover, the analogy between mass and heat transfer is investigated by comparing Sh to the equivalent Nusselt number.

Optofluidic droplet coalescence on a microfluidic chip

NASA Astrophysics Data System (ADS)

Jung, Jin Ho; Lee, Kyung Heon; Lee, Kang Soo; Cho, Hyunjun; Ha, Byung Hang; Destgeer, Ghulam; Sung, Hyung Jin

2013-11-01

Coalescence is the procedure that two or more droplets fuse during contact to form a larger droplet. Optofluidic droplet coalescence on a microfluidic chip was demonstrated with theoretical and experimental approaches. Droplets were produced in a T-junction geometry and their velocities and sizes were adjusted by flow rate. In order to bring them in a direct contact of coalescence, optical gradient force was used to trap the droplets. A theoretical modeling of the coalescence was derived by combining the optical force and drag force on the droplet. The analytical expression of the optical force on a sphere droplet was employed to estimate the trapping efficiency in the ray optics regime. The drag force acting on the droplet was calculated in terms of the fluid velocity, viscosity and the geometrical parameters of a microfluidic channel. The droplet coalescence was conducted in a microfluidic setup equipped with a 1064 CW laser, focusing optics, a syringe pump, a custom-made stage and a sCMOS camera. The droplets were successfully coalesced using the optical gradient force. The experimental data of coalescence were in good agreement with the prediction. This work was supported by the Creative Research Initiatives program (No.2013-003364) of the National Research Foundation of Korea (MSIP).

Vaporization dynamics of volatile perfluorocarbon droplets: A theoretical model and in vitro validation

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

Doinikov, Alexander A., E-mail: doinikov@bsu.by; Bouakaz, Ayache; Sheeran, Paul S.

2014-10-15

Purpose: Perfluorocarbon (PFC) microdroplets, called phase-change contrast agents (PCCAs), are a promising tool in ultrasound imaging and therapy. Interest in PCCAs is motivated by the fact that they can be triggered to transition from the liquid state to the gas state by an externally applied acoustic pulse. This property opens up new approaches to applications in ultrasound medicine. Insight into the physics of vaporization of PFC droplets is vital for effective use of PCCAs and for anticipating bioeffects. PCCAs composed of volatile PFCs (with low boiling point) exhibit complex dynamic behavior: after vaporization by a short acoustic pulse, a PFCmore » droplet turns into a vapor bubble which undergoes overexpansion and damped radial oscillation until settling to a final diameter. This behavior has not been well described theoretically so far. The purpose of our study is to develop an improved theoretical model that describes the vaporization dynamics of volatile PFC droplets and to validate this model by comparison with in vitro experimental data. Methods: The derivation of the model is based on applying the mathematical methods of fluid dynamics and thermodynamics to the process of the acoustic vaporization of PFC droplets. The used approach corrects shortcomings of the existing models. The validation of the model is carried out by comparing simulated results with in vitro experimental data acquired by ultrahigh speed video microscopy for octafluoropropane (OFP) and decafluorobutane (DFB) microdroplets of different sizes. Results: The developed theory allows one to simulate the growth of a vapor bubble inside a PFC droplet until the liquid PFC is completely converted into vapor, and the subsequent overexpansion and damped oscillations of the vapor bubble, including the influence of an externally applied acoustic pulse. To evaluate quantitatively the difference between simulated and experimental results, the L2-norm errors were calculated for all cases where the simulated and experimental results are compared. These errors were found to be in the ranges of 0.043–0.067 and 0.037–0.088 for OFP and DFB droplets, respectively. These values allow one to consider agreement between the simulated and experimental results as good. This agreement is attained by varying only 2 of 16 model parameters which describe the material properties of gaseous and liquid PFCs and the liquid surrounding the PFC droplet. The fitting parameters are the viscosity and the surface tension of the surrounding liquid. All other model parameters are kept invariable. Conclusions: The good agreement between the theoretical and experimental results suggests that the developed model is able to correctly describe the key physical processes underlying the vaporization dynamics of volatile PFC droplets. The necessity of varying the parameters of the surrounding liquid for fitting the experimental curves can be explained by the fact that the parts of the initial phospholipid shell of PFC droplets remain on the surface of vapor bubbles at the oscillatory stage and their presence affects the bubble dynamics.« less

Computational Modeling Approaches to Multiscale Design of Icephobic Surfaces

NASA Technical Reports Server (NTRS)

Tallman, Aaron; Wang, Yan; Vargas, Mario

2017-01-01

To aid in the design of surfaces that prevent icing, a model and computational simulation of impact ice formation at the single droplet scale was implemented. The nucleation of a single supercooled droplet impacting on a substrate, in rime ice conditions, was simulated. Open source computational fluid dynamics (CFD) software was used for the simulation. To aid in the design of surfaces that prevent icing, a model of impact ice formation at the single droplet scale was proposed•No existing model simulates simultaneous impact and freezing of a single super-cooled water droplet•For the 10-week project, a low-fidelity feasibility study was the goal.

An algorithm for modeling entrainment and naturally and chemically dispersed oil droplet size distribution under surface breaking wave conditions.

PubMed

Li, Zhengkai; Spaulding, Malcolm L; French-McCay, Deborah

2017-06-15

A surface oil entrainment model and droplet size model have been developed to estimate the flux of oil under surface breaking waves. Both equations are expressed in dimensionless Weber number (We) and Ohnesorge number (Oh, which explicitly accounts for the oil viscosity, density, and oil-water interfacial tension). Data from controlled lab studies, large-scale wave tank tests, and field observations have been used to calibrate the constants of the two independent equations. Predictions using the new algorithm compared well with the observed amount of oil removed from the surface and the sizes of the oil droplets entrained in the water column. Simulations with the new algorithm, implemented in a comprehensive spill model, show that entrainment rates increase more rapidly with wind speed than previously predicted based on the existing Delvigne and Sweeney's (1988) model, and a quasi-stable droplet size distribution (d<~50μm) is developed in the near surface water. Copyright © 2017 Elsevier Ltd. All rights reserved.

Dynamics of acoustic droplet vaporization in gas embolotherapy

NASA Astrophysics Data System (ADS)

Qamar, Adnan; Wong, Zheng Z.; Fowlkes, J. Brian; Bull, Joseph L.

2010-04-01

Acoustic droplet vaporization is investigated in a theoretical model. This work is motivated by gas embolotherapy, a developmental cancer treatment involving tumor infarction with gas microbubbles that are selectively formed from liquid droplets. The results indicate that there exists a threshold value for initial droplet size below which the bubble evolution is oscillatory and above which it is smooth and asymptotic, and show that the vaporization process affects the subsequent microbubble expansion. Dampening of the bubble expansion is observed for higher viscosity and surface tension, with effects more pronounced for droplet size less than 6 μm in radius.

Marangoni flow in an evaporating water droplet

NASA Astrophysics Data System (ADS)

Xu, Xuefeng; Luo, Jianbin

2007-09-01

Marangoni effect has been observed in many liquids, but its existence in pure water is still a debated problem. In the present work, the Marangoni flow in evaporating water droplets has been observed by using fluorescent nanoparticles. Flow patterns indicate that a stagnation point where the surface flow, the surface tension gradient, and the surface temperature gradient change their directions exists at the droplet surface. The deduced nonmonotonic variation of the droplet surface temperature, which is different from that in some previous works, is explained by a heat transfer model considering the adsorbed thin film of the evaporating liquid droplet.

Influence of droplet spacing on drag coefficient in nonevaporating, monodisperse streams

NASA Astrophysics Data System (ADS)

Mulholland, J. A.; Srivastava, R. K.; Wendt, J. O. L.

1988-10-01

Trajectory measurements on single, monodisperse, nonevaporating droplet streams whose droplet size, velocity, and spacing were varied to yield initial Re numbers in the 90-290 range are presently used to ascertain the influence of droplet spacing on the drag coefficient of individual drops injected into a quiescent environment. A trajectory model containing the local drag coefficient was fitted to the experimental data by a nonlinear regression; over 40 additional trajectories were predicted with acceptable accuracy. This formulation will aid the computation of waste-droplet drag in flames for improved combustion-generated pollutant predictions.

Formation and Levitation of Unconfined Droplet Clusters

NASA Technical Reports Server (NTRS)

Liu, S.; Ruff, G. A.

1999-01-01

Combustion experiments using arrays of droplets seek to provide a link between single droplet combustion phenomena and the behavior of complex spray combustion systems. Both single droplet and droplet array studies have been conducted in microgravity to better isolate the droplet interaction phenomena and eliminate or reduce the confounding effects of buoyancy-induced convection. In most experiments involving droplet arrays, the droplets are supported on fibers to keep them stationary and close together before the combustion event. The presence of the fiber, however, disturbs the combustion process by introducing a source of heat transfer and asymmetry into the configuration. As the number of drops in a droplet array increases, supporting the drops on fibers becomes less practical because of the cumulative effect of the fibers on the combustion process. The overall objective of this research is to study the combustion of well-characterized drop clusters in a microgravity environment. Direct experimental observations and measurements of the combustion of droplet clusters would fill a large gap in our current understanding of droplet and spray combustion and provide unique experimental data for the verification and improvement of spray combustion models. This paper describes current work on the design and performance of an apparatus to generate and stabilize droplet clusters using acoustic and electrostatic forces.

Measurements and predictions of a liquid spray from an air-assist nozzle

NASA Technical Reports Server (NTRS)

Bulzan, Daniel L.; Levy, Yeshayahou; Aggarwal, Suresh K.; Chitre, Susheel

1991-01-01

Droplet size and gas velocity were measured in a water spray using a two-component Phase/Doppler Particle Analyzer. A complete set of measurements was obtained at axial locations from 5 to 50 cm downstream of the nozzle. The nozzle used was a simple axisymmetric air-assist nozzle. The sprays produced, using the atomizer, were extremely fine. Sauter mean diameters were less than 20 microns at all locations. Measurements were obtained for droplets ranging from 1 to 50 microns. The gas phase was seeded with micron sized droplets, and droplets having diameters of 1.4 microns and less were used to represent gas-phase properties. Measurements were compared with predictions from a multi-phase computer model. Initial conditions for the model were taken from measurements at 5 cm downstream. Predictions for both the gas phase and the droplets showed relatively good agreement with the measurements.

Dynamics of contact line depinning during droplet evaporation based on thermodynamics.

PubMed

Yu, Dong In; Kwak, Ho Jae; Doh, Seung Woo; Ahn, Ho Seon; Park, Hyun Sun; Kiyofumi, Moriyama; Kim, Moo Hwan

2015-02-17

For several decades, evaporation phenomena have been intensively investigated for a broad range of applications. However, the dynamics of contact line depinning during droplet evaporation has only been inductively inferred on the basis of experimental data and remains unclear. This study focuses on the dynamics of contact line depinning during droplet evaporation based on thermodynamics. Considering the decrease in the Gibbs free energy of a system with different evaporation modes, a theoretical model was developed to estimate the receding contact angle during contact line depinning as a function of surface conditions. Comparison of experimentally measured and theoretically modeled receding contact angles indicated that the dynamics of contact line depinning during droplet evaporation was caused by the most favorable thermodynamic process encountered during constant contact radius (CCR mode) and constant contact angle (CCA mode) evaporation to rapidly reach an equilibrium state during droplet evaporation.

Effect of interactions between multiple interfaces on the rheological characteristics of double emulsions

NASA Astrophysics Data System (ADS)

Choi, Se Bin; Park, Jae Yong; Moon, Ji Young; Lee, Joon Sang

2018-06-01

In this study, we analyzed the rheological characteristics of double emulsions by using a three-dimensional lattice Boltzmann model. Numerical simulations indicate that interactions between multiple interfaces play a vital role in determining the shear stress on interfaces and affect deformations, which influence the relative viscosity of double emulsions. The large shear stress induced by droplets in contact increases the relative viscosity for high volume fractions. The double emulsions also show shear-thinning behavior, which corresponds with the Carreau model. The interfacial interference between the core and the deforming shell cause the relative viscosity to increase with increasing core-droplet radius. Finally, we investigated the dependence of the double-emulsion viscosity on the core-droplet viscosity. At high shear rates, the relative viscosity increases with increasing core-droplet viscosity. However, the trend is opposite at low shear rates, which results from the high inward flow (Marangoni flow) at low core-droplet viscosity.

Compositional evolution of particle-phase reaction products and water in the heterogeneous OH oxidation of model aqueous organic aerosols

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

Chim, Man Mei; Cheng, Chiu Tung; Davies, James F.

Organic compounds present at or near the surface of aqueous droplets can be efficiently oxidized by gas-phase OH radicals, which alter the molecular distribution of the reaction products within the droplet. A change in aerosol composition affects the hygroscopicity and leads to a concomitant response in the equilibrium amount of particle-phase water. The variation in the aerosol water content affects the aerosol size and physicochemical properties, which in turn governs the oxidation kinetics and chemistry. To attain better knowledge of the compositional evolution of aqueous organic droplets during oxidation, this work investigates the heterogeneous OH-radical-initiated oxidation of aqueous methylsuccinic acid (C 5Hmore » 8O 4) droplets, a model compound for small branched dicarboxylic acids found in atmospheric aerosols, at a high relative humidity of 85 % through experimental and modeling approaches. Aerosol mass spectra measured by a soft atmospheric pressure ionization source (Direct Analysis in Real Time, DART) coupled with a high-resolution mass spectrometer reveal two major products: a five carbon atom (C 5) hydroxyl functionalization product (C 5H 8O 5) and a C 4 fragmentation product (C 4H 6O 3). These two products likely originate from the formation and subsequent reactions (intermolecular hydrogen abstraction and carbon–carbon bond scission) of tertiary alkoxy radicals resulting from the OH abstraction occurring at the methyl-substituted carbon site. Based on the identification of the reaction products, a kinetic model of oxidation (a two-product model) coupled with the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model is built to simulate the size and compositional changes of aqueous methylsuccinic acid droplets during oxidation. Model results show that at the maximum OH exposure, the droplets become slightly more hygroscopic after oxidation, as the mass fraction of water is predicted to increase from 0.362 to 0.424; however, the diameter of the droplets decreases by 6.1 %. This can be attributed to the formation of volatile fragmentation products that partition to the gas phase, leading to a net loss of organic species and associated particle-phase water, and thus a smaller droplet size. Overall, fragmentation and volatilization processes play a larger role than the functionalization process in determining the evolution of aerosol water content and droplet size at high-oxidation stages.« less

Compositional evolution of particle-phase reaction products and water in the heterogeneous OH oxidation of model aqueous organic aerosols

DOE PAGES

Chim, Man Mei; Cheng, Chiu Tung; Davies, James F.; ...

2017-12-05

Organic compounds present at or near the surface of aqueous droplets can be efficiently oxidized by gas-phase OH radicals, which alter the molecular distribution of the reaction products within the droplet. A change in aerosol composition affects the hygroscopicity and leads to a concomitant response in the equilibrium amount of particle-phase water. The variation in the aerosol water content affects the aerosol size and physicochemical properties, which in turn governs the oxidation kinetics and chemistry. To attain better knowledge of the compositional evolution of aqueous organic droplets during oxidation, this work investigates the heterogeneous OH-radical-initiated oxidation of aqueous methylsuccinic acid (C 5Hmore » 8O 4) droplets, a model compound for small branched dicarboxylic acids found in atmospheric aerosols, at a high relative humidity of 85 % through experimental and modeling approaches. Aerosol mass spectra measured by a soft atmospheric pressure ionization source (Direct Analysis in Real Time, DART) coupled with a high-resolution mass spectrometer reveal two major products: a five carbon atom (C 5) hydroxyl functionalization product (C 5H 8O 5) and a C 4 fragmentation product (C 4H 6O 3). These two products likely originate from the formation and subsequent reactions (intermolecular hydrogen abstraction and carbon–carbon bond scission) of tertiary alkoxy radicals resulting from the OH abstraction occurring at the methyl-substituted carbon site. Based on the identification of the reaction products, a kinetic model of oxidation (a two-product model) coupled with the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model is built to simulate the size and compositional changes of aqueous methylsuccinic acid droplets during oxidation. Model results show that at the maximum OH exposure, the droplets become slightly more hygroscopic after oxidation, as the mass fraction of water is predicted to increase from 0.362 to 0.424; however, the diameter of the droplets decreases by 6.1 %. This can be attributed to the formation of volatile fragmentation products that partition to the gas phase, leading to a net loss of organic species and associated particle-phase water, and thus a smaller droplet size. Overall, fragmentation and volatilization processes play a larger role than the functionalization process in determining the evolution of aerosol water content and droplet size at high-oxidation stages.« less

Combustion of interacting droplet arrays in a microgravity environment

NASA Technical Reports Server (NTRS)

Dietrich, Daniel L.; Haggard, John B.

1993-01-01

This research program involves the study of one and two dimensional arrays of droplets in a buoyant-free environment. The purpose of the work is to extend the database and theories that exist for single droplets into the regime where droplet interactions are important. The eventual goal being to use the results of this work as inputs to models on spray combustion where droplets seldom burn individually; instead the combustion history of a droplet is strongly influenced by the presence of the neighboring droplets. The emphasis of the present investigation is experimental, although comparison will be made to existing theoretical and numerical treatments when appropriate. Both normal gravity and low gravity testing will be employed, and the results compared. The work to date will be summarized in the next section, followed by a section detailing the future plans.

Wettability control of droplet deposition and detachment.

PubMed

Baret, Jean-Christophe; Brinkmann, Martin

2006-04-14

The conditions for droplet deposition on plane substrates are studied using electrowetting to continuously modulate the surface wettability. Droplets of controlled volume attached to the tip of a pipette are brought into contact with the surface. During retraction of the pipette the droplets are deposited or detach completely depending on volume and contact angle. The experimental limit of deposition in the contact angle or volume plane is in good agreement with analytical and numerical predictions obtained within the capillary model.

Optical caustics associated with the primary and the secondary rainbows of oblate droplets

NASA Astrophysics Data System (ADS)

Yu, Haitao; Shen, Jianqi; Tropea, Cameron

2014-08-01

The vector ray tracing (VRT) model is used to simulate the optical caustic structures near the primary and the secondary rainbow angles of oblate water droplets. The evolution process of the optical caustic structures in response to shape deformation of the water droplet is discussed. The dependence of the caustic structures on equatorial radius, refractive index and aspect ratio of the droplet are studied and the curvatures of the two rainbow fringes are calculated.

Characterization of organic residues of size-resolved fog droplets and their atmospheric implications

NASA Astrophysics Data System (ADS)

Chakraborty, Abhishek; Ervens, Barbara; Gupta, Tarun; Tripathi, Sachchida N.

2016-04-01

Size-resolved fog water samples were collected in two consecutive winters at Kanpur, a heavily polluted urban area of India. Samples were analyzed by an aerosol mass spectrometer after drying and directly in other instruments. Residues of fine fog droplets (diameter: 4-16 µm) are found to be more enriched with oxidized (oxygen to carbon ratio, O/C = 0.88) and low volatility organics than residues of coarse (diameter > 22 µm) and medium size (diameter: 16-22 µm) droplets with O/C of 0.68 and 0.74, respectively. These O/C ratios are much higher than those observed for background ambient organic aerosols, indicating efficient oxidation in fog water. Accompanying box model simulations reveal that longer residence times, together with high aqueous OH concentrations in fine droplets, can explain these trends. High aqueous OH concentrations in smaller droplets are caused by their highest surface-volume ratio and high Fe and Cu concentrations, allowing more uptake of gas phase OH and enhanced Fenton reaction rates, respectively. Although some volatile organic species may have escaped during droplet evaporation, these findings indicate that aqueous processing of dissolved organics varies with droplet size. Therefore, large (regional, global)-scale models need to consider the variable reaction rates, together with metal-catalyzed radical formation throughout droplet populations for accurately predicting aqueous secondary organic aerosol formation.

Ground deposition of liquid droplets released from a point source in the atmospheric surface layer

NASA Astrophysics Data System (ADS)

Panneton, Bernard

1989-05-01

A series of field experiments is presented in which the ground deposition of liquid droplets, 120 and 150 micrometers in diameter, released from a point source at 7 meters above the ground level, was measured. A detailed description of the experimental technique is provided, and the results are presented and compared to the predictions of a few models. A new rotating droplet generator is described. Droplets are produced by the forced breakup of capillary liquid jets and droplet coalescence is inhibited by the rotational motion of the spray head. A system for analyzing spray samples was developed. This is a specialized image analysis system based on an electronic digitizing camera which measures the area and perimeter of stains left by dyed droplets collected on Kromekote cards. A complete set of meteorological data supports the ground-deposition data. The turbulent air velocities at two levels above the ground and the temperature of the air at one level were measured with one sonic anemometer and a sonic anemometer-thermometer. The vertical heat and momentum fluxes were estimated using the eddy-correlation technique. The two-dimensional deposition patterns are presented in the form of plots of contours of constant density, normalized arcwise distributions and crosswind integrated distributions. Models of the crosswind integrated deposit from Godson, Csanady, Walker, Bache and Sayer, and Wilson et al., are evaluated. The results indicate that the Wilson et al random walk model is adequate for predicting the ground deposition of the 150 micrometer droplets.

Mechanism of Water Droplet Breakup Near the Leading Edge of an Airfoil

NASA Technical Reports Server (NTRS)

Vargas, Mario; Sor, Suthyvann; Magarino, Adelaida, Garcia

2012-01-01

This work presents results of an experimental study on droplet deformation and breakup near the leading edge of an airfoil. The experiment was conducted in the rotating rig test cell at the Instituto Nacional de Tecnica Aeroespacial (INTA) in Madrid, Spain. The airfoil model was placed at the end of the rotating arm and a monosize droplet generator produced droplets that fell from above, perpendicular to the path of the airfoil. The interaction between the droplets and the airfoil was captured with high speed imaging and allowed observation of droplet deformation and breakup as the droplet approached the airfoil near the stagnation line. Image processing software was used to measure the position of the droplet centroid, equivalent diameter, perimeter, area, and the major and minor axes of an ellipse superimposed over the deforming droplet. The horizontal and vertical displacement of each droplet against time was also measured, and the velocity, acceleration, Weber number, Bond number, Reynolds number, and the drag coefficients were calculated along the path of the droplet to the beginning of breakup. Droplet deformation is defined and studied against main parameters. The high speed imaging allowed observation of the actual mechanism of breakup and identification of the sequence of configurations from the initiation of the breakup to the disintegration of the droplet. Results and comparisons are presented for droplets of diameters in the range of 500 to 1800 microns, and airfoil velocities of 70 and 90 m/sec.

Unidirectional Fast Growth and Forced Jumping of Stretched Droplets on Nanostructured Microporous Surfaces.

PubMed

Aili, Abulimiti; Li, Hongxia; Alhosani, Mohamed H; Zhang, TieJun

2016-08-24

Superhydrophobic nanostructured surfaces have demonstrated outstanding capability in energy and water applications by promoting dropwise condensation, where fast droplet growth and efficient condensate removal are two key parameters. However, these parameters remain contradictory. Although efficient droplet removal is easily obtained through coalescence jumping on uniform superhydrophobic surfaces, simultaneously achieving fast droplet growth is still challenging. Also, on such surfaces droplets can grow to larger sizes without restriction if there is no coalescence. In this work, we show that superhydrophobic nanostructured microporous surfaces can manipulate the droplet growth and jumping. Microporous surface morphology effectively enhances the growth of droplets in pores owing to large solid-liquid contact area. At low supersaturations, the upward growth rate (1-1.5 μm/s) of these droplets in pores is observed to be around 15-25 times that of the droplets outside the pores. Meanwhile, their top curvature radius increases relatively slowly (∼0.25 μm/s) due to pore confinement, which results in a highly stretched droplet surface. We also observed forced jumping of stretched droplets in pores either through coalescence with spherical droplets outside pores or through self-pulling without coalescence. Both experimental observation and theoretical modeling reveal that excess surface free energy stored in the stretched droplet surface and micropore confinement are responsible for this pore-scale-forced jumping. These findings reveal the insightful physics of stretched droplet dynamics and offer guidelines for the design and fabrication of novel super-repellent surfaces with microporous morphology.

Mechanism of Water Droplet Breakup near the Leading Edge of an Airfoil

NASA Technical Reports Server (NTRS)

Vargas, Mario; Sor, Suthyvann; Magarino, Adelaida Garcia

2012-01-01

This work presents results of an experimental study on droplet deformation and breakup near the leading edge of an airfoil. The experiment was conducted in the rotating rig test cell at the Instituto Nacional de T cnica Aeroespacial (INTA) in Madrid, Spain. The airfoil model was placed at the end of the rotating arm and a monosize droplet generator produced droplets that fell from above, perpendicular to the path of the airfoil. The interaction between the droplets and the airfoil was captured with high speed imaging and allowed observation of droplet deformation and breakup as the droplet approached the airfoil near the stagnation line. Image processing software was used to measure the position of the droplet centroid, equivalent diameter, perimeter, area, and the major and minor axes of an ellipse superimposed over the deforming droplet. The horizontal and vertical displacement of each droplet against time was also measured, and the velocity, acceleration, Weber number, Bond number, Reynolds number, and the drag coefficients were calculated along the path of the droplet to the beginning of breakup. Droplet deformation is defined and studied against main parameters. The high speed imaging allowed observation of the actual mechanism of breakup and identification of the sequence of configurations from the initiation of the breakup to the disintegration of the droplet. Results and comparisons are presented for droplets of diameters in the range of 500 to 1800 micrometers, and airfoil velocities of 70 and 90 meters/second.

Comparison of numerical simulations to experiments for atomization in a jet nebulizer.

PubMed

Lelong, Nicolas; Vecellio, Laurent; Sommer de Gélicourt, Yann; Tanguy, Christian; Diot, Patrice; Junqua-Moullet, Alexandra

2013-01-01

The development of jet nebulizers for medical purposes is an important challenge of aerosol therapy. The performance of a nebulizer is characterized by its output rate of droplets with a diameter under 5 µm. However the optimization of this parameter through experiments has reached a plateau. The purpose of this study is to design a numerical model simulating the nebulization process and to compare it with experimental data. Such a model could provide a better understanding of the atomization process and the parameters influencing the nebulizer output. A model based on the Updraft nebulizer (Hudson) was designed with ANSYS Workbench. Boundary conditions were set with experimental data then transient 3D calculations were run on a 4 µm mesh with ANSYS Fluent. Two air flow rate (2 L/min and 8 L/min, limits of the operating range) were considered to account for different turbulence regimes. Numerical and experimental results were compared according to phenomenology and droplet size. The behavior of the liquid was compared to images acquired through shadowgraphy with a CCD Camera. Three experimental methods, laser diffractometry, phase Doppler anemometry (PDA) and shadowgraphy were used to characterize the droplet size distributions. Camera images showed similar patterns as numerical results. Droplet sizes obtained numerically are overestimated in relation to PDA and diffractometry, which only consider spherical droplets. However, at both flow rates, size distributions extracted from numerical image processing were similar to distributions obtained from shadowgraphy image processing. The simulation then provides a good understanding and prediction of the phenomena involved in the fragmentation of droplets over 10 µm. The laws of dynamics apply to droplets down to 1 µm, so we can assume the continuity of the distribution and extrapolate the results for droplets between 1 and 10 µm. So, this model could help predicting nebulizer output with defined geometrical and physical parameters.

Comparison of Numerical Simulations to Experiments for Atomization in a Jet Nebulizer

PubMed Central

Lelong, Nicolas; Vecellio, Laurent; Sommer de Gélicourt, Yann; Tanguy, Christian; Diot, Patrice; Junqua-Moullet, Alexandra

2013-01-01

The development of jet nebulizers for medical purposes is an important challenge of aerosol therapy. The performance of a nebulizer is characterized by its output rate of droplets with a diameter under 5 µm. However the optimization of this parameter through experiments has reached a plateau. The purpose of this study is to design a numerical model simulating the nebulization process and to compare it with experimental data. Such a model could provide a better understanding of the atomization process and the parameters influencing the nebulizer output. A model based on the Updraft nebulizer (Hudson) was designed with ANSYS Workbench. Boundary conditions were set with experimental data then transient 3D calculations were run on a 4 µm mesh with ANSYS Fluent. Two air flow rate (2 L/min and 8 L/min, limits of the operating range) were considered to account for different turbulence regimes. Numerical and experimental results were compared according to phenomenology and droplet size. The behavior of the liquid was compared to images acquired through shadowgraphy with a CCD Camera. Three experimental methods, laser diffractometry, phase Doppler anemometry (PDA) and shadowgraphy were used to characterize the droplet size distributions. Camera images showed similar patterns as numerical results. Droplet sizes obtained numerically are overestimated in relation to PDA and diffractometry, which only consider spherical droplets. However, at both flow rates, size distributions extracted from numerical image processing were similar to distributions obtained from shadowgraphy image processing. The simulation then provides a good understanding and prediction of the phenomena involved in the fragmentation of droplets over 10 µm. The laws of dynamics apply to droplets down to 1 µm, so we can assume the continuity of the distribution and extrapolate the results for droplets between 1 and 10 µm. So, this model could help predicting nebulizer output with defined geometrical and physical parameters. PMID:24244334

Recent Advances In Science Support For Isolated Droplet Combustion Experiments

NASA Technical Reports Server (NTRS)

Dryer, F. L.; Kazakov, A.; Urban, B. D.; Kroenlein, K.

2003-01-01

In a joint program involving Prof. F.A. Williams of the University of California, San Diego and Dr. V. Nayagam of the National Center for Microgravity Research, the combustion characteristics of isolated liquid fuel droplets of n-heptane, n-decane, methanol, methanol-water, ethanol and ethanol-water having initial diameters between about 1 mm and 6 mm continues to be investigated. The objectives of the work are to improve fundamental knowledge of droplet combustion dynamics for pure fuels and fuel-water mixtures through microgravity experiments and theoretical analyses. The Princeton contributions support the engineering design, data analysis, and data interpretation requirements for the study of initially single component, spherically symmetric, isolated droplet combustion studies through experiments and numerical modeling. UCSD contributions are described in a companion communication in this conference. The Princeton effort also addresses the analyses of Fiber Supported Droplet Combustion (FSDC) experiments conducted with the above fuels and collaborative work with others who are investigating droplet combustion in the presence of steady convection. A thorough interpretation of droplet burning behavior for n-heptane and n-decane over a relatively wide range of conditions also involves the influences of sooting on the combustion behavior, and this particular aspect on isolated burning of droplets is under consideration in a collaborative program underway with Drexel University. This collaboration is addressed in another communication at this conference. The one-dimensional, time-dependent, numerical modeling approach that we have continued to evolve for analyzing isolated, quiescent droplet combustion data has been further applied to investigate several facets of isolated droplet burning of simple alcohols, n-heptane, and n-decane. Some of the new results are described below.

Modeling the Effects of Nanopatterned Surfaces on Wetting States of Droplets

NASA Astrophysics Data System (ADS)

Xiao, Ke; Zhao, Yanping; Ouyang, Gang; Li, Xinlei

2017-04-01

An analytic thermodynamic model has been established to quantitatively investigate the wetting states of droplets on nanopatterned surfaces. Based on the calculations for the free energies of droplets with the Wenzel state and the Cassie-Baxter state, it is found that the size and shape of nanostructured surfaces play crucial roles in wetting states. In detail, for nanohole-patterned surfaces, the deep and thin nanoholes lead to the Cassie-Baxter state, and contrarily, the shallow and thick nanoholes result in the Wenzel state. However, the droplets have the Wenzel state on the patterned surfaces with small height and radii nanopillars and have the Cassie-Baxter state when the height and radii of nanopillars are large. Furthermore, the intuitive phase diagrams of the wetting states of the droplet in the space of surface geometrical parameters are obtained. The theoretical results are in good agreement with the experimental observations and reveal physical mechanisms involved in the effects of nanopatterned surfaces on wetting states, which implies that these studies may provide useful guidance to the conscious design of patterned surfaces to control the wetting states of droplets.

Volatility of organic aerosol: evaporation of ammonium sulfate/succinic acid aqueous solution droplets.

PubMed

Yli-Juuti, Taina; Zardini, Alessandro A; Eriksson, Axel C; Hansen, Anne Maria K; Pagels, Joakim H; Swietlicki, Erik; Svenningsson, Birgitta; Glasius, Marianne; Worsnop, Douglas R; Riipinen, Ilona; Bilde, Merete

2013-01-01

Condensation and evaporation modify the properties and effects of atmospheric aerosol particles. We studied the evaporation of aqueous succinic acid and succinic acid/ammonium sulfate droplets to obtain insights on the effect of ammonium sulfate on the gas/particle partitioning of atmospheric organic acids. Droplet evaporation in a laminar flow tube was measured in a Tandem Differential Mobility Analyzer setup. A wide range of droplet compositions was investigated, and for some of the experiments the composition was tracked using an Aerosol Mass Spectrometer. The measured evaporation was compared to model predictions where the ammonium sulfate was assumed not to directly affect succinic acid evaporation. The model captured the evaporation rates for droplets with large organic content but overestimated the droplet size change when the molar concentration of succinic acid was similar to or lower than that of ammonium sulfate, suggesting that ammonium sulfate enhances the partitioning of dicarboxylic acids to aqueous particles more than currently expected from simple mixture thermodynamics. If extrapolated to the real atmosphere, these results imply enhanced partitioning of secondary organic compounds to particulate phase in environments dominated by inorganic aerosol.

Volatility of Organic Aerosol: Evaporation of Ammonium Sulfate/Succinic Acid Aqueous Solution Droplets

PubMed Central

2013-01-01

Condensation and evaporation modify the properties and effects of atmospheric aerosol particles. We studied the evaporation of aqueous succinic acid and succinic acid/ammonium sulfate droplets to obtain insights on the effect of ammonium sulfate on the gas/particle partitioning of atmospheric organic acids. Droplet evaporation in a laminar flow tube was measured in a Tandem Differential Mobility Analyzer setup. A wide range of droplet compositions was investigated, and for some of the experiments the composition was tracked using an Aerosol Mass Spectrometer. The measured evaporation was compared to model predictions where the ammonium sulfate was assumed not to directly affect succinic acid evaporation. The model captured the evaporation rates for droplets with large organic content but overestimated the droplet size change when the molar concentration of succinic acid was similar to or lower than that of ammonium sulfate, suggesting that ammonium sulfate enhances the partitioning of dicarboxylic acids to aqueous particles more than currently expected from simple mixture thermodynamics. If extrapolated to the real atmosphere, these results imply enhanced partitioning of secondary organic compounds to particulate phase in environments dominated by inorganic aerosol. PMID:24107221

Deformation of an Elastic Substrate Due to a Resting Sessile Droplet

NASA Astrophysics Data System (ADS)

Bardall, Aaron; Daniels, Karen; Shearer, Michael

2017-11-01

On a sufficiently soft substrate, a resting fluid droplet will cause significant deformation of the substrate. This deformation is driven by a combination of capillary forces at the contact line and the fluid pressure at the solid surface. These forces are balanced at the surface by the solid traction stress induced by the substrate deformation. Young's Law, which predicts the equilibrium contact angle of the droplet, also indicates an a priori radial force balance for rigid substrates, but not necessarily for soft substrates which deform under loading. It remains an open question whether the contact line transmits a non-zero force tangent to the substrate surface in addition to the conventional normal force. This talk will present a model for the static deformation of the substrate that includes a non-zero tangential contact line force as well as general interfacial energy conditions governing the angle of a two-dimensional droplet. We discuss extensions of this model to non-symmetric droplets and their effect on the static configuration of the droplet/substrate system. NSF #DMS-1517291.

Self-Assembly of Emulsion Droplets into Polymer Chains

NASA Astrophysics Data System (ADS)

Bargteil, Dylan; McMullen, Angus; Brujic, Jasna

We experimentally investigate `beads-on-a-string' models of polymers using the spontaneous assembly of emulsion droplets into linear chains. Droplets functionalized with surface-mobile DNA allow for programmable 'monomers' through which we can influence the three-dimensional structure of the assembled 'polymer'. Such model polymers can be used to study conformational changes of polypeptides and the principles governing protein folding. In our system, we find that droplets bind via complementary DNA strands that are recruited into adhesion patches. Recruitment is driven by the DNA hybridization energy, and is limited by the energy cost of surface deformation and the entropy loss of the mobile linkers, yielding adhesion patches of a characteristic size with a given number of linkers. By tuning the initial surface coverage of linkers, we control valency between the droplets to create linear or branched polymer chains. We additionally control the flexibility of the model polymers by varying the salt concentration and study their dynamics between extended and collapsed states. This system opens the possibility of programming stable three-dimensional structures, such as those found within folded proteins.

Smoothed particle hydrodynamics study of the roughness effect on contact angle and droplet flow

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

Shigorina, Elena; Kordilla, Jannes; Tartakovsky, Alexandre M.

We employ a pairwise force Smoothed Particle Hydrodynamics (PF-SPH) model to simulate sessile and transient droplets on rough hydrophobic and hydrophilic surfaces. PF-SPH allows for modeling of free surface flow without discretizing the air phase, which is achieved by imposing the surface tension and dynamic contact angles with pairwise interaction forces. We use the PF-SPH model to study the effect of surface roughness and microscopic contact angle on the effective contact angle and droplet dynamics. In the first part of this work, we investigate static contact angles of sessile droplets on rough surfaces in a shape of a sinusoidal functionmore » and made of rectangular bars placed on top of a flat surface. We find that the effective static contact angles of Cassie and Wenzel droplets on a rough surface are greater than the corresponding microscale static contact angles. As a result, microscale hydrophobic rough surfaces also show effective hydrophobic behavior. On the other hand, microscale hydrophilic surfaces may be macroscopically hydrophilic or hydrophobic, depending on the type of roughness. Next, we study the impact of the roughness orientation (i.e., an anisotropic roughness) and surface inclination on droplet flow velocities. Simulations show that droplet flow velocities are lower if the surface roughness is oriented perpendicular to the flow direction. If the predominant elements of surface roughness are in alignment with the flow direction, the flow velocities increase compared to smooth surfaces, which can be attributed to the decrease in fluid-solid contact area similar to the classical lotus effect. We demonstrate that linear scaling relationships between Bond and capillary number for droplet flow on flat surfaces also hold for flow on rough surfaces.« less

Reduced Gravity Studies of Soret Transport Effects in Liquid Fuel Combustion

NASA Technical Reports Server (NTRS)

Shaw, Benjamin D.

2004-01-01

Soret transport, which is mass transport driven by thermal gradients, can be important in practical flames as well as laboratory flames by influencing transport of low molecular weight species (e.g., monatomic and diatomic hydrogen). In addition, gas-phase Soret transport of high molecular weight fuel species that are present in practical liquid fuels (e.g., octane or methanol) can be significant in practical flames (Rosner et al., 2000; Dakhlia et al., 2002) and in high pressure droplet evaporation (Curtis and Farrell, 1992), and it has also been shown that Soret transport effects can be important in determining oxygen diffusion rates in certain classes of microgravity droplet combustion experiments (Aharon and Shaw, 1998). It is thus useful to obtain information on flames under conditions where Soret effects can be clearly observed. This research is concerned with investigating effects of Soret transport on combustion of liquid fuels, in particular liquid fuel droplets. Reduced-gravity is employed to provide an ideal (spherically-symmetrical) experimental model with which to investigate effects of Soret transport on combustion. The research will involve performing reduced-gravity experiments on combustion of liquid fuel droplets in environments where Soret effects significantly influence transport of fuel and oxygen to flame zones. Experiments will also be performed where Soret effects are not expected to be important. Droplets initially in the 0.5 to 1 mm size range will be burned. Data will be obtained on influences of Soret transport on combustion characteristics (e.g., droplet burning rates, droplet lifetimes, gas-phase extinction, and transient flame behaviors) under simplified geometrical conditions that are most amenable to theoretical modeling (i.e., spherical symmetry). The experiments will be compared with existing theoretical models as well as new models that will be developed. Normal gravity experiments will also be performed.

Analysis of the vapor-liquid-solid mechanism for nanowire growth and a model for this mechanism.

PubMed

Mohammad, S Noor

2008-05-01

The vapor-liquid-solid (VLS) mechanism is most widely employed to grow nanowires (NWs). The mechanism uses foreign element catalytic agent (FECA) to mediate the growth. Because of this, it is believed to be very stable with the FECA-mediated droplets not consumed even when reaction conditions change. Recent experiments however differ, which suggest that even under cleanest growth conditions, VLS mechanism may not produce long, thin, uniform, single-crystal nanowires of high purity. The present investigation has addressed various issues involving fundamentals of VLS growth. While addressing these issues, it has taken into consideration the influence of the electrical, hydrodynamic, thermodynamic, and surface tension effects on NW growth. It has found that parameters such as mesoscopic effects on nanoparticle seeds, charge distribution in FECA-induced droplets, electronegativity of the droplet with respect to those of reactive nanowire vapor species, growth temperature, and chamber pressure play important role in the VLS growth. On the basis of an in-depth analysis of various issues, a simple, novel, malleable (SNM) model has been presented for the VLS mechanism. The model appears to explain the formation and observed characteristics of a wide variety of nanowires, including elemental and compound semiconductor nanowires. Also it provides an understanding of the influence of the dynamic behavior of the droplets on the NW growth. This study finds that increase in diameter with time of the droplet of tapered nanowires results primarily from gradual incorporation of oversupplied nanowire species into the FECA-mediated droplet, which is supported by experiments. It finds also that optimum compositions of the droplet constituents are crucial for VLS nanowire growth. An approximate model presented to exemplify the parametric dependency of VLS growth provides good description of NW growth rate as a function of temperature.

Boundary conditions for a one-sided numerical model of evaporative instabilities in sessile drops of ethanol on heated substrates

NASA Astrophysics Data System (ADS)

Semenov, Sergey; Carle, Florian; Medale, Marc; Brutin, David

2017-12-01

The work is focused on obtaining boundary conditions for a one-sided numerical model of thermoconvective instabilities in evaporating pinned sessile droplets of ethanol on heated substrates. In the one-sided model, appropriate boundary conditions for heat and mass transfer equations are required at the droplet surface. Such boundary conditions are obtained in the present work based on a derived semiempirical theoretical formula for the total droplet's evaporation rate, and on a two-parametric nonisothermal approximation of the local evaporation flux. The main purpose of these boundary conditions is to be applied in future three-dimensional (3D) one-sided numerical models in order to save a lot of computational time and resources by solving equations only in the droplet domain. Two parameters, needed for the nonisothermal approximation of the local evaporation flux, are obtained by fitting computational results of a 2D two-sided numerical model. Such model is validated here against parabolic flight experiments and the theoretical value of the total evaporation rate. This study combines theoretical, experimental, and computational approaches in convective evaporation of sessile droplets. The influence of the gravity level on evaporation rate and contributions of different mechanisms of vapor transport (diffusion, Stefan flow, natural convection) are shown. The qualitative difference (in terms of developing thermoconvective instabilities) between steady-state and unsteady numerical approaches is demonstrated.

Phase behavior and kinetics of phase separation of a nonionic microemulsion of C12E5/water/1-chlorotetradecane upon a temperature quench.

PubMed

Roshan Deen, G; Oliveira, Cristiano L P; Pedersen, Jan Skov

2009-05-21

The phase behavior and phase separation kinetics of a model ternary nonionic microemulsion system composed of pentaethylene glycol dodecyl ether (C12E5), water, and 1-chlorotetradecane were studied. With increasing temperature, the microemulsion exhibits the following rich phase behavior: oil-in-water phase (L1+O), droplet microemulsion phase (L1), lamellar liquid crystalline phase (Lproportional), and sponge-like (liquid) phase (L3). The microemulsion with a fixed surfactant-to-oil volume fraction ratio (Phis/Phio) of 0.81 and droplet volume fraction of 0.087 was perturbed from equilibrium by a temperature quench from the L1 region (24 degrees C) to an unstable region L1+O (13 degrees C), where the excess oil phase is in equilibrium with the microemulsion droplets. The process of phase separation in the unstable region was followed by time-resolved small-angle X-ray scattering (TR-SAXS) and time-resolved turbidity methods. Due to the large range of scattering vector (q=0.004-0.22 A(-1)) that is possible to access with the TR-SAXS method, the growth of the oil droplets and shrinking of the microemulsion droplets as a result of phase separation could be studied simultaneously. By using an advanced polydisperse ellipsoidal hard-sphere model, the experimental curves have been quantitatively analyzed. The microemulsion droplets were modeled as polydisperse core-shell ellipsoidal particles, using molecular constraints, and the oil droplets are modeled as polydisperse spheres. The radius of gyration (Rg) of the growing oil droplets, volume fraction of oil in the microemulsion droplets, and polydispersity were obtained from the fit parameters. The volume equivalent radius at the neutral plane between the surfactant head and tail of the microemulsion droplet decreased from 76 to 51 A, while the radius of oil drop increased to 217 A within the 160 min of the experiment. After about 48 min from the temperature quench, the system reaches a steady state and continues to coarsen at a constant fraction of the oil of 0.51 in the oil phase by Ostwald ripening with the power law dependence of Roil proportional, variant t1/3. The size of the oil droplets determined by the time-resolved turbidity method is in good agreement with that of the TR-SAXS, highlighting the usefulness of the method in the size determination of oil-in-water microemulsions on an absolute scale.

Smoothed particle hydrodynamics study of the roughness effect on contact angle and droplet flow.

PubMed

Shigorina, Elena; Kordilla, Jannes; Tartakovsky, Alexandre M

2017-09-01

We employ a pairwise force smoothed particle hydrodynamics (PF-SPH) model to simulate sessile and transient droplets on rough hydrophobic and hydrophilic surfaces. PF-SPH allows modeling of free-surface flows without discretizing the air phase, which is achieved by imposing the surface tension and dynamic contact angles with pairwise interaction forces. We use the PF-SPH model to study the effect of surface roughness and microscopic contact angle on the effective contact angle and droplet dynamics. In the first part of this work, we investigate static contact angles of sessile droplets on different types of rough surfaces. We find that the effective static contact angles of Cassie and Wenzel droplets on a rough surface are greater than the corresponding microscale static contact angles. As a result, microscale hydrophobic rough surfaces also show effective hydrophobic behavior. On the other hand, microscale hydrophilic surfaces may be macroscopically hydrophilic or hydrophobic, depending on the type of roughness. We study the dependence of the transition between Cassie and Wenzel states on roughness and droplet size, which can be linked to the critical pressure for the given fluid-substrate combination. We observe good agreement between simulations and theoretical predictions. Finally, we study the impact of the roughness orientation (i.e., an anisotropic roughness) and surface inclination on droplet flow velocities. Simulations show that droplet flow velocities are lower if the surface roughness is oriented perpendicular to the flow direction. If the predominant elements of surface roughness are in alignment with the flow direction, the flow velocities increase compared to smooth surfaces, which can be attributed to the decrease in fluid-solid contact area similar to the lotus effect. We demonstrate that classical linear scaling relationships between Bond and capillary numbers for droplet flow on flat surfaces also hold for flow on rough surfaces.

Transport and Retention of Emulsion Droplets in Sandy Porous Media

NASA Astrophysics Data System (ADS)

Esahani, S. G.; Muller, K.; Chapra, S. C.; Ramsburg, A.

2014-12-01

Emulsions are commonly used as amendments during remediation; yet, the processes controlling the distribution of droplets within the subsurface are not well understood. Given that inadequate spatial and/or temporal delivery of amendments often leads to ineffective treatment, there is a need to better understand emulsion transport. Experiments were conducted to evaluate the transport and retention of emulsion droplets in columns containing Ottawa sands. Breakthrough curves and deposition profiles from these experiments were interrogated using a mathematical model capable of describing attachment, detachment, and straining to begin to elucidate the physical processes controlling delivery. Emulsions were constructed by stabilizing soybean oil droplets within a continuous aqueous phase. Physical properties of the resulting oil-in-water emulsions were favorable for subsurface delivery (nominal properties: 1 g/mL density; 10 cP viscosity; and 1.5 μm droplet d50). Emulsions were introduced to the columns for approximately two pore volumes and followed by an extended flush of background solution. Effluent droplet size distributions did not vary significantly over the course of the experiment and remained similar to those measured for the influent emulsion. Emulsion breakthrough curves exhibited tailing, and deposition profiles were found to be hyper-exponential and unaffected by extended periods of background flow. Depending on emulsion composition and flow characteristics, 10-30% of the injected emulsion was retained on the sand suggesting a non-negligible influence on accessible porosity over the course of the experiment. Experimental results were further interpreted using a droplet transport model that accounts for temporal and spatial variation in porosity due to the retention of the emulsion droplets. At present the model assumes a uniform size distribution of inelastic emulsion droplets which are transported by advection and dispersion, and exchanged with the solid phase through attachment, detachment, and straining processes. Results examine the relative roles of attachment-detachment and straining in reducing the accessible porosity. Evaluation of how the porosity change influences the flow regime for moderately and slightly clogged media is currently under investigation.

Controlled Gelation of Particle Suspensions Using Controlled Solvent Removal in Picoliter Droplets

NASA Astrophysics Data System (ADS)

Vuong, Sharon; Walker, Lynn; Anna, Shelley

2013-11-01

Droplets in microfluidic devices have proven useful as uniform picoliter reactors for nanoparticle synthesis and as components in tunable emulsions. However, there can be significant transport between the component phases depending on solubility and other factors. In the present talk, we show that water droplets trapped within a microfluidic device for tens of hours slowly dehydrate, concentrating the contents encapsulated within. We use this slow dehydration along with control of the initial droplet composition to monitor gelation of aqueous suspensions of spherical silica particles (Ludox) and disk-shaped clay particles (Laponite). Droplets are generated in a microfluidic device containing small wells that trap the droplets. We monitor the concentration process through size and shape changes of these droplets as a function of time in tens of droplets and use the large number of individual reactors to generate statistics regarding the gelation process. We also examine changes in suspension viscosity through fluorescent particle tracking as a function of dehydration rate, initial suspension concentration and initial droplet volume, and added salt, and compare the results with the Krieger-Dougherty model in which viscosity increases dramatically with particle volume fraction.

Optical droplet vaporization of micron-sized perfluorocarbon droplets and their photoacoustic detection

NASA Astrophysics Data System (ADS)

Strohm, Eric; Rui, Min; Gorelikov, Ivan; Matsuura, Naomi; Kolios, Michael

2011-03-01

An acoustic and photoacoustic characterization of micron-sized perfluorocarbon (PFC) droplets is presented. PFC droplets are currently being investigated as acoustic and photoacoustic contrast agents and as cancer therapy agents. Pulse echo measurements at 375 MHz were used to determine the diameter, ranging from 3.2 to 6.5 μm, and the sound velocity, ranging from 311 to 406 m/s of nine droplets. An average sound velocity of 379 +/- 18 m/s was calculated for droplets larger than the ultrasound beam width of 4.0 μm. Optical droplet vaporization, where vaporization of a single droplet occurred upon laser irradiation of sufficient intensity, was verified using pulse echo acoustic methods. The ultrasonic backscatter amplitude, acoustic impedance and attenuation increased after vaporization, consistent with a phase change from a liquid to gas core. Photoacoustic measurements were used to compare the spectra of three droplets ranging in diameter from 3.0 to 6.2 μm to a theoretical model. Good agreement in the spectral features was observed over the bandwidth of the 375 MHz transducer.

Design and characterization of a cough simulator.

PubMed

Zhang, Bo; Zhu, Chao; Ji, Zhiming; Lin, Chao-Hsin

2017-02-23

Expiratory droplets from human coughing have always been considered as potential carriers of pathogens, responsible for respiratory infectious disease transmission. To study the transmission of disease by human coughing, a transient repeatable cough simulator has been designed and built. Cough droplets are generated by different mechanisms, such as the breaking of mucus, condensation and high-speed atomization from different depths of the respiratory tract. These mechanisms in coughing produce droplets of different sizes, represented by a bimodal distribution of 'fine' and 'coarse' droplets. A cough simulator is hence designed to generate transient sprays with such bimodal characteristics. It consists of a pressurized gas tank, a nebulizer and an ejector, connected in series, which are controlled by computerized solenoid valves. The bimodal droplet size distribution is characterized for the coarse droplets and fine droplets, by fibrous collection and laser diffraction, respectively. The measured size distributions of coarse and fine droplets are reasonably represented by the Rosin-Rammler and log-normal distributions in probability density function, which leads to a bimodal distribution. To assess the hydrodynamic consequences of coughing including droplet vaporization and polydispersion, a Lagrangian model of droplet trajectories is established, with its ambient flow field predetermined from a computational fluid dynamics simulation.

Modeling Evaporation and Particle Assembly in Colloidal Droplets.

PubMed

Zhao, Mingfei; Yong, Xin

2017-06-13

Evaporation-induced assembly of nanoparticles in a drying droplet is of great importance in many engineering applications, including printing, coating, and thin film processing. The investigation of particle dynamics in evaporating droplets can provide fundamental hydrodynamic insight for revealing the processing-structure relationship in the particle self-organization induced by solvent evaporation. We develop a free-energy-based multiphase lattice Boltzmann method coupled with Brownian dynamics to simulate evaporating colloidal droplets on solid substrates with specified wetting properties. The influence of interface-bound nanoparticles on the surface tension and evaporation of a flat liquid-vapor interface is first quantified. The results indicate that the particles at the interface reduce surface tension and enhance evaporation flux. For evaporating particle-covered droplets on substrates with different wetting properties, we characterize the increase of evaporate rate via measuring droplet volume. We find that droplet evaporation is determined by the number density and circumferential distribution of interfacial particles. We further correlate particle dynamics and assembly to the evaporation-induced convection in the bulk and on the surface of droplet. Finally, we observe distinct final deposits from evaporating colloidal droplets with bulk-dispersed and interface-bound particles. In addition, the deposit pattern is also influenced by the equilibrium contact angle of droplet.

Droplet-based gene expression analysis using a device with magnetic force-based-droplet-handling system.

PubMed

Okochi, Mina; Tsuchiya, Hiroyoshi; Kumazawa, Fumitaka; Shikida, Mitsuhiro; Honda, Hiroyuki

2010-02-01

A droplet-based cell lysis and reverse transcription-polymerase chain reaction (PCR) were performed on-chip employing magnetic force-based-droplet-handling system. The actuation with a magnet offers a simple system for droplet manipulation; it does not need mechanical fluidic systems such as pumps and valves for handling solutions. It can be used as a powerful tool for various biochemical applications by moving and coalescing sample droplets using magnetic beads immersed in mineral oil. The droplet containing magnetic beads and the cells were manipulated with the magnet located underneath the channel, and coalesced with a droplet of lysis buffer. Using K562 cells as the leukemia model, the cell lysis, cDNA synthesis, and amplification of WT1 gene that is known as the prognostic factor for acute leukemia were successfully performed from a single cell. Copyright (c) 2009 The Society for Biotechnology, Japan. Published by Elsevier B.V. All rights reserved.

Assessment of water droplet evaporation mechanisms on hydrophobic and superhydrophobic substrates.

PubMed

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

2013-12-23

Evaporation rates are predicted and important transport mechanisms identified for evaporation of water droplets on hydrophobic (contact angle ~110°) and superhydrophobic (contact angle ~160°) substrates. Analytical models for droplet evaporation in the literature are usually simplified to include only vapor diffusion in the gas domain, and the system is assumed to be isothermal. In the comprehensive model developed in this study, evaporative cooling of the interface is accounted for, and vapor concentration is coupled to local temperature at the interface. Conjugate heat and mass transfer are solved in the solid substrate, liquid droplet, and surrounding gas. Buoyancy-driven convective flows in the droplet and vapor domains are also simulated. The influences of evaporative cooling and convection on the evaporation characteristics are determined quantitatively. The liquid-vapor interface temperature drop induced by evaporative cooling suppresses evaporation, while gas-phase natural convection acts to enhance evaporation. While the effects of these competing transport mechanisms are observed to counterbalance for evaporation on a hydrophobic surface, the stronger influence of evaporative cooling on a superhydrophobic surface accounts for an overprediction of experimental evaporation rates by ~20% with vapor diffusion-based models. The local evaporation fluxes along the liquid-vapor interface for both hydrophobic and superhydrophobic substrates are investigated. The highest local evaporation flux occurs at the three-phase contact line region due to proximity to the higher temperature substrate, rather than at the relatively colder droplet top; vapor diffusion-based models predict the opposite. The numerically calculated evaporation rates agree with experimental results to within 2% for superhydrophobic substrates and 3% for hydrophobic substrates. The large deviations between past analytical models and the experimental data are therefore reconciled with the comprehensive model developed here.

CFD simulation of pesticide spray from air-assisted sprayers in an apple orchard: Tree deposition and off-target losses

NASA Astrophysics Data System (ADS)

Hong, Se-Woon; Zhao, Lingying; Zhu, Heping

2018-02-01

The ultimate goal of a pesticide spraying system is to provide adequate coverage on intended canopies with a minimum amount of spray materials and off-target waste. Better spray coverage requires an understanding of the fate and transport of spray droplets carried by turbulent airflows in orchards. In this study, an integrated computational fluid dynamics (CFD) model was developed to predict displacement of pesticide spray droplets discharged from an air-assisted sprayer, depositions onto tree canopies, and off-target deposition and airborne drift in an apple orchard. Pesticide droplets discharged from a moving sprayer were tracked using the Lagrangian particle transport model, and the deposition model was applied to droplets entering porous canopy zones. Measurements of the droplet deposition and drift in the same orchard were used to validate the model simulations. Good agreement was found between the measured and simulated spray concentrations inside tree canopies and off-target losses (ground deposition and airborne drifts) with the overall relative errors of 22.1% and 40.6%, respectively, under three growth stages. The CFD model was able to estimate the mass balance of pesticide droplets in the orchard, which was practically difficult to investigate by measurements in field conditions. As the foliage of trees became denser, spray deposition inside canopies increased from 8.5% to 65.8% and airborne drift and ground deposition decreased from 25.8% to 7.0% and 47.8% to 21.2%, respectively. Higher wind speed also increased the spray airborne drift downwind of the orchard. This study demonstrates that CFD model can be used to evaluate spray application performance and design and operate sprayers with increased spray efficiencies and reduced drift potentials.

Experiments on Spray from a Rolling Tire

NASA Astrophysics Data System (ADS)

Radovich, Charles; Browand, Fred

2010-11-01

A novel laboratory apparatus has been built to understand the mechanisms and statistics of droplet production for spray emerging from a rolling tire. Using high-speed imaging, water passing through a single circumferential groove was observed to leave the tire contact patch in the form of a liquid sheet of non-uniform thickness. The sheet breaks into droplets as a result of several, organized instabilities. Measurements for the breakup length of the liquid sheet showed a dependence on Weber number proportional to We-1/6, for Weber numbers of 2700, 10900 and 24400. A technique to identify and size water droplets was developed and the distribution of droplet sizes was determined as a function of Weber number. At We = 2700, droplet sizes between 80 and 9000μm were detected, with a mean diameter near 800μm. Both the range of droplet sizes and the mean diameter were found to decrease with increasing Weber number as (approximately) We-1/2. Correlation Image Velocimetry (CIV) was used to estimate the distribution of droplet velocities as a function of droplet size. The spread of droplet velocities about the tire peripheral speed is strongly correlated with droplet size. The spread can be estimated by a simple physical model incorporating rigid droplets subject to gravity and drag.

Computational Study of Droplet Trains Impacting a Smooth Solid Surface

NASA Astrophysics Data System (ADS)

Markt, David, Jr.; Pathak, Ashish; Raessi, Mehdi; Lee, Seong-Young; Zhao, Emma

2017-11-01

The study of droplet impingement is vital to understanding the fluid dynamics of fuel injection in modern internal combustion engines. One widely accepted model was proposed by Yarin and Weiss (JFM, 1995), developed from experiments of single trains of ethanol droplets impacting a substrate. The model predicts the onset of splashing and the mass ejected upon splashing. In this study, using an in-house 3D multiphase flow solver, the experiments of Yarin and Weiss were computationally simulated. The experimentally observed splashing threshold was captured by the simulations, thus validating the solver's ability to accurately simulate the splashing dynamics. Then, we performed simulations of cases with multiple droplet trains, which have high relevance to dense fuel sprays, where droplets impact within the spreading diameters of their neighboring droplets, leading to changes in splashing dynamics due to interactions of spreading films. For both single and multi-train simulations the amount of splashed mass was calculated as a function of time, allowing a quantitative comparison between the two cases. Furthermore, using a passive scalar the amount of splashed mass per impinging droplet was also calculated. This work is supported by the Department of Energy, Office of Energy Efficiency and Renewable Energy (EERE) and the Department of Defense, Tank and Automotive Research, Development, and Engineering Center (TARDEC), under Award Number DE-EE0007292.

Numerical simulations of an impinging liquid spray in a cross-flow

NASA Astrophysics Data System (ADS)

Gomatam, Sreekar; Vengadesan, S.; Chakravarthy, S. R.

2017-11-01

The characteristics of a liquid spray in a uniform cross-flow field are numerically simulated in this study. A hollow cone liquid spray is injected perpendicular to the air stream flowing through a rectangular duct under room temperature and pressure. An Eulerian-Lagrangian framework is adopted to simulate the spray in cross-flow phenomenon. The cross-flow velocity is varied from 6-12 m/s while the liquid injection pressure is varied from 0.3-0.6 MPa. The liquid droplets from the injected spray undergo breakup and/or coalescence further in the cross-flow. Moreover, the spray injected into the cross-flow impinges on the opposite wall resulting in the formation of a liquid film. This liquid film disintegrates further into discrete droplets because of the impingement of the droplets from the spray and the shear from the cross-flow. The overall distribution of the droplets in the cross-flow for varying conditions is studied in detail. The evolution of the liquid film with space and time for varying conditions is also investigated. Suitable sub-models are used to numerically model the droplet break-up, coalescence, liquid film formation and disintegration, splashing of the droplets on the film and subsequent formation of daughter droplets. Department of Applied Mechanics, Indian Inst of Tech-Madras.

Validated Numerical Models for the Convective Extinction of Fuel Droplets (CEFD)

NASA Technical Reports Server (NTRS)

Gogos, George; Bowen, Brent; Nickerson, Jocelyn S.

2002-01-01

The NASA Nebraska Space Grant (NSGC) & EPSCoR programs have continued their effort to support outstanding research endeavors by funding the Numerical Simulation of the Combustion of Fuel Droplets study at the University of Nebraska at Lincoln (UNL). This team of researchers has developed a transient numerical model to study the combustion of suspended and moving droplets. The engines that propel missiles, jets, and many other devices are dependent upon combustion. Therefore, data concerning the combustion of fuel droplets is of immediate relevance to aviation and aeronautical personnel, especially those involved in flight operations. The experiments being conducted by Dr. Gogos and Dr. Nayagam s research teams, allow investigators to gather data for comparison with theoretical predictions of burning rates, flame structures, and extinction conditions. The consequent improved fundamental understanding of droplet combustion may contribute to the clean and safe utilization of fossil fuels (Williams, Dryer, Haggard & Nayagam, 1997, f 2). The present state of knowledge on convective extinction of fuel droplets derives from experiments conducted under normal gravity conditions. However, any data obtained with suspended droplets under normal gravity are grossly affected by gravity. The need to obtain experimental data under microgravity conditions is therefore well justified and addresses one of the goals of NASA's Human Exploration and Development of Space (HEDS) microgravity combustion experiment.

Impact of Viscous Droplets on Superamphiphobic Surfaces

NASA Astrophysics Data System (ADS)

Zhao, Binyu; Chen, Longquan; Deng, Xu

2016-11-01

Superamphiphobic coating is promising for various applications in industry, e.g. self-cleaning windows, where the impingement of droplets on surfaces is commonly encountered. In this work, we experimentally investigated the impact of droplets with similar surface tension (63-72 mN/m) but much different viscosity (1-150 mPa s) on superamphiphobic surfaces. We found that droplets can rebound from the superamphiphobic surfaces when the impact velocity is larger than a critical value, which linearly increases with the liquid viscosity. Droplet with higher viscosity spreads, retracts slower, and eventually rebounds lower and fewer times than that of low viscous droplet. These findings have important implications for surface engineers to use superamphiphobic coatings. Furthermore, we measured the maximum spreading factors for droplet impact on superamphiphobic surfaces and proposed a simple model based on energy conversation to describe its relationship to the Weber number and Reynolds number.

Bouncing droplets on a billiard table.

PubMed

Shirokoff, David

2013-03-01

In a set of experiments, Couder et al. demonstrate that an oscillating fluid bed may propagate a bouncing droplet through the guidance of the surface waves. I present a dynamical systems model, in the form of an iterative map, for a droplet on an oscillating bath. I examine the droplet bifurcation from bouncing to walking, and prescribe general requirements for the surface wave to support stable walking states. I show that in addition to walking, there is a region of large forcing that may support the chaotic motion of the droplet. Using the map, I then investigate the droplet trajectories in a square (billiard ball) domain. I show that in large domains, the long time trajectories are either non-periodic dense curves or approach a quasiperiodic orbit. In contrast, in small domains, at low forcing, trajectories tend to approach an array of circular attracting sets. As the forcing increases, the attracting sets break down and the droplet travels throughout space.

Co-Compartmentation of Terpene Biosynthesis and Storage via Synthetic Droplet.

PubMed

Zhao, Cheng; Kim, YongKyoung; Zeng, Yining; Li, Man; Wang, Xin; Hu, Cheng; Gorman, Connor; Dai, Susie Y; Ding, Shi-You; Yuan, Joshua S

2018-03-16

Traditional bioproduct engineering focuses on pathway optimization, yet is often complicated by product inhibition, downstream consumption, and the toxicity of certain products. Here, we present the co-compartmentation of biosynthesis and storage via a synthetic droplet as an effective new strategy to improve the bioproduct yield, with squalene as a model compound. A hydrophobic protein was designed and introduced into the tobacco chloroplast to generate a synthetic droplet for terpene storage. Simultaneously, squalene biosynthesis enzymes were introduced to chloroplasts together with the droplet-forming protein to co-compartmentalize the biosynthesis and storage of squalene. The strategy has enabled a record yield of squalene at 2.6 mg/g fresh weight without compromising plant growth. Confocal fluorescent microscopy imaging, stimulated Raman scattering microscopy, and droplet composition analysis confirmed the formation of synthetic storage droplet in chloroplast. The co-compartmentation of synthetic storage droplet with a targeted metabolic pathway engineering represents a new strategy for enhancing bioproduct yield.

Complex Dynamics of Droplet Traffic in a Bifurcating Microfluidic Channel: Periodicity, Multistability, and Selection Rules

NASA Astrophysics Data System (ADS)

Sessoms, D. A.; Amon, A.; Courbin, L.; Panizza, P.

2010-10-01

The binary path selection of droplets reaching a T junction is regulated by time-delayed feedback and nonlinear couplings. Such mechanisms result in complex dynamics of droplet partitioning: numerous discrete bifurcations between periodic regimes are observed. We introduce a model based on an approximation that makes this problem tractable. This allows us to derive analytical formulae that predict the occurrence of the bifurcations between consecutive regimes, establish selection rules for the period of a regime, and describe the evolutions of the period and complexity of droplet pattern in a cycle with the key parameters of the system. We discuss the validity and limitations of our model which describes semiquantitatively both numerical simulations and microfluidic experiments.

A theoretical study of the spheroidal droplet evaporation in forced convection

NASA Astrophysics Data System (ADS)

Li, Jie; Zhang, Jian

2014-11-01

In many applications, the shape of a droplet may be assumed to be an oblate spheroid. A theoretical study is conducted on the evaporation of an oblate spheroidal droplet under forced convection conditions. Closed-form analytical expressions of the mass evaporation rate for an oblate spheroid are derived, in the regime of controlled mass-transfer and heat-transfer, respectively. The variation of droplet size during the evaporation process is presented in the regime of shrinking dynamic model. Comparing with the droplets having the same surface area, an increase in the aspect ratio enhances the mass evaporation rate and prolongs the burnout time.

Manifestations of Dynamical Localization in the Disordered XXZ Spin Chain

NASA Astrophysics Data System (ADS)

Elgart, Alexander; Klein, Abel; Stolz, Günter

2018-04-01

We study disordered XXZ spin chains in the Ising phase exhibiting droplet localization, a single cluster localization property we previously proved for random XXZ spin chains. It holds in an energy interval I near the bottom of the spectrum, known as the droplet spectrum. We establish dynamical manifestations of localization in the energy window I, including non-spreading of information, zero-velocity Lieb-Robinson bounds, and general dynamical clustering. Our results do not rely on knowledge of the dynamical characteristics of the model outside the droplet spectrum. A byproduct of our analysis is that for random XXZ spin chains this droplet localization can happen only inside the droplet spectrum.

Cloud Coverage Enhancement and Nocturnal Drizzle Suppression in Stratocumulus by Aerosols

NASA Technical Reports Server (NTRS)

Ackerman, Andrew S.; Toon, Owen B.; Stevens, David E.; Coakley, James A., Jr.; Gore, Warren J. (Technical Monitor)

2002-01-01

Recent satellite observations of ship tracks surprisingly indicate that cloud water decreases with increasing droplet concentrations. However, we find by analyzing detailed simulations of stratocumulus that the reported trend is likely an artifact of sampling, only overcast clouds. The simulations instead show cloud coverage increasing with droplet concentrations, accounting for 25% of cloud albedo increase at moderate droplet concentrations. Our simulations also show that increases in cloud water from drizzle suppression (by increasing droplet concentrations) are favored only at night or at extremely low droplet concentrations, suggesting that the indirect aerosol forcing is overestimated in climate change projections by many general circulation models.

Artificial tektites: an experimental technique for capturing the shapes of spinning drops

NASA Astrophysics Data System (ADS)

Baldwin, Kyle A.; Butler, Samuel L.; Hill, Richard J. A.

2015-01-01

Determining the shapes of a rotating liquid droplet bound by surface tension is an archetypal problem in the study of the equilibrium shapes of a spinning and charged droplet, a problem that unites models of the stability of the atomic nucleus with the shapes of astronomical-scale, gravitationally-bound masses. The shapes of highly deformed droplets and their stability must be calculated numerically. Although the accuracy of such models has increased with the use of progressively more sophisticated computational techniques and increases in computing power, direct experimental verification is still lacking. Here we present an experimental technique for making wax models of these shapes using diamagnetic levitation. The wax models resemble splash-form tektites, glassy stones formed from molten rock ejected from asteroid impacts. Many tektites have elongated or `dumb-bell' shapes due to their rotation mid-flight before solidification, just as we observe here. Measurements of the dimensions of our wax `artificial tektites' show good agreement with equilibrium shapes calculated by our numerical model, and with previous models. These wax models provide the first direct experimental validation for numerical models of the equilibrium shapes of spinning droplets, of importance to fundamental physics and also to studies of tektite formation.

Effects of isotropic and anisotropic slip on droplet impingement on a superhydrophobic surface

NASA Astrophysics Data System (ADS)

Clavijo, Cristian E.; Crockett, Julie; Maynes, Daniel

2015-12-01

The dynamics of single droplet impingement on micro-textured superhydrophobic surfaces with isotropic and anisotropic slip are investigated. While several analytical models exist to predict droplet impact on superhydrophobic surfaces, no previous model has rigorously considered the effect of the shear-free region above the gas cavities resulting in an apparent slip that is inherent for many of these surfaces. This paper presents a model that accounts for slip during spreading and recoiling. A broad range of Weber numbers and slip length values were investigated at low Ohnesorge numbers. The results show that surface slip exerts negligible influence throughout the impingement process for low Weber numbers but can exert significant influence for high Weber numbers (on the order of 102). When anisotropic slip prevails, the droplet exhibits an elliptical shape at the point of maximum spread, with greater eccentricity for increasing slip and increasing Weber number. Experiments were performed on isotropic and anisotropic micro-structured superhydrophobic surfaces and the agreement between the experimental results and the model is very good.

Lipidomic and proteomic analysis of Caenorhabditis elegans lipid droplets and identification of ACS-4 as a lipid droplet-associated protein

DOE PAGES

Vrablik, Tracy L.; Petyuk, Vladislav A.; Larson, Emily M.; ...

2015-06-27

Lipid droplets are cytoplasmic organelles that store neutral lipids for membrane synthesis and energy reserves. In this study, we characterized the lipid and protein composition of purified Caenorhabditis elegans lipid droplets. These lipid droplets are composed mainly of triacylglycerols, surrounded by a phospholipid monolayer composed primarily of phosphatidylcholine and phosphatidylethanolamine. The fatty acid composition of the triacylglycerols is rich in fatty acid species obtained from the dietary Escherichia coli, including cyclopropane fatty acids and cis-vaccenic acid. Unlike other organisms, C. elegans lipid droplets contain very little cholesterol or cholesterol esters. Comparison of the lipid droplet proteomes of wild type andmore » high-fat daf-2 mutant strains shows a very similar proteome in both strains, except that the most abundant protein in the C. elegans lipid droplet proteome, MDT-28, is relatively less abundant in lipid droplets isolated from daf-2 mutants. Functional analysis of lipid droplet proteins identified in our proteomic studies indicated an enrichment of proteins required for growth and fat homeostasis in C. elegans. Finally, we confirmed the localization of one of the newly identified lipid droplet proteins, ACS-4. We found that ACS-4 localizes to the surface of lipid droplets in the C. elegans intestine and skin. This study bolsters C. elegans as a model to study the dynamics and functions of lipid droplets in a multicellular organism.« less

Lattice Boltzmann study of chemically-driven self-propelled droplets.

PubMed

Fadda, F; Gonnella, G; Lamura, A; Tiribocchi, A

2017-12-19

We numerically study the behavior of self-propelled liquid droplets whose motion is triggered by a Marangoni-like flow. This latter is generated by variations of surfactant concentration which affect the droplet surface tension promoting its motion. In the present paper a model for droplets with a third amphiphilic component is adopted. The dynamics is described by Navier-Stokes and convection-diffusion equations, solved by the lattice Boltzmann method coupled with finite-difference schemes. We focus on two cases. First, the study of self-propulsion of an isolated droplet is carried on and, then, the interaction of two self-propelled droplets is investigated. In both cases, when the surfactant migrates towards the interface, a quadrupolar vortex of the velocity field forms inside the droplet and causes the motion. A weaker dipolar field emerges instead when the surfactant is mainly diluted in the bulk. The dynamics of two interacting droplets is more complex and strongly depends on their reciprocal distance. If, in a head-on collision, droplets are close enough, the velocity field initially attracts them until a motionless steady state is achieved. If the droplets are vertically shifted, the hydrodynamic field leads to an initial reciprocal attraction followed by a scattering along opposite directions. This hydrodynamic interaction acts on a separation of some droplet radii otherwise it becomes negligible and droplets motion is only driven by the Marangoni effect. Finally, if one of the droplets is passive, this latter is generally advected by the fluid flow generated by the active one.

Why droplet dimension can be larger than, equal to, or smaller than the nanowire dimension

NASA Astrophysics Data System (ADS)

Mohammad, S. Noor

2009-11-01

Droplets play central roles in the nanowire (NW) growth by vapor phase mechanisms. These mechanisms include vapor-liquid-solid (VLS), vapor-solid-solid or vapor-solid (VSS), vapor-quasisolid-solid or vapor-quasiliquid-solid (VQS), oxide-assisted growth (OAG), and self-catalytic growth (SCG) mechanisms. Fundamentals of the shape, size, characteristics, and dynamics of droplets and the impacts of them on the NW growth, have been studied. The influence of growth techniques, growth parameters (e.g., growth temperature, partial pressure, gas flow rates, etc.), thermodynamic conditions, surface and interface energy, molar volume, chemical potentials, etc. have been considered on the shapes and sizes of droplets. A model has been presented to explain why droplets can be larger than, equal to, or smaller than the associated NWs. Various growth techniques have been analyzed to understand defects created in NWs. Photoluminescence characteristics have been presented to quantify the roles of droplets in the creation of NW defects. The study highlights the importance of the purity of the droplet material. It attests to the superiority of the SCG mechanism, and clarifies the differences between the VSS, VQS, VLS, and SCG mechanisms. It explains why droplets produced by some mechanisms are visible but droplets produced by some other mechanisms are not visible. It elucidates the formation mechanisms of very large and very small droplets, and discusses the ground rules for droplets creating necked NWs. It puts forth reasons to demonstrate that very large droplets may not behave as droplets.

Numerical investigation of impact of relative humidity on droplet accumulation and film cooling on compressor blades

NASA Astrophysics Data System (ADS)

Bugarin, Luz Irene

During the summer, high inlet temperatures affect the power output of gas turbine systems. Evaporative coolers have gained popularity as an inlet cooling method for these systems. Wet compression has been one of the common evaporative cooling methods implemented to increase power output of gas turbine systems due to its simple installation and low cost. This process involves injection of water droplets into the continuous phase of compressor to reduce the temperature of the flow entering the compressor and in turn increase the power output of the whole gas turbine system. This study focused on a single stage rotor-stator compressor model with varying inlet temperature between 300K and 320K, as well as relative humidity between 0% and 100%. The simulations are carried out using the commercial CFD tool ANSYS: FLUENT. The study modeled the interaction between the two phases including mass and heat transfer, given different inlet relative humidity (RH) and temperature conditions. The Reynolds Averaged Navier-Stokes (RANS) equations with k-epsilon turbulence model were applied as well as the droplet coalescence and droplet breakup model considered in the simulation. Sliding mesh theory was implemented to simulate the compressor movement in 2-D. The interaction between the blade and droplets were modeled to address all possible interactions; which include: stick spread, splash, or rebound and compared to an interaction of only reflect. The goal of this study is to quantify the relation between RH, inlet temperature, overall heat transfer coefficient, and the heat transferred from the droplets to the blades surface. The result of this study lead to further proof that wet compression yields higher pressure ratios and lower temperatures in the domain under all of the cases. Additionally, droplet-wall interaction has an interesting effect on the heat transfer coefficient at the compressor blades.

Soot agglomeration in isolated, free droplet combustion

NASA Technical Reports Server (NTRS)

Choi, M. Y.; Dryer, F. L.; Green, G. J.; Sangiovanni, J. J.

1993-01-01

Under the conditions of an isolated, free droplet experiment, hollow, carbonaceous structures, called soot spheres, were observed to form during the atmospheric pressure, low Reynolds number combustion of 1-methylnaphthalene. These structures which are agglomerates composed of smaller spheroidal units result from both thermophoretic effects induced by the envelope flame surrounding each drop and aerodynamic effects caused by changes in the relative gas/drop velocities. A chemically reacting flow model was used to analyze the process of sootshell formation during microgravity droplet combustion. The time-dependent temperature and gas property field surrounding the droplet was determined, and the soot cloud location for microgravity combustion of n-heptane droplets was predicted. Experiments showed that the sooting propensity of n-alkane fuel droplets can be varied through diluent substitution, oxygen-index variations, and ambient pressure reductions.

Conditions for super-adiabatic droplet growth after entrainment mixing

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

Yang, Fan; Shaw, Raymond; Xue, Huiwen

Cloud droplet response to entrainment and mixing between a cloud and its environment is considered, accounting for subsequent droplet growth during adiabatic ascent following a mixing event. The vertical profile for liquid water mixing ratio after a mixing event is derived analytically, allowing the reduction to be predicted from the mixing fraction and from the temperature and humidity for both the cloud and environment. It is derived for the limit of homogeneous mixing. The expression leads to a critical height above the mixing level: at the critical height the cloud droplet radius is the same for both mixed and unmixedmore » parcels, and the critical height is independent of the updraft velocity and mixing fraction. Cloud droplets in a mixed parcel are larger than in an unmixed parcel above the critical height, which we refer to as the “super-adiabatic” growth region. Analytical results are confirmed with a bin microphysics cloud model. Using the model, we explore the effects of updraft velocity, aerosol source in the environmental air, and polydisperse cloud droplets. Results show that the mixed parcel is more likely to reach the super-adiabatic growth region when the environmental air is humid and clean. It is also confirmed that the analytical predictions are matched by the volume-mean cloud droplet radius for polydisperse size distributions. Lastly, these findings have implications for the origin of large cloud droplets that may contribute to onset of collision–coalescence in warm clouds.« less

SO 2 oxidation in an entraining cloud model with explicit microphysics

NASA Astrophysics Data System (ADS)

Bower, K. N.; Hill, T. A.; Coe, H.; Choularton, T. W.

A model of the chemical evolution of the droplets in a hill-cap cloud is presented. The chemistry of individual droplets forming on cloud condensation nuclei of differing size and chemical composition is considered, and the take-up of species from the gas phase by the droplets is treated explicity for the droplet population. Oxidation of S(IV) dissolved in cloud droplets is assumed to be dominated by hydrogen peroxide and ozone. Hydrogen peroxide is normally found to be the dominant oxidant for the oxidation of sulphur dioxide (except in the presence of substantial concentrations of ammonia gas, which increases droplet pH and the contribution made by the oxidant ozone). The entrainment of hydrogen peroxide from above the cloud top increases the amount of sulphate produced in conditions where the reaction is otherwise oxidant limited by the availability hydrogen peroxide. These conditions occur when there are high concentrations of sulphur dioxide accompanied by low cloudwater pH values. Within droplets formed on sodium chloride aerosol, reduced levels of acidity lead to an increase in sulphate production as a result of an enhanced reaction between SO 2 and the oxidant ozone. This results in an overall higher increase in cloudwater sulphate than would be expected assuming an even distribution of all reactants amongst the droplets. In addition, concentrations of the hydrogen sulphite ion predicted to occur in the cloudwater can be substantially in excess of those predicted from the bulk cloudwater pH. This is consistent with recent observations.

Conditions for super-adiabatic droplet growth after entrainment mixing

DOE PAGES

Yang, Fan; Shaw, Raymond; Xue, Huiwen

2016-07-29

Cloud droplet response to entrainment and mixing between a cloud and its environment is considered, accounting for subsequent droplet growth during adiabatic ascent following a mixing event. The vertical profile for liquid water mixing ratio after a mixing event is derived analytically, allowing the reduction to be predicted from the mixing fraction and from the temperature and humidity for both the cloud and environment. It is derived for the limit of homogeneous mixing. The expression leads to a critical height above the mixing level: at the critical height the cloud droplet radius is the same for both mixed and unmixedmore » parcels, and the critical height is independent of the updraft velocity and mixing fraction. Cloud droplets in a mixed parcel are larger than in an unmixed parcel above the critical height, which we refer to as the “super-adiabatic” growth region. Analytical results are confirmed with a bin microphysics cloud model. Using the model, we explore the effects of updraft velocity, aerosol source in the environmental air, and polydisperse cloud droplets. Results show that the mixed parcel is more likely to reach the super-adiabatic growth region when the environmental air is humid and clean. It is also confirmed that the analytical predictions are matched by the volume-mean cloud droplet radius for polydisperse size distributions. Lastly, these findings have implications for the origin of large cloud droplets that may contribute to onset of collision–coalescence in warm clouds.« less

Characterizing Droplet Formation from Non-Linear Slosh in a Propellant Tank

NASA Technical Reports Server (NTRS)

Brodnick, Jacob; Yang, Hong; West, Jeffrey

2015-01-01

The Fluid Dynamics Branch (ER42) at the Marshall Space Flight Center (MSFC) was tasked with characterizing the formation and evolution of liquid droplets resulting from nonlinear propellant slosh in a storage tank. Lateral excitation of propellant tanks can produce high amplitude nonlinear slosh waves through large amplitude excitations and or excitation frequencies near a resonance frequency of the tank. The high amplitude slosh waves become breaking waves upon attaining a certain amplitude or encountering a contracting geometry such as the upper dome section of a spherical tank. Inherent perturbations in the thinning regions of breaking waves result in alternating regions of high and low pressure within the fluid. Droplets form once the force from the local pressure differential becomes larger than the force maintaining the fluid interface shape due to surface tension. Droplets released from breaking waves in a pressurized tank may lead to ullage collapse given the appropriate conditions due to the increased liquid surface area and thus heat transfer between the fluids. The goal of this project is to create an engineering model that describes droplet formation as a function of propellant slosh for use in the evaluation of ullage collapse during a sloshing event. The Volume of Fluid (VOF) model in the production level Computational Fluid Dynamics (CFD) code Loci-Stream was used to predict droplet formation from breaking waves with realistic surface tension characteristics. Various excitation frequencies and amplitudes were investigated at multiple fill levels for a single storage tank to create the engineering model of droplet formation from lateral propellant slosh.

The effect of different spectral shape parameterizations of cloud droplet size distribution on first and second aerosol indirect effects in NACR CAM5 and evaluation with satellite data

NASA Astrophysics Data System (ADS)

Wang, M.; Peng, Y.; Xie, X.; Liu, Y.

2017-12-01

Aerosol cloud interaction continues to constitute one of the most significant uncertainties for anthropogenic climate perturbations. The parameterization of cloud droplet size distribution and autoconversion process from large scale cloud to rain can influence the estimation of first and second aerosol indirect effects in global climate models. We design a series of experiments focusing on the microphysical cloud scheme of NCAR CAM5 (Community Atmospheric Model Version 5) in transient historical run with realistic sea surface temperature and sea ice. We investigate the effect of three empirical, two semi-empirical and one analytical expressions for droplet size distribution on cloud properties and explore the statistical relationships between aerosol optical thickness (AOT) and simulated cloud variables, including cloud top droplet effective radius (CDER), cloud optical depth (COD), cloud water path (CWP). We also introduce the droplet spectral shape parameter into the autoconversion process to incorporate the effect of droplet size distribution on second aerosol indirect effect. Three satellite datasets (MODIS Terra/ MODIS Aqua/ AVHRR) are used to evaluate the simulated aerosol indirect effect from the model. Evident CDER decreasing with significant AOT increasing is found in the east coast of China to the North Pacific Ocean and the east coast of USA to the North Atlantic Ocean. Analytical and semi-empirical expressions for spectral shape parameterization show stronger first aerosol indirect effect but weaker second aerosol indirect effect than empirical expressions because of the narrower droplet size distribution.

The effects of detailed chemistry and transport on microgravity droplet combustion

NASA Technical Reports Server (NTRS)

Marchese, A. J.; Lee, J. C.; Held, T. J.; Dryer, F. L.

1995-01-01

A brief overview of recent advances in the theoretical study of microgravity droplet combustion is presented. Much of this work has centered on the development and utilization of sphero-symmetric transient numerical models which consider detailed gas phase chemistry and transport as well as energy and/or species transport within a regressing condensed phase. Numerical results for microgravity combustion and vaporization of methanol, methanol/water, heptane, and heptane/hexadecane droplets are summarized along with refinements in chemical kinetics and the development of a new two-dimensional axi-symmetric model.

Thermocapillary flow contribution to dropwise condensation heat transfer

NASA Astrophysics Data System (ADS)

Phadnis, Akshay; Rykaczewski, Konrad

2017-11-01

With recent developments of durable hydrophobic materials potentially enabling industrial applications of dropwise condensation, accurate modeling of heat transfer during this phase change process is becoming increasingly important. Classical steady state models of dropwise condensation are based on the integration of heat transfer through individual droplets over the entire drop size distribution. These models consider only the conduction heat transfer inside the droplets. However, simple scaling arguments suggest that thermocapillary flows might exist in such droplets. In this work, we used Finite Element heat transfer model to quantify the effect of Marangoni flow on dropwise condensation heat transfer of liquids with a wide range of surface tensions ranging from water to pentane. We confirmed that the Marangoni flow is present for a wide range of droplet sizes, but only has quantifiable effects on heat transfer in drops larger than 10 µm. By integrating the single drop heat transfer simulation results with drop size distribution for the cases considered, we demonstrated that Marangoni flow contributes a 10-30% increase in the overall heat transfer coefficient over conduction only model.

A study on post impingement effects of urea-water solution spray on the heated wall of automotive SCR systems

NASA Astrophysics Data System (ADS)

Shahariar, G. M. H.; Wardana, M. K. A.; Lim, O. T.

2018-04-01

The post impingement effects of urea-water solution spray on the heated wall of automotive SCR systems was numerically investigated in a constant volume chamber using STAR CCM+ CFD code. The turbulence flow was modelled by realizable k-ε two-layer model together with standard wall function and all y+ treatment was applied along with two-layer approach. The Eulerian-Lagrangian approach was used for the modelling of multi phase flow. Urea water solution (UWS) was injected onto the heated wall for the wall temperature of 338, 413, 473, 503 & 573 K. Spray development after impinging on the heated wall was visualized and measured. Droplet size distribution and droplet evaporation rates were also measured, which are vital parameters for the system performance but still not well researched. Specially developed user defined functions (UDF) are implemented to simulate the desired conditions and parameters. The investigation reveals that wall temperature has a great impact on spray development after impingement, droplet size distribution and evaporation. Increasing the wall temperature leads to longer spray front projection length, smaller droplet size and faster droplet evaporation which are preconditions for urea crystallization reduction. The numerical model and parameters are validated comparing with experimental data.

Gas Atomization of Molten Metal: Part I. Numerical Modeling Conception

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

Leon, Genaro Perez-de; Lamberti, Vincent E.; Seals, Roland D.

This numerical analysis study entails creating and assessing a model that is capable of simulating molten metal droplets and the production of metal powder during the Gas Atomization (GA) method. The essential goal of this research aims to gather more information on simulating the process of creating metal powder. The model structure and perspective was built through the application of governing equations and aspects that utilized factors such as gas dynamics, droplet dynamics, energy balance, heat transfer, fluid mechanics and thermodynamics that were proposed from previous studies. The model is very simple and can be broken down into having amore » set of inputs to produce outputs. The inputs are the processing parameters such as the initial temperature of the metal alloy, the gas pressure and the size of the droplets. Additional inputs include the selection of the metal alloy and the atomization gas and factoring in their properties. The outputs can be designated by the velocity and thermal profiles of the droplet and gas. These profiles illustrate the speed of both as well as the rate of temperature change or cooling rate of the droplets. Here, the main focus is the temperature change and finding the right parameters to ensure that the metal powder is efficiently produced. Once the model was conceptualized and finalized, it was employed to verify the results of other previous studies.« less

Gas Atomization of Molten Metal: Part I. Numerical Modeling Conception

DOE PAGES

Leon, Genaro Perez-de; Lamberti, Vincent E.; Seals, Roland D.; ...

2016-02-01

This numerical analysis study entails creating and assessing a model that is capable of simulating molten metal droplets and the production of metal powder during the Gas Atomization (GA) method. The essential goal of this research aims to gather more information on simulating the process of creating metal powder. The model structure and perspective was built through the application of governing equations and aspects that utilized factors such as gas dynamics, droplet dynamics, energy balance, heat transfer, fluid mechanics and thermodynamics that were proposed from previous studies. The model is very simple and can be broken down into having amore » set of inputs to produce outputs. The inputs are the processing parameters such as the initial temperature of the metal alloy, the gas pressure and the size of the droplets. Additional inputs include the selection of the metal alloy and the atomization gas and factoring in their properties. The outputs can be designated by the velocity and thermal profiles of the droplet and gas. These profiles illustrate the speed of both as well as the rate of temperature change or cooling rate of the droplets. Here, the main focus is the temperature change and finding the right parameters to ensure that the metal powder is efficiently produced. Once the model was conceptualized and finalized, it was employed to verify the results of other previous studies.« less

CFD Modeling of the Multipurpose Hydrogen Test Bed (MHTB) Self-Pressurization and Spray Bar Mixing Experiments in Normal Gravity: Effect of the Accommodation Coefficient on the Tank Pressure

NASA Technical Reports Server (NTRS)

Kartuzova, Olga; Kassemi, Mohammad

2015-01-01

A CFD model for simulating the self-pressurization of a large scale liquid hydrogen storage tank is utilized in this paper to model the MHTB self-pressurization experiment. The kinetics-based Schrage equation is used to account for the evaporative and condensi ng interfacial mass flows in this model. The effect of the accommodation coefficient for calculating the interfacial mass transfer rate on the tank pressure during tank selfpressurization is studied. The values of the accommodation coefficient which were considered in this study vary from 1.0e-3 to 1.0e-1 for the explicit VOF model and from 1.0e-4 to 1.0e-3 for the implicit VOF model. The ullage pressure evolutions are compared against experimental data. A CFD model for controlling pressure in cryogenic storage tanks by spraying cold liquid into the ullage is also presented. The Euler-Lagrange approach is utilized for tracking the spray droplets and for modeling the interaction between the droplets and the continuous phase (ullage). The spray model is coupled with the VOF model by performing particle tracking in the ullage, removing particles from the ullage when they reach the interface, and then adding their contributions to the liquid. Droplet-ullage heat and mass transfer are modeled. The flow, temperature, and interfacial mass flux, as well as droplets trajectories, size distribution and temperatures predicted by the model are presented. The ul lage pressure and vapor temperature evolutions are compared with experimental data obtained from the MHTB spray bar mixing experiment. The effect of the accommodation coefficient for calculating the interfacial and droplet mass transfer rates on the tank pressure during mixing of the vapor using spray is studied. The values used for the accommodation coefficient at the interface vary from 1.0e-5 to 1.0e-2. The droplet accommodation coefficient values vary from 2.0e-6 to 1.0e-4.

Gas dynamics and mixture formation in swirled flows with precession of air flow

NASA Astrophysics Data System (ADS)

Tretyakov, V. V.; Sviridenkov, A. A.

2017-10-01

The effect of precessing air flow on the processes of mixture formation in the wake of the front winding devices of the combustion chambers is considered. Visual observations have shown that at different times the shape of the atomized jet is highly variable and has signs of precessing motion. The experimental data on the distribution of the velocity and concentration fields of the droplet fuel in the working volume of the flame tube of a typical combustion chamber are obtained. The method of calculating flows consisted in integrating the complete system of Reynolds equations written in Euler variables and closed with the two-parameter model of turbulence k-ε. Calculation of the concentration fields of droplet and vapor fuel is based on the use of models for disintegration into droplets of fuel jets, fragmentation of droplets and analysis of motion and evaporation of individual droplets in the air flow. Comparison of the calculation results with experimental data showed their good agreement.

One Dimensional Analysis Model of a Condensing Spray Chamber Including Rocket Exhaust Using SINDA/FLUINT and CEA

NASA Technical Reports Server (NTRS)

Sakowski, Barbara; Edwards, Daryl; Dickens, Kevin

2014-01-01

Modeling droplet condensation via CFD codes can be very tedious, time consuming, and inaccurate. CFD codes may be tedious and time consuming in terms of using Lagrangian particle tracking approaches or particle sizing bins. Also since many codes ignore conduction through the droplet and or the degradating effect of heat and mass transfer if noncondensible species are present, the solutions may be inaccurate. The modeling of a condensing spray chamber where the significant size of the water droplets and the time and distance these droplets take to fall, can make the effect of droplet conduction a physical factor that needs to be considered in the model. Furthermore the presence of even a relatively small amount of noncondensible has been shown to reduce the amount of condensation [Ref 1]. It is desirable then to create a modeling tool that addresses these issues. The path taken to create such a tool is illustrated. The application of this tool and subsequent results are based on the spray chamber in the Spacecraft Propulsion Research Facility (B2) located at NASA's Plum Brook Station that tested an RL-10 engine. The platform upon which the condensation physics is modeled is SINDAFLUINT. The use of SINDAFLUINT enables the ability to model various aspects of the entire testing facility, including the rocket exhaust duct flow and heat transfer to the exhaust duct wall. The ejector pumping system of the spray chamber is also easily implemented via SINDAFLUINT. The goal is to create a transient one dimensional flow and heat transfer model beginning at the rocket, continuing through the condensing spray chamber, and finally ending with the ejector pumping system. However the model of the condensing spray chamber may be run independently of the rocket and ejector systems detail, with only appropriate mass flow boundary conditions placed at the entrance and exit of the condensing spray chamber model. The model of the condensing spray chamber takes into account droplet conduction as well as the degrading effect of mass and heat transfer due to the presence of noncondensibles. The one dimension model of the condensing spray chamber makes no presupposition on the pressure profile within the chamber, allowing the implemented droplet physics of heat and mass transfer coupled to the SINDAFLUINT solver to determine a transient pressure profile of the condensing spray chamber. Model results compare well to the RL-10 engine pressure test data.

Topography- and topology-driven spreading of non-Newtonian power-law liquids on a flat and a spherical substrate

NASA Astrophysics Data System (ADS)

Iwamatsu, Masao

2017-10-01

The spreading of a cap-shaped spherical droplet of non-Newtonian power-law liquids on a flat and a spherical rough and textured substrate is theoretically studied in the capillary-controlled spreading regime. A droplet whose scale is much larger than that of the roughness of substrate is considered. The equilibrium contact angle on a rough substrate is modeled by the Wenzel and the Cassie-Baxter model. Only the viscous energy dissipation within the droplet volume is considered, and that within the texture of substrate by imbibition is neglected. Then, the energy balance approach is adopted to derive the evolution equation of the contact angle. When the equilibrium contact angle vanishes, the relaxation of dynamic contact angle θ of a droplet obeys a power-law decay θ ˜t-α except for the Newtonian and the non-Newtonian shear-thinning liquid of the Wenzel model on a spherical substrate. The spreading exponent α of the non-Newtonian shear-thickening liquid of the Wenzel model on a spherical substrate is larger than others. The relaxation of the Newtonian liquid of the Wenzel model on a spherical substrate is even faster showing the exponential relaxation. The relaxation of the non-Newtonian shear-thinning liquid of Wenzel model on a spherical substrate is fastest and finishes within a finite time. Thus, the topography (roughness) and the topology (flat to spherical) of substrate accelerate the spreading of droplet.

Modeling and measurement of electrostatic spray behavior in a rectangular throat of Pease-Anthony venturi scrubber.

PubMed

Yang, H T; Viswanathan, S; Balachandran, W; Ray, M B

2003-06-01

This paper presents the simulation and experimental results of the distribution of droplets produced by electrostatic nozzles inside a venturi scrubber. The simulation model takes into account initial liquid momentum, hydrodynamic, gravitational and electric forces, and eddy diffusion. The velocity and concentration profile of charged droplets injected from an electrostatic nozzle in the scrubber under the combined influence of hydrodynamic and electric fields were simulated. The effects of operating parameters, such as gas velocity, diameter of the scrubbing droplets, charge-to-mass ratio, and liquid-to-gas ratio on the distribution of the water droplets within the scrubber, were also investigated. The flux distribution of scrubbing liquid in the presence of electric field is improved considerably over a conventional venturi scrubber, and the effect increases with the increase in charge-to-mass ratio. Improved flux distribution using charged droplets increases the calculated overall collection efficiency of the submicron particles. However, the effect of an electric field on the droplet distribution pattern for small drop sizes in strong hydrodynamic field conditions is negligible. Simulated results are in good agreement with the experimental data obtained in the laboratory.

An evaporation model of colloidal suspension droplets

NASA Astrophysics Data System (ADS)

Sartori, Silvana; Li\\ Nán, Amable; Lasheras, Juan C.

2009-11-01

Colloidal suspensions of polymers in water or other solvents are widely used in the pharmaceutical industry to coat tablets with different agents. These allow controlling the rate at which the drug is delivered, taste or physical appearance. The coating is performed by simultaneously spraying and drying the tablets with the colloidal suspension at moderately high temperatures. The spreading of the coating on the pills surface depends on the droplet Webber and Reynolds numbers, angle of impact, but more importantly on the rheological properties of the drop. We present a model for the evaporation of a colloidal suspension droplet in a hot air environment with temperatures substantially lower than the boiling temperature of the carrier fluid. As the liquid vaporizes from the surface, a compacting front advances into the droplet faster than the liquid surface regresses, forming a shell of a porous medium where the particles reach their maximum packing density. While the surface regresses, the evaporation rate is determined by both the rate at which heat is transported to the droplet surface and the rate at which liquid vapor is diffused away from it. This regime continues until the compacting front reaches the center of the droplet, at which point the evaporation rate is drastically reduced.

Investigating the evolution of the phase behavior of AOT-based w/o microemulsions in dodecane as a function of droplet volume fraction.

PubMed

Ganguly, R; Choudhury, N

2012-04-15

AOT-based water in oil (w/o) microemulsions are one of the most extensively studied reverse micellar systems because of their rich phase behavior and their ability to form in the absence of any co-surfactant. The aggregation characteristics and interaction of the microemulsion droplets in these systems are known to be governed by AOT-oil compatibility and water to AOT molar ratio (w). In this manuscript by using Dynamic Light Scattering (DLS) and viscometry techniques, we show that droplet volume fraction too plays an important role in shaping the phase behavior of these microemulsions in dodecane. The phase separation characteristics and the evolution of the viscosity and the hydrodynamic radius of the microemulsion droplets on approaching the cloud points have thus been found to undergo complete transformation as one goes from low to high droplet volume fraction even at a fixed 'w'. Modeling of the DLS data attributes this to the weakening of inter droplet attractive interaction caused by the growing dominance of the excluded volume effect with increase in droplet volume fraction. In the literature, the inter droplet attractive interaction driven phase separation in these microemulsions is explained based on gas-liquid type phase transition, conceptualized in the framework of Baxter adhesive hard sphere theory. The modeling of our viscosity data, however, does not support such proposition as the characteristic stickiness parameter (τ(-1)) of the microemulsion droplets in this system remains much lower than the critical value (τ(c)(-1)≈10.25) required to enforce such phase transition. Copyright © 2012 Elsevier Inc. All rights reserved.

MODELING OF SO2 REMOVAL IN SPRAY-DRYER FLUE-GAS DESULFURIZATION SYSTEM

EPA Science Inventory

The report presents a comprehensive mathematical model of the SO2 removal process in a spray-dryer flue-gas desulfurization system. Simultaneous evaporation of a sorbent droplet and absorption/reaction of SO2 in the droplet are described by the corresponding heat- and mass-transf...

Lipid droplets form from distinct regions of the cell in the fission yeast Schizosaccharomyces pombe

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

Meyers, Alex; del Rio, Zuania P.; Beaver, Rachael A.

Eukaryotic cells store cholesterol/sterol esters (SEs) and triacylglycerols (TAGs) in lipid droplets, which form from the contiguous endoplasmic reticulum (ER) network. However, it is not known if droplets preferentially form from certain regions of the ER over others. Here, we used fission yeast Schizosaccharomyces pombe cells where the nuclear and cortical/peripheral ER domains are distinguishable by light microscopy to show that SE-enriched lipid droplets form away from the nucleus at the cell tips, whereas TAG-enriched lipid droplets form around the nucleus. Sterols localize to the regions of the cells where droplets enriched in SEs are observed. TAG droplet formation aroundmore » the nucleus appears to be a strong function of diacylglycerol (DAG) homeostasis with Cpt1p, which coverts DAG into phosphatidylcholine and phosphatidylethanolamine localized exclusively to the nuclear ER. Also, Dgk1p, which converts DAG into phosphatidic acid localized strongly to the nuclear ER over the cortical/peripheral ER. We also show that TAG more readily translocates from the ER to lipid droplets than do SEs. Lastly, the results augment the standard lipid droplet formation model, which has SEs and TAGs flowing into the same nascent lipid droplet regardless of its biogenesis point in the cell.« less

Simulation of air-droplet mixed phase flow in icing wind-tunnel

NASA Astrophysics Data System (ADS)

Mengyao, Leng; Shinan, Chang; Menglong, Wu; Yunhang, Li

2013-07-01

Icing wind-tunnel is the main ground facility for the research of aircraft icing, which is different from normal wind-tunnel for its refrigeration system and spraying system. In stable section of icing wind-tunnel, the original parameters of droplets and air are different, for example, to keep the nozzles from freezing, the droplets are heated while the temperature of air is low. It means that complex mass and heat transfer as well as dynamic interactive force would happen between droplets and air, and the parameters of droplet will acutely change along the passageway. Therefore, the prediction of droplet-air mixed phase flow is necessary in the evaluation of icing researching wind-tunnel. In this paper, a simplified droplet-air mixed phase flow model based on Lagrangian method was built. The variation of temperature, diameter and velocity of droplet, as well as the air flow field, during the flow process were obtained under different condition. With calculating three-dimensional air flow field by FLUENT, the droplet could be traced and the droplet distribution could also be achieved. Furthermore, the patterns about how initial parameters affect the parameters in test section were achieved. The numerical simulation solving the flow and heat and mass transfer characteristics in the mixing process is valuable for the optimization of experimental parameters design and equipment adjustment.

Experimental Observations on the Deformation and Breakup of Water Droplets Near the Leading Edge of an Airfoil

NASA Technical Reports Server (NTRS)

Vargas, Mario; Feo, Alex

2011-01-01

This work presents the results of an experimental study on droplet deformation and breakup near the leading edge of an airfoil. The experiment was conducted in the rotating rig test cell at the Instituto Nacional de Tecnica Aeroespacial (INTA) in Madrid, Spain. An airfoil model placed at the end of the rotating arm was moved at speeds of 50 to 90 m/sec. A monosize droplet generator was employed to produce droplets that were allowed to fall from above, perpendicular to the path of the airfoil at a given location. High speed imaging was employed to observe the interaction between the droplets and the airfoil. The high speed imaging allowed observation of droplet deformation and breakup as the droplet approached the airfoil near the stagnation line. A tracking software program was used to measure from the high speed movies the horizontal and vertical displacement of the droplet against time. The velocity, acceleration, Weber number, Bond number, Reynolds number, and the drag coefficients were calculated along the path of a given droplet from beginning of deformation to breakup and/or hitting the airfoil. Results are presented for droplets with a diameter of 490 micrometers at airfoil speeds of 50, 60, 70, 80 and 90 m/sec

Lipid droplets form from distinct regions of the cell in the fission yeast Schizosaccharomyces pombe

DOE PAGES

Meyers, Alex; del Rio, Zuania P.; Beaver, Rachael A.; ...

2016-04-29

Eukaryotic cells store cholesterol/sterol esters (SEs) and triacylglycerols (TAGs) in lipid droplets, which form from the contiguous endoplasmic reticulum (ER) network. However, it is not known if droplets preferentially form from certain regions of the ER over others. Here, we used fission yeast Schizosaccharomyces pombe cells where the nuclear and cortical/peripheral ER domains are distinguishable by light microscopy to show that SE-enriched lipid droplets form away from the nucleus at the cell tips, whereas TAG-enriched lipid droplets form around the nucleus. Sterols localize to the regions of the cells where droplets enriched in SEs are observed. TAG droplet formation aroundmore » the nucleus appears to be a strong function of diacylglycerol (DAG) homeostasis with Cpt1p, which coverts DAG into phosphatidylcholine and phosphatidylethanolamine localized exclusively to the nuclear ER. Also, Dgk1p, which converts DAG into phosphatidic acid localized strongly to the nuclear ER over the cortical/peripheral ER. We also show that TAG more readily translocates from the ER to lipid droplets than do SEs. Lastly, the results augment the standard lipid droplet formation model, which has SEs and TAGs flowing into the same nascent lipid droplet regardless of its biogenesis point in the cell.« less

Ultrasensitive detection of protein translocated through toxin pores in droplet-interface bilayers

PubMed Central

Fischer, Audrey; Holden, Matthew A.; Pentelute, Brad L.; Collier, R. John

2011-01-01

Many bacterial toxins form proteinaceous pores that facilitate the translocation of soluble effector proteins across cellular membranes. With anthrax toxin this process may be monitored in real time by electrophysiology, where fluctuations in ionic current through these pores inserted in model membranes are used to infer the translocation of individual protein molecules. However, detecting the minute quantities of translocated proteins has been a challenge. Here, we describe use of the droplet-interface bilayer system to follow the movement of proteins across a model membrane separating two submicroliter aqueous droplets. We report the capture and subsequent direct detection of as few as 100 protein molecules that have translocated through anthrax toxin pores. The droplet-interface bilayer system offers new avenues of approach to the study of protein translocation. PMID:21949363

Droplet states in quantum XXZ spin systems on general graphs

NASA Astrophysics Data System (ADS)

Fischbacher, C.; Stolz, G.

2018-05-01

We study XXZ spin systems on general graphs. In particular, we describe the formation of droplet states near the bottom of the spectrum in the Ising phase of the model, where the Z-term dominates the XX-term. As key tools, we use particle number conservation of XXZ systems and symmetric products of graphs with their associated adjacency matrices and Laplacians. Of particular interest to us are strips and multi-dimensional Euclidean lattices, for which we discuss the existence of spectral gaps above the droplet regime. We also prove a Combes-Thomas bound which shows that the eigenstates in the droplet regime are exponentially small perturbations of strict (classical) droplets.

Acoustic Streaming and Microparticle Enrichment within a Microliter Droplet Using a Lamb-Wave Resonator Array

NASA Astrophysics Data System (ADS)

Zhang, Hongxiang; Tang, Zifan; Wang, Zhan; Pan, Shuting; Han, Ziyu; Sun, Chongling; Zhang, Menglun; Duan, Xuexin; Pang, Wei

2018-06-01

We report the nonlinear acoustic streaming effect and the fast manipulation of microparticles by microelectromechanical Lamb-wave resonators in a microliter droplet. The device, consisting of four Lamb-wave resonators on a silicon die, generates cylindrical traveling waves in a liquid and efficiently drives nine horizontal vortices within a 1 -μ l droplet; the performance of the device coincides with the numerical model prediction. Experimentally, the particles are enriched at the stagnation center of the main vortex on the free surface of the droplet in open space without microfluidic channels. In addition, the trajectories of the particles in the droplet can be controlled by the excitation power.

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

Zhang, Jie; He, Yunteng; Kong, Wei, E-mail: wei.kong@oregonstate.edu

We report electron diffraction of ferrocene doped in superfluid helium droplets. By taking advantage of the velocity slip in our pulsed droplet beam using a pulsed electron gun, and by doping with a high concentration of ferrocene delivered via a pulsed valve, we can obtain high quality diffraction images from singly doped droplets. Under the optimal doping conditions, 80% of the droplets sampled in the electron beam are doped with just one ferrocene molecule. Extension of this size selection method to dopant clusters has also been demonstrated. However, incomplete separation of dopant clusters might require deconvolution and modeling of themore » doping process. This method can be used for studies of nucleation processes in superfluid helium droplets.« less

Dynamic Model of Basic Oxygen Steelmaking Process Based on Multi-zone Reaction Kinetics: Model Derivation and Validation

NASA Astrophysics Data System (ADS)

Rout, Bapin Kumar; Brooks, Geoff; Rhamdhani, M. Akbar; Li, Zushu; Schrama, Frank N. H.; Sun, Jianjun

2018-04-01

A multi-zone kinetic model coupled with a dynamic slag generation model was developed for the simulation of hot metal and slag composition during the basic oxygen furnace (BOF) operation. The three reaction zones (i) jet impact zone, (ii) slag-bulk metal zone, (iii) slag-metal-gas emulsion zone were considered for the calculation of overall refining kinetics. In the rate equations, the transient rate parameters were mathematically described as a function of process variables. A micro and macroscopic rate calculation methodology (micro-kinetics and macro-kinetics) were developed to estimate the total refining contributed by the recirculating metal droplets through the slag-metal emulsion zone. The micro-kinetics involves developing the rate equation for individual droplets in the emulsion. The mathematical models for the size distribution of initial droplets, kinetics of simultaneous refining of elements, the residence time in the emulsion, and dynamic interfacial area change were established in the micro-kinetic model. In the macro-kinetics calculation, a droplet generation model was employed and the total amount of refining by emulsion was calculated by summing the refining from the entire population of returning droplets. A dynamic FetO generation model based on oxygen mass balance was developed and coupled with the multi-zone kinetic model. The effect of post-combustion on the evolution of slag and metal composition was investigated. The model was applied to a 200-ton top blowing converter and the simulated value of metal and slag was found to be in good agreement with the measured data. The post-combustion ratio was found to be an important factor in controlling FetO content in the slag and the kinetics of Mn and P in a BOF process.

A smoothed particle hydrodynamics model for droplet and film flow on smooth and rough fracture surfaces

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

Kordilla, Jannes; Tartakovsky, Alexandre M.; Geyer, Tobias

2013-09-01

Flow on fracture surfaces has been identified by many authors as an important flow process in unsaturated fractured rock formations. Given the complexity of flow dynamics on such small scales, robust numerical methods have to be employed in order to capture the highly dynamic interfaces and flow intermittency. In this work we present microscale free-surface flow simulations using a three-dimensional multiphase Smoothed Particle Hydrodynamics (SPH) code. Pairwise solid-fluid and fluid-fluid interaction forces are used to control the wetting behavior and cover a wide range of static and transient contact angles as well as Reynolds numbers encountered in droplet flow onmore » rock surfaces. We validate our model via comparison with existing empirical and semi-analyical solutions for droplet flow. We use the model to investigate the occurence of adsorbed trailing films of droplets under various flow conditions and its importance for the flow dynamics when films and droplets coexist. We show that flow velocities are higher on prewetted surfaces covered by a thin film which is qualitatively attributed to the enhanced dynamic wetting and dewetting at the trailing and advancing contact line.« less

Aspects of Cool-Flame Supported Droplet Combustion in Microgravity

NASA Technical Reports Server (NTRS)

Nayagam, Vedha; Dietrich, Daniel L.; Williams, Forman A.

2015-01-01

Droplet combustion experiments performed on board the International Space Station have shown that normal-alkane fuels with negative temperature coefficient (NTC) chemistry can support quasi-steady, low-temperature combustion without any visible flame. Here we review the results for n-decane, n-heptane, and n-octane droplets burning in carbon dioxidehelium diluted environments at different pressures and initial droplet sizes. Experimental results for cool-flame burning rates, flame standoff ratios, and extinction diameters are compared against simplified theoretical models of the phenomenon. A simplified quasi-steady model based on the partial-burning regime of Lin predicts the burning rate, and flame standoff ratio reasonably well for all three normal alkanes. The second-stage cool-flame burning and extinction following the first-stage hot-flame combustion, however, shows a small dependence on the initial droplet size, thus deviating from the quasi-steady results. An asymptotic model that estimates the oxygen depletion by the hot flame and its influence on cool-flame burning rates is shown to correct the quasi-steady results and provide a better comparison with the measured burning-rate results.This work was supported by the NASA Space Life and Physical Sciences Research and Applications Program and the International Space Station Program.

Coalescence-induced jumping of nanoscale droplets on super-hydrophobic surfaces

NASA Astrophysics Data System (ADS)

Liang, Zhi; Keblinski, Pawel; Nanoscale Science; Engineering Center Team

The coalescence-induced jumping of tens of microns size droplets on super-hydrophobic surfaces has been observed in both experiments and simulations. However, whether the coalescence-induced jumping would occur for smaller, particularly nanoscale droplets, is an open question. Using molecular dynamics simulations, we demonstrate that in spite of the large internal viscous dissipation, coalescence of two nanoscale droplets on a super-hydrophobic surface can result in a jumping of the coalesced droplet from the surface with a speed of a few m/s. Similar to the coalescence-induced jumping of microscale droplets, we observe that the bridge between the coalescing nano-droplets expands and impacts the solid surface, which leads to an acceleration of the coalesced droplet by the pressure force from the solid surface. We observe that the jumping velocity decreases with the droplet size and its ratio to the inertial-capillary velocity is a constant of about 0.126, which is close to the minimum value of 0.111 predicted by continuum-level modeling of Enright et al. [R. Enright, N. Miljkovic, J. Sprittles, K. Nolan, R. Mitchell, and E. N. Wang, ACS Nano 8, 10352 (2014)].

Clustering and entrainment effects on the evaporation of dilute droplets in a turbulent jet

NASA Astrophysics Data System (ADS)

Dalla Barba, Federico; Picano, Francesco

2018-03-01

The evaporation of droplets within turbulent sprays involves unsteady, multiscale, and multiphase processes which make its comprehension and modeling capabilities still limited. The present work aims to investigate the dynamics of droplet vaporization within a turbulent spatial developing jet in dilute, nonreacting conditions. We address the problem considering a turbulent jet laden with acetone droplets and using the direct numerical simulation framework based on a hybrid Eulerian-Lagrangian approach and the point droplet approximation. A detailed statistical analysis of both phases is presented. In particular, we show how crucial is the preferential sampling of the vapor phase induced by the inhomogeneous localization of the droplets through the flow. Strong droplet preferential segregation develops suddenly downstream from the inflow section both within the turbulent core and the jet mixing layer. Two distinct mechanisms have been found to drive this phenomenon: the inertial small-scale clustering in the jet core and the intermittent dynamics of droplets across the turbulent-nonturbulent interface in the mixing layer, where dry air entrainment occurs. These phenomenologies strongly affect the overall vaporization process and lead to an impressive widening of the droplet size and vaporization rate distributions in the downstream evolution of the turbulent spray.

Imparting Icephobicity with Substrate Flexibility

NASA Astrophysics Data System (ADS)

Schutzius, Thomas; Vasileiou, Thomas; Poulikakos, Dimos

2017-11-01

Ice accumulation poses serious safety and performance issues for modern infrastructure. Rationally designed superhydrophobic surfaces have demonstrated potential as a passive means to mitigate ice accretion; however, further studies on solutions that reduce impalement and contact time for impacting supercooled droplets are urgently needed. Here we demonstrate the collaborative effect of substrate flexibility and surface texture on enhancing icephobicity and repelling viscous droplets. We first investigate the influence of increased viscosity on impalement resistance and droplet-substrate contact time. Then we examine the effect of droplet partial solidification on recoil by impacting supercooled water droplets onto surfaces containing ice nucleation promoters. We demonstrate a passive method for shedding partially solidified droplets that does not rely on the classic recoil mechanism. Using an energy-based model, we identify a previously unexplored mechanism whereby the substrate oscillation governs the rebound process by efficiently absorbing the droplet kinetic energy and rectifying it back, allowing for droplet recoil. This mechanism applies for a range of droplet viscosities and ice slurries, which do not rebound from rigid superhydrophobic substrates. Partial support of the Swiss National Science Foundation under Grant No. 162565 and the European Research Council under Advanced Grant No. 669908 (INTICE) is acknowledged.

Study of the Fine-Scale Structure of Cumulus Clouds.

NASA Astrophysics Data System (ADS)

Rodi, Alfred R.

Small cumulus clouds are studied using data from an instrumented aircraft. Two aspects of the role of turbulence and mixing in these couds are examined: (1) the effect of mixing on the droplet size distribution, and (2) the effect of turbulence on the spread of ice crystal plumes artificially generated with cloud seeding agents. The data were collected in the course of the Bureau of Reclamation's High Plains Cooperative Experiment (HIPLEX) in Montana in the summers of 1978-80 by the University of Wyoming King Air aircraft. The shape of the cloud droplet spectrum as measured by the Particle Measuring Systems (PMS) Forward Scattering Spectrometer Probe (FSSP) is found to be very sensitive to entrainment of dry environmental air into the cloud. The narrowest cloud droplet spectra, the highest droplet concentrations, and the largest sized droplets are found in the cloud parcels which are least affected by entrainment. The most dilute regions of cloud exhibit the broadest spectra which are frequently bimodal. A procedure for measuring cloud inhomogeneity from FSSP is developed. The data shows that the clouds are extremely inhomogeneous in structure. Current models of inhomogeneous mixing are shown to be inadequate in explaining droplet spectrum effects. However, the inhomogeneous models characterize the data far better than classical models of droplet spectrum evolution. High resolution measurements of ice crystals from the PMS two dimensional imaging probe are used to characterize the spread of the ice crystal plume in seeded clouds. Plume spread is found to be a very complicated process which is in some cases dominated by organized motions in the cloud. As a result, classical diffusion theory is often inadequate to predict plume growth. The turbulent diffusion that occurs is shown to be best modeled using the relative diffusion concept of Richardson. Procedures for adapting aircraft data to the relative diffusion model are developed, including techniques for converting the aircraft Eulerian data into estimates of Lagrangian correlations. Predictions of the model are compared with observations of plume growth. A detailed analysis of errors in the air motion sensing system on the aircraft is presented. A procedure is developed to estimate the errors due to aircraft gyroscope sensitivity to horizontal accelerations.

Movement of liquid droplets containing polymers on substrate

NASA Astrophysics Data System (ADS)

Hu, Guohui; Wang, Heng

2016-11-01

It is of both fundamental and practical interests to study the flow physics in the manipulation of droplets. As a microreactor, the macromolecules or particles inside the droplets might have significant influences on their movement. In the present study, the many-body dissipative particle dynamics (MDPD) is utilized to investigate the translocation of droplets containing polymer on a substrate driven by the wettability gradient, where the polymer is modelled as worm-like chain (WLC). The internal flows of the droplets are analyzed, as well as the comparison to the polymer-free moving droplets. The effects of physical parameters, such as the interaction potential between liquid particle and polymer beads, the mass of the beads, on the translocation speed are also addressed in the present study. These results might be helpful to the optimization in design of the microfluidic systems.

The Liquid Droplet Radiator - an Ultralightweight Heat Rejection System for Efficient Energy Conversion in Space

NASA Technical Reports Server (NTRS)

Mattick, A. T.; Hertzberg, A.

1984-01-01

A heat rejection system for space is described which uses a recirculating free stream of liquid droplets in place of a solid surface to radiate waste heat. By using sufficiently small droplets ( 100 micron diameter) of low vapor pressure liquids the radiating droplet sheet can be made many times lighter than the lightest solid surface radiators (heat pipes). The liquid droplet radiator (LDR) is less vulnerable to damage by micrometeoroids than solid surface radiators, and may be transported into space far more efficiently. Analyses are presented of LDR applications in thermal and photovoltaic energy conversion which indicate that fluid handling components (droplet generator, droplet collector, heat exchanger, and pump) may comprise most of the radiator system mass. Even the unoptimized models employed yield LDR system masses less than heat pipe radiator system masses, and significant improvement is expected using design approaches that incorporate fluid handling components more efficiently. Technical problems (e.g., spacecraft contamination and electrostatic deflection of droplets) unique to this method of heat rejectioon are discussed and solutions are suggested.

The liquid droplet radiator - An ultralightweight heat rejection system for efficient energy conversion in space

NASA Technical Reports Server (NTRS)

Mattick, A. T.; Hertzberg, A.

1981-01-01

A heat rejection system for space is described which uses a recirculating free stream of liquid droplets in place of a solid surface to radiate waste heat. By using sufficiently small droplets (less than about 100 micron diameter) of low vapor pressure liquids (tin, tin-lead-bismuth eutectics, vacuum oils) the radiating droplet sheet can be made many times lighter than the lightest solid surface radiators (heat pipes). The liquid droplet radiator (LDR) is less vulnerable to damage by micrometeoroids than solid surface radiators, and may be transported into space far more efficiently. Analyses are presented of LDR applications in thermal and photovoltaic energy conversion which indicate that fluid handling components (droplet generator, droplet collector, heat exchanger, and pump) may comprise most of the radiator system mass. Even the unoptimized models employed yield LDR system masses less than heat pipe radiator system masses, and significant improvement is expected using design approaches that incorporate fluid handling components more efficiently. Technical problems (e.g., spacecraft contamination and electrostatic deflection of droplets) unique to this method of heat rejection are discussed and solutions are suggested.

Measurements of droplet size in shear-driven atomization using ultra-small angle x-ray scattering

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

Kastengren, A.; Ilavsky, J.; Viera, Juan Pablo

Measurements of droplet size in optically-thick, non-evaporating, shear-driven sprays have been made using ultra-small angle x-ray scattering (USAXS). The sprays are produced by orifice-type nozzles coupled to diesel injectors, with measurements conducted from 1 – 24 mm from the orifice, spanning from the optically-dense near-nozzle region to more dilute regions where optical diagnostics are feasible. The influence of nozzle diameter, liquid injection pressure, and ambient density were examined. The USAXS measurements reveal few if any nanoscale droplets, in conflict with a popular computational model of diesel spray breakup. The average droplet diameter rapidly decreases with downstream distance from the nozzlemore » until a plateau value is reached, after which only small changes are seen in droplet diameter. This plateau droplet size is consistent with the droplets being small enough to be stable with respect to further breakup. As a result, liquid injection pressure and nozzle diameter have the biggest impact on droplet size, while ambient density has a smaller effect.« less

Measurements of droplet size in shear-driven atomization using ultra-small angle x-ray scattering

DOE PAGES

Kastengren, A.; Ilavsky, J.; Viera, Juan Pablo; ...

2017-03-16

Measurements of droplet size in optically-thick, non-evaporating, shear-driven sprays have been made using ultra-small angle x-ray scattering (USAXS). The sprays are produced by orifice-type nozzles coupled to diesel injectors, with measurements conducted from 1 – 24 mm from the orifice, spanning from the optically-dense near-nozzle region to more dilute regions where optical diagnostics are feasible. The influence of nozzle diameter, liquid injection pressure, and ambient density were examined. The USAXS measurements reveal few if any nanoscale droplets, in conflict with a popular computational model of diesel spray breakup. The average droplet diameter rapidly decreases with downstream distance from the nozzlemore » until a plateau value is reached, after which only small changes are seen in droplet diameter. This plateau droplet size is consistent with the droplets being small enough to be stable with respect to further breakup. As a result, liquid injection pressure and nozzle diameter have the biggest impact on droplet size, while ambient density has a smaller effect.« less

Modeling of Fume Formation from Shielded Metal Arc Welding Process

NASA Astrophysics Data System (ADS)

Sivapirakasam, S. P.; Mohan, Sreejith; Santhosh Kumar, M. C.; Surianarayanan, M.

2017-04-01

In this study, a semi-empirical model of fume formation rate (FFR) from a shielded metal arc welding (SMAW) process has been developed. The model was developed for a DC electrode positive (DCEP) operation and involves the calculations of droplet temperature, surface area of the droplet, and partial vapor pressures of the constituents of the droplet to predict the FFR. The model was further extended for predicting FFR from nano-coated electrodes. The model estimates the FFR for Fe and Mn assuming constant proportion of other elements in the electrode. Fe FFR was overestimated, while Mn FFR was underestimated. The contribution of spatters and other mechanism in the arc responsible for fume formation were neglected. A good positive correlation was obtained between the predicted and experimental FFR values which highlighted the usefulness of the model.

Lattice Boltzmann modeling of contact angle and its hysteresis in two-phase flow with large viscosity difference.

PubMed

Liu, Haihu; Ju, Yaping; Wang, Ningning; Xi, Guang; Zhang, Yonghao

2015-09-01

Contact angle hysteresis is an important physical phenomenon omnipresent in nature and various industrial processes, but its effects are not considered in many existing multiphase flow simulations due to modeling complexity. In this work, a multiphase lattice Boltzmann method (LBM) is developed to simulate the contact-line dynamics with consideration of the contact angle hysteresis for a broad range of kinematic viscosity ratios. In this method, the immiscible two-phase flow is described by a color-fluid model, in which the multiple-relaxation-time collision operator is adopted to increase numerical stability and suppress unphysical spurious currents at the contact line. The contact angle hysteresis is introduced using the strategy proposed by Ding and Spelt [Ding and Spelt, J. Fluid Mech. 599, 341 (2008)JFLSA70022-112010.1017/S0022112008000190], and the geometrical wetting boundary condition is enforced to obtain the desired contact angle. This method is first validated by simulations of static contact angle and dynamic capillary intrusion process on ideal (smooth) surfaces. It is then used to simulate the dynamic behavior of a droplet on a nonideal (inhomogeneous) surface subject to a simple shear flow. When the droplet remains pinned on the surface due to hysteresis, the steady interface shapes of the droplet quantitatively agree well with the previous numerical results. Four typical motion modes of contact points, as observed in a recent study, are qualitatively reproduced with varying advancing and receding contact angles. The viscosity ratio is found to have a notable impact on the droplet deformation, breakup, and hysteresis behavior. Finally, this method is extended to simulate the droplet breakup in a microfluidic T junction, with one half of the wall surface ideal and the other half nonideal. Due to the contact angle hysteresis, the droplet asymmetrically breaks up into two daughter droplets with the smaller one in the nonideal branch channel, and the behavior of daughter droplets is significantly different in both branch channels. Also, it is found that the contact angle hysteresis is strengthened with decreasing the viscosity ratio, leading to an earlier droplet breakup and a decrease in the maximum length that the droplet can reach before the breakup. These simulation results manifest that the present multiphase LBM can be a useful substitute to Ba et al. [Phys. Rev. E 88, 043306 (2013)PLEEE81539-375510.1103/PhysRevE.88.043306] for modeling the contact angle hysteresis, and it can be easily implemented with higher computational efficiency.

Simulation of drift of pesticides: development and validation of a model.

PubMed

Brusselman, E; Spanoghe, P; Van der Meeren, P; Gabriels, D; Steurbaut, W

2003-01-01

Over the last decade drift of pesticides has been recognized as a major problem for the environment. High fractions of pesticides can be transported through the air and deposited in neighbouring ecosystems during and after application. A new computer-two steps-drift model is developed: FYDRIMO or F(ph)Ysical DRift MOdel. In the first step the droplet size spectrum of a nozzle is analysed. In this way the volume percentage of droplets with a certain size is known. In the second step the model results in a prediction of deposition of each droplet with a certain size. This second part of the model runs in MATLAB and is grounded on a combination of two physical factors: gravity force and friction forces. In this stage of development corrections are included for evaporation and wind force following a certain measured wind profile. For validation wind tunnel experiments were performed. Salt solutions were sprayed at two wind velocities and variable distance above the floor. Small gutters in the floor filled with filter paper were used to collect the sprayed droplets. After analysing and comparing the wind tunnel results with the model predictions, FYDRIMO seems to have good predicting capacities.

The dynamics of milk droplet-droplet collisions

NASA Astrophysics Data System (ADS)

Finotello, Giulia; Kooiman, Roeland F.; Padding, Johan T.; Buist, Kay A.; Jongsma, Alfred; Innings, Fredrik; Kuipers, J. A. M.

2018-01-01

Spray drying is an important industrial process to produce powdered milk, in which concentrated milk is atomized into small droplets and dried with hot gas. The characteristics of the produced milk powder are largely affected by agglomeration, combination of dry and partially dry particles, which in turn depends on the outcome of a collision between droplets. The high total solids (TS) content and the presence of milk proteins cause a relatively high viscosity of the fed milk concentrates, which is expected to largely influence the collision outcomes of drops inside the spray. It is therefore of paramount importance to predict and control the outcomes of binary droplet collisions. Only a few studies report on droplet collisions of high viscous liquids and no work is available on droplet collisions of milk concentrates. The current study therefore aims to obtain insight into the effect of viscosity on the outcome of binary collisions between droplets of milk concentrates. To cover a wide range of viscosity values, three milk concentrates (20, 30 and 46% TS content) are investigated. An experimental set-up is used to generate two colliding droplet streams with consistent droplet size and spacing. A high-speed camera is used to record the trajectories of the droplets. The recordings are processed by Droplet Image Analysis in MATLAB to determine the relative velocities and the impact geometries for each individual collision. The collision outcomes are presented in a regime map dependent on the dimensionless impact parameter and Weber ( We) number. The Ohnesorge ( Oh) number is introduced to describe the effect of viscosity from one liquid to another and is maintained constant for each regime map by using a constant droplet diameter ( d ˜ 700 μ m). In this work, a phenomenological model is proposed to describe the boundaries demarcating the coalescence-separation regimes. The collision dynamics and outcome of milk concentrates are compared with aqueous glycerol solutions experiments. While milk concentrates have complex chemical composition and rheology, glycerol solutions are Newtonian fluids and therefore easy to characterize. The collision morphologies of glycerol solutions and milk concentrates are similar, and the regime maps can be described by the same phenomenological model developed in this work. The regime of bouncing, however, was not observed for any of the milk concentrates.

Analysis of isothermal and cooling-rate-dependent immersion freezing by a unifying stochastic ice nucleation model

NASA Astrophysics Data System (ADS)

Alpert, Peter A.; Knopf, Daniel A.

2016-02-01

Immersion freezing is an important ice nucleation pathway involved in the formation of cirrus and mixed-phase clouds. Laboratory immersion freezing experiments are necessary to determine the range in temperature, T, and relative humidity, RH, at which ice nucleation occurs and to quantify the associated nucleation kinetics. Typically, isothermal (applying a constant temperature) and cooling-rate-dependent immersion freezing experiments are conducted. In these experiments it is usually assumed that the droplets containing ice nucleating particles (INPs) all have the same INP surface area (ISA); however, the validity of this assumption or the impact it may have on analysis and interpretation of the experimental data is rarely questioned. Descriptions of ice active sites and variability of contact angles have been successfully formulated to describe ice nucleation experimental data in previous research; however, we consider the ability of a stochastic freezing model founded on classical nucleation theory to reproduce previous results and to explain experimental uncertainties and data scatter. A stochastic immersion freezing model based on first principles of statistics is presented, which accounts for variable ISA per droplet and uses parameters including the total number of droplets, Ntot, and the heterogeneous ice nucleation rate coefficient, Jhet(T). This model is applied to address if (i) a time and ISA-dependent stochastic immersion freezing process can explain laboratory immersion freezing data for different experimental methods and (ii) the assumption that all droplets contain identical ISA is a valid conjecture with subsequent consequences for analysis and interpretation of immersion freezing. The simple stochastic model can reproduce the observed time and surface area dependence in immersion freezing experiments for a variety of methods such as: droplets on a cold-stage exposed to air or surrounded by an oil matrix, wind and acoustically levitated droplets, droplets in a continuous-flow diffusion chamber (CFDC), the Leipzig aerosol cloud interaction simulator (LACIS), and the aerosol interaction and dynamics in the atmosphere (AIDA) cloud chamber. Observed time-dependent isothermal frozen fractions exhibiting non-exponential behavior can be readily explained by this model considering varying ISA. An apparent cooling-rate dependence of Jhet is explained by assuming identical ISA in each droplet. When accounting for ISA variability, the cooling-rate dependence of ice nucleation kinetics vanishes as expected from classical nucleation theory. The model simulations allow for a quantitative experimental uncertainty analysis for parameters Ntot, T, RH, and the ISA variability. The implications of our results for experimental analysis and interpretation of the immersion freezing process are discussed.

Lattice Boltzmann simulations of multiple-droplet interaction dynamics.

PubMed

Zhou, Wenchao; Loney, Drew; Fedorov, Andrei G; Degertekin, F Levent; Rosen, David W

2014-03-01

A lattice Boltzmann (LB) formulation, which is consistent with the phase-field model for two-phase incompressible fluid, is proposed to model the interface dynamics of droplet impingement. The interparticle force is derived by comparing the macroscopic transport equations recovered from LB equations with the governing equations of the continuous phase-field model. The inconsistency between the existing LB implementations and the phase-field model in calculating the relaxation time at the phase interface is identified and an approximation is proposed to ensure the consistency with the phase-field model. It is also shown that the commonly used equilibrium velocity boundary for the binary fluid LB scheme does not conserve momentum at the wall boundary and a modified scheme is developed to ensure the momentum conservation at the boundary. In addition, a geometric formulation of the wetting boundary condition is proposed to replace the popular surface energy formulation and results show that the geometric approach enforces the prescribed contact angle better than the surface energy formulation in both static and dynamic wetting. The proposed LB formulation is applied to simulating droplet impingement dynamics in three dimensions and results are compared to those obtained with the continuous phase-field model, the LB simulations reported in the literature, and experimental data from the literature. The results show that the proposed LB simulation approach yields not only a significant speed improvement over the phase-field model in simulating droplet impingement dynamics on a submillimeter length scale, but also better accuracy than both the phase-field model and the previously reported LB techniques when compared to experimental data. Upon validation, the proposed LB modeling methodology is applied to the study of multiple-droplet impingement and interactions in three dimensions, which demonstrates its powerful capability of simulating extremely complex interface phenomena.

A Novel Tool for Simulating Aerosol-cloud Interactions with a Sectional Model Implemented to a Large-Eddy Simulator

NASA Astrophysics Data System (ADS)

Tonttila, J.; Romakkaniemi, S.; Kokkola, H.; Maalick, Z.; Korhonen, H.; Liqing, H.

2015-12-01

A new cloud-resolving model setup for studying aerosol-cloud interactions, with a special emphasis on partitioning and wet deposition of semi-volatile aerosol species, is presented. The model is based on modified versions of two well-established model components: the Large-Eddy Simulator (LES) UCLALES, and the sectional aerosol model SALSA, previously employed in the ECHAM climate model family. Implementation of the UCLALES-SALSA is described in detail. As the basis for this work, SALSA has been extended to include a sectional representation of the size distributions of cloud droplets and precipitation. Microphysical processes operating on clouds and precipitation have also been added. Given our main motivation, the cloud droplet size bins are defined according to the dry particle diameter. The droplet wet diameter is solved dynamically through condensation equations, but represents an average droplet diameter inside each size bin. This approach allows for accurate tracking of the aerosol properties inside clouds, but minimizes the computational cost. Since the actual cloud droplet diameter is not fully resolved inside the size bins, processes such as precipitation formation rely on parameterizations. For realistic growth of drizzle drops to rain, which is critical for the aerosol wet deposition, the precipitation size bins are defined according to the actual drop size. With these additions, the implementation of the SALSA model replaces most of the microphysical and thermodynamical components within the LES. The cloud properties and aerosol-cloud interactions simulated by the model are analysed and evaluated against detailed cloud microphysical boxmodel results and in-situ aerosol-cloud interaction observations from the Puijo measurement station in Kuopio, Finland. The ability of the model to reproduce the impacts of wet deposition on the aerosol population is demonstrated.

A zero-gravity demonstration of the collision and coalescence of water droplets

NASA Technical Reports Server (NTRS)

Hung, R. J.; Vaughan, O. H.; Smith, R. E.

1974-01-01

The mechanics of the collision and coalescence of liquid droplets is one of the main research areas in the fields of nuclear physics, astrophysics, meteorology and fluid mechanics. The crew members on the Skylab 3 and 4 missions were requested to perform demonstrations of the collision and coalescence of water droplets under the low gravity environment at orbital altitude. In Skylab 4 two water droplets with equal volumes, 30 cu cm each, were used. A dark colored droplet (contaminated with grape drink) moving with a velocity of 3.14 cm/sec collided with a stationary pink colored droplet (contaminated with strawberry drink) and coalescence occurred. Theoretical models are proposed to study the various stages of the collision-coalescence processes. Special considerations are concentrated in the investigation of the bounce-coalescence and coalescence-instability processes. The surface tension of the coalesced droplets was calculated to be 52 dynes/cm in perfect agreement with laboratory measurements made after the flight using a reproduction of the liquids.

Co-Compartmentation of Terpene Biosynthesis and Storage via Synthetic Droplet

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

Zhao, Cheng; Kim, YongKyoung; Zeng, Yining

Traditional bioproduct engineering focuses on pathway optimization, yet is often complicated by product inhibition, downstream consumption, and the toxicity of certain products. Here, we present the co-compartmentation of biosynthesis and storage via a synthetic droplet as an effective new strategy to improve the bioproduct yield, with squalene as a model compound. A hydrophobic protein was designed and introduced into the tobacco chloroplast to generate a synthetic droplet for terpene storage. Simultaneously, squalene biosynthesis enzymes were introduced to chloroplasts together with the droplet-forming protein to co-compartmentalize the biosynthesis and storage of squalene. The strategy has enabled a record yield of squalenemore » at 2.6 mg/g fresh weight without compromising plant growth. Confocal fluorescent microscopy imaging, stimulated Raman scattering microscopy, and droplet composition analysis confirmed the formation of synthetic storage droplet in chloroplast. The co-compartmentation of synthetic storage droplet with a targeted metabolic pathway engineering represents a new strategy for enhancing bioproduct yield.« less

Example of a Fluid-Phase Change Examined with MD Simulation: Evaporative Cooling of a Nanoscale Droplet.

PubMed

Ao, Takashi; Matsumoto, Mitsuhiro

2017-10-24

We carried out a series of molecular dynamics simulations in order to examine the evaporative cooling of a nanoscale droplet of a Lennard-Jones liquid. After thermally equilibrating a droplet at a temperature T ini /T t ≃ 1.2 (T t is the triple-point temperature), we started the evaporation into vacuum by removing vaporized particles and monitoring the change in droplet size and the temperature inside. As free evaporation proceeds, the droplet reaches a deep supercooled liquid state of T/T t ≃ 0.7. The temperature was found to be uniform in spite of the fast evaporative cooling on the surface. The time evolution of the evaporating droplet properties was satisfactorily explained with a simple one-dimensional phase-change model. After a sufficiently long run, the supercooled droplet was crystallized into a polycrystalline fcc structure. The crystallization is a stochastic nucleation process. The time and the temperature of inception were evaluated over 42 samples, which indicate the existence of a stability limit.

Growth and assembly of cobalt oxide nanoparticle rings at liquid nanodroplets with solid junction.

PubMed

Zhou, Yilong; Powers, Alexander S; Zhang, Xiaowei; Xu, Tao; Bustillo, Karen; Sun, Litao; Zheng, Haimei

2017-09-28

Using liquid cell TEM, we imaged the formation of CoO nanoparticle rings. Nanoparticles nucleated and grew tracing the perimeter of droplets sitting on the SiN x solid substrate, and finally formed necklace-like rings. By tracking single nanoparticle trajectories during the ring formation and an estimation of the forces between droplets and nanoparticles using a simplified model, we found the junction of liquid nanodroplets with a solid substrate is the attractive site for CoO nanoparticles. Coalescing droplets were capable of pushing nanoparticles to the perimeter of the new droplet and nanoparticles on top of the droplets rolled off toward the perimeter. We propose that the curved surface morphology of the droplets created a force gradient that contributed to the assembly of nanoparticles at the droplet perimeter. Revealing the dynamics of nanoparticle movements and the interactions of nanoparticles with the liquid nanodroplet provides insights on developing novel self-assembly strategies for building precisely defined nanostructures on solid substrates.

Numerical analysis of deposition frequency for successive droplets coalescence dynamics

NASA Astrophysics Data System (ADS)

Cheng, Xiaoding; Zhu, Yunlong; Zhang, Lei; Zhang, Dingyi; Ku, Tao

2018-04-01

A pseudopotential based multi-relaxation-time lattice Boltzmann model is employed to investigate the dynamic behaviors of successive droplets' impact and coalescence on a solid surface. The effects of deposition frequency on the morphology of the formed line are investigated with a zero receding contact angle by analyzing the droplet-to-droplet coalescence dynamics. Two collision modes (in-phase mode and out-of-phase mode) between the pre-deposited bead and the subsequent droplet are identified depending on the deposition frequency. A uniform line can be obtained at the optimal droplet spacing in the in-phase mode (Δt* < 1.875). However, a scalloped line pattern is formed in the out-of-phase mode (Δt* > 1.875). It is found that decreasing the droplet spacing or advancing contact angle can improve the smoothness of line in the out-of-phase mode. Furthermore, the effects of deposition frequency on the morphology of the formed lines are validated to be applicable to cases with a finite receding contact angle.

Co-Compartmentation of Terpene Biosynthesis and Storage via Synthetic Droplet

DOE PAGES

Zhao, Cheng; Kim, YongKyoung; Zeng, Yining; ...

2018-02-13

Traditional bioproduct engineering focuses on pathway optimization, yet is often complicated by product inhibition, downstream consumption, and the toxicity of certain products. Here, we present the co-compartmentation of biosynthesis and storage via a synthetic droplet as an effective new strategy to improve the bioproduct yield, with squalene as a model compound. A hydrophobic protein was designed and introduced into the tobacco chloroplast to generate a synthetic droplet for terpene storage. Simultaneously, squalene biosynthesis enzymes were introduced to chloroplasts together with the droplet-forming protein to co-compartmentalize the biosynthesis and storage of squalene. The strategy has enabled a record yield of squalenemore » at 2.6 mg/g fresh weight without compromising plant growth. Confocal fluorescent microscopy imaging, stimulated Raman scattering microscopy, and droplet composition analysis confirmed the formation of synthetic storage droplet in chloroplast. The co-compartmentation of synthetic storage droplet with a targeted metabolic pathway engineering represents a new strategy for enhancing bioproduct yield.« less

Sintering of polydisperse viscous droplets

NASA Astrophysics Data System (ADS)

Wadsworth, Fabian B.; Vasseur, Jérémie; Llewellin, Edward W.; Dingwell, Donald B.

2017-03-01

Sintering—or coalescence—of compacts of viscous droplets is driven by the interfacial tension between the droplets and the interstitial gas phase. The process, which occurs in a range of industrial and natural settings, such as the manufacture of ceramics and the welding of volcanic ash, causes the compact to densify, to become stronger, and to become less permeable. We investigate the role of droplet polydispersivity in sintering dynamics by conducting experiments in which populations of glass spheres with different size distributions are heated to temperatures above the glass transition interval. We quantify the progress of sintering by tracking changes in porosity with time. The sintering dynamics is modeled by treating the system as a random distribution of interstitial gas bubbles shrinking under the action of interfacial tension only. We identify the scaling between the polydispersivity of the initial droplets and the dynamics of bulk densification. The framework that we develop allows the sintering dynamics of arbitrary polydisperse populations of droplets to be predicted if the initial droplet (or particle) size distribution is known.

Mechanism of Supercooled Water Droplet Breakup near the Leading Edge of an Airfoil

NASA Technical Reports Server (NTRS)

Veras-Alba, Belen; Palacios, Jose; Vargas, Mario; Ruggeri, Charles; Bartkus, Tadas P.

2017-01-01

This work presents the results of an experimental study on supercooled droplet deformation and breakup near the leading edge of an airfoil. The results are compared to prior room temperature droplet deformation results to explore the effects of droplet supercooling. The experiments were conducted in the Adverse Environment Rotor Test Stand (AERTS) at The Pennsylvania State University. An airfoil model placed at the end of the rotor blades mounted onto the hub in the AERTS chamber was moved at speeds ranging between 50 and 80 m/sec. The temperature of the chamber was set at -20°C. A monotonic droplet generator was used to produce droplets that fell from above, perpendicular to the path of the airfoil. The supercooled state of the droplets was determined by measurement of the temperature of the drops at various locations below the droplet generator exit. A temperature prediction code was also used to estimate the temperature of the droplets based on vertical velocity and the distance traveled by droplets from the droplet generator to the airfoil stagnation line. High speed imaging was employed to observe the interaction between the droplets and the airfoil. The high speed imaging provided droplet deformation information as the droplet approached the airfoil near the stagnation line. A tracking software program was used to measure the horizontal and vertical displacement of the droplet against time. It was demonstrated that to compare the effects of water supercooling on droplet deformation, the ratio of the slip velocity and the initial droplet velocity must be equal. A case with equal slip velocity to initial velocity ratios was selected for room temperature and supercooled droplet conditions. The airfoil velocity was 60 m/s and the slip velocity for both sets of data was 40 m/s. In these cases, the deformation of the weakly supercooled and warm droplets did not present different trends. The similar behavior for both environmental conditions indicates that water supercooling has no effect on particle deformation for the limited range of the weak supercooling of the droplets tested and the selected impact velocity. The assumption of a constant surface tension value was further supported by the equal trend of the Bond number obtained for supercooled and room temperature droplets.

Ice Accretion Modeling using an Eulerian Approach for Droplet Impingement

NASA Technical Reports Server (NTRS)

Kim, Joe Woong; Garza, Dennis P.; Sankar, Lakshmi N.; Kreeger, Richard E.

2012-01-01

A three-dimensional Eulerian analysis has been developed for modeling droplet impingement on lifting bodes. The Eulerian model solves the conservation equations of mass and momentum to obtain the droplet flow field properties on the same mesh used in CFD simulations. For complex configurations such as a full rotorcraft, the Eulerian approach is more efficient because the Lagrangian approach would require a significant amount of seeding for accurate estimates of collection efficiency. Simulations are done for various benchmark cases such as NACA0012 airfoil, MS317 airfoil and oscillating SC2110 airfoil to illustrate its use. The present results are compared with results from the Lagrangian approach used in an industry standard analysis called LEWICE.

Satellite Remote Sensing of the Liquid Water Sensitivity in Water Clouds

NASA Technical Reports Server (NTRS)

Han, Qing-Yuan; Rossow, William B.; Welch, Ronald; Zeng, Jane; Jansen, James E. (Technical Monitor)

2001-01-01

In estimation of the aerosol indirect effect, cloud liquid water path is considered either constant (Twomey effect) or increasing with enhanced droplet number concentrations (drizzle-suppression effect, or Albrecht effect) if cloud microphysics is the prevailing mechanism during the aerosol-cloud interactions. On the other hand, if cloud thermodynamics and dynamics are considered, the cloud liquid water path may be decreased with increasing droplet number concentration, which is predicted by model calculations and observed in ship-track and urban influence studies. This study is to examine the different responses of cloud liquid water path to changes of cloud droplet number concentration. Satellite data (January, April, July and October 1987) are used to retrieve the cloud liquid water sensitivity, defined as the changes of liquid water path versus changes of column droplet number concentrations. The results of a global survey reveal that 1) in at least one third of the cases the cloud liquid water sensitivity is negative, and the regional and seasonal variations of the negative liquid water sensitivity are consistent with other observations; 2) cloud droplet sizes are always inversely proportional to column droplet number concentrations. Our results suggest that an increase of cloud droplet number concentration leads to reduced cloud droplet size and enhanced evaporation, which weakens the coupling between water clouds and boundary layer in warm zones, decreases water supply from surface and desiccates cloud liquid water. Our results also suggest that the current evaluations of negative aerosol indirect forcing by global climate models (GCM), which are based on Twomey effect or Albrecht effect, may be overestimated.

Potential of lattice Boltzmann to model droplets on chemically stripe-patterned substrates

NASA Astrophysics Data System (ADS)

Patrick Jansen, H.; Sotthewes, K.; Zandvliet, Harold J. W.; Kooij, E. Stefan

2016-01-01

Lattice Boltzmann modelling (LBM) has recently been applied to a range of different wetting situations. Here we demonstrate its potential in representing complex kinetic effects encountered in droplets on chemically stripe-patterned surfaces. An ultimate example of the power of LBM is provided by comparing simulations and experiments of impacting droplets with varying Weber numbers. Also, the shape evolution of droplets is discussed in relation to their final shape. The latter can then be compared to Surface Evolver (SE) results, since under the proper boundary conditions both approaches should yield the same configuration in a static state. During droplet growth in LBM simulations, achieved by increasing the density within the droplet, the contact line initially advances in the direction parallel to the stripes, therewith increasing its aspect ratio. Once the volume becomes too large the droplet starts wetting additional stripes, leading to a lower aspect ratio. The maximum aspect ratio is shown to be a function of the width ratio of the hydrophobic and hydrophilic stripes and also their absolute widths. In the limit of sufficiently large stripe widths the aspect ratio is solely dependent on the relative stripe widths. The maximum droplet aspect ratio in the LBM simulations is compared to SE simulations and results are shown to be in good agreement. Additionally, we also show the ability of LBM to investigate single stripe wetting, enabling determination of the maximum aspect ratio that can be achieved in the limit of negligible hydrophobic stripe width, under the constraint that the stripe widths are large enough such that they are not easily crossed.

Electrohydrodynamic coalescence of droplets using an embedded potential flow model

NASA Astrophysics Data System (ADS)

Garzon, M.; Gray, L. J.; Sethian, J. A.

2018-03-01

The coalescence, and subsequent satellite formation, of two inviscid droplets is studied numerically. The initial drops are taken to be of equal and different sizes, and simulations have been carried out with and without the presence of an electrical field. The main computational challenge is the tracking of a free surface that changes topology. Coupling level set and boundary integral methods with an embedded potential flow model, we seamlessly compute through these singular events. As a consequence, the various coalescence modes that appear depending upon the relative ratio of the parent droplets can be studied. Computations of first stage pinch-off, second stage pinch-off, and complete engulfment are analyzed and compared to recent numerical studies and laboratory experiments. Specifically, we study the evolution of bridge radii and the related scaling laws, the minimum drop radii evolution from coalescence to satellite pinch-off, satellite sizes, and the upward stretching of the near cylindrical protrusion at the droplet top. Clear evidence of partial coalescence self-similarity is presented for parent droplet ratios between 1.66 and 4. This has been possible due to the fact that computational initial conditions only depend upon the mother droplet size, in contrast with laboratory experiments where the difficulty in establishing the same initial physical configuration is well known. The presence of electric forces changes the coalescence patterns, and it is possible to control the satellite droplet size by tuning the electrical field intensity. All of the numerical results are in very good agreement with recent laboratory experiments for water droplet coalescence.

Surface tension prevails over solute effect in organic-influenced cloud droplet activation.

PubMed

Ovadnevaite, Jurgita; Zuend, Andreas; Laaksonen, Ari; Sanchez, Kevin J; Roberts, Greg; Ceburnis, Darius; Decesari, Stefano; Rinaldi, Matteo; Hodas, Natasha; Facchini, Maria Cristina; Seinfeld, John H; O' Dowd, Colin

2017-06-29

The spontaneous growth of cloud condensation nuclei (CCN) into cloud droplets under supersaturated water vapour conditions is described by classic Köhler theory. This spontaneous activation of CCN depends on the interplay between the Raoult effect, whereby activation potential increases with decreasing water activity or increasing solute concentration, and the Kelvin effect, whereby activation potential decreases with decreasing droplet size or increases with decreasing surface tension, which is sensitive to surfactants. Surface tension lowering caused by organic surfactants, which diminishes the Kelvin effect, is expected to be negated by a concomitant reduction in the Raoult effect, driven by the displacement of surfactant molecules from the droplet bulk to the droplet-vapour interface. Here we present observational and theoretical evidence illustrating that, in ambient air, surface tension lowering can prevail over the reduction in the Raoult effect, leading to substantial increases in cloud droplet concentrations. We suggest that consideration of liquid-liquid phase separation, leading to complete or partial engulfing of a hygroscopic particle core by a hydrophobic organic-rich phase, can explain the lack of concomitant reduction of the Raoult effect, while maintaining substantial lowering of surface tension, even for partial surface coverage. Apart from the importance of particle size and composition in droplet activation, we show by observation and modelling that incorporation of phase-separation effects into activation thermodynamics can lead to a CCN number concentration that is up to ten times what is predicted by climate models, changing the properties of clouds. An adequate representation of the CCN activation process is essential to the prediction of clouds in climate models, and given the effect of clouds on the Earth's energy balance, improved prediction of aerosol-cloud-climate interactions is likely to result in improved assessments of future climate change.

Ignition and combustion of metallized propellants

NASA Technical Reports Server (NTRS)

Turns, Stephen R.

1991-01-01

The overall objective is the development of a fundamental understanding of the ignition and combustion of aluminum-based slurry (or gel) propellant droplets using a combination of experiment and analysis. Specific objectives are the following: (1) The development and application of a burner/spray rig and single particle optical diagnosis to study the detailed ignition and combustion behavior of small droplets; (2) Understanding the role of surfactants and gellants (or other additives) in promoting or inhibiting secondary atomization of propellant droplets; and (3) The extension of previously developed analytical models and the development of new models to address the phenomena associated with microexplosions (secondary atomization).

Bimodal SLD Ice Accretion on a NACA 0012 Airfoil Model

NASA Technical Reports Server (NTRS)

Potapczuk, Mark; Tsao, Jen-Ching; King-Steen, Laura

2016-01-01

This presentation describes the results of ice accretion measurements on a NACA 0012 airfoil model, from the NASA Icing Research Tunnel, using an icing cloud composed of a bimodal distribution of Supercooled Large Droplets. The data consists of photographs, laser scans of the ice surface, and measurements of the mass of ice for each icing condition. The results of ice shapes accumulated as a result of exposure to an icing cloud with a bimodal droplet distribution were compared to the ice shapes resulting from an equivalent cloud composed of a droplet distribution with a standard bell curve shape.

Modeling the Bergeron-Findeisen Process Using PDF Methods With an Explicit Representation of Mixing

NASA Astrophysics Data System (ADS)

Jeffery, C.; Reisner, J.

2005-12-01

Currently, the accurate prediction of cloud droplet and ice crystal number concentration in cloud resolving, numerical weather prediction and climate models is a formidable challenge. The Bergeron-Findeisen process in which ice crystals grow by vapor deposition at the expense of super-cooled droplets is expected to be inhomogeneous in nature--some droplets will evaporate completely in centimeter-scale filaments of sub-saturated air during turbulent mixing while others remain unchanged [Baker et al., QJRMS, 1980]--and is unresolved at even cloud-resolving scales. Despite the large body of observational evidence in support of the inhomogeneous mixing process affecting cloud droplet number [most recently, Brenguier et al., JAS, 2000], it is poorly understood and has yet to be parameterized and incorporated into a numerical model. In this talk, we investigate the Bergeron-Findeisen process using a new approach based on simulations of the probability density function (PDF) of relative humidity during turbulent mixing. PDF methods offer a key advantage over Eulerian (spatial) models of cloud mixing and evaporation: the low probability (cm-scale) filaments of entrained air are explicitly resolved (in probability space) during the mixing event even though their spatial shape, size and location remain unknown. Our PDF approach reveals the following features of the inhomogeneous mixing process during the isobaric turbulent mixing of two parcels containing super-cooled water and ice, respectively: (1) The scavenging of super-cooled droplets is inhomogeneous in nature; some droplets evaporate completely at early times while others remain unchanged. (2) The degree of total droplet evaporation during the initial mixing period depends linearly on the mixing fractions of the two parcels and logarithmically on Damköhler number (Da)---the ratio of turbulent to evaporative time-scales. (3) Our simulations predict that the PDF of Lagrangian (time-integrated) subsaturation (S) goes as S-1 at high Da. This behavior results from a Gaussian mixing closure and requires observational validation.

Dynamic Roughness Ratio-Based Framework for Modeling Mixed Mode of Droplet Evaporation.

PubMed

Gunjan, Madhu Ranjan; Raj, Rishi

2017-07-18

The spatiotemporal evolution of an evaporating sessile droplet and its effect on lifetime is crucial to various disciplines of science and technology. Although experimental investigations suggest three distinct modes through which a droplet evaporates, namely, the constant contact radius (CCR), the constant contact angle (CCA), and the mixed, only the CCR and the CCA modes have been modeled reasonably. Here we use experiments with water droplets on flat and micropillared silicon substrates to characterize the mixed mode. We visualize that a perfect CCA mode after the initial CCR mode is an idealization on a flat silicon substrate, and the receding contact line undergoes intermittent but recurring pinning (CCR mode) as it encounters fresh contaminants on the surface. The resulting increase in roughness lowers the contact angle of the droplet during these intermittent CCR modes until the next depinning event, followed by the CCA mode of evaporation. The airborne contaminants in our experiments are mostly loosely adhered to the surface and travel along with the receding contact line. The resulting gradual increase in the apparent roughness and hence the extent of CCR mode over CCA mode forces appreciable decrease in the contact angle observed during the mixed mode of evaporation. Unlike loosely adhered airborne contaminants on flat samples, micropillars act as fixed roughness features. The apparent roughness fluctuates about the mean value as the contact line recedes between pillars. Evaporation on these surfaces exhibits stick-jump motion with a short-duration mixed mode toward the end when the droplet size becomes comparable to the pillar spacing. We incorporate this dynamic roughness into a classical evaporation model to accurately predict the droplet evolution throughout the three modes, for both flat and micropillared silicon surfaces. We believe that this framework can also be extended to model the evaporation of nanofluids and the coffee-ring effect, among others.

A test-tube model for rainfall

NASA Astrophysics Data System (ADS)

Wilkinson, Michael

2014-05-01

If the temperature of a cell containing two partially miscible liquids is changed very slowly, so that the miscibility is decreased, microscopic droplets nucleate, grow and migrate to the interface due to their buoyancy. The system may show an approximately periodic variation of the turbidity of the mixture, as the mean droplet size fluctuates. These precipitation events are analogous to rainfall. This paper considers a theoretical model for these experiments. After nucleation the initial growth is by Ostwald ripening, followed by a finite-time runaway growth of droplet sizes due to larger droplets sweeping up smaller ones. The model predicts that the period \\Delta t and the temperature sweep rate ξ are related by \\Delta t\\sim C \\xi^{-3/7} , and is in good agreement with experiments. The coefficient C has a power-law divergence approaching the critical point of the miscibility transition: C\\sim (T-T_{\\text{c}})^{-\\eta} , and the critical exponent η is determined. It is argued that while the mechanism does not provide a quantitative description of terrestrial rainfall, it may be a faithful model for precipitation on other planets.

Kinetic Model of the Initial Stage of the Nanowire Growth

NASA Astrophysics Data System (ADS)

Filimonov, S. N.; Hervieu, Yu. Yu.

2018-03-01

A kinetic model of the formation of pyramid-like bulges (pedestals) at the bases of vertical nanowires is proposed. The formation of the pedestals at the early stage of the nanowire growth is assumed to be induced by a higher nucleation rate of two-dimensional islands under the catalyst droplet, as compared to the nucleation rate at the non-activated surface areas. Kinetics of the nucleation and propagation of the steps in the pyramid is described with a model of the multilayer growth, taking into account that the catalyst droplet at the nanowire top is a strong sink for adatoms. It is shown that the transition from the growth of the pyramid to the axial growth of the nanowire is possible if the appearance of a nucleus of the new layer under the catalyst droplet results in a partial dissolution of the underlying layer. In this case a segment of the nanowire sidewall is formed, preventing the lateral growth of the layers generated by the droplet.

Thermal management of metallic surfaces: evaporation of sessile water droplets on polished and patterned stainless steel

NASA Astrophysics Data System (ADS)

Czerwiec, T.; Tsareva, S.; Andrieux, A.; Bortolini, G. A.; Bolzan, P. H.; Castanet, G.; Gradeck, M.; Marcos, G.

2017-10-01

This communication focus on the evaporation of sessile water droplets on different states of austenitic stainless steel surfaces: mirror polished, mirror polished and aged and patterned by sputtering. The evolution of the contact angle and of the droplet diameter is presented as a function of time at room temperature. For all the surface states, a constant diameter regime (CCR) is observed. An important aging effect on the contact angle is measured on polished surfaces due to atmospheric contamination. The experimental observations are compared to a quasi-static evaporation model assuming spherical caps. The evolution of the droplet volume as a function of time is almost linear with the evaporation time for all the observed surfaces. This is in accordance with the model prediction for the CCR mode for small initial contact angles. In our experiments, the evaporation time is found to be linearly dependent on the initial contact angle. This dependence is not correctly described by the evaporation model

Direct Numerical Simulation of Turbulent Condensation in Clouds

NASA Technical Reports Server (NTRS)

Shariff, K.; Paoli, R.

2004-01-01

In this brief, we investigate the turbulent condensation of a population of droplets by means of a direct numerical simulation. To that end, a coupled Navier-Stokes/Lagrangian solver is used where each particle is tracked and its growth by water vapor condensation is monitored exactly. The main goals of the study are to find out whether turbulence broadens the droplet size distribution, as observed in in situ measurements. The second issue is to understand if and for how long a correlation between the droplet radius and the local supersaturation exists for the purpose of modeling sub-grid scale microphysics in cloud-resolving codes. This brief is organized as follows. In Section 2 the governing equations are presented, including the droplet condensation model. The implementation of the forcing procedure is described in Section 3. The simulation results are presented in Section 4 together with a sketch of a simple stochastic model for turbulent condensation. Conclusions and the main outcomes of the study are given in Section 5.

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

Shi-chune, Y.; Liu, L.

The evaporation, heating, and burning of single coal-water slurry droplets are studied. The coal selected in this study is Pittsburgh Seam number 8 coal which is a medium volatile caking bituminous coal. The droplet is suspended on a microthermocouple and exposed to a hot gas stream. Temperature measurement and microscopic observation are performed in the parametric studies. The duration of water evaporation in CWS droplets decreases with the reduction of the droplet size, increasing of coal weight fraction, and increasing of gas temperature and velocity. The duration of heat-up is always significant due to the agglomeration. The CWS droplets aremore » generally observed to swell like popcorn during heating. A model for the formation of the popped swelling is proposed and discussed.« less

Droplet Translation Actuated by Photoelectrowetting.

PubMed

Palma, Cesar; Deegan, Robert D

2018-03-13

In traditional electrowetting-on-dielectric (EWOD) devices, droplets are moved about a substrate using electric fields produced by an array of discrete electrodes. In this study, we show that a drop can be driven across a substrate with a localized light beam by exploiting the photoelectrowetting (PEW) effect, a light-activated variant of EWOD. Droplet transport actuated by PEW eliminates the need for electrode arrays and the complexities entailed in their fabrication and control, and offers a new approach for designing lab-on-a-chip applications. We report measurements of the maximum droplet speed as a function of frequency and magnitude of the applied bias, intensity of illumination, volume of the droplet, and viscosity and also introduce a model that reproduces these data.

Model of formation of droplets during electric arc surfacing of functional coatings

NASA Astrophysics Data System (ADS)

Sarychev, Vladimir D.; Granovskii, Alexei Yu; Nevskii, Sergey A.; Gromov, Victor E.

2016-01-01

The mathematical model was developed for the initial stage of formation of an electrode metal droplet in the process of arc welding. Its essence lies in the fact that the presence of a temperature gradient in the boundary layer of the molten metal causes thermo-capillary instability, which leads to the formation of electrode metal droplets. A system of equations including Navier-Stokes equations, heat conduction and Maxwell's equations was solved as well as the boundary conditions for the system electrodes-plasma. Dispersion equation for thermo-capillary waves in the linear approximation for the plane layer was received and analyzed. The values of critical wavelengths, at which thermo-capillary instability appears in the nanometer wavelength range, were found. The parameters at which the mode of a fine-droplet transfer of the material takes place were theoretically defined.

Shear driven droplet shedding and coalescence on a superhydrophobic surface

NASA Astrophysics Data System (ADS)

Moghtadernejad, S.; Tembely, M.; Jadidi, M.; Esmail, N.; Dolatabadi, A.

2015-03-01

The interest on shedding and coalescence of sessile droplets arises from the importance of these phenomena in various scientific problems and industrial applications such as ice formation on wind turbine blades, power lines, nacelles, and aircraft wings. It is shown recently that one of the ways to reduce the probability of ice accretion on industrial components is using superhydrophobic coatings due to their low adhesion to water droplets. In this study, a combined experimental and numerical approach is used to investigate droplet shedding and coalescence phenomena under the influence of air shear flow on a superhydrophobic surface. Droplets with a size of 2 mm are subjected to various air speeds ranging from 5 to 90 m/s. A numerical simulation based on the Volume of Fluid method coupled with the Large Eddy Simulation turbulent model is carried out in conjunction with the validating experiments to shed more light on the coalescence of droplets and detachment phenomena through a detailed analysis of the aerodynamics forces and velocity vectors on the droplet and the streamlines around it. The results indicate a contrast in the mechanism of two-droplet coalescence and subsequent detachment with those related to the case of a single droplet shedding. At lower speeds, the two droplets coalesce by attracting each other with successive rebounds of the merged droplet on the substrate, while at higher speeds, the detachment occurs almost instantly after coalescence, with a detachment time decreasing exponentially with the air speed. It is shown that coalescence phenomenon assists droplet detachment from the superhydrophobic substrate at lower air speeds.

The electroosmotic droplet switch: countering capillarity with electrokinetics.

PubMed

Vogel, Michael J; Ehrhard, Peter; Steen, Paul H

2005-08-23

Electroosmosis, originating in the double-layer of a small liquid-filled pore (size R) and driven by a voltage V, is shown to be effective in pumping against the capillary pressure of a larger liquid droplet (size B) provided the dimensionless parameter sigmaR(2)/epsilon|zeta|VB is small enough. Here sigma is surface tension of the droplet liquid/gas interface, epsilon is the liquid dielectric constant, and zeta is the zeta potential of the solid/liquid pair. As droplet size diminishes, the voltage required to pump electroosmotically scales as V approximately R(2)/B. Accordingly, the voltage needed to pump against smaller higher-pressure droplets can actually decrease provided the pump poresize scales down with droplet size appropriately. The technological implication of this favorable scaling is that electromechanical transducers made of moving droplets, so-called "droplet transducers," become feasible. To illustrate, we demonstrate a switch whose bistable energy landscape derives from the surface energy of a droplet-droplet system and whose triggering derives from the electroosmosis effect. The switch is an electromechanical transducer characterized by individual addressability, fast switching time with low voltage, and no moving solid parts. We report experimental results for millimeter-scale droplets to verify key predictions of a mathematical model of the switch. With millimeter-size water droplets and micrometer-size pores, 5 V can yield switching times of 1 s. Switching time scales as B(3)/VR(2). Two possible "grab-and-release" applications of arrays of switches are described. One mimics the controlled adhesion of an insect, the palm beetle; the other uses wettability to move a particle along a trajectory.

Simultaneous measurement of monocomponent droplet temperature/refractive index, size and evaporation rate with phase rainbow refractometry

NASA Astrophysics Data System (ADS)

Wu, Yingchun; Crua, Cyril; Li, Haipeng; Saengkaew, Sawitree; Mädler, Lutz; Wu, Xuecheng; Gréhan, Gérard

2018-07-01

The accurate measurements of droplet temperature, size and evaporation rate are of great importance to characterize the heat and mass transfer during evaporation/condensation processes. The nanoscale size change of a micron-sized droplet exactly describes its transient mass transfer, but is difficult to measure because it is smaller than the resolutions of current size measurement techniques. The Phase Rainbow Refractometry (PRR) technique is developed and applied to measure droplet temperature, size and transient size changes and thereafter evaporation rate simultaneously. The measurement principle of PRR is theoretically derived, and it reveals that the phase shift of the time-resolved ripple structures linearly depends on, and can directly yield, nano-scale size changes of droplets. The PRR technique is first verified through the simulation of rainbows of droplets with changing size, and results show that PRR can precisely measure droplet refractive index, absolute size, as well as size change with absolute and relative errors within several nanometers and 0.6%, respectively, and thus PRR permits accurate measurements of transient droplet evaporation rates. The evaporations of flowing single n-nonane droplet and mono-dispersed n-heptane droplet stream are investigated by two PRR systems with a high speed linear CCD and a low speed array CCD, respectively. Their transient evaporation rates are experimentally determined and quantitatively agree well with the theoretical values predicted by classical Maxwell and Stefan-Fuchs models. With the demonstration of evaporation rate measurement of monocomponent droplet in this work, PRR is an ideal tool for measurements of transient droplet evaporation/condensation processes, and can be extended to multicomponent droplets in a wide range of industrially-relevant applications.

Combustion of Unconfined Droplet Clusters in Microgravity

NASA Technical Reports Server (NTRS)

Ruff, G. A.; Liu, S.

2001-01-01

Combustion experiments using arrays of droplets seek to provide a link between single droplet combustion phenomena and the behavior of complex spray combustion systems. Both single droplet and droplet array studies have been conducted in microgravity to better isolate the droplet interaction phenomena and eliminate or reduce the confounding effects of buoyancy-induced convection. In most experiments involving droplet arrays, the droplets are supported on fibers to keep them stationary and close together before the combustion event. The presence of the fiber, however, disturbs the combustion process by introducing a source of heat transfer and asymmetry into the configuration. As the number of drops in a droplet array increases, supporting the drops on fibers becomes less practical because of the cumulative effect of the fibers on the combustion process. To eliminate the effect of the fiber, several researchers have conducted microgravity experiments using unsupported droplets. Jackson and Avedisian investigated single, unsupported drops while Nomura et al. studied droplet clouds formed by a condensation technique. The overall objective of this research is to extend the study of unsupported drops by investigating the combustion of well-characterized drop clusters in a microgravity environment. Direct experimental observations and measurements of the combustion of droplet clusters would fill a large gap in our current understanding of droplet and spray combustion and provide unique experimental data for the verification and improvement of spray combustion models. In this work, the formation of drop clusters is precisely controlled using an acoustic levitation system so that dilute, as well as dense clusters can be created and stabilized before combustion in microgravity is begun. This paper describes the design and performance of the 1-g experimental apparatus, some preliminary 1-g results, and plans for testing in microgravity.

Shapes of rotating superfluid helium nanodroplets

DOE PAGES

Bernando, Charles; Tanyag, Rico Mayro P.; Jones, Curtis; ...

2017-02-16

Rotating superfluid He droplets of approximately 1 μm in diameter were obtained in a free nozzle beam expansion of liquid He in vacuum and were studied by single-shot coherent diffractive imaging using an x-ray free electron laser. The formation of strongly deformed droplets is evidenced by large anisotropies and intensity anomalies (streaks) in the obtained diffraction images. The analysis of the images shows that in addition to previously described axially symmetric oblate shapes, some droplets exhibit prolate shapes. Forward modeling of the diffraction images indicates that the shapes of rotating superfluid droplets are very similar to their classical counterparts, givingmore » direct access to the droplet angular momenta and angular velocities. Here, the analyses of the radial intensity distribution and appearance statistics of the anisotropic images confirm the existence of oblate metastable superfluid droplets with large angular momenta beyond the classical bifurcation threshold.« less

Shapes of rotating superfluid helium nanodroplets

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

Bernando, Charles; Tanyag, Rico Mayro P.; Jones, Curtis

Rotating superfluid He droplets of approximately 1 μm in diameter were obtained in a free nozzle beam expansion of liquid He in vacuum and were studied by single-shot coherent diffractive imaging using an x-ray free electron laser. The formation of strongly deformed droplets is evidenced by large anisotropies and intensity anomalies (streaks) in the obtained diffraction images. The analysis of the images shows that in addition to previously described axially symmetric oblate shapes, some droplets exhibit prolate shapes. Forward modeling of the diffraction images indicates that the shapes of rotating superfluid droplets are very similar to their classical counterparts, givingmore » direct access to the droplet angular momenta and angular velocities. Here, the analyses of the radial intensity distribution and appearance statistics of the anisotropic images confirm the existence of oblate metastable superfluid droplets with large angular momenta beyond the classical bifurcation threshold.« less

Spontaneous Spreading of a Droplet: The Role of Solid Continuity and Advancing Contact Angle.

PubMed

Jiang, Youhua; Sun, Yujin; Drelich, Jaroslaw W; Choi, Chang-Hwan

2018-05-01

Spontaneous spreading of a droplet on a solid surface is poorly understood from a macroscopic level down to a molecular level. Here, we investigate the effect of surface topography and wettability on spontaneous spreading of a water droplet. Spreading force is measured for a suspended droplet that minimizes interference of kinetic energy in the spontaneous spreading during its contact with solid surfaces of discontinuous (pillar) and continuous (pore) patterns with various shapes and dimensions. Results show that a droplet cannot spread spontaneously on pillared surfaces regardless of their shapes or dimensions because of the solid discontinuity. On the contrary, a droplet on pored surfaces can undergo spontaneous spreading whose force increases with a decrease in the advancing contact angle. Theoretical models based on both the system free energy and capillary force along the contact line validate the direct and universal dependency of the spontaneous spreading force on the advancing contact angle.

Composition measurements of binary mixture droplets by rainbow refractometry.

PubMed

Wilms, J; Weigand, B

2007-04-10

So far, refractive index measurements by rainbow refractometry have been used to determine the temperature of single droplets and ensembles of droplets. Rainbow refractometry is, for the first time, to the best of our knowledge, applied to measure composition histories of evaporating, binary mixture droplets. An evaluation method is presented that makes use of Airy theory and the simultaneous size measurement by Mie scattering imaging. The method further includes an empirical correction function for a certain diameter and refractive index range. The measurement uncertainty was investigated by numerical simulations with Lorenz-Mie theory. For the experiments, an optical levitation setup was used allowing for long measurement periods. Temperature measurements of single-component droplets at different temperature levels are shown to demonstrate the accuracy of rainbow refractometry. Measurements of size and composition histories of binary mixture droplets are presented for two different mixtures. Experimental results show good agreement with numerical results using a rapid-mixing model.

High-resolution liquid patterns via three-dimensional droplet shape control.

PubMed

Raj, Rishi; Adera, Solomon; Enright, Ryan; Wang, Evelyn N

2014-09-25

Understanding liquid dynamics on surfaces can provide insight into nature's design and enable fine manipulation capability in biological, manufacturing, microfluidic and thermal management applications. Of particular interest is the ability to control the shape of the droplet contact area on the surface, which is typically circular on a smooth homogeneous surface. Here, we show the ability to tailor various droplet contact area shapes ranging from squares, rectangles, hexagons, octagons, to dodecagons via the design of the structure or chemical heterogeneity on the surface. We simultaneously obtain the necessary physical insights to develop a universal model for the three-dimensional droplet shape by characterizing the droplet side and top profiles. Furthermore, arrays of droplets with controlled shapes and high spatial resolution can be achieved using this approach. This liquid-based patterning strategy promises low-cost fabrication of integrated circuits, conductive patterns and bio-microarrays for high-density information storage and miniaturized biochips and biosensors, among others.

Simulating the Surface Relief of Nanoaerosols Obtained via the Rapid Cooling of Droplets

NASA Astrophysics Data System (ADS)

Tovbin, Yu. K.; Zaitseva, E. S.; Rabinovich, A. B.

2018-03-01

An approach is formulated that theoretically describes the structure of a rough surface of small aerosol particles obtained from a liquid droplet upon its rapid cooling. The problem consists of two stages. In the first stage, a concentration profile of the droplet-vapor transition region is calculated. In the second stage, local fractions of vacant sites and their pairs are found on the basis of this profile, and the rough structure of a frozen droplet surface transitioning to the solid state is calculated. Model parameters are the temperature of the initial droplet and those of the lateral interaction between droplet atoms. Information on vacant sites inside the region of transition allows us to identify adsorption centers and estimate the monolayer capacity, compared to that of the total space of the region of transition. The approach is oriented toward calculating adsorption isotherms on real surfaces.

Predictive model for ice formation on superhydrophobic surfaces.

PubMed

Bahadur, Vaibhav; Mishchenko, Lidiya; Hatton, Benjamin; Taylor, J Ashley; Aizenberg, Joanna; Krupenkin, Tom

2011-12-06

The prevention and control of ice accumulation has important applications in aviation, building construction, and energy conversion devices. One area of active research concerns the use of superhydrophobic surfaces for preventing ice formation. The present work develops a physics-based modeling framework to predict ice formation on cooled superhydrophobic surfaces resulting from the impact of supercooled water droplets. This modeling approach analyzes the multiple phenomena influencing ice formation on superhydrophobic surfaces through the development of submodels describing droplet impact dynamics, heat transfer, and heterogeneous ice nucleation. These models are then integrated together to achieve a comprehensive understanding of ice formation upon impact of liquid droplets at freezing conditions. The accuracy of this model is validated by its successful prediction of the experimental findings that demonstrate that superhydrophobic surfaces can fully prevent the freezing of impacting water droplets down to surface temperatures of as low as -20 to -25 °C. The model can be used to study the influence of surface morphology, surface chemistry, and fluid and thermal properties on dynamic ice formation and identify parameters critical to achieving icephobic surfaces. The framework of the present work is the first detailed modeling tool developed for the design and analysis of surfaces for various ice prevention/reduction strategies. © 2011 American Chemical Society

Imparting Icephobicity with Substrate Flexibility.

PubMed

Vasileiou, Thomas; Schutzius, Thomas M; Poulikakos, Dimos

2017-07-11

Ice accumulation hinders the performance of, and poses safety threats for, infrastructure both on the ground and in the air. Previously, rationally designed superhydrophobic surfaces have demonstrated some potential as a passive means to mitigate ice accretion; however, further studies on material solutions that reduce impalement and the contact time for impacting supercooled droplets (high viscosity) and can also repel droplets that freeze during surface contact are urgently needed. Here we demonstrate the collaborative effect of substrate flexibility and surface micro/nanotexture on enhancing both icephobicity and the repellency of viscous droplets (typical of supercooled water). We first investigate the influence of increased viscosity (spanning from 0.9 to 1078 mPa·s using water-glycerol mixtures) on impalement resistance and the droplet-substrate contact time after impact. Then we examine the effect of droplet partial solidification on recoil and simulate more challenging icing conditions by impacting supercooled water droplets (down to -15 °C) onto flexible and rigid surfaces containing ice nucleation promoters (AgI). We demonstrate a passive mechanism for shedding partially solidified (recalescent) droplets-under conditions where partial solidification occurs much faster than the natural droplet oscillation-which does not rely on converting droplet surface energy into kinetic energy (classic recoil mechanism). Using an energy-based model (kinetic-elastic-capillary), we identify a previously unexplored mechanism whereby the substrate oscillation and velocity govern the rebound process, with low areal density and moderately stiff substrates acting to efficiently absorb the incoming droplet kinetic energy and rectify it back, allowing droplets to overcome adhesion and gravitational forces, and recoil. This mechanism applies for a range of droplet viscosities, spanning from low- to high-viscosity fluids and even ice slurries, which do not rebound from rigid superhydrophobic substrates. For a low-viscosity fluid, i.e., water, if the substrate oscillates faster than the droplet spreading and retraction, the action of the substrate is decoupled from the droplet oscillation, resulting in a reduction in the droplet-substrate contact time.

Oscillations and uniaxial mechanochemical waves in a model of an active poroelastic medium: Application to deformation patterns in protoplasmic droplets of Physarum polycephalum

NASA Astrophysics Data System (ADS)

Alonso, Sergio; Strachauer, Ulrike; Radszuweit, Markus; Bär, Markus; Hauser, Marcus J. B.

2016-04-01

Self-organization in cells often manifests itself in oscillations and waves. Here, we address deformation waves in protoplasmic droplets of the plasmodial slime mould Physarum polycephalum by modelling and experiments. In particular, we extend a one-dimensional model that considered the cell as a poroelastic medium, where active tension caused mechanochemical waves that were regulated by an inhibitor (Radszuweit et al., 2013). Our extension consists of a simple, qualitative chemical reaction-diffusion model (Brusselator) that describes the regulation of the inhibitor by another biochemical species. The biochemical reaction enhances the formation of mechanochemical waves if the reaction rates and input concentrations are near or inside an oscillatory regime. The period of the waves is found to be controlled by the characteristic oscillation period, whereas their wavelength is set by mechanical parameters. The model also allows for a systematic study of the chemical activity at the onset of mechanochemical waves. We also present examples for pattern formation in protoplasmic droplets of Physarum polycephalum including global oscillations where the central region of the droplets is in antiphase to the boundary zone, as well as travelling and standing wave-like uniaxial patterns. Finally, we apply our model to reproduce these experimental results by identifying the active tension inhibitor with the intracellular calcium concentration in the Physarum droplets and by using parameter values from mechanical experiments, respectively knowledge about the properties of calcium oscillations in Physarum. The simulation results are then found to be in good agreement with the experimental observations.

The mechanisms of filter feeding on oil droplets: Theoretical considerations.

PubMed

Mehrabian, Sasan; Letendre, Francis; Cameron, Christopher B

2018-04-01

Filter feeding animals capture food particles and oil droplets from the fluid environment using cilia or appendages composed of arrays of fibers. Here we review the theoretical models that have provided a foundation for observations on the efficiency of particle capture. We then provide the mathematical theoretical framework to characterize the efficient filtration of oil droplets. In the aquatic and marine environments oil droplets are released from the decay of organisms or as hydrocarbons. Droplet size and flow velocity, oil-to-water viscosity ratio, oil-water interfacial tension, oil and water density difference, and the surface wettability, or surface texture, of the filter fiber are the key parameters for oil droplet capture. Following capture, capillary force maintains the droplet at its location due to the oil-water interfacial tension. If the oil-coated fiber is subject to any external force such as viscous or gravitational forces, it may deform and separate from the fiber and re-enter the fluid stream. We show oil droplet capture in Daphnia and the barnacle Balanus glandula, and outline some of the ecological unknowns regarding oil capture in the oceans. Awareness of these mechanisms and their interrelationships will provide a foundation for investigations into the efficiency of various modes of filter feeding on oil droplets. Copyright © 2018 Elsevier Ltd. All rights reserved.

New generation of amino coumarin methyl sulfonate-based fluorogenic substrates for amidase assays in droplet-based microfluidic applications.

PubMed

Woronoff, Gabrielle; El Harrak, Abdeslam; Mayot, Estelle; Schicke, Olivier; Miller, Oliver J; Soumillion, Patrice; Griffiths, Andrew D; Ryckelynck, Michael

2011-04-15

Droplet-based microfluidics is a powerful tool for biology and chemistry as it allows the production and the manipulation of picoliter-size droplets acting as individual reactors. In this format, high-sensitivity assays are typically based on fluorescence, so fluorophore exchange between droplets must be avoided. Fluorogenic substrates based on the coumarin leaving group are widely used to measure a variety of enzymatic activities, but their application in droplet-based microfluidic systems is severely impaired by the fast transport of the fluorescent product between compartments. Here we report the synthesis of new amidase fluorogenic substrates based on 7-aminocoumarin-4-methanesulfonic acid (ACMS), a highly water-soluble dye, and their suitability for droplet-based microfluidics applications. Both substrate and product had the required spectral characteristics and remained confined in droplets from hours to days. As a model experiment, a phenylacetylated ACMS was synthesized and used as a fluorogenic substrate of Escherichia coli penicillin G acylase. Kinetic parameters (k(cat) and K(M)) measured in bulk and in droplets on-chip were very similar, demonstrating the suitability of this synthesis strategy to produce a variety of ACMS-based substrates for assaying amidase activities both in microtiter plate and droplet-based microfluidic formats. © 2011 American Chemical Society

Modelling droplet collision outcomes for different substances and viscosities

NASA Astrophysics Data System (ADS)

Sommerfeld, Martin; Kuschel, Matthias

2016-12-01

The main objective of the present study is the derivation of models describing the outcome of binary droplet collisions for a wide range of dynamic viscosities in the well-known collision maps (i.e. normalised lateral droplet displacement at collision, called impact parameter, versus collision Weber number). Previous studies by Kuschel and Sommerfeld (Exp Fluids 54:1440, 2013) for different solution droplets having a range of solids contents and hence dynamic viscosities (here between 1 and 60 mPa s) revealed that the locations of the triple point (i.e. coincidence of bouncing, stretching separation and coalescence) and the critical Weber number (i.e. condition for the transition from coalescence to separation for head-on collisions) show a clear dependence on dynamic viscosity. In order to extend these findings also to pure liquids and to provide a broader data basis for modelling the viscosity effect, additional binary collision experiments were conducted for different alcohols (viscosity range 1.2-15.9 mPa s) and the FVA1 reference oil at different temperatures (viscosity range 3.0-28.2 mPa s). The droplet size for the series of alcohols was around 365 and 385 µm for the FVA1 reference oil, in each case with fixed diameter ratio at Δ= 1. The relative velocity between the droplets was varied in the range 0.5-3.5 m/s, yielding maximum Weber numbers of around 180. Individual binary droplet collisions with defined conditions were generated by two droplet chains each produced by vibrating orifice droplet generators. For recording droplet motion and the binary collision process with good spatial and temporal resolution high-speed shadow imaging was employed. The results for varied relative velocity and impact angle were assembled in impact parameter-Weber number maps. With increasing dynamic viscosity a characteristic displacement of the regimes for the different collision scenarios was also observed for pure liquids similar to that observed for solutions. This displacement could be described on a physical basis using the similarity number and structure parameter K which was obtained through flow process evaluation and optimal proportioning of momentum and energy by Naue and Bärwolff (Transportprozesse in Fluiden. Deutscher Verlag für Grundstoffindustrie GmbH, Leipzig 1992). Two correlations including the structure parameter K could be derived which describe the location of the triple point and the critical We number. All fluids considered, pure liquids and solutions, are very well fitted by these physically based correlations. The boundary model of Jiang et al. (J Fluid Mech 234:171-190, 1992) for distinguishing between coalescence and stretching separation could be adapted to go through the triple point by the two involved model parameters C a and C b, which were correlated with the relaxation velocity u_{{relax}} = {σ/μ}. Based on the predicted critical Weber number, denoting the onset of reflexive separation, the model of Ashgriz and Poo (J Fluid Mech 221:183-204, 1990) was adapted accordingly. The proper performance of the new generalised models was validated based on the present and previous measurements for a wide range of dynamic viscosities (i.e. 1-60 mPa s) and liquid properties. Although the model for the lower boundary of bouncing (Estrade et al. in J Heat Fluid Flow 20:486-491, 1999) could be adapted through the shape factor, it was found not suitable for the entire range of Weber numbers and viscosities.

Photoacoustic measurements of photokinetics in single optically trapped aerosol droplets

NASA Astrophysics Data System (ADS)

Covert, Paul; Cremer, Johannes; Signorell, Ruth; Thaler, Klemens; Haisch, Christoph

2017-04-01

It is well established that interaction of light with atmospheric aerosols has a large impact on the Earth's climate. However, uncertainties in the magnitude of this impact remain large, due in part to broad distributions of aerosol size, composition, and chemical reactivity. In this context, photoacoustic spectroscopy is commonly used to measure light absorption by aerosols. Here, we present photoacoustic measurements of single, optically-trapped nanodroplets to reveal droplet size-depencies of photochemical and physical processes. Theoretical considerations have pointed to a size-dependence in the magnitude and phase of the photoacoustic response from aerosol droplets. This dependence is thought to originate from heat transfer processes that are slow compared to the acoustic excitation frequency. In the case of a model aerosol, our measurements of single particle absorption cross-section versus droplet size confirm these theoretical predictions. In a related study, using the same model aerosol, we also demonstrate a droplet size-dependence of photochemical reaction rates [1]. Within sub-micron sized particles, photolysis rates were observed to be an order of magnitude greater than those observed in larger droplets. [1] J. W. Cremer, K. M. Thaler, C. Haisch, and R. Signorell. Photoacoustics of single laser-trapped nanodroplets for the direct observation of nanofocusing in aerosol photokinetics. Nat. Commun., 7:10941, 2016.

Simulation of High-Speed Droplet Impact Against Dry Substrates with Partial Velocity Slip

NASA Astrophysics Data System (ADS)

Kondo, Tomoki; Ando, Keita

2017-11-01

High-speed droplet impact can be used to clean substrates such as silicon wafers. Radially spreading shear flow after the impact may allow for mechanically removing contaminant particles at substrate surfaces. Since it is a big challenge to experimentally explore such complicated flow that exhibits contact line motion and water hammer, its flow feature is not well understood. Here, we aim to numerically evaluate shear flow caused by the impact of a spherical water droplet (of submillimeter sizes) at high speed (up to 50 m/s) against a dry rigid wall. We model the flow based on compressible Navier-Stokes equations with Stokes' hypothesis and solve them by a high-order-accurate finite volume method equipped with shock and interface capturing. To treat the motion of a contact line between the three phases (the droplet, the rigid wall, and the ambient air) in a robust manner, we permit velocity slip at the wall with Navier's model, for wall slip is known to come into play under steep velocity gradients that can arise from high-speed droplet impact. In our presentation, we will examine radially spreading flow after the droplet impact and the resulting wall shear stress generation from the simulation. This work was supported by JSPS KAKENHI Grant Number JP17J02211.

Time constants for the evolution of sea spray droplets

NASA Astrophysics Data System (ADS)

Andreas, Edgar L.

1990-11-01

Sea spray droplets start with the same temperature as the ocean surface from which they form. In high-latitude, polar-low conditions, they therefore cool and evaporate in a relatively cold wind and may alter the air sea exchange of heat and moisture. This paper presents equations that model the thermal and size (moisture) evolution of a spray droplet from the time it forms until it reaches equilibrium with its environment. The model does well when tested against some of the scanty data available on the evolution of saline droplets. We parameterize the thermal and size evolution of spray droplets with the time constants τT and τr, which are, respectively, the times required for a droplet to come to within e1 of its equilibrium temperature and within e1 of its equilibrium radius. τr is always about three orders of magnitude larger than τT; the thermal exchange is thus complete before the moisture exchange even starts. Consequently, the ambient humidity has little effect on the thermal exchange rate, and the initial droplet temperature has negligible effect on the moisture exchange rate. We also parameterize the gravitational settling of droplets and their potential for turbulent suspension with the time scales τf and τw, respectively. Comparing the four time scales, we see that spray droplets with initial radii less than 10μm reach both thermal and size equilibrium with the ambient air. Droplets with initial radii greater than 300μm, on the other hand, fall back into the sea before exchanging appreciable heat or moisture; they thus have little impact on air sea exchange. In the mid-range, droplets with initial radii between 10 and 300μm, the physics is more complex. Even after comparing τT and τr with τf and τw, we still cannot say unequivocally which process is fastest because of the rudimentary nature of the τw estimates. Future work must thus focus on the generation and turbulent transport of droplets of this size if we are to understand how sea spray affects air sea exchange.

Hydrodynamic clustering of droplets in turbulence

NASA Astrophysics Data System (ADS)

Kunnen, Rudie; Yavuz, Altug; van Heijst, Gertjan; Clercx, Herman

2017-11-01

Small, inertial particles are known to cluster in turbulent flows: particles are centrifuged out of eddies and gather in the strain-dominated regions. This so-called preferential concentration is reflected in the radial distribution function (RDF; a quantitative measure of clustering). We study clustering of water droplets in a loudspeaker-driven turbulence chamber. We track the motion of droplets in 3D and calculate the RDF. At moderate scales (a few Kolmogorov lengths) we find the typical power-law scaling of preferential concentration in the RDF. However, at even smaller scales (a few droplet diameters), we encounter a hitherto unobserved additional clustering. We postulate that the additional clustering is due to hydrodynamic interactions, an effect which is typically disregarded in modeling. Using a perturbative expansion of inertial effects in a Stokes-flow description of two interacting spheres, we obtain an expression for the RDF which indeed includes the additional clustering. The additional clustering enhances the collision probability of droplets, which enhances their growth rate due to coalescence. The additional clustering is thus an essential effect in precipitation modeling.

Combustion of liquid fuel droplets in supercritical conditions

NASA Technical Reports Server (NTRS)

Shuen, J. S.; Yang, Vigor

1991-01-01

A comprehensive analysis of liquid-fuel droplet combustion in both sub- and super-critical environments has been conducted. The formulation is based on the complete conservation equations for both gas and liquid phases, and accommodates finite-rate chemical kinetics and a full treatment of liquid-vapor phase equilibrium at the droplet surface. The governing equations and the associated interface boundary conditions are solved numerically using a fully coupled, implicit scheme with the dual time-stepping integration technique. The model is capable of treating the entire droplet history, including the transition from the subcritical to the supercritical state. As a specific example, the combustion of n-pentane fuel droplets in air is studied for pressures of 5-140 atm. Results indicate that the ambient gas pressure exerts significant control of droplet gasification and burning processes through its influences on the fluid transport, gas/liquid interface thermodynamics, and chemical reactions. The droplet gasification rate increases progressively with pressure. However, the data for the overall burnout time exhibits a significant variation near the critical burning pressure, mainly as a result of reduced mass-diffusion rate and latent heat of vaporization with increased pressure. The influence of droplet size on the burning characteristics is also noted.

Mechanical vibration of viscoelastic liquid droplets

NASA Astrophysics Data System (ADS)

Sharp, James; Harrold, Victoria

2014-03-01

The resonant vibrations of viscoelastic sessile droplets supported on different substrates were monitored using a simple laser light scattering technique. In these experiments, laser light was reflected from the surfaces of droplets of high Mw poly acrylamide-co-acrylic acid (PAA) dissolved in water. The scattered light was allowed to fall on the surface of a photodiode detector and a mechanical impulse was applied to the drops using a vibration motor mounted beneath the substrates. The mechanical impulse caused the droplets to vibrate and the scattered light moved across the surface of the photodiode. The resulting time dependent photodiode signal was then Fourier transformed to obtain the mechanical vibrational spectra of the droplets. The frequencies and widths of the resonant peaks were extracted for droplets containing different concentrations of PAA and with a range of sizes. This was repeated for PAA loaded water drops on surfaces which displayed different values of the three phase contact angle. The results were compared to a simple model of droplet vibration which considers the formation of standing wave states on the surface of a viscoelastic droplet. We gratefully acknowledge the support of the Leverhulme trust under grant number RPG-2012-702.

Electrohydrodynamic assisted droplet alignment for lens fabrication by droplet evaporation

NASA Astrophysics Data System (ADS)

Wang, Guangxu; Deng, Jia; Guo, Xing

2018-04-01

Lens fabrication by droplet evaporation has attracted a lot of attention since the fabrication approach is simple and moldless. Droplet position accuracy is a critical parameter in this approach, and thus it is of great importance to use accurate methods to realize the droplet position alignment. In this paper, we propose an electrohydrodynamic (EHD) assisted droplet alignment method. An electrostatic force was induced at the interface between materials to overcome the surface tension and gravity. The deviation of droplet position from the center region was eliminated and alignment was successfully realized. We demonstrated the capability of the proposed method theoretically and experimentally. First, we built a simulation model coupled with the three-phase flow formulations and the EHD equations to study the three-phase flowing process in an electric field. Results show that it is the uneven electric field distribution that leads to the relative movement of the droplet. Then, we conducted experiments to verify the method. Experimental results are consistent with the numerical simulation results. Moreover, we successfully fabricated a crater lens after applying the proposed method. A light emitting diode module packaging with the fabricated crater lens shows a significant light intensity distribution adjustment compared with a spherical cap lens.

Experimental determination of interfacial tension by different dynamical methods under simple shear flow conditions with a novel computer-controlled parallel band apparatus.

PubMed

Megías-Alguacil, David; Fischer, Peter; Windhab, Erich J

2004-06-15

We present experimental investigations on droplet deformation under simple shear flow conditions, using a computer-controlled parallel band apparatus and an optical device which allows us to record the time dependence of the droplet shape. Several methods are applied to determine the interfacial tension from the observed shape and relaxation mechanism. Specific software developed in our laboratory allows the droplet to be fixed in a certain position for extended times, in fact, indefinite. This is an advantage over most other work done in this area, where only limited time is available. In our experiments, the transient deformation of sheared droplets can be observed to reach the steady state. The measured systems were Newtonian, both droplet and fluid phase. Droplet deformation, orientation angle and retraction were studied and compared to several models. The interfacial tension of the different systems was calculated using the theories of Taylor, Rallison, and Hinch and Acrivos. The results obtained from the analysis of the droplet deformation were in very good agreement with drop detachment experiments of Feigl and co-workers. The study of orientation angle shows qualitative agreement to the theory of Hinch and Acrivos but reveals larger quantitative discrepancies for several empirical fitting parameters of the used model. Analysis of the relaxation of sheared drops provided estimates of the interfacial tension that were in very good agreement with the steady-state measurements.

Predicting Droplet Formation on Centrifugal Microfluidic Platforms

NASA Astrophysics Data System (ADS)

Moebius, Jacob Alfred

Centrifugal microfluidics is a widely known research tool for biological sample and water quality analysis. Currently, the standard equipment used for such diagnostic applications include slow, bulky machines controlled by multiple operators. These machines can be condensed into a smaller, faster benchtop sample-to-answer system. Sample processing is an important step taken to extract, isolate, and convert biological factors, such as nucleic acids or proteins, from a raw sample to an analyzable solution. Volume definition is one such step. The focus of this thesis is the development of a model predicting monodispersed droplet formation and the application of droplets as a technique for volume definition. First, a background of droplet microfluidic platforms is presented, along with current biological analysis technologies and the advantages of integrating such technologies onto microfluidic platforms. Second, background and theories of centrifugal microfluidics is given, followed by theories relevant to droplet emulsions. Third, fabrication techniques for centrifugal microfluidic designs are discussed. Finally, the development of a model for predicting droplet formation on the centrifugal microfluidic platform are presented for the rest of the thesis. Predicting droplet formation analytically based on the volumetric flow rates of the continuous and dispersed phases, the ratios of these two flow rates, and the interfacial tension between the continuous and dispersed phases presented many challenges, which will be discussed in this work. Experimental validation was completed using continuous phase solutions of different interfacial tensions. To conclude, prospective applications are discussed with expected challenges.

Marangoni Effect on the Shape of Freely Receding Evaporating Sessile Droplets of Perfectly Wetting Liquids

NASA Astrophysics Data System (ADS)

Tsoumpas, Yannis; Dehaeck, Sam; Rednikov, Alexey; Colinet, Pierre

2014-11-01

Freely receding evaporating sessile droplets of perfectly wetting liquids (HFE-7100, 7200 and 7500), with small finite contact angles induced by evaporation, are studied with a Mach-Zehnder interferometer. Surprisingly, the experimentally obtained profiles turn out to deviate from the classical macroscopic static shape of a sessile droplet (as determined by gravity and capillarity), often used when modeling evaporating droplets. These deviations can be seen in two ways. Namely, either the droplet appears to be inflated as compared to the classical static shape assuming the same contact angle and contact radius, or the apparent contact angle appears lower than the classical static one assuming the same volume and contact radius. In reality, the experimental profiles exhibit a local decrease of the slope near the contact line, which we attribute to the Marangoni effect in an evaporating sessile droplet. In this case, the radially inward (along the liquid-air interface) direction of the flow delivers more liquid to the center of the droplet making it appear inflated. When the Marangoni effect is weak, as in the case of the poorly volatile HFE-7500, no significant influence is noticed on the drop shape. The experimental results are compared with the predictions of a lubrication-type theoretical model that incorporates the evaporation-induced Marangoni flow. Financial support of FP7 Marie Curie MULTIFLOW Network (PITN-GA-2008-214919), ESA/BELSPO-PRODEX, BELSPO- μMAST (IAP 7/38) & FRS-FNRS is gratefully acknowledged.

"Micro to macro (M2M)"--A novel approach for intravenous delivery of propofol.

PubMed

Damitz, Robert; Chauhan, Anuj

2015-10-15

Propofol emulsions have limited shelf life and safety concerns for injection. Microemulsions of propofol are thermodynamically stable and simpler to manufacture, but cause additional pain on injection. We propose a novel micro to macro (M2M) approach of destabilizing a microemulsion immediately prior to injection. Microemulsions of propofol were prepared at two to three times the drug loadings of commercial formulations. We determined suitable microemulsion compositions which destabilize into macroemulsions after two or three fold dilutions with water. Droplet growth after dilution was measured with dynamic light scattering. Increasing solution turbidity after dilution was also measured optically with millisecond resolution. Experimental data was analyzed in the context of a coalescence model. Microemulsions rapidly coalesce into larger droplet size macroemulsions after dilution according to the phase diagram shift. The resulting macroemulsions are metastable retaining their droplet size for several hours. Droplet growth occurs on the order of seconds and a metastable size of about 1 micron is reached in minutes. Rates of droplet growth and metastable droplet sizes depend on the surfactant composition. The coalescence model predicts droplet growth with good agreement but only after accounting for the finite probability of coalescence from each collision. The M2M concept has been demonstrated for the anesthetic drug propofol which may improve stability and manufacturability in addition to reducing pain on injection. This approach could be adapted to other hydrophobic vesicant drugs as well. Copyright © 2015 Elsevier B.V. All rights reserved.

Gas Atomization of Molten Metal: Part II. Applications

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

Abu-Lebdeh, Taher M.; Leon, Genaro Perez-de; Hamoush, Sameer A.

A numerical model was derived to obtain results for two alloys during the Gas Atomization (GA) method. The model equations and governing equations were implemented through the application of part I data. Aspects such as heat transfer, fluid mechanics, thermodynamics and law of motions were taken into account for the formulation of equations that take gas dynamics, droplet dynamics and energy balance or conservation into consideration. The inputs of the model include: Processing parameters such as the size of the droplets, characteristics of the metal alloy, initial temperature of the molten metal, properties and fractions of the atomization gas andmore » the gas pressure. The outputs include velocity and thermal profiles of the droplet and gas. Velocity profiles illustrate the velocity of both droplet and gas, while thermal profiles illustrate cooling rate and the rate of temperature change of the droplets. The alloys are gamma-Titanium Aluminide (γ-TiAl) and Al-3003-O. These alloys were selected due to the vast amount of applications both can have in several industries. Certain processing parameters were held constant, while others were altered. Furthermore, the main focus of this study was to gain insight into which optimal parameters should be utilized within the GA method for these alloys and to provide insight into the behavior of these alloys« less

Effects of Droplet Size on Intrusion of Sub-Surface Oil Spills

NASA Astrophysics Data System (ADS)

Adams, Eric; Chan, Godine; Wang, Dayang

2014-11-01

We explore effects of droplet size on droplet intrusion and transport in sub-surface oil spills. Negatively buoyant glass beads released continuously to a stratified ambient simulate oil droplets in a rising multiphase plume, and distributions of settled beads are used to infer signatures of surfacing oil. Initial tests used quiescent conditions, while ongoing tests simulate currents by towing the source and a bottom sled. Without current, deposited beads have a Gaussian distribution, with variance increasing with decreasing particle size. Distributions agree with a model assuming first order particle loss from an intrusion layer of constant thickness, and empirically determined flow rate. With current, deposited beads display a parabolic distribution similar to that expected from a source in uniform flow; we are currently comparing observed distributions with similar analytical models. Because chemical dispersants have been used to reduce oil droplet size, our study provides one measure of their effectiveness. Results are applied to conditions from the `Deep Spill' field experiment, and the recent Deepwater Horizon oil spill, and are being used to provide ``inner boundary conditions'' for subsequent far field modeling of these events. This research was made possible by grants from Chevron Energy Technology Co., through the Chevron-MITEI University Partnership Program, and BP/The Gulf of Mexico Research Initiative, GISR.

Polycrystalline structures formed in evaporating droplets as a parameter to test the action of Zincum metallicum 30c in a wheat seed model.

PubMed

Kokornaczyk, Maria Olga; Baumgartner, Stephan; Betti, Lucietta

2016-05-01

Polycrystalline structures formed inside evaporating droplets of different biological fluids have been shown sensitive towards various influences, including ultra high dilutions (UHDs), representing so a new approach potentially useful for basic research in homeopathy. In the present study we tested on a wheat seed model Zincum metallicum 30c efficacy versus lactose 30c and water. Stressed and non-stressed wheat seeds were watered with the three treatments. Seed-leakage droplets were evaporated and the polycrystalline structures formed inside the droplet residues were analyzed for their local connected fractal dimensions (LCFDs) (measure of complexity) using the software ImageJ. We have found significant differences in LCFD values of polycrystalline structures obtained from stressed seeds following the treatments (p<0.0001); Zincum metallicum 30c lowered the structures' complexity compared to lactose 30c and water. In non-stressed seeds no significant differences were found. The droplet evaporation method (DEM) might represent a potentially useful tool in basic research in homeopathy. Furthermore our results suggest a sensitization of the stressed model towards the treatment action, which is conforming to previous findings. Copyright © 2015 The Faculty of Homeopathy. Published by Elsevier Ltd. All rights reserved.

Gas Atomization of Molten Metal: Part II. Applications

DOE PAGES

Abu-Lebdeh, Taher M.; Leon, Genaro Perez-de; Hamoush, Sameer A.; ...

2016-02-01

A numerical model was derived to obtain results for two alloys during the Gas Atomization (GA) method. The model equations and governing equations were implemented through the application of part I data. Aspects such as heat transfer, fluid mechanics, thermodynamics and law of motions were taken into account for the formulation of equations that take gas dynamics, droplet dynamics and energy balance or conservation into consideration. The inputs of the model include: Processing parameters such as the size of the droplets, characteristics of the metal alloy, initial temperature of the molten metal, properties and fractions of the atomization gas andmore » the gas pressure. The outputs include velocity and thermal profiles of the droplet and gas. Velocity profiles illustrate the velocity of both droplet and gas, while thermal profiles illustrate cooling rate and the rate of temperature change of the droplets. The alloys are gamma-Titanium Aluminide (γ-TiAl) and Al-3003-O. These alloys were selected due to the vast amount of applications both can have in several industries. Certain processing parameters were held constant, while others were altered. Furthermore, the main focus of this study was to gain insight into which optimal parameters should be utilized within the GA method for these alloys and to provide insight into the behavior of these alloys« less

Scavenging dissolved oxygen via acoustic droplet vaporization.

PubMed

Radhakrishnan, Kirthi; Holland, Christy K; Haworth, Kevin J

2016-07-01

Acoustic droplet vaporization (ADV) of perfluorocarbon emulsions has been explored for diagnostic and therapeutic applications. Previous studies have demonstrated that vaporization of a liquid droplet results in a gas microbubble with a diameter 5-6 times larger than the initial droplet diameter. The expansion factor can increase to a factor of 10 in gassy fluids as a result of air diffusing from the surrounding fluid into the microbubble. This study investigates the potential of this process to serve as an ultrasound-mediated gas scavenging technology. Perfluoropentane droplets diluted in phosphate-buffered saline (PBS) were insonified by a 2 MHz transducer at peak rarefactional pressures lower than and greater than the ADV pressure amplitude threshold in an in vitro flow phantom. The change in dissolved oxygen (DO) of the PBS before and after ADV was measured. A numerical model of gas scavenging, based on conservation of mass and equal partial pressures of gases at equilibrium, was developed. At insonation pressures exceeding the ADV threshold, the DO of air-saturated PBS decreased with increasing insonation pressures, dropping as low as 25% of air saturation within 20s. The decrease in DO of the PBS during ADV was dependent on the volumetric size distribution of the droplets and the fraction of droplets transitioned during ultrasound exposure. Numerically predicted changes in DO from the model agreed with the experimentally measured DO, indicating that concentration gradients can explain this phenomenon. Using computationally modified droplet size distributions that would be suitable for in vivo applications, the DO of the PBS was found to decrease with increasing concentrations. This study demonstrates that ADV can significantly decrease the DO in an aqueous fluid, which may have direct therapeutic applications and should be considered for ADV-based diagnostic or therapeutic applications. Copyright © 2016 Elsevier B.V. All rights reserved.

Scavenging dissolved oxygen via acoustic droplet vaporization

PubMed Central

Radhakrishnan, Kirthi; Holland, Christy K.; Haworth, Kevin J.

2016-01-01

Acoustic droplet vaporization (ADV) of perfluorocarbon emulsions has been explored for diagnostic and therapeutic applications. Previous studies have demonstrated that vaporization of a liquid droplet results in a gas microbubble with a diameter 5 to 6 times larger than the initial droplet diameter. The expansion factor can increase to a factor of 10 in gassy fluids as a result of air diffusing from the surrounding fluid into the microbubble. This study investigates the potential of this process to serve as an ultrasound-mediated gas scavenging technology. Perfluoropentane droplets diluted in phosphate-buffered saline (PBS) were insonified by a 2 MHz transducer at peak rarefactional pressures lower than and greater than the ADV pressure amplitude threshold in an in vitro flow phantom. The change in dissolved oxygen (DO) of the PBS before and after ADV was measured. A numerical model of gas scavenging, based on conservation of mass and equal partial pressures of gases at equilibrium, was developed. At insonation pressures exceeding the ADV threshold, the DO of air-saturated PBS decreased with increasing insonation pressures, dropping as low as 25% of air saturation within 20 s. The decrease in DO of the PBS during ADV was dependent on the volumetric size distribution of the droplets and the fraction of droplets transitioned during ultrasound exposure. Numerically predicted changes in DO from the model agreed with the experimentally measured DO, indicating that concentration gradients can explain this phenomenon. Using computationally modified droplet size distributions that would be suitable for in vivo applications, the DO of the PBS was found to decrease with increasing concentrations. This study demonstrates that ADV can significantly decrease the DO in an aqueous fluid, which may have direct therapeutic applications and should be considered for ADV-based diagnostic or therapeutic applications. PMID:26964964

Efficient demulsification of oil-in-water emulsions using a zeolitic imidazolate framework: Adsorptive removal of oil droplets from water.

PubMed

Lin, Kun-Yi Andrew; Chen, Yu-Chien; Phattarapattamawong, Songkeart

2016-09-15

To demulsify oil-in-water (O/W) emulsions, a zinc-based zeolitic imidazolate framework (ZIF-8) was employed for the first time to remove oil droplets from water. ZIF-8 exhibits a high surface area and positive surface charges, making it a suitable adsorbent to adsorb negatively-charged oil droplets. Adsorption behaviors of oil droplets to ZIF-8 were studied by analyzing the adsorption kinetics and isotherm with theoretical models. The activation energy of adsorption of oil droplets to ZIF-8 was determined as 24.1kJmol(-1). The Langmuir-Freundlich (L-F) model was found to be most applicable to interpret the isotherm data and the predicated maximum adsorption capacity of ZIF-8 can reach 6633mgg(-1), revealing a promising capability of ZIF-8 for demulsification. Factors influencing the adsorption of oil droplets to ZIF-8 were investigated including temperature, pH, salt and surfactants. The adsorption capacity of ZIF-8 for oil was improved at elevated temperatures, whereas alkaline condition was unfavorable for the adsorption of oil droplets due to the electrostatic repulsion at high pH. The adsorption capacity of ZIF-8 remained similar in the presence of NaCl but it was reduced in the presence of surfactants. ZIF-8 was regenerated by a simple ethanol-washing method; the regenerated ZIF-8 exhibited more than 85% of regeneration efficiency over six cycles. Its crystalline structure also remained intact after the regeneration. These characteristics indicate that ZIF-8 can be a promising and effective adsorbent to remove oil droplets for demulsification of O/W emulsions. Copyright © 2016 Elsevier Inc. All rights reserved.

Equilibrium Droplets on Deformable Substrates: Equilibrium Conditions.

PubMed

Koursari, Nektaria; Ahmed, Gulraiz; Starov, Victor M

2018-05-15

Equilibrium conditions of droplets on deformable substrates are investigated, and it is proven using Jacobi's sufficient condition that the obtained solutions really provide equilibrium profiles of both the droplet and the deformed support. At the equilibrium, the excess free energy of the system should have a minimum value, which means that both necessary and sufficient conditions of the minimum should be fulfilled. Only in this case, the obtained profiles provide the minimum of the excess free energy. The necessary condition of the equilibrium means that the first variation of the excess free energy should vanish, and the second variation should be positive. Unfortunately, the mentioned two conditions are not the proof that the obtained profiles correspond to the minimum of the excess free energy and they could not be. It is necessary to check whether the sufficient condition of the equilibrium (Jacobi's condition) is satisfied. To the best of our knowledge Jacobi's condition has never been verified for any already published equilibrium profiles of both the droplet and the deformable substrate. A simple model of the equilibrium droplet on the deformable substrate is considered, and it is shown that the deduced profiles of the equilibrium droplet and deformable substrate satisfy the Jacobi's condition, that is, really provide the minimum to the excess free energy of the system. To simplify calculations, a simplified linear disjoining/conjoining pressure isotherm is adopted for the calculations. It is shown that both necessary and sufficient conditions for equilibrium are satisfied. For the first time, validity of the Jacobi's condition is verified. The latter proves that the developed model really provides (i) the minimum of the excess free energy of the system droplet/deformable substrate and (ii) equilibrium profiles of both the droplet and the deformable substrate.

The relation between moisture and liquid water content in fog - an experimental approach

NASA Astrophysics Data System (ADS)

Gonser, S.; Griessbaum, F.; Chang, S.-C.; Chu, H.-S.; Hsia, Y.-J.; Klemm, O.

2010-07-01

In July 2009, microphysical measurements of orographic fog were performed above a montane cloud forest in north eastern Taiwan (Chilan mountain site). At this location, orographic fog appears almost every day. The goal of this work was to study the short term variations of the droplet size distribution (DSD), temperature, and relative humidity (RH), with a temporal resolution of 3 Hz. The relative humidity was calculated from precise temperature readings and absolute humidity (AH) measurements, recorded by a temperature sensor with an accuracy of 0.002°C (Model TR-1050, RBR Ltd.) and an infrared gas analyzer (Model Li-7500, LI-COR Biosciences), respectively. DSD's were measured by an optical droplet spectrometer (Model FM100, Droplet Measurement Technologies). It provides droplet spectra between 2 and 50 µm diameter. The liquid water content (LWC) of the fog was deduced from the measured DSD's. Data showed that orographic fog is composed of various air parcels of different size, RH and DSD. Three general types of fog parcels have been identified via the recorded DSD’s. DSD-type 1 is characterized by narrow spectra with maximum concentrations in the smallest size class and a continuous decrease towards greater diameters, DSD-type 2 represents slightly broader spectra with a plateau or second peak around 10 µm, and DSD-type 3 exhibits broad spectra with the droplet number concentrations peaking around 15 µm diameter. The appearance of the three different DSD-types is strongly related to RH and the general evolutional state of the fog. At the onset of a fog event, DSD’s are largely dominated by small droplets (DSD-type 1). Later on the spectra tend to become broader, RH shows relative low values, and DSD-type 3 is dominating the DSD’s. A statistical analysis of the characteristics of these parcels was performed and yielded large variability in persistence, RH, and LWC. DSD-type 2 showed the shortest durations and can, therefore, be regarded as a transitional state between the other two DSD-types. Further, the 3 Hz data revealed an inverse relation between RH and LWC. In principle, this finding can be explained by the diffusional growth theory for droplets containing soluble or insoluble materials. Droplets with greater diameters are able to exist at lower ambient RH’s than smaller droplets. If, on the other hand, the large observed variabilities of both LWC and RH can be explained through these considerations, remains unclear so far. Likely, local disequilibrium between the fog droplets and the bulk gas phase during turbulent transport is a cause for the observed effects. More analyses including fog droplet chemistry and dynamic microphysical modeling are required to further analyze these findings. To our knowledge, this is the first experimental field observation of the anti-correlation between RH and LWC in fog.

Inertial migration of deformable droplets in a microchannel

NASA Astrophysics Data System (ADS)

Chen, Xiaodong; Xue, Chundong; Zhang, Li; Hu, Guoqing; Jiang, Xingyu; Sun, Jiashu

2014-11-01

The microfluidic inertial effect is an effective way of focusing and sorting droplets suspended in a carrier fluid in microchannels. To understand the flow dynamics of microscale droplet migration, we conduct numerical simulations on the droplet motion and deformation in a straight microchannel. The results are compared with preliminary experiments and theoretical analysis. In contrast to most existing literature, the present simulations are three-dimensional and full length in the streamwise direction and consider the confinement effects for a rectangular cross section. To thoroughly examine the effect of the velocity distribution, the release positions of single droplets are varied in a quarter of the channel cross section based on the geometrical symmetries. The migration dynamics and equilibrium positions of the droplets are obtained for different fluid velocities and droplet sizes. Droplets with diameters larger than half of the channel height migrate to the centerline in the height direction and two equilibrium positions are observed between the centerline and the wall in the width direction. In addition to the well-known Segré-Silberberg equilibrium positions, new equilibrium positions closer to the centerline are observed. This finding is validated by preliminary experiments that are designed to introduce droplets at different initial lateral positions. Small droplets also migrate to two equilibrium positions in the quarter of the channel cross section, but the coordinates in the width direction are between the centerline and the wall. The equilibrium positions move toward the centerlines with increasing Reynolds number due to increasing deformations of the droplets. The distributions of the lift forces, angular velocities, and the deformation parameters of droplets along the two confinement direction are investigated in detail. Comparisons are made with theoretical predictions to determine the fundamentals of droplet migration in microchannels. In addition, existence of the inner equilibrium position is linked to the quartic velocity distribution in the width direction through a simple model for the slip angular velocities of droplets.

Size resolved fog water chemistry and its atmospheric implications

NASA Astrophysics Data System (ADS)

Chakraborty, Abhishek; Gupta, Tarun; Tripathi, Sachchida; Ervens, Barbara; Bhattu, Deepika

2015-04-01

Fog is a natural meteorological phenomenon that occurs throughout the world. It usually contains substantial quantity of liquid water and results in severe visibility reduction leading to disruption of normal life. Fog is generally seen as a natural cleansing agent but it also has the potential to form Secondary Organic Aerosol (SOA) via aqueous processing of ambient aerosols. Size- resolved fog water chemistry for inorganics were reported in previous studies but processing of organics inside the fog water and quantification of aqSOA remained a challenge. To assess the organics processing via fog aqueous processing, size resolved fog water samples were collected in two consecutive winter seasons (2012-13, 2013-14) at Kanpur, a heavily polluted urban area of India. Caltech 3 stage fog collector was used to collect the fog droplets in 3 size fraction; coarse (droplet diameter > 22 µm), medium (22> droplet diameter >16 µm) and fine (16> droplet diameter >4 µm). Collected samples were atomized into various instruments such as Aerosol Mass Spectrometer (AMS), Cloud Condensation Nucleus Counter (CCNc), Total Organic Carbon (TOC) and a thermo denuder (TD) for the physico-chemical characterization of soluble constituents. Fine droplets are found to be more enriched with different aerosol species and interestingly contain more aged and less volatile organics compared to other coarser sizes. Organics inside fine droplets have an average O/C = 0.87 compared to O/C of 0.67 and 0.74 of coarse and medium droplets. Metal chemistry and higher residence time of fine droplets are seemed to be the two most likely reasons for this outcome from as the results of a comprehensive modeling carried out on the observed data indicate. CCN activities of the aerosols from fine droplets are also much higher than that of coarse or medium droplets. Fine droplets also contain light absorbing material as was obvious from their 'yellowish' solution. Source apportionment of fog water organics via PMF (Positive matrix factorization) revealed presence of some very highly oxidized OA inside fog water samples. From PMF results a method for aqSOA estimation is developed and aqSOA was found to be substantially contributing to total SOA. These findings indicate that light fog with large number of fine droplets can process the ambient aerosols more efficiently than very dense fog with larger droplets where scavenging becomes more important. These findings also highlight the need of incorporating fog size resolved chemistry along with metal chemistry into global models for accurately predicting aqSOA formation and contribution to total organic aerosol loading.

Enhancing droplet deposition through in-situ precipitation

PubMed Central

Damak, Maher; Mahmoudi, Seyed Reza; Hyder, Md Nasim; Varanasi, Kripa K.

2016-01-01

Retention of agricultural sprays on plant surfaces is an important challenge. Bouncing of sprayed pesticide droplets from leaves is a major source of soil and groundwater pollution and pesticide overuse. Here we report a method to increase droplet deposition through in-situ formation of hydrophilic surface defects that can arrest droplets during impact. Defects are created by simultaneously spraying oppositely charged polyelectrolytes that induce surface precipitation when two droplets come into contact. Using high-speed imaging, we study the coupled dynamics of drop impact and surface precipitate formation. We develop a physical model to estimate the energy dissipation by the defects and predict the transition from bouncing to sticking. We demonstrate macroscopic enhancements in spray retention and surface coverage for natural and synthetic non-wetting surfaces and provide insights into designing effective agricultural sprays. PMID:27572948

Cavitation-induced fragmentation of an acoustically-levitated droplet

NASA Astrophysics Data System (ADS)

Gonzalez Avila, Silvestre Roberto; Ohl, Claus-Dieter

2015-12-01

In this paper we investigate the initial sequence of events that lead to the fragmentation of a millimetre sized water droplets when interacting with a focused ns-laser pulse. The experimental results show complex processes that result from the reflection of an initial shock wave from plasma generation with the soft boundary of the levitating droplet; furthermore, when the reflected waves from the walls of the droplet refocus they leave behind a trail of microbubbles that later act as cavitation inception regions. Numerical simulations of a shock wave impacting and reflecting from a soft boundary are also reported; the simulated results show that the lowest pressure inside the droplet occurs at the equatorial plane. The results of the numerical model display good agreement with the experimental results both in time and in space.

Kinetic control of the coverage of oil droplets by DNA-functionalized colloids

PubMed Central

Joshi, Darshana; Bargteil, Dylan; Caciagli, Alessio; Burelbach, Jerome; Xing, Zhongyang; Nunes, André S.; Pinto, Diogo E. P.; Araújo, Nuno A. M.; Brujic, Jasna; Eiser, Erika

2016-01-01

We report a study of reversible adsorption of DNA-coated colloids on complementary functionalized oil droplets. We show that it is possible to control the surface coverage of oil droplets using colloidal particles by exploiting the fact that, during slow adsorption, compositional arrest takes place well before structural arrest occurs. As a consequence, we can prepare colloid-coated oil droplets with a “frozen” degree of loading but with fully ergodic colloidal dynamics on the droplets. We illustrate the equilibrium nature of the adsorbed colloidal phase by exploring the quasi–two-dimensional phase behavior of the adsorbed colloids under the influence of depletion interactions and present simulations of a simple model that illustrates the nature of the compositional arrest and the structural ergodicity. PMID:27532053

Enhancing droplet deposition through in-situ precipitation

NASA Astrophysics Data System (ADS)

Damak, Maher; Mahmoudi, Seyed Reza; Hyder, Md Nasim; Varanasi, Kripa K.

2016-08-01

Retention of agricultural sprays on plant surfaces is an important challenge. Bouncing of sprayed pesticide droplets from leaves is a major source of soil and groundwater pollution and pesticide overuse. Here we report a method to increase droplet deposition through in-situ formation of hydrophilic surface defects that can arrest droplets during impact. Defects are created by simultaneously spraying oppositely charged polyelectrolytes that induce surface precipitation when two droplets come into contact. Using high-speed imaging, we study the coupled dynamics of drop impact and surface precipitate formation. We develop a physical model to estimate the energy dissipation by the defects and predict the transition from bouncing to sticking. We demonstrate macroscopic enhancements in spray retention and surface coverage for natural and synthetic non-wetting surfaces and provide insights into designing effective agricultural sprays.

Liquid Droplet Dynamics in Gravity Compensating High Magnetic Field

NASA Technical Reports Server (NTRS)

Bojarevics, V.; Easter, S.; Pericleous, K.

2012-01-01

Numerical models are used to investigate behavior of liquid droplets suspended in high DC magnetic fields of various configurations providing microgravity-like conditions. Using a DC field it is possible to create conditions with laminar viscosity and heat transfer to measure viscosity, surface tension, electrical and thermal conductivities, and heat capacity of a liquid sample. The oscillations in a high DC magnetic field are quite different for an electrically conducting droplet, like liquid silicon or metal. The droplet behavior in a high magnetic field is the subject of investigation in this paper. At the high values of magnetic field some oscillation modes are damped quickly, while others are modified with a considerable shift of the oscillating droplet frequencies and the damping constants from the non-magnetic case.

Investigation of critical burning of fuel droplets

NASA Technical Reports Server (NTRS)

Faeth, G. M.

1979-01-01

The general problem of spray combustion was investigated. The combustion of bipropellent droplets; combustion of hydrozine fuels; and combustion of sprays were studied. A model was developed to predict mean velocities and temperatures in a combusting gas jet.

Yeast Droplets

NASA Astrophysics Data System (ADS)

Nguyen, Baochi; Upadhyaya, Arpita; van Oudenaarden, Alexander; Brenner, Michael

2002-11-01

It is well known that the Young's law and surface tension govern the shape of liquid droplets on solid surfaces. Here we address through experiments and theory the shape of growing aggregates of yeast on agar substrates, and assess whether these ideas still hold. Experiments are carried out on Baker's yeast, with different levels of expressions of an adhesive protein governing cell-cell and cell-substrate adhesion. Changing either the agar concentration or the expression of this protein modifies the local contact angle of a yeast droplet. When the colony is small, the shape is a spherical cap with the contact angle obeying Young's law. However, above a critical volume this structure is unstable, and the droplet becomes nonspherical. We present a theoretical model where this instability is caused by bulk elastic effects. The model predicts that the transition depends on both volume and contact angle, in a manner quantitatively consistent with our experiments.

Pattern-Directed Ordering of Spin-Dewetted Liquid Crystal Micro- or Nanodroplets as Pixelated Light Reflectors and Locomotives.

PubMed

Ravi, Bolleddu; Chakraborty, Snigdha; Bhattacharjee, Mitradip; Mitra, Shirsendu; Ghosh, Abir; Gooh Pattader, Partho Sarathi; Bandyopadhyay, Dipankar

2017-01-11

Chemical pattern directed spin-dewetting of a macroscopic droplet composed of a dilute organic solution of liquid crystal (LC) formed an ordered array of micro- and nanoscale LC droplets. Controlled evaporation of the spin-dewetted droplets through vacuum drying could further miniaturize the size to the level of ∼90 nm. The size, periodicity, and spacing of these mesoscale droplets could be tuned with the variations in the initial loading of LC in the organic solution, the strength of the centripetal force on the droplet, and the duration of the evaporation. A simple theoretical model was developed to predict the spacing between the spin-dewetted droplets. The patterned LC droplets showed a reversible phase transition from nematic to isotropic and vice versa with the periodic exposure of a solvent vapor and its removal. A similar phase transition behavior was also observed with the periodic increase or reduction of temperature, suggesting their usefulness as vapor or temperature sensors. Interestingly, when the spin-dewetted droplets were confined between a pair of electrodes and an external electric field was applied, the droplets situated at the hydrophobic patches showed light-reflecting properties under the polarization microscopy highlighting their importance in the development of micro- or nanoscale LC displays. The digitized LC droplets, which were stationary otherwise, showed dielectrophoretic locomotion under the guidance of the external electric field beyond a threshold intensity of the field. Remarkably, the motion of these droplets could be restricted to the hydrophilic zones, which were confined between the hydrophobic patches of the chemically patterned surface. The findings could significantly contribute in the development of futuristic vapor or temperature sensors, light reflectors, and self-propellers using the micro- or nanoscale digitized LC droplets.

Droplet Vaporization In A Levitating Acoustic Field

NASA Technical Reports Server (NTRS)

Ruff, G. A.; Liu, S.; Ciobanescu, I.

2003-01-01

Combustion experiments using arrays of droplets seek to provide a link between single droplet combustion phenomena and the behavior of complex spray combustion systems. Both single droplet and droplet array studies have been conducted in microgravity to better isolate the droplet interaction phenomena and eliminate or reduce the effects of buoyancy-induced convection. In most experiments involving droplet arrays, the droplets are supported on fibers to keep them stationary and close together before the combustion event. The presence of the fiber, however, disturbs the combustion process by introducing a source of heat transfer and asymmetry into the configuration. As the number of drops in a droplet array increases, supporting the drops on fibers becomes less practical because of the cumulative effect of the fibers on the combustion process. To eliminate the effect of the fiber, several researchers have conducted microgravity experiments using unsupported droplets. Jackson and Avedisian investigated single, unsupported drops while Nomura et al. studied droplet clouds formed by a condensation technique. The overall objective of this research is to extend the study of unsupported drops by investigating the combustion of well-characterized drop clusters in a microgravity environment. Direct experimental observations and measurements of the combustion of droplet clusters would provide unique experimental data for the verification and improvement of spray combustion models. In this work, the formation of drop clusters is precisely controlled using an acoustic levitation system so that dilute, as well as dense clusters can be created and stabilized before combustion in microgravity is begun. While the low-gravity test facility is being completed, tests have been conducted in 1-g to characterize the effect of the acoustic field on the vaporization of single and multiple droplets. This is important because in the combustion experiment, the droplets will be formed and levitated prior to ignition. Therefore, the droplets will begin to vaporize in the acoustic field thus forming the "initial conditions" for the combustion process. Understanding droplet vaporization in the acoustic field of this levitator is a necessary step that will help to interpret the experimental results obtained in low-gravity.

Impact of Metal Droplets: A Numerical Approach to Solidification

NASA Astrophysics Data System (ADS)

Koldeweij, Robin; Mandamparambil, Rajesh; Lohse, Detlef

2016-11-01

Layer-wise deposition of material to produce complex products is a subject of increasing technological relevance. Subsequent deposition of droplets is one of the possible 3d printing technologies to accomplish this. The shape of the solidified droplet is crucial for product quality. We employ the volume-of-fluid method (in the form of the open-source code Gerris) to study liquid metal (in particular tin) droplet impact. Heat transfer has been implemented based on the enthalpy approach for the liquid-solid phase. Solidification is modeled by adding a sink term to the momentum equations, reducing Navier-Stokes to Darcy's law for high solid fraction. Good agreement is found when validating the results against experimental data. We then map out a phase diagram in which we distinguish between solidification behavior based on Weber and Stefan number. In an intermediate impact regime impact, solidification due to a retracting phase occurs. In this regime the maximum spreading diameter almost exclusively depends on Weber number. Droplet shape oscillations lead to a broad variation of the morphology of the solidified droplet and determine the final droplet height. TNO.

MRI of perfluorocarbon emulsion kinetics in rodent mammary tumours

NASA Astrophysics Data System (ADS)

Fan, Xiaobing; River, Jonathan N.; Muresan, Adrian S.; Popescu, Carmen; Zamora, Marta; Culp, Rita M.; Karczmar, Gregory S.

2006-01-01

Perfluorocarbon (PFC) emulsions can be imaged directly by fluorine-19 MRI. We developed an optimized protocol for preparing PFC droplets of uniform size, evaluated use of the resulting droplets as blood pool contrast agents, studied their uptake by tumours and determined the spatial resolution with which they can be imaged at 4.7 T. Perfluorocarbon droplets of three different average sizes (324, 293 and 225 nm) were prepared using a microemulsifier. Images of PFC droplets with good signal-to-noise ratio were acquired with 625 µm in-plane resolution, 3 mm slice thickness and acquisition time of ~4.5 min per image. Kinetics of washout were determined using a simple mathematical model. The maximum uptake of the PFC droplets was three times greater at the tumour rim than in muscle, but the washout rate was two to three times slower in the tumour. The results are consistent with leakage of the droplets into the tumour extravascular space due to the hyper-permeability of tumour capillaries. PFC droplets may allow practical and quantitative measurements of blood volume and capillary permeability in tumours with reasonable spatial resolution.

Autonomous model protocell division driven by molecular replication.

PubMed

Taylor, J W; Eghtesadi, S A; Points, L J; Liu, T; Cronin, L

2017-08-10

The coupling of compartmentalisation with molecular replication is thought to be crucial for the emergence of the first evolvable chemical systems. Minimal artificial replicators have been designed based on molecular recognition, inspired by the template copying of DNA, but none yet have been coupled to compartmentalisation. Here, we present an oil-in-water droplet system comprising an amphiphilic imine dissolved in chloroform that catalyses its own formation by bringing together a hydrophilic and a hydrophobic precursor, which leads to repeated droplet division. We demonstrate that the presence of the amphiphilic replicator, by lowering the interfacial tension between droplets of the reaction mixture and the aqueous phase, causes them to divide. Periodic sampling by a droplet-robot demonstrates that the extent of fission is increased as the reaction progresses, producing more compartments with increased self-replication. This bridges a divide, showing how replication at the molecular level can be used to drive macroscale droplet fission.Coupling compartmentalisation and molecular replication is essential for the development of evolving chemical systems. Here the authors show an oil-in-water droplet containing a self-replicating amphiphilic imine that can undergo repeated droplet division.

Vitrification and levitation of a liquid droplet on liquid nitrogen.

PubMed

Song, Young S; Adler, Douglas; Xu, Feng; Kayaalp, Emre; Nureddin, Aida; Anchan, Raymond M; Maas, Richard L; Demirci, Utkan

2010-03-09

The vitrification of a liquid occurs when ice crystal formation is prevented in the cryogenic environment through ultrarapid cooling. In general, vitrification entails a large temperature difference between the liquid and its surrounding medium. In our droplet vitrification experiments, we observed that such vitrification events are accompanied by a Leidenfrost phenomenon, which impedes the heat transfer to cool the liquid, when the liquid droplet comes into direct contact with liquid nitrogen. This is distinct from the more generally observed Leidenfrost phenomenon that occurs when a liquid droplet is self-vaporized on a hot plate. In the case of rapid cooling, the phase transition from liquid to vitrified solid (i.e., vitrification) and the levitation of droplets on liquid nitrogen (i.e., Leidenfrost phenomenon) take place simultaneously. Here, we investigate these two simultaneous physical events by using a theoretical model containing three dimensionless parameters (i.e., Stefan, Biot, and Fourier numbers). We explain theoretically and observe experimentally a threshold droplet radius during the vitrification of a cryoprotectant droplet in the presence of the Leidenfrost effect.

Vitrification and levitation of a liquid droplet on liquid nitrogen

PubMed Central

Song, Young S.; Adler, Douglas; Xu, Feng; Kayaalp, Emre; Nureddin, Aida; Anchan, Raymond M.; Maas, Richard L.; Demirci, Utkan

2010-01-01

The vitrification of a liquid occurs when ice crystal formation is prevented in the cryogenic environment through ultrarapid cooling. In general, vitrification entails a large temperature difference between the liquid and its surrounding medium. In our droplet vitrification experiments, we observed that such vitrification events are accompanied by a Leidenfrost phenomenon, which impedes the heat transfer to cool the liquid, when the liquid droplet comes into direct contact with liquid nitrogen. This is distinct from the more generally observed Leidenfrost phenomenon that occurs when a liquid droplet is self-vaporized on a hot plate. In the case of rapid cooling, the phase transition from liquid to vitrified solid (i.e., vitrification) and the levitation of droplets on liquid nitrogen (i.e., Leidenfrost phenomenon) take place simultaneously. Here, we investigate these two simultaneous physical events by using a theoretical model containing three dimensionless parameters (i.e., Stefan, Biot, and Fourier numbers). We explain theoretically and observe experimentally a threshold droplet radius during the vitrification of a cryoprotectant droplet in the presence of the Leidenfrost effect. PMID:20176969

Impact of viscous droplets on different wettable surfaces: Impact phenomena, the maximum spreading factor, spreading time and post-impact oscillation.

PubMed

Lin, Shiji; Zhao, Binyu; Zou, Song; Guo, Jianwei; Wei, Zheng; Chen, Longquan

2018-04-15

In this paper, we experimentally investigated the impact dynamics of different viscous droplets on solid surfaces with diverse wettabilities. We show that the outcome of an impinging droplet is dependent on the physical property of the droplet and the wettability of the surface. Whereas only deposition was observed on lyophilic surfaces, more impact phenomena were identified on lyophobic and superlyophobic surfaces. It was found that none of the existing theoretical models can well describe the maximum spreading factor, revealing the complexity of the droplet impact dynamics and suggesting that more factors need to be considered in the theory. By using the modified capillary-inertial time, which considers the effects of liquid viscosity and surface wettability on droplet spreading, a universal scaling law describing the spreading time was obtained. Finally, we analyzed the post-impact droplet oscillation with the theory for damped harmonic oscillators and interpreted the effects of liquid viscosity and surface wettability on the oscillation by simple scaling analyses. Copyright © 2017 Elsevier Inc. All rights reserved.

High pressure droplet burning experiments in reduced gravity

NASA Technical Reports Server (NTRS)

Chauveau, Christian; Goekalp, Iskender

1995-01-01

A parametric investigation of single droplet gasification regimes is helpful in providing the necessary physical ideas for sub-grid models used in spray combustion numerical prediction codes. A research program has been initiated at the LCSR to explore the vaporization regimes of single and interacting hydrocarbon and liquid oxygen droplets under high pressure conditions. This paper summarizes the status of the LCSR program on the high pressure burning of single fuel droplets; recent results obtained under normal and reduced gravity conditions with suspended droplets are presented. In the work described here, parabolic flights of the CNES Caravelle is used to create a reduced gravity environment of the order of 10(exp -2) g(sub O). For all the droplet burning experiments reported here, the suspended droplet initial diameters are scattered around 1.5 mm; and the ambient air temperature is 300 K. The ambient pressure is varied between 0.1 MPa and 12 MPa. Four fuels are investigated: methanol (Pc = 7.9 MPa), n-heptane (Pc = 2.74 MPa), n-hexane (Pc = 3.01 MPa) and n-octane (Pc = 2.48 MPa).

Repressive effects of oat extracts on intracellular lipid-droplet formation in adipocytes and a three-dimensional subcutaneous adipose tissue model.

PubMed

Kato, Shinya; Kato, Yuko; Shibata, Hiroki; Saitoh, Yasukazu; Miwa, Nobuhiko

2015-04-01

We assessed the repression of lipid-droplet formation in mouse mesenchymal stromal preadipocytes OP9 by specified oat extracts (Hatomugi, Coix lacryma-jobi var. ma-yuen) named "SPH" which were proteolytically and glucosyl-transferredly prepared from finely-milled oat whole-grain. Stimulation of OP9 preadipocytes with insulin-containing serum-replacement promoted differentiation to adipocytes, concurrently with an increase in the intracellular lipid droplets by 51.5%, which were repressed by SPH-bulk or SPH-water-extract at 840ppm, to 33.5% or 46.9%, respectively, but not by SPH-ethanol-extract at the same dose, showing the hydrophilic property of the anti-adipogenetic ingredients. The intracellular lipid droplets were scanty for intact preadipocytes, small-sized but abundant for the SPH-unadministered adipocytes, and large-sized but few for SPH-bulk-administered adipocytes being coexistent with many lipid-droplet-lacking viable cells, suggesting "the all-or-none rule" for lipid-droplet generation in cell-to-cell. Hydrogen-peroxide-induced cell death in human epidermal keratinocytes HaCaT was prevented by SPH-bulk at 100 or 150ppm by 5.6-8.1%, being consistent with higher viabilities of SPH-bulk-administered OP9 cells, together with repressions of both cell shrinkage and cell detachment from the culture substratum. In three-dimensional subcutaneous adipose tissue models reconstructed with HaCaT-keratinocytes and OP9-preadipocytes, lipid droplets were accumulated in dermal OP9-cell-parts, and repressed to 43.5% by SPH-bulk at 840ppm concurrently with marked diminishment of huge aggregates of lipid droplets. Thus SPH-bulk suppresses adipogenesis-associated lipid-droplet accumulation during differentiation of OP9 preadipocytes together with lowered cytotoxicity to either HaCaT keratinocytes or the preadipocytes. Copyright © 2015 Elsevier B.V. All rights reserved.

Analysis of isothermal and cooling-rate-dependent immersion freezing by a unifying stochastic ice nucleation model

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

Alpert, Peter A.; Knopf, Daniel A.

Immersion freezing is an important ice nucleation pathway involved in the formation of cirrus and mixed-phase clouds. Laboratory immersion freezing experiments are necessary to determine the range in temperature, T, and relative humidity, RH, at which ice nucleation occurs and to quantify the associated nucleation kinetics. Typically, isothermal (applying a constant temperature) and cooling-rate-dependent immersion freezing experiments are conducted. In these experiments it is usually assumed that the droplets containing ice nucleating particles (INPs) all have the same INP surface area (ISA); however, the validity of this assumption or the impact it may have on analysis and interpretation of the experimentalmore » data is rarely questioned. Descriptions of ice active sites and variability of contact angles have been successfully formulated to describe ice nucleation experimental data in previous research; however, we consider the ability of a stochastic freezing model founded on classical nucleation theory to reproduce previous results and to explain experimental uncertainties and data scatter. A stochastic immersion freezing model based on first principles of statistics is presented, which accounts for variable ISA per droplet and uses parameters including the total number of droplets, N tot, and the heterogeneous ice nucleation rate coefficient, J het( T). This model is applied to address if (i) a time and ISA-dependent stochastic immersion freezing process can explain laboratory immersion freezing data for different experimental methods and (ii) the assumption that all droplets contain identical ISA is a valid conjecture with subsequent consequences for analysis and interpretation of immersion freezing. The simple stochastic model can reproduce the observed time and surface area dependence in immersion freezing experiments for a variety of methods such as: droplets on a cold-stage exposed to air or surrounded by an oil matrix, wind and acoustically levitated droplets, droplets in a continuous-flow diffusion chamber (CFDC), the Leipzig aerosol cloud interaction simulator (LACIS), and the aerosol interaction and dynamics in the atmosphere (AIDA) cloud chamber. Observed time-dependent isothermal frozen fractions exhibiting non-exponential behavior can be readily explained by this model considering varying ISA. An apparent cooling-rate dependence of J het is explained by assuming identical ISA in each droplet. When accounting for ISA variability, the cooling-rate dependence of ice nucleation kinetics vanishes as expected from classical nucleation theory. Finally, the model simulations allow for a quantitative experimental uncertainty analysis for parameters N tot, T, RH, and the ISA variability. We discuss the implications of our results for experimental analysis and interpretation of the immersion freezing process.« less

Analysis of isothermal and cooling-rate-dependent immersion freezing by a unifying stochastic ice nucleation model

DOE PAGES

Alpert, Peter A.; Knopf, Daniel A.

2016-02-24

Immersion freezing is an important ice nucleation pathway involved in the formation of cirrus and mixed-phase clouds. Laboratory immersion freezing experiments are necessary to determine the range in temperature, T, and relative humidity, RH, at which ice nucleation occurs and to quantify the associated nucleation kinetics. Typically, isothermal (applying a constant temperature) and cooling-rate-dependent immersion freezing experiments are conducted. In these experiments it is usually assumed that the droplets containing ice nucleating particles (INPs) all have the same INP surface area (ISA); however, the validity of this assumption or the impact it may have on analysis and interpretation of the experimentalmore » data is rarely questioned. Descriptions of ice active sites and variability of contact angles have been successfully formulated to describe ice nucleation experimental data in previous research; however, we consider the ability of a stochastic freezing model founded on classical nucleation theory to reproduce previous results and to explain experimental uncertainties and data scatter. A stochastic immersion freezing model based on first principles of statistics is presented, which accounts for variable ISA per droplet and uses parameters including the total number of droplets, N tot, and the heterogeneous ice nucleation rate coefficient, J het( T). This model is applied to address if (i) a time and ISA-dependent stochastic immersion freezing process can explain laboratory immersion freezing data for different experimental methods and (ii) the assumption that all droplets contain identical ISA is a valid conjecture with subsequent consequences for analysis and interpretation of immersion freezing. The simple stochastic model can reproduce the observed time and surface area dependence in immersion freezing experiments for a variety of methods such as: droplets on a cold-stage exposed to air or surrounded by an oil matrix, wind and acoustically levitated droplets, droplets in a continuous-flow diffusion chamber (CFDC), the Leipzig aerosol cloud interaction simulator (LACIS), and the aerosol interaction and dynamics in the atmosphere (AIDA) cloud chamber. Observed time-dependent isothermal frozen fractions exhibiting non-exponential behavior can be readily explained by this model considering varying ISA. An apparent cooling-rate dependence of J het is explained by assuming identical ISA in each droplet. When accounting for ISA variability, the cooling-rate dependence of ice nucleation kinetics vanishes as expected from classical nucleation theory. Finally, the model simulations allow for a quantitative experimental uncertainty analysis for parameters N tot, T, RH, and the ISA variability. We discuss the implications of our results for experimental analysis and interpretation of the immersion freezing process.« less

Swimming with a cage: low-Reynolds-number locomotion inside a droplet.

PubMed

Reigh, Shang Yik; Zhu, Lailai; Gallaire, François; Lauga, Eric

2017-05-03

Inspired by recent experiments using synthetic microswimmers to manipulate droplets, we investigate the low-Reynolds-number locomotion of a model swimmer (a spherical squirmer) encapsulated inside a droplet of a comparable size in another viscous fluid. Meditated solely by hydrodynamic interactions, the encaged swimmer is seen to be able to propel the droplet, and in some situations both remain in a stable co-swimming state. The problem is tackled using both an exact analytical theory and a numerical implementation based on a boundary element method, with a particular focus on the kinematics of the co-moving swimmer and the droplet in a concentric configuration, and we obtain excellent quantitative agreement between the two. The droplet always moves slower than a swimmer which uses purely tangential surface actuation but when it uses a particular combination of tangential and normal actuations, the squirmer and droplet are able to attain the same velocity and stay concentric for all times. We next employ numerical simulations to examine the stability of their concentric co-movement, and highlight several stability scenarios depending on the particular gait adopted by the swimmer. Furthermore, we show that the droplet reverses the nature of the far-field flow induced by the swimmer: a droplet cage turns a pusher swimmer into a puller, and vice versa. Our work sheds light on the potential development of droplets as self-contained carriers of both chemical content and self-propelled devices for controllable and precise drug deliveries.

Anvil Glaciation in a Deep Cumulus Updraught over Florida Simulated with the Explicit Microphysics Model. I: Impact of Various Nucleation Processes

NASA Technical Reports Server (NTRS)

Phillips, Vaughan T. J.; Andronache, Constantin; Sherwood, Steven C.; Bansemer, Aaron; Conant, William C.; Demott, Paul J.; Flagan, Richard C.; Heymsfield, Andy; Jonsson, Haflidi; Poellot, Micheal;

Transient deformation of a droplet near a microfluidic constriction: A quantitative analysis

NASA Astrophysics Data System (ADS)

Trégouët, Corentin; Salez, Thomas; Monteux, Cécile; Reyssat, Mathilde

2018-05-01

We report on experiments that consist of deforming a collection of monodisperse droplets produced by a microfluidic chip through a flow-focusing device. We show that a proper numerical modeling of the flow is necessary to access the stress applied by the latter on the droplet along its trajectory through the chip. This crucial step enables the full integration of the differential equation governing the dynamical deformation, and consequently the robust measurement of the interfacial tension by fitting the experiments with the calculated deformation. Our study thus demonstrates the feasibility of quantitative in situ rheology in microfluidic flows involving, e.g., droplets, capsules, or cells.

Amplifying and attenuating the coffee-ring effect in drying sessile nanofluid droplets

NASA Astrophysics Data System (ADS)

Crivoi, A.; Duan, Fei

2013-04-01

Experiments and simulations to promote or attenuate the “coffee-ring effect” for pinned sessile nanofluid droplets are presented. The addition of surfactant inside a water suspension of aluminum oxide nanoparticles results in coffee-ring formation after the pinned sessile droplets are fully dried on a substrate, while droplets of the same suspension without the surfactant produce a fine uniform coverage. A mathematical model based on diffusion-limited cluster-cluster aggregation has been developed to explain the observed difference in the experiments. The simulations show that the particle sticking probability is a crucial factor on the morphology of finally dried structures.

Fluid Dynamics and Solidification of Molten Solder Droplets Impacting on a Substrate in Microgravity

NASA Technical Reports Server (NTRS)

Megardis, C. M.; Poulikakos, D.; Diversiev, G.; Boomsma, K.; Xiong, B.; Nayagam, V.

1999-01-01

This program investigates the fluid dynamics and simultaneous solidification of molten solder droplets impacting on a flat smooth substrate. The problem of interest is directly relevant to the printing of microscopic solder droplets in surface mounting of microelectronic devices. The study consists of a theoretical and an experimental component. The theoretical work uses axisymmetric Navier-Stokes models based on finite element techniques. The experimental work will be ultimately performed in microgravity in order to allow for the use of larger solder droplets which make feasible the performance of accurate measurements, while maintaining similitude of the relevant fluid dynamics groups (Re, We).

Fluid Dynamics and Solidification of Molten Solder Droplets Impacting on a Substrate in Microgravity

NASA Technical Reports Server (NTRS)

Poulikakos, Dimos; Megaridis, Constantine M.; Vedha-Nayagam, M.

1996-01-01

This program investigates the fluid dynamics and simultaneous solidification of molten solder droplets impacting on a flat substrate. The problem of interest is directly relevant to the printing of microscopic solder droplets in surface mounting of microelectronic devices. The study consists of a theoretical and an experimental component. The theoretical work uses axisymmetric Navier-Stokes models based on finite element techniques. The experimental work is performed in microgravity to allow for the use of larger solder droplets that make feasible the performance of accurate measurements while maintaining similitude of the relevant fluid dynamics groups (Re, We) and keeping the effect of gravity negligible.

Lox droplet vaporization in a supercritical forced convective environment

NASA Technical Reports Server (NTRS)

Hsiao, Chia-Chun; Yang, Vigor

1994-01-01

A systematic investigation has been conducted to study the effects of ambient flow conditions (i.e. pressure and velocity) on supercritical droplet gasification in a forced-convective environment. The model is based on the time-dependent conservation equations in axisymmetric coordinates, and accommodates thermodynamic nonidealities and transport anomalies. In addition, an efficient scheme for evaluating thermophysical properties over the entire range of fluid thermodynamic states is established. The analysis allows a thorough examination of droplet behavior during its entire lifetime, including transient gasification, dynamic deformation, and shattering. A parametric study of droplet vaporization rate in terms of ambient pressure and Reynolds number is also conducted.

Absorption of charged particulate surfactants in microfluidics

NASA Astrophysics Data System (ADS)

Kong, Tiantian; Liu, Zhou; Yao, Xiaoxue; Liu, Yaming

2017-11-01

We use microfluidics to uncouple the generation of Pickering emulsion droplets and stability analysis against coalescence. By designing the microchannels, we control the packing time for charged particles arriving at the droplet interfaces, and subsequently test the droplet stability in a coalescence chamber. The critical particle coverage on interfaces that prevents coalescence are estimated by an adsorption model. We further investigate the dependence of the critical particle coverage on its properties such as particle sizes, surface charge densities, and bulk concentrations. Our studies are potentially beneficial to the applications involving particle-stabilized droplets including cosmetics, food products, and oil recovery. NSFC 11504238,JCYJ20160308092144035,2016A050503048.

Nucleation of an oil phase in a nonionic microemulsion-containing chlorinated oil upon systematic temperature quench.

PubMed

Deen, G Roshan; Pedersen, Jan Skov

2010-06-17

A clear and stable nonionic model microemulsion consisting of pentaoxyethylene glycol dodecyl ether (C(12)E(5)), water, and 1-chlorotetradecane (CLTD) was prepared. This system was subjected to a systematic temperature quench (perturbation out of equilibrium) in steps of 1.0 degrees C from 20.4 to 15.3 degrees C in the unstable region of its phase diagram. The change in turbidity (for droplet volume fractions of 0.02 and 0.08) and hydrodynamic radius (R(h)) (for a droplet volume fraction of 0.02) of the system on its way to its new equilibrium was measured at each quench temperature. For small systematic temperature quenches just below the emulsification failure boundary (EFB) the turbidity decreases and remains constant indicating quick changes in the microstructures. Further lowering of temperature brings the system to the unstable region where the turbidity and light scattering increase sharply as function of time because of expulsion of excess oil from the microemulsion droplets. The newly formed oil-rich droplets grow in size as a function of time. These observations indicate the existence of a narrow but observable metastable region en route to the new equilibrium where both microemulsion droplets and larger oil-rich droplets coexist. The region in which microemulsion droplets are metastable is very narrow and is concentration-dependent. The presence of a metastable region is as for other similar systems attributed to the presence of a free energy barrier for the formation of the larger oil-rich droplets associated with curvature free energy of the surfactant film. The turbidity-time curves were converted to the radius-time curves using a model assuming monodisperse spherical droplets. The obtained results are in good agreement with the results for the hydrodynamic radius. The observed average radius from both type of measurements decreases in the metastable region. By performing calculation of the influence of eccentricity and size polydispersity on the observed radius, we have shown that the distribution of the microemulsion droplets becomes more homogeneous in the metastable region.

Optical diagnostics and computational modeling of reacting and non-reacting single and multiphase flows

NASA Astrophysics Data System (ADS)

Basu, Saptarshi

Three critical problem domains namely water transport in PEM fuel cell, interaction of vortices with diffusion flames and laminar diffusion layers and thermo-physical processes in droplets heated by a plasma or monochromatic radiation have been analyzed in this dissertation. The first part of the dissertation exhibits a unique, in situ, line-of-sight measurements of water vapor partial pressure and temperature in single and multiple gas channels on the cathode side of an operating PEM fuel cell. Tunable diode laser absorption spectroscopy was employed for these measurements for which water transitions sensitive to temperature and partial pressure were utilized. The technique was demonstrated in a PEM fuel cell operating under both steady state and time-varying load conditions. The second part of the dissertation is dedicated to the study of vortex interaction with laminar diffusion flame and non-reacting diffusion layers. For the non-reacting case, a detailed computational study of scalar mixing in a laminar vortex is presented for vortices generated between two gas streams. A detailed parametric study was conducted to determine the effects of vortex strength, convection time, and non-uniform temperature on scalar mixing characteristics. For the reacting case, an experimental study of the interaction of a planar diffusion flame with a line vortex is presented. The flame-vortex interactions are diagnosed by laser induced incandescence for soot yield and by particle image velocimetry for vortex flow characterization. The soot topography was studied as a function of the vortex strength, residence time, flame curvature and the reactant streams from which vortices are initiated. The third part of the dissertation is modeling of thermo-physical processes in liquid ceramic precursor droplets injected into plasma as used in the thermal spray industry to generate thermal barrier coatings on high value materials. Models include aerodynamic droplet break-up process, mixing of droplets in the high temperature plasma, heat and mass transfer within individual droplets as well as droplet precipitation and internal pressurization. The last part of the work is also concerned with the modeling of thermo-physical processes in liquid ceramic precursor droplets heated by monochromatic radiation. Purpose of this work was to evaluate the feasibility of studying precipitation kinetics and morphological changes in a droplet by mimicking similar heating rates as the plasma.

Proteome Analysis of Cytoplasmatic and Plastidic β-Carotene Lipid Droplets in Dunaliella bardawil1[OPEN

PubMed Central

Davidi, Lital; Levin, Yishai; Ben-Dor, Shifra; Pick, Uri

2015-01-01

The halotolerant green alga Dunaliella bardawil is unique in that it accumulates under stress two types of lipid droplets: cytoplasmatic lipid droplets (CLD) and β-carotene-rich (βC) plastoglobuli. Recently, we isolated and analyzed the lipid and pigment compositions of these lipid droplets. Here, we describe their proteome analysis. A contamination filter and an enrichment filter were utilized to define core proteins. A proteome database of Dunaliella salina/D. bardawil was constructed to aid the identification of lipid droplet proteins. A total of 124 and 42 core proteins were identified in βC-plastoglobuli and CLD, respectively, with only eight common proteins. Dunaliella spp. CLD resemble cytoplasmic droplets from Chlamydomonas reinhardtii and contain major lipid droplet-associated protein and enzymes involved in lipid and sterol metabolism. The βC-plastoglobuli proteome resembles the C. reinhardtii eyespot and Arabidopsis (Arabidopsis thaliana) plastoglobule proteomes and contains carotene-globule-associated protein, plastid-lipid-associated protein-fibrillins, SOUL heme-binding proteins, phytyl ester synthases, β-carotene biosynthesis enzymes, and proteins involved in membrane remodeling/lipid droplet biogenesis: VESICLE-INDUCING PLASTID PROTEIN1, synaptotagmin, and the eyespot assembly proteins EYE3 and SOUL3. Based on these and previous results, we propose models for the biogenesis of βC-plastoglobuli and the biosynthesis of β-carotene within βC-plastoglobuli and hypothesize that βC-plastoglobuli evolved from eyespot lipid droplets. PMID:25404729

Cool-flame Extinction During N-Alkane Droplet Combustion in Microgravity

NASA Technical Reports Server (NTRS)

Nayagam, Vedha; Dietrich, Daniel L.; Hicks, Michael C.; Williams, Forman A.

2014-01-01

Recent droplet combustion experiments onboard the International Space Station (ISS) have revealed that large n-alkane droplets can continue to burn quasi-steadily following radiative extinction in a low-temperature regime, characterized by negative-temperaturecoefficient (NTC) chemistry. In this study we report experimental observations of n-heptane, n-octane, and n-decane droplets of varying initial sizes burning in oxygen/nitrogen/carbon dioxide and oxygen/helium/nitrogen environments at 1.0, 0.7, and 0.5 atmospheric pressures. The oxygen concentration in these tests varied in the range of 14% to 25% by volume. Large n-alkane droplets exhibited quasi-steady low-temperature burning and extinction following radiative extinction of the visible flame while smaller droplets burned to completion or disruptively extinguished. A vapor-cloud formed in most cases slightly prior to or following the "cool flame" extinction. Results for droplet burning rates in both the hot-flame and cool-flame regimes as well as droplet extinction diameters at the end of each stage are presented. Time histories of radiant emission from the droplet captured using broadband radiometers are also presented. Remarkably the "cool flame" extinction diameters for all the three n-alkanes follow a trend reminiscent of the ignition delay times observed in previous studies. The similarities and differences among the n-alkanes during "cool flame" combustion are discussed using simplified theoretical models of the phenomenon

Modeling of SSME fuel preburner ASI

NASA Technical Reports Server (NTRS)

Liang, Pak-Yan

1992-01-01

The Augmented Spark Ignitor (ASI) is a LOX/H2/electrical spark system that functions as an ignition source and sustainer for stable combustion. It is used in the Space Shuttle Main Engine (SSME) preburner combustor, the SMME main combustion chamber, the J-1 and J-2 engines, as well as proposed designs of the Space Transportation Main Engine (STME) main combustor and gas generators. An undertaking to characterize the flow of the ASI is documented. The code consists of a marriage of the Implicit-Continuous Eulerian/Arbitrary Lagrangian Code (ICE-ALE) Navier-Stokes solver with the Volume-of-Fluid (VOF) Methodology for tracking of two immiscible fluids with sharp discontinuities. Spray droplets are represented by discrete numerical parcels tracked in a Lagrangian fashion. Numerous physical sub-models are also incorporated to describe the processes of atomization, droplet collision, droplet breakup, evaporation, and droplet and gas phase turbulence. An equilibrium chemistry model accounting for 8 active gaseous species is also used. Taking advantage of this symmetry plane, half of the actual ASI is modeled with a 3-D grid that geometrically resolves the LOX ports, the spark plug locations, and the hydrogen injection slots.

Computational and Experimental Investigation of Interfacial Area in Near-Field Diesel Spray Simulation

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

Pandal, Adrian; Pastor, Jose M.; Payri, Raul

The dense spray region in the near-field of diesel fuel injection remains an enigma. This region is difficult to interrogate with light in the visible range and difficult to model due to the rapid interaction between liquid and gas. In particular, modeling strategies that rely on Lagrangian particle tracking of droplets have struggled in this area. To better represent the strong interaction between phases, Eulerian modeling has proven particularly useful. Models built on the concept of surface area density are advantageous where primary and secondary atomization have not yet produced droplets, but rather form more complicated liquid structures. Surface areamore » density, a more general concept than Lagrangian droplets, naturally represents liquid structures, no matter how complex. These surface area density models, however, have not been directly experimentally validated in the past due to the inability of optical methods to elucidate such a quantity. Optical diagnostics traditionally measure near-spherical droplet size far downstream, where the spray is optically thin. Using ultra-small-angle x-ray scattering (USAXS) measurements to measure the surface area and x-ray radiography to measure the density, we have been able to test one of the more speculative parts of Eulerian spray modeling. In conclusion, the modeling and experimental results have been combined to provide insight into near-field spray dynamics.« less

Computational and Experimental Investigation of Interfacial Area in Near-Field Diesel Spray Simulation

DOE PAGES

Pandal, Adrian; Pastor, Jose M.; Payri, Raul; ...

2017-03-28

The dense spray region in the near-field of diesel fuel injection remains an enigma. This region is difficult to interrogate with light in the visible range and difficult to model due to the rapid interaction between liquid and gas. In particular, modeling strategies that rely on Lagrangian particle tracking of droplets have struggled in this area. To better represent the strong interaction between phases, Eulerian modeling has proven particularly useful. Models built on the concept of surface area density are advantageous where primary and secondary atomization have not yet produced droplets, but rather form more complicated liquid structures. Surface areamore » density, a more general concept than Lagrangian droplets, naturally represents liquid structures, no matter how complex. These surface area density models, however, have not been directly experimentally validated in the past due to the inability of optical methods to elucidate such a quantity. Optical diagnostics traditionally measure near-spherical droplet size far downstream, where the spray is optically thin. Using ultra-small-angle x-ray scattering (USAXS) measurements to measure the surface area and x-ray radiography to measure the density, we have been able to test one of the more speculative parts of Eulerian spray modeling. In conclusion, the modeling and experimental results have been combined to provide insight into near-field spray dynamics.« less

Condensational Droplet Growth in Rarefied Quiescent Vapor and Forced Convective Conditions

NASA Astrophysics Data System (ADS)

Anand, Sushant

Multiphase Heat transfer is ubiquitous in diverse fields of application such as cooling systems, micro and mini power systems and many chemical processes. By now, single phase dynamics are mostly understood in their applications in vast fields, however multiphase systems especially involving phase changes are still a challenge. Present study aims to enhance understanding in this domain especially in the field of condensation heat transfer. Of special relevance to present studies is study of condensation phenomenon for detection of airborne nanoparticles using heterogeneous nucleation. Detection of particulate matter in the environment via heterogeneous condensation is based on the droplet growth phenomenon where seeding particles in presence of supersaturated vapor undergo condensation on their surface and amplify in size to micrometric ranges, thereby making them optically visible. Previous investigations show that condensation is a molecular exchange process affected by mean free path of vapor molecules (lambda) in conjunction with size of condensing droplet (d), which is measured in terms of Knudsen number (Kn=lambda/ d). In an event involving heterogeneous nucleation with favorable thermodynamic conditions for condensation to take place, the droplet growth process begins with accretion of vapor molecules on a surface through random molecular collision (Kn>1) until diffusive forces start dominating the mass transport process (Kn<<1). Knowledge of droplet growth thus requires understanding of mass transport in both of these regimes. Present study aims to understand the dynamics of the Microthermofluidic sensor which has been developed, based on above mentioned fundamentals. Using continuum approach, numerical modeling was carried to understand the effect of various system parameters for improving the device performance to produce conditions which can lead to conditions abetting condensational growth. The study reveals that the minimum size of nanoparticle which can be detected is critically dependent upon controlling wall geometry and size, wall temperature, flow rate and relative humidity of nanoparticle laden air stream. Droplet growths rates and sizes have been predicted based on different models. The efficacy of the device under various conditions has been measured in terms of its ability to activate nanoparticles of different sizes. Since the condensation mechanism is dependent upon the Knudsen regime in which droplets are growing via condensation, special consideration was made to understand their behavior in large Knudsen number conditions. For this purpose, ESEM was used to study condensation on a bare surface. Droplet growth obtained as a function of time reveals that the rate of growth decreases as the droplet increases in size. The experimental results obtained from these experiments were matched with theoretical description provided by a model based on framework of kinetic theory. Evidence was also found which establishes the presence of submicroscopic droplets nucleating and growing in between microscopic droplets for partially wetting case.

Composition measurements of binary mixture droplets by rainbow refractometry

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

Wilms, J.; Weigand, B

2007-04-10

So far, refractive index measurements by rainbow refractometry have been used to determine the temperature of single droplets and ensembles of droplets. Rainbow refractometry is, for the first time, to the best of our knowledge, applied to measure composition histories of evaporating, binary mixture droplets. An evaluation method is presented that makes use of Airy theory and the simultaneous size measurement by Mie scattering imaging. The method further includes an empirical correction function for a certain diameter and refractive index range. The measurement uncertainty was investigated by numerical simulations with Lorenz-Mie theory. For the experiments, an optical levitation setup wasmore » used allowing for long measurement periods. Temperature measurements of single-component droplets at different temperature levels are shown to demonstrate the accuracy of rainbow refractometry. Measurements of size and composition histories of binary mixture droplets are presented for two different mixtures. Experimental results show good agreement with numerical results using a rapid-mixing model.« less

Modeling the Influence of Injection Modes on the Evolution of Solution Sprays in a Plasma Jet

NASA Astrophysics Data System (ADS)

Shan, Y.; Coyle, T. W.; Mostaghimi, J.

2010-01-01

Solution precursor plasma spraying (SPPS) is a novel technology with great potential for depositing finely structured ceramic coatings with nano- and sub-micrometric features. The solution is injected into the plasma jet either as a liquid stream or gas atomized droplets. Solution droplets or the stream interact with the plasma jet and break up into fine droplets. The solvent vaporizes very fast as the droplets travel downstream. Solid particles are finally formed, and the particle are heated up and accelerated to the substrate to generate the coating. The deposition process and the properties of coatings obtained are extremely sensitive to the process parameters, such as torch operating conditions, injection modes, injection parameters, and substrate temperatures. This article numerically investigates the effect of injection modes, a liquid stream injection and a gas-blast injection, on the size distribution of injected droplets. The particle/droplet size, temperature, and position distributions on the substrate are predicted for different injection modes.

Taking Advantage of Reduced Droplet-surface Interaction to Optimize Transport of Bioanalytes in Digital Microfluidics

PubMed Central

Freire, Sergio L. S.; Thorne, Nathaniel; Wutkowski, Michael; Dao, Selina

2014-01-01

Digital microfluidics (DMF), a technique for manipulation of droplets, is a promising alternative for the development of “lab-on-a-chip” platforms. Often, droplet motion relies on the wetting of a surface, directly associated with the application of an electric field; surface interactions, however, make motion dependent on droplet contents, limiting the breadth of applications of the technique. Some alternatives have been presented to minimize this dependence. However, they rely on the addition of extra chemical species to the droplet or its surroundings, which could potentially interact with droplet moieties. Addressing this challenge, our group recently developed Field-DW devices to allow the transport of cells and proteins in DMF, without extra additives. Here, the protocol for device fabrication and operation is provided, including the electronic interface for motion control. We also continue the studies with the devices, showing that multicellular, relatively large, model organisms can also be transported, arguably unaffected by the electric fields required for device operation. PMID:25407533

Role of Arsenic During Aluminum Droplet Etching of Nanoholes in AlGaAs

NASA Astrophysics Data System (ADS)

Heyn, Christian; Zocher, Michel; Schnüll, Sandra; Hansen, Wolfgang

2016-09-01

Self-assembled nanoholes are drilled into (001) AlGaAs surfaces during molecular beam epitaxy (MBE) using local droplet etching (LDE) with Al droplets. It is known that this process requires a small amount of background arsenic for droplet material removal. The present work demonstrates that the As background can be supplied by both a small As flux to the surface as well as by the topmost As layer in an As-terminated surface reconstruction acting as a reservoir. We study the temperature-dependent evaporation of the As topmost layer with in situ electron diffraction and determine an activation energy of 2.49 eV. After thermal removal of the As topmost layer droplet etching is studied under well-defined As supply. We observe with decreasing As flux four regimes: planar growth, uniform nanoholes, non-uniform holes, and droplet conservation. The influence of the As supply is discussed quantitatively on the basis of a kinetic rate model.

Bioeffects due to acoustic droplet vaporization

NASA Astrophysics Data System (ADS)

Bull, Joseph

2015-11-01

Encapsulated micro- and nano-droplets can be vaporized via ultrasound, a process termed acoustic droplet vaporization. Our interest is primarily motivated by a developmental gas embolotherapy technique for cancer treatment. In this methodology, infarction of tumors is induced by selectively formed vascular gas bubbles that arise from the acoustic vaporization of vascular microdroplets. Additionally, the microdroplets may be used as vehicles for localized drug delivery, with or without flow occlusion. In this talk, we examine the dynamics of acoustic droplet vaporization through experiments and theoretical/computational fluid mechanics models, and investigate the bioeffects of acoustic droplet vaporization on endothelial cells and in vivo. Early timescale vaporization events, including phase change, are directly visualized using ultra-high speed imaging, and the influence of acoustic parameters on droplet/bubble dynamics is discussed. Acoustic and fluid mechanics parameters affecting the severity of endothelial cell bioeffects are explored. These findings suggest parameter spaces for which bioeffects may be reduced or enhanced, depending on the objective of the therapy. This work was supported by NIH grant R01EB006476.

Quantifying the role of noise on droplet decisions in bifurcating microchannels

NASA Astrophysics Data System (ADS)

Norouzi Darabad, Masoud; Vaughn, Mark; Vanapalli, Siva

2017-11-01

While many aspects of path selection of droplets flowing through a bifurcating microchannel have been studied, there are still unaddressed issues in predicting and controlling droplet traffic. One of the more important is understanding origin of aperiodic patterns. As a new tool to investigate this phenomena we propose monitoring the continuous time response of pressure fluctuations at different locations. Then we use time-series analysis to investigate the dynamics of the system. We suggest that natural system noise is the cause of irregularity in the traffic patterns. Using a mathematical model, we investigate the effect of noise on droplet decisions at the junction. Noise can be derived from different sources including droplet size variation, droplet spacing, and pump induced velocity fluctuation. By analyzing different situations we explain system behavior. We also investigate the ``memory'' of a microfluidic system in terms of the resistance to perturbations that quantify the allowable deviation in operating condition before the system changes state.

Influence of droplet size, pH and ionic strength on endotoxin-triggered ordering transitions in liquid crystalline droplets

PubMed Central

Miller, Daniel S.; Abbott, Nicholas L.

2012-01-01

We report an investigation of ordering transitions that are induced in water-dispersed, micrometer-sized droplets of a thermotropic liquid crystal (LC) by the bacterial lipopolysaccharide endotoxin. We reveal that the ordering transitions induced by endotoxin – from a bipolar state of the droplets to a radial state – are strongly dependent on the size of the LC droplets. Specifically, as the diameters of the LC droplets increase from 2 μm to above 10 μm (in phosphate buffered saline with an ionic strength of 90 mM and a pH of 7.2), we measured the percentage of droplets exhibiting a radial configuration in the presence of 100 pg/mL endotoxin to decrease from 98 ± 1 % to 3 ± 2 %. In addition, we measured a decrease in either the ionic strength or pH of the aqueous phase to reduce the percentage of droplets exhibiting a radial configuration in the presence of endotoxin. These results, when interpreted within the context of a simple thermodynamic model that incorporates the contributions of elasticity and surface anchoring to the free energies of the LC droplets, lead us to conclude that (i) the elastic constant K24 plays a central role in determining the size-dependent response of the LC droplets to endotoxin, and (ii) endotoxin-triggered ordering transitions occur only under solution conditions (pH, ionic strength) where the combined contributions of elasticity and surface anchoring to the free energies of the bipolar and radial configurations of the LC droplets are similar in magnitude. Our analysis also suggests that the presence of endotoxin perturbs the free energies of the LC droplets by ~10−17 J/droplet, which is comparable to the standard free energy of self-association of ~103 endotoxin molecules. These results, when combined with prior reports of localization of endotoxin at the center of LC droplets, are consistent with the hypothesis that self-assembly of endotoxin within micrometer-sized LC droplets provides the driving force for the ordering transitions. Overall, these results advance our understanding of ordering transitions triggered by the interactions of analytes with LC droplets and, more broadly, provide guidance to the design of LC droplet systems as the basis of stimuli-responsive soft materials. PMID:23675387

Release of Native-like Gaseous Proteins from Electrospray Droplets via the Charged Residue Mechanism: Insights from Molecular Dynamics Simulations.

PubMed

McAllister, Robert G; Metwally, Haidy; Sun, Yu; Konermann, Lars

2015-10-07

The mechanism whereby gaseous protein ions are released from charged solvent droplets during electrospray ionization (ESI) remains a matter of debate. Also, it is unclear to what extent electrosprayed proteins retain their solution structure. Molecular dynamics (MD) simulations offer insights into the temporal evolution of protein systems. Surprisingly, there have been no all-atom simulations of the protein ESI process to date. The current work closes this gap by investigating the behavior of protein-containing aqueous nanodroplets that carry excess positive charge. We focus on "native ESI", where proteins initially adopt their biologically active solution structures. ESI proceeds while the protein remains entrapped within the droplet. Protein release into the gas phase occurs upon solvent evaporation to dryness. Droplet shrinkage is accompanied by ejection of charge carriers (Na(+) for the conditions chosen here), keeping the droplet at ∼85% of the Rayleigh limit throughout its life cycle. Any remaining charge carriers bind to the protein as the final solvent molecules evaporate. The outcome of these events is largely independent of the initial protein charge and the mode of charge carrier binding. ESI charge states and collision cross sections of the MD structures agree with experimental data. Our results confirm the Rayleigh/charged residue model (CRM). Field emission of excess Na(+) plays an ancillary role by governing the net charge of the shrinking droplet. Models that envision protein ejection from the droplet are not supported. Most nascent CRM ions retain native-like conformations. For unfolded proteins ESI likely proceeds along routes that are different from the native state mechanism explored here.

Microfluidic passive permeability assay using nanoliter droplet interface lipid bilayers.

PubMed

Nisisako, Takasi; Portonovo, Shiva A; Schmidt, Jacob J

2013-11-21

Membrane permeability assays play an important role in assessing drug transport activities across biological membranes. However, in conventional parallel artificial membrane permeability assays (PAMPA), the membrane model used is dissimilar to biological membranes physically and chemically. Here, we describe a microfluidic passive permeability assay using droplet interface bilayers (DIBs). In a microfluidic network, nanoliter-sized donor and acceptor aqueous droplets are alternately formed in cross-flowing oil containing phospholipids. Subsequently, selective removal of oil through hydrophobic pseudo-porous sidewalls induces the contact of the lipid monolayers, creating arrayed planar DIBs between the donor and acceptor droplets. Permeation of fluorescein from the donor to the acceptor droplets was fluorometrically measured. From the measured data and a simple diffusion model we calculated the effective permeabilities of 5.1 × 10(-6) cm s(-1), 60.0 × 10(-6) cm s(-1), and 87.6 × 10(-6) cm s(-1) with donor droplets at pH values of 7.5, 6.4 and 5.4, respectively. The intrinsic permeabilities of specific monoanionic and neutral fluorescein species were obtained similarly. We also measured the permeation of caffeine in 10 min using UV microspectroscopy, obtaining a permeability of 20.8 × 10(-6) cm s(-1). With the small solution volumes, short measurement time, and ability to measure a wide range of compounds, this device has considerable potential as a platform for high-throughput drug permeability assays.

Droplet Combustion in a Slow Convective Flow

NASA Technical Reports Server (NTRS)

Nayagam, V.; Hicks, M. C.; Ackerman, M.; Haggard, J. B., Jr.; Williams, F. A.

2003-01-01

The influences of slow convective flow on droplet combustion, particularly in the low Reynolds number regime, have received very little attention in the past. Most studies in the literature are semi-empirical in nature and they were motivated by spray combustion applications in the moderate to high Reynolds number regime. None of the limited number of fundamental theoretical studies applicable to low Reynolds numbers have been verified by rigorous experimental data. Moreover, many unsteady phenomena associated with fluid-dynamic unsteadiness, such as impulsive starting or stopping of a burning droplet, or flow acceleration/deceleration effects, have not been investigated despite their importance in practical applications. In this study we investigate the effects of slow convection on droplet burning dynamics both experimentally and theoretically. The experimental portion of the study involves both ground-based experiments in the drop towers and future flight experiments on board the International Space Station. Heptane and methanol are used as test fuels, and this choice complements the quiescent-environment studies of the Droplet Combustion Experiment (DCE). An analytical model that employs the method of matched asymptotic expansions and uses the ratio of the convective velocity far from the droplet to the Stefan velocity at its surface as the small parameter for expansion has also been developed as a part of this investigation. Results from the ground-based experiments and comparison with the analytical model are presented in this report.

Thermocapillary droplet actuation on structured solid surfaces

NASA Astrophysics Data System (ADS)

Karapetsas, George; Chamakos, Nikolaos T.; Papathanasiou, Athanasios G.

2017-11-01

The present work investigates, through 2D and 3D finite element simulations, the thermocapillary-driven flow inside a droplet which resides on a non-uniformly heated patterned surface. We employ a recently proposed sharp-interface scheme capable of efficiently modelling the flow over complicate surfaces and consider a wide range of substrate wettabilities, i.e. from hydrophilic to super-hydrophobic surfaces. Our simulations indicate that due to the presence of the solid structures and the induced effect of contact angle hysteresis, inherently predicted by our model, a critical thermal gradient arises beyond which droplet migration is possible, in line with previous experimental observations. The migration velocity as well as the direction of motion depends on the combined action of the net mechanical force along the contact line and the thermocapillary induced flow at the liquid-air interface. We also show that through a proper control and design of the substrate wettability, the contact angle hysteresis and the induced flow field it is possible to manipulate the droplet dynamics, e.g. controlling its motion along a predefined track or entrapping by a wetting defect a droplet based on its size as well as providing appropriate conditions for enhanced mixing inside the droplet. Funding from the European Research Council under the Europeans Community's Seventh Framework Programme (FP7/2007-2013)/ERC Grant agreement no. [240710] is acknowledged.

Modelling NOx emissions of single droplet combustion

NASA Astrophysics Data System (ADS)

Moesl, Klaus G.; Schwing, Joachim E.; Sattelmayer, Thomas

2012-02-01

An approach for modelling and simulation of the generation of nitrogen oxide (NOx) in the gas phase surrounding single burning droplets is presented. Assuming spherical symmetry (no gravity, no forced convection), the governing equations are derived first. Then simplifications are introduced and it is proven that they are appropriate. The influences of the initial droplet diameter, the ambient conditions, and the droplet pre-vapourisation on NOx are investigated. The fuel of choice is n-decane (C10H22) as it resembles kerosene and diesel fuel best, and the complexity of the reaction mechanism is manageable. Combinations of C10H22 mechanisms and well-established NOx kinetics are evaluated in detail and validated for their applicability in the context of this work. The conducted simulations of droplet combustion in an atmosphere of hot exhaust gas show that NOx formation (by mass of fuel) increases linearly with the droplet diameter. There is a trade-off between available oxygen and ambient temperature. Increasing the equivalence ratio of the exhaust gas leads to higher NOx emissions in the very lean regime, but to lower emissions if the equivalence ratio exceeds 0.85. Pre-vapourisation of fuel at ambient conditions becomes beneficial with respect to NOx emissions only if the degree of vapourisation is above a minimum limit. If less fuel is vapourised before ignition, the NOx emissions remain almost unaffected.

Lipid droplets of arbuscular mycorrhizal fungi emerge in concert with arbuscule collapse.

PubMed

Kobae, Yoshihiro; Gutjahr, Caroline; Paszkowski, Uta; Kojima, Tomoko; Fujiwara, Toru; Hata, Shingo

2014-11-01

Plants share photosynthetically fixed carbon with arbuscular mycorrhizal (AM) fungi to maintain their growth and nutrition. AM fungi are oleogenic fungi that contain numerous lipid droplets in their syncytial mycelia during most of their life cycle. These lipid droplets are probably used for supporting growth of extraradical mycelia and propagation; however, when and where the lipid droplets are produced remains unclear. To address these issues, we investigated the correlation between intracellular colonization stages and the appearance of fungal lipid droplets in roots by a combination of vital staining of fungal structures, selective staining of lipids and live imaging. We discovered that a surge of lipid droplets coincided with the collapse of arbuscular branches, indicating that arbuscule collapse and the emergence of lipid droplets may be associated processes. This phenomenon was observed in the model AM fungus Rhizophagus irregularis and the ancestral member of AM fungi Paraglomus occultum. Because the collapsing arbuscules were metabolically inactive, the emerged lipid droplets are probably derived from preformed lipids but not de novo synthesized. Our observations highlight a novel mode of lipid release by AM fungi. © The Author 2014. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oup.com.

Droplet collisions and interaction with the turbulent flow within a two-phase wind tunnel

NASA Astrophysics Data System (ADS)

Bordás, Róbert; Hagemeier, Thomas; Wunderlich, Bernd; Thévenin, Dominique

2011-08-01

Experiments in wind tunnels concerning meteorological issues are not very frequent in the literature. However, such experiments might be essential, for instance for a careful investigation of droplet-droplet interactions in turbulent flows. This issue is crucial for many configurations, in particular to understand warm rain initiation. It is clearly impossible to completely reproduce cloud turbulence within a wind tunnel due to the enormous length scales involved. Nevertheless, it is not necessary to recover the whole spectrum in order to quantify droplet interactions. It is sufficient for this purpose to account correctly for the relevant properties only. In the present paper, these properties and a methodology for setting those in a two-phase wind tunnel are first described. In particular, droplet size and number density, velocities, turbulent kinetic energy, k, and its dissipation rate, ɛ, are suitably reproduced, as demonstrated by non-intrusive measurement techniques. A complete experimental characterization of the air and droplet properties is freely available in a database accessible at http://www.ovgu.de/isut/lss/metstroem. Finally, quantifications of droplet collision rates and comparisons with theoretical predictions are presented, showing that measured collision rates are higher, typically by a factor of 2 to 5. These results demonstrate that model modifications are needed to estimate correctly droplet collision probabilities in turbulent flows

Immersed Boundary Simulations of Active Fluid Droplets

PubMed Central

Hawkins, Rhoda J.

2016-01-01

We present numerical simulations of active fluid droplets immersed in an external fluid in 2-dimensions using an Immersed Boundary method to simulate the fluid droplet interface as a Lagrangian mesh. We present results from two example systems, firstly an active isotropic fluid boundary consisting of particles that can bind and unbind from the interface and generate surface tension gradients through active contractility. Secondly, a droplet filled with an active polar fluid with homeotropic anchoring at the droplet interface. These two systems demonstrate spontaneous symmetry breaking and steady state dynamics resembling cell motility and division and show complex feedback mechanisms with minimal degrees of freedom. The simulations outlined here will be useful for quantifying the wide range of dynamics observable in these active systems and modelling the effects of confinement in a consistent and adaptable way. PMID:27606609

Chemical consequences of the initial diffusional growth of cloud droplets - A clean marine case

NASA Technical Reports Server (NTRS)

Twohy, C. H.; Charlson, R. J.; Austin, P. H.

1989-01-01

A simple microphysical cloud parcel model and a simple representation of the background marine aerosol are used to predict the concentrations and compositions of droplets of various sizes near cloud base. The aerosol consists of an externally-mixed ammonium bisulfate accumulation mode and a sea-salt coarse particle mode. The difference in diffusional growth rates between the small and large droplets as well as the differences in composition between the two aerosol modes result in substantial differences in solute concentration and composition with size of droplets in the parcel. The chemistry of individual droplets is not, in general, representative of the bulk (volume-weighted mean) cloud water sample. These differences, calculated to occur early in the parcel's lifetime, should have important consequences for chemical reactions such as aqueous phase sulfate production.

Spark Ignition of Monodisperse Fuel Sprays. Ph.D. Thesis

NASA Technical Reports Server (NTRS)

Danis, Allen M.; Cernansky, Nicholas P.; Namer, Izak

1987-01-01

A study of spark ignition energy requirements was conducted with a monodisperse spray system allowing independent control of droplet size, equivalent ratio, and fuel type. Minimum ignition energies were measured for n-heptane and methanol sprays characterized at the spark gap in terms of droplet diameter, equivalence ratio (number density) and extent of prevaporization. In addition to sprays, minimum ignition energies were measured for completely prevaporized mixtures of the same fuels over a range of equivalence ratios to provide data at the lower limit of droplet size. Results showed that spray ignition was enhanced with decreasing droplet size and increasing equivalence ratio over the ranges of the parameters studied. By comparing spray and prevaporized ignition results, the existence of an optimum droplet size for ignition was indicated for both fuels. Fuel volatility was seen to be a critical factor in spray ignition. The spray ignition results were analyzed using two different empirical ignition models for quiescent mixtures. Both models accurately predicted the experimental ignition energies for the majority of the spray conditions. Spray ignition was observed to be probabilistic in nature, and ignition was quantified in terms of an ignition frequency for a given spark energy. A model was developed to predict ignition frequencies based on the variation in spark energy and equivalence ratio in the spark gap. The resulting ignition frequency simulations were nearly identical to the experimentally observed values.

Do the contact angle and line tension of surface-attached droplets depend on the radius of curvature?

PubMed

Das, Subir K; Egorov, Sergei A; Virnau, Peter; Winter, David; Binder, Kurt

2018-06-27

Results from Monte Carlo simulations of wall-attached droplets in the three-dimensional Ising lattice gas model and in a symmetric binary Lennard-Jones fluid, confined by antisymmetric walls, are analyzed, with the aim to estimate the dependence of the contact angle [Formula: see text] on the droplet radius [Formula: see text] of curvature. Sphere-cap shape of the wall-attached droplets is assumed throughout. An approach, based purely on 'thermodynamic' observables, e.g. chemical potential, excess density due to the droplet, etc, is used, to avoid ambiguities in the decision which particles belong (or do not belong, respectively) to the droplet. It is found that the results are compatible with a variation [Formula: see text], [Formula: see text] being the contact angle in the thermodynamic limit ([Formula: see text]). The possibility to use such results to estimate the excess free energy related to the contact line of the droplet, namely the line tension, at the wall, is discussed. Various problems that hamper this approach and were not fully recognized in previous attempts to extract the line tension are identified. It is also found that the dependence of wall tensions on the difference of chemical potential of the droplet from that at the bulk coexistence provides effectively a change of the contact angle of similar magnitude. The simulation approach yields precise estimates for the excess density due to wall-attached droplets and the corresponding free energy excess, relative to a system without a droplet at the same chemical potential. It is shown that this information suffices to estimate nucleation barriers, not affected by ambiguities on droplet shape, contact angle and line tension.

Inhalation of expiratory droplets in aircraft cabins.

PubMed

Gupta, J K; Lin, C-H; Chen, Q

2011-08-01

Airliner cabins have high occupant density and long exposure time, so the risk of airborne infection transmission could be high if one or more passengers are infected with an airborne infectious disease. The droplets exhaled by an infected passenger may contain infectious agents. This study developed a method to predict the amount of expiratory droplets inhaled by the passengers in an airliner cabin for any flight duration. The spatial and temporal distribution of expiratory droplets for the first 3 min after the exhalation from the index passenger was obtained using the computational fluid dynamics simulations. The perfectly mixed model was used for beyond 3 min after the exhalation. For multiple exhalations, the droplet concentration in a zone can be obtained by adding the droplet concentrations for all the exhalations until the current time with a time shift via the superposition method. These methods were used to determine the amount of droplets inhaled by the susceptible passengers over a 4-h flight under three common scenarios. The method, if coupled with information on the viability and the amount of infectious agent in the droplet, can aid in evaluating the infection risk. The distribution of the infectious agents contained in the expiratory droplets of an infected occupant in an indoor environment is transient and non-uniform. The risk of infection can thus vary with time and space. The investigations developed methods to predict the spatial and temporal distribution of expiratory droplets, and the inhalation of these droplets in an aircraft cabin. The methods can be used in other indoor environments to assess the relative risk of infection in different zones, and suitable measures to control the spread of infection can be adopted. Appropriate treatment can be implemented for the zone identified as high-risk zones. © 2011 John Wiley & Sons A/S.

A numerical cloud model for the support of laboratory experimentation

NASA Technical Reports Server (NTRS)

Hagen, D. E.

1979-01-01

A numerical cloud model is presented which can describe the evolution of a cloud starting from moist aerosol-laden air through the diffusional growth regime. The model is designed for the direct support of cloud chamber laboratory experimentation, i.e., experiment preparation, real-time control and data analysis. In the model the thermodynamics is uncoupled from the droplet growth processes. Analytic solutions for the cloud droplet growth equations are developed which can be applied in most laboratory situations. The model is applied to a variety of representative experiments.

Investigation of water droplet dynamics in PEM fuel cell gas channels

NASA Astrophysics Data System (ADS)

Gopalan, Preethi

Water management in Proton Exchange Membrane Fuel Cell (PEMFC) has remained one of the most important issues that need to be addressed before its commercialization in automotive applications. Accumulation of water on the gas diffusion layer (GDL) surface in a PEMFC introduces a barrier for transport of reactant gases through the GDL to the catalyst layer. Despite the fact that the channel geometry is one of the key design parameters of a fluidic system, very limited research is available to study the effect of microchannel geometry on the two-phase flow structure. In this study, the droplet-wall dynamics and two-phase pressure drop across the water droplet present in a typical PEMFC channel, were examined in auto-competitive gas channel designs (0.4 x 0.7 mm channel cross section). The liquid water flow pattern inside the gas channel was analyzed for different air velocities. Experimental data was analyzed using the Concus-Finn condition to determine the wettability characteristics in the corner region. It was confirmed that the channel angle along with the air velocity and the channel material influences the water distribution and holdup within the channel. Dynamic contact angle emerged as an important parameter in controlling the droplet-wall interaction. Experiments were also performed to understand how the inlet location of the liquid droplet on the GDL surface affects the droplet dynamic behavior in the system. It was found that droplets emerging near the channel wall or under the land lead to corner filling of the channel. Improvements in the channel design has been proposed based on the artificial channel roughness created to act as capillary grooves to transport the liquid water away from the land area. For droplets emerging near the center of the channel, beside the filling and no-filling behavior reported in the literature, a new droplet jumping behavior was observed. As droplets grew and touched the sidewalls, they jumped off to the sidewall leaving the whole GDL exposed for gases to diffuse to the catalyst layer. A theoretical model was developed and a criterion was proposed to predict the droplet jumping behavior in the gas channel. A theoretical force balance model was proposed to predict the pressure force and air velocity required to remove the droplet from the channel to avoid complete channel blockage. The overall goal of this work was to identify the gas channel configuration that provides efficient water removal with a lower pressure drop in the system efficiency while meeting the US Department of Energy's specifications for a PEMFC for automotive application.

Atomization of liquids in a Pease-Anthony Venturi scrubber. Part II. Droplet dispersion.

PubMed

Gonçalves, J A S; Costa, M A M; Aguiar, M L; Coury, J R

2004-12-10

Droplet distribution is of fundamental importance to the performance of a Venturi scrubber. Ensuring good liquid distribution can increase performance at minimal liquid usage. In this study, droplet dispersion in a rectangular Pease-Anthony Venturi scrubber, operating horizontally, was examined both theoretically and experimentally. The Venturi throat cross-section was 24 mm x 35 mm, and the throat length varied from 63 to 140 mm. Liquid was injected through a single orifice (1.0 mm diameter) on the throat wall. This arrangement allowed the study of the influence of jet penetration on droplet distribution. Gas velocity at the throat was 58.3 and 74.6 m/s, and the liquid flow rate was 286, 559 and 853 ml/min. A probe with a 2.7 mm internal diameter was used to isokinetically remove liquid from several positions inside the equipment. It was possible to study liquid distribution close to the injection point. A new model for droplet dispersion, which incorporates the new description of the jet atomization process developed by the present authors in the first article of this series, is proposed and evaluated. The model predicted well the experimental data.

Numerical study of droplet evaporation in an acoustic levitator

NASA Astrophysics Data System (ADS)

Bänsch, Eberhard; Götz, Michael

2018-03-01

We present a finite element method for the simulation of all relevant processes of the evaporation of a liquid droplet suspended in an acoustic levitation device. The mathematical model and the numerical implementation take into account heat and mass transfer across the interface between the liquid and gaseous phase and the influence of acoustic streaming on this process, as well as the displacement and deformation of the droplet due to acoustic radiation pressure. We apply this numerical method to several theoretical and experimental examples and compare our results with the well-known d2-law for the evaporation of spherical droplets and with theoretical predictions for the acoustic streaming velocity. We study the influence of acoustic streaming on the distribution of water vapor and temperature in the levitation device, with special attention to the vapor distribution in the emerging toroidal vortices. We also compare the evaporation rate of a droplet with and without acoustic streaming, as well as the evaporation rates in dependence of different temperatures and sound pressure levels. Finally, a simple model of protein inactivation due to heat damage is considered and studied for different evaporation settings and their respective influence on protein damage.

Linear Stability Analysis of an Acoustically Vaporized Droplet

NASA Astrophysics Data System (ADS)

Siddiqui, Junaid; Qamar, Adnan; Samtaney, Ravi

2015-11-01

Acoustic droplet vaporization (ADV) is a phase transition phenomena of a superheat liquid (Dodecafluoropentane, C5F12) droplet to a gaseous bubble, instigated by a high-intensity acoustic pulse. This approach was first studied in imaging applications, and applicable in several therapeutic areas such as gas embolotherapy, thrombus dissolution, and drug delivery. High-speed imaging and theoretical modeling of ADV has elucidated several physical aspects, ranging from bubble nucleation to its subsequent growth. Surface instabilities are known to exist and considered responsible for evolving bubble shapes (non-spherical growth, bubble splitting and bubble droplet encapsulation). We present a linear stability analysis of the dynamically evolving interfaces of an acoustically vaporized micro-droplet (liquid A) in an infinite pool of a second liquid (liquid B). We propose a thermal ADV model for the base state. The linear analysis utilizes spherical harmonics (Ynm, of degree m and order n) and under various physical assumptions results in a time-dependent ODE of the perturbed interface amplitudes (one at the vapor/liquid A interface and the other at the liquid A/liquid B interface). The perturbation amplitudes are found to grow exponentially and do not depend on m. Supported by KAUST Baseline Research Funds.

Development and evaluation of a dimensionless mechanistic pan coating model for the prediction of coated tablet appearance.

PubMed

Niblett, Daniel; Porter, Stuart; Reynolds, Gavin; Morgan, Tomos; Greenamoyer, Jennifer; Hach, Ronald; Sido, Stephanie; Karan, Kapish; Gabbott, Ian

2017-08-07

A mathematical, mechanistic tablet film-coating model has been developed for pharmaceutical pan coating systems based on the mechanisms of atomisation, tablet bed movement and droplet drying with the main purpose of predicting tablet appearance quality. Two dimensionless quantities were used to characterise the product properties and operating parameters: the dimensionless Spray Flux (relating to area coverage of the spray droplets) and the Niblett Number (relating to the time available for drying of coating droplets). The Niblett Number is the ratio between the time a droplet needs to dry under given thermodynamic conditions and the time available for the droplet while on the surface of the tablet bed. The time available for drying on the tablet bed surface is critical for appearance quality. These two dimensionless quantities were used to select process parameters for a set of 22 coating experiments, performed over a wide range of multivariate process parameters. The dimensionless Regime Map created can be used to visualise the effect of interacting process parameters on overall tablet appearance quality and defects such as picking and logo bridging. Copyright © 2017 Elsevier B.V. All rights reserved.

New Method for Quantitation of Lipid Droplet Volume From Light Microscopic Images With an Application to Determination of PAT Protein Density on the Droplet Surface.

PubMed

Dejgaard, Selma Y; Presley, John F

2018-06-01

Determination of lipid droplet (LD) volume has depended on direct measurement of the diameter of individual LDs, which is not possible when LDs are small or closely apposed. To overcome this problem, we describe a new method in which a volume-fluorescence relationship is determined from automated analysis of calibration samples containing well-resolved LDs. This relationship is then used to estimate total cellular droplet volume in experimental samples, where the LDs need not be individually resolved, or to determine the volumes of individual LDs. We describe quantitatively the effects of various factors, including image noise, LD crowding, and variation in LD composition on the accuracy of this method. We then demonstrate this method by utilizing it to address a scientifically interesting question, to determine the density of green fluorescent protein (GFP)-tagged Perilipin-Adipocyte-Tail (PAT) proteins on the LD surface. We find that PAT proteins cover only a minority of the LD surface, consistent with models in which they primarily serve as scaffolds for binding of regulatory proteins and enzymes, but inconsistent with models in which their major function is to sterically block access to the droplet surface.

Dropwise Condensation on Soft Hydrophobic Coatings.

PubMed

Phadnis, Akshay; Rykaczewski, Konrad

2017-10-31

Promoting dropwise condensation (DWC) could improve the efficiency of many industrial systems. Consequently, a lot of effort has been dedicated to finding durable materials that could sustainably promote DWC as well as finding routes to enhance the heat transfer rate during this phase change process. Motivated by previous reports of substrate softening increasing droplet nucleation rate, here we investigated how mechanical properties of a substrate impact relevant droplet-surface interactions and DWC heat transfer rate. Specifically, we experimentally quantified the effect of hydrophobic elastomer's shear modulus on droplet nucleation density and shedding radius. To quantify the impact of substrate softening on heat transfer through individual droplets, we combined analytical solution of elastomer deformation induced by droplets with finite element modeling of the heat transfer process. By substituting these experimentally and theoretically derived values into DWC heat transfer model, we quantified the compounding effect of the substrate's mechanical properties on the overall heat transfer rate. Our results show that softening of the substrates below a shear modulus of 500 kPa results in a significant reduction in the condensation heat transfer rate. This trend is primarily driven by additional thermal resistance of the liquid posed by depression of the soft substrate.

Numerical simulation of the flow field and fuel sprays in an IC engine

NASA Technical Reports Server (NTRS)

Nguyen, H. L.; Schock, H. J.; Ramos, J. I.; Carpenter, M. H.; Stegeman, J. D.

1987-01-01

A two-dimensional model for axisymmetric piston-cylinder configurations is developed to study the flow field in two-stroke direct-injection Diesel engines under motored conditions. The model accounts for turbulence by a two-equation model for the turbulence kinetic energy and its rate of dissipation. A discrete droplet model is used to simulate the fuel spray, and the effects of the gas phase turbulence on the droplets is considered. It is shown that a fluctuating velocity can be added to the mean droplet velocity every time step if the step is small enough. Good agreement with experimental data is found for a range of ambient pressures in Diesel engine-type microenvironments. The effects of the intake swirl angle in the spray penetration, vaporization, and mixing in a uniflow-scavenged two-stroke Diesel engine are analyzed. It is found that the swirl increases the gas phase turbulence levels and the rates of vaporization.

Influence of water mist on propagation and suppression of laminar premixed flame

NASA Astrophysics Data System (ADS)

Belyakov, Nikolay S.; Babushok, Valeri I.; Minaev, Sergei S.

2018-03-01

The combustion of premixed gas mixtures containing micro droplets of water was studied using one-dimensional approximation. The dependencies of the burning velocity and flammability limits on the initial conditions and on the properties of liquid droplets were analyzed. Effects of droplet size and concentration of added liquid were studied. It was demonstrated that the droplets with smaller diameters are more effective in reducing the flame velocity. For droplets vaporizing in the reaction zone, the burning velocity is independent of droplet size, and it depends only on the concentration of added liquid. With further increase of the droplet diameter the droplets are passing through the reaction zone with completion of vaporization in the combustion products. It was demonstrated that for droplets above a certain size there are two stable stationary modes of flame propagation with transition of hysteresis type. The critical conditions of the transition are due to the appearance of the temperature maximum at the flame front and the temperature gradient with heat losses from the reaction zone to the products, as a result of droplet vaporization passing through the reaction zone. The critical conditions are similar to the critical conditions of the classical flammability limits of flame with the thermal mechanism of flame propagation. The maximum decrease in the burning velocity and decrease in the combustion temperature at the critical turning point corresponds to predictions of the classical theories of flammability limits of Zel'dovich and Spalding. The stability analysis of stationary modes of flame propagation in the presence of water mist showed the lack of oscillatory processes in the frames of the assumed model.

Numerical modeling for dilute and dense sprays

NASA Technical Reports Server (NTRS)

Chen, C. P.; Kim, Y. M.; Shang, H. M.; Ziebarth, J. P.; Wang, T. S.

1992-01-01

We have successfully implemented a numerical model for spray-combustion calculations. In this model, the governing gas-phase equations in Eulerian coordinate are solved by a time-marching multiple pressure correction procedure based on the operator-splitting technique. The droplet-phase equations in Lagrangian coordinate are solved by a stochastic discrete particle technique. In order to simplify the calculation procedure for the circulating droplets, the effective conductivity model is utilized. The k-epsilon models are utilized to characterize the time and length scales of the gas phase in conjunction with turbulent modulation by droplets and droplet dispersion by turbulence. This method entails random sampling of instantaneous gas flow properties and the stochastic process requires a large number of computational parcels to produce the satisfactory dispersion distributions even for rather dilute sprays. Two major improvements in spray combustion modelings were made. Firstly, we have developed a probability density function approach in multidimensional space to represent a specific computational particle. Secondly, we incorporate the Taylor Analogy Breakup (TAB) model for handling the dense spray effects. This breakup model is based on the reasonable assumption that atomization and drop breakup are indistinguishable processes within a dense spray near the nozzle exit. Accordingly, atomization is prescribed by injecting drops which have a characteristic size equal to the nozzle exit diameter. Example problems include the nearly homogeneous and inhomogeneous turbulent particle dispersion, and the non-evaporating, evaporating, and burning dense sprays. Comparison with experimental data will be discussed in detail.

DWH MC 252: Subsurface Oil Transport

NASA Astrophysics Data System (ADS)

Beegle-Krause, C. J.; Boyer, T.; Murray, D.

2010-12-01

Before reaching the ocean surface, the oil and gas released from the DWH MC 252 blowout at 1500 m moves as a buoyant plume until the trapping depth and plume transition point are reached (Zheng et al 2002). At the transition point, the oil droplets and bubbles move independently of each other, and rise at a rate related to their diameter. The oil density, droplet size distribution and currents primarily determine the distribution of the oil between: Large droplets that rise quickly and create a surface expression of the oil. Moderate size droplets that rise over the course of days, and so spread out quite differently than the surface oil, and commonly do not reach the surface in large enough quantities to create a surface sheen. These droplets separate in the currents, particularly in the strong current shear in upper 500 m currents. Very tiny droplets that rise very slowly, over the course or weeks to months, and may be removed by dissolution, biodegradation or marine snow before ever reaching the surface. Modeling and observations (Joint Analysis Group, 2010) confirm the presence of a deep layer of oil and gas between approximately 1100 and 1300 m over the release location and spreading out along the isopycnal surfaces. Later in the event, a small oxygen depression was a proxy for where oil and gas had been. The DWH MC252 well is located at intermediate depth in the Gulf of Mexico (GoM). The water mass is Antarctic Intermediate Water, which enters and exits the GoM through the Yucatan Straits. Surface influences, such as Loop Current Frontal Eddies (e.g. Berger et al 2000) can reach down to these depths, and alter the flow within De Soto Canyon. The water mass containing the deep layer of oil droplets changes depth within the GoM, but does not reach above a depth of about 900 m. There are no physical processes that could cause this deep layer of oil to reach the continental shelf or the Florida Straits. Observed and historical hydrographic data, observations, previous research and modeling were combined to tell the story of the DWH MC 252 from the subsurface perspective. The Comprehensive Deepwater Oil and Gas model (CDOG, Yapa and Xie, 2005), and the General NOAA Operational Modeling Environment (GNOME, Beegle-Krause, 1999) were used with the NOAA Gulf of Mexico Model nowcast/forecast model to understand the 3D evolution of the subsurface spill. Model/observational comparisons are favorable, though limitations of the available models are apparent. Historical perspective on Thunder Horse (a deepwater well incident that was a dress-rehearsal for the DWH MC 252, Beegle-Krause and Walton, 2004), transitioning models from research to operations, and research needs will also be discussed.

Probing the Evaporation Dynamics of Ethanol/Gasoline Biofuel Blends Using Single Droplet Manipulation Techniques.

PubMed

Corsetti, Stella; Miles, Rachael E H; McDonald, Craig; Belotti, Yuri; Reid, Jonathan P; Kiefer, Johannes; McGloin, David

2015-12-24

Using blends of bioethanol and gasoline as automotive fuel leads to a net decrease in the production of harmful emission compared to the use of pure fossil fuel. However, fuel droplet evaporation dynamics change depending on the mixing ratio. Here we use single particle manipulation techniques to study the evaporation dynamics of ethanol/gasoline blend microdroplets. The use of an electrodynamic balance enables measurements of the evaporation of individual droplets in a controlled environment, while optical tweezers facilitate studies of the behavior of droplets inside a spray. Hence, the combination of both methods is perfectly suited to obtain a complete picture of the evaporation process. The influence of adding varied amounts of ethanol to gasoline is investigated, and we observe that droplets with a greater fraction of ethanol take longer to evaporate. Furthermore, we find that our methods are sensitive enough to observe the presence of trace amounts of water in the droplets. A theoretical model, predicting the evaporation of ethanol and gasoline droplets in dry nitrogen gas, is used to explain the experimental results. Also a theoretical estimation of the saturation of the environment, with other aerosols, in the tweezers is carried out.

Theoretical analysis for the optical deformation of emulsion droplets.

PubMed

Tapp, David; Taylor, Jonathan M; Lubansky, Alex S; Bain, Colin D; Chakrabarti, Buddhapriya

2014-02-24

We propose a theoretical framework to predict the three-dimensional shapes of optically deformed micron-sized emulsion droplets with ultra-low interfacial tension. The resulting shape and size of the droplet arises out of a balance between the interfacial tension and optical forces. Using an approximation of the laser field as a Gaussian beam, working within the Rayleigh-Gans regime and assuming isotropic surface energy at the oil-water interface, we numerically solve the resulting shape equations to elucidate the three-dimensional droplet geometry. We obtain a plethora of shapes as a function of the number of optical tweezers, their laser powers and positions, surface tension, initial droplet size and geometry. Experimentally, two-dimensional droplet silhouettes have been imaged from above, but their full side-on view has not been observed and reported for current optical configurations. This experimental limitation points to ambiguity in differentiating between droplets having the same two-dimensional projection but with disparate three-dimensional shapes. Our model elucidates and quantifies this difference for the first time. We also provide a dimensionless number that indicates the shape transformation (ellipsoidal to dumbbell) at a value ≈ 1.0, obtained by balancing interfacial tension and laser forces, substantiated using a data collapse.

Dynamics of submicron aerosol droplets in a robust optical trap formed by multiple Bessel beams

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

Thanopulos, Ioannis; Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, Athens 11635; Luckhaus, David

In this paper, we model the three-dimensional escape dynamics of single submicron-sized aerosol droplets in optical multiple Bessel beam traps. Trapping in counter-propagating Bessel beams (CPBBs) is compared with a newly proposed quadruple Bessel beam (QBB) trap, which consists of two perpendicularly arranged CPBB traps. Calculations are performed for perfectly and imperfectly aligned traps. Mie-theory and finite-difference time-domain methods are used to calculate the optical forces. The droplet escape kinetics are obtained from the solution of the Langevin equation using a Verlet algorithm. Provided the traps are perfectly aligned, the calculations indicate very long lifetimes for droplets trapped either inmore » the CPBB or in the QBB trap. However, minor misalignments that are hard to control experimentally already severely diminish the stability of the CPBB trap. By contrast, such minor misalignments hardly affect the extended droplet lifetimes in a QBB trap. The QBB trap is found to be a stable, robust optical trap, which should enable the experimental investigation of submicron droplets with radii down to 100 nm. Optical binding between two droplets and its potential role in preventing coagulation when loading a CPBB trap is briefly addressed.« less

Entrainment, Drizzle, and the Indirect Effect in Stratiform Clouds

NASA Technical Reports Server (NTRS)

Ackerman, Andrew

2005-01-01

Activation of some fraction of increased concentrations of sub-micron soluble aerosol particles lead to enhanced cloud droplet concentrations and hence smaller droplets, increasing their total cross sectional area and thus reflecting solar radiation more efficiently (the Twomey, or first indirect, effect). However, because of competition during condensational growth, droplet distributions tend to broaden as numbers increase, reducing the sensitivity of cloud albedo to droplet concentration on the order of 10%. Also, smaller droplets less effectively produce drizzle through collisions and coalescence, and it is widely expected (and found in large-scale models) that decreased precipitation leads to clouds with more cloud water on average (the so-called cloud lifetime, or second indirect, effect). Much of the uncertainty regarding the overall indirect aerosol effect stems from inadequate understanding of such changes in cloud water. Detailed simulations based on FIRE-I, ASTEX, and DYCOMS-II conditions show that suppression of precipitation from increased droplet concentrations leads to increased cloud water only when sufficient precipitation reaches the surface, a condition favored when the overlying air is-humid or droplet concentrations are very low. Otherwise, aerosol induced suppression of precipitation enhances entrainment of overlying dry air, thereby reducing cloud water and diminishing the indirect climate forcing.

On the relevance of droplet sedimentation in stratocumulus-top mixing

NASA Astrophysics Data System (ADS)

Mellado, Juan Pedro; de Lozar, Alberto

2017-11-01

The interaction between droplet sedimentation, turbulent mixing, evaporative cooling, and radiative cooling at the top of stratocumulus clouds has been studied using direct numerical simulations. This interaction is important to determine the mixing rate of the cloud and dry air above it, which eventually determines the cloud lifetime. By investigating the entrainment-rate equation, which is an analytical relationship between the contributions to cloud-top entrainment from the phenomena indicated above, we have found that the reduction of entrainment velocity by droplet sedimentation can be 2 to 3 times larger than previously conjectured. The reason is twofold. First, the reduction of evaporative cooling as droplets fall out of the inversion is stronger than previously observed in large-eddy simulations, where excessive mixing by turbulence models and numerical artifacts may have partially masked this effect of sedimentation on entrainment. Second, there is a non-negligible direct contribution from mass loading, as falling droplets leave behind more buoyant air in the inversion. This contribution is proportional to the fifth moment of the droplet-size distribution, which provides further evidence for the need to better understand the evolution of the droplet-size distribution.

Caustics and the growth of droplets

NASA Astrophysics Data System (ADS)

Govindarajan, Rama; Ravichandran, S.; Ray, Samriddhi; Deepu, P.

Caustics are formed when inertial particles of very different velocities collide in a flow, and are a consequence of the dissipative nature of particle motion in a suspension. Using a model vortex-dominated flow with heavy droplets in a saturated environment, we suggest that sling caustics form only within a neighbourhood around a vortex, the square of whose radius is proportional to the product of circulation and particle inertia. Droplets starting close to this critical radius congregate very close together, resulting in large spikes in (Lagrangian) number density. Allowing for merger when droplets collide, we show that droplets starting out close to the critical radius display a much more rapid growth in size than those starting elsewhere, and a large fraction of the large droplets are those that originate within the caustics-forming region. We test these predictions in a two-dimensional simulation of turbulent flow. We hope that our study will be of interest in long-standing problems of physical interest such as the mechanism of broadening of droplet spectra in a turbulent flow. Support from the Ministry of Earth Sciences, Government of India for the project Coupled physical processes in the Bay of Bengal and monsoon air-sea interaction under OMM is gratefully acknowledged.

Collective oscillations and coupled modes in confined microfluidic droplet arrays

NASA Astrophysics Data System (ADS)

Schiller, Ulf D.; Fleury, Jean-Baptiste; Seemann, Ralf; Gompper, Gerhard

Microfluidic droplets have a wide range of applications ranging from analytic assays in cellular biology to controlled mixing in chemical engineering. Ensembles of microfluidic droplets are interesting model systems for non-equilibrium many-body phenomena. When flowing in a microchannel, trains of droplets can form microfluidic crystals whose dynamics are governed by long-range hydrodynamic interactions and boundary effects. In this contribution, excitation mechanisms for collective waves in dense and confined microfluidic droplet arrays are investigated by experiments and computer simulations. We demonstrate that distinct modes can be excited by creating specific `defect' patterns in flowing droplet trains. While longitudinal modes exhibit a short-lived cascade of pairs of laterally displacing droplets, transversely excited modes form propagating waves that behave like microfluidic phonons. We show that the confinement induces a coupling between longitudinal and transverse modes. We also investigate the life time of the collective oscillations and discuss possible mechanisms for the onset of instabilities. Our results demonstrate that microfluidic phonons can exhibit effects beyond the linear theory, which can be studied particularly well in dense and confined systems. This work was supported by Deutsche Forschungsgemeinschaft under Grant No. SE 1118/4.

Patterned gradient surface for spontaneous droplet transportation and water collection: simulation and experiment

NASA Astrophysics Data System (ADS)

Tan, Xianhua; Zhu, Yiying; Shi, Tielin; Tang, Zirong; Liao, Guanglan

2016-11-01

We demonstrate spontaneous droplet transportation and water collection on wedge-shaped gradient surfaces consisting of alternating hydrophilic and hydrophobic regions. Droplets on the surfaces are modeled and simulated to analyze the Gibbs free energy and free energy gradient distributions. Big half-apex angle and great wettability difference result in considerable free energy gradient, corresponding to large driving force for spontaneous droplet transportation, thus causing the droplets to move towards the open end of the wedge-shaped hydrophilic regions, where the Gibbs free energy is low. Gradient surfaces are then fabricated and tested. Filmwise condensation begins on the hydrophilic regions, forming wedge-shaped tracks for water collection. Dropwise condensation occurs on the hydrophobic regions, where the droplet size distribution and departure diameters are controlled by the width of the regions. Condensate water from both the hydrophilic and hydrophobic regions are collected directionally to the open end of the wedge-shaped hydrophilic regions, agreeing with the simulations. Directional droplet transport and controllable departure diameters make the branched gradient surfaces more efficient than smooth surfaces for water collection, which proves that gradient surfaces are potential in water collection, microfluidic devices, anti-fogging and self-cleaning.

Theoretical Analysis for the Optical Shaping of Emulsion Droplets

NASA Astrophysics Data System (ADS)

Tapp, David; Taylor, Jonathan; Lubanksy, Alex; Bain, Colin; Chakrabarti, Buddhapriya

2014-03-01

Motivated by recent experimental observations, I discuss a theoretical framework to predict the three-dimensional shapes of optically deformed micron-sized emulsion droplets with ultra-low interfacial tension. The resulting shape and size of the droplet arises out of a balance between the interfacial tension and optical forces. Using an approximation of the laser field as a Gaussian beam, working within the Rayleigh-Gans regime and beyond, and assuming isotropic surface energy at the oil-water interface, the resulting shape equations are numerically solved to elucidate the three-dimensional droplet geometry. A plethora of shapes as a function of the number of optical tweezers, their laser powers and positions, surface tension, initial droplet size and geometry are obtained. Experimentally, two-dimensional emulsion droplet silhouettes have been imaged from above, but their full side-on view has not been observed and reported for current optical configurations. This experimental limitation points to ambiguity in differentiating between droplets having the same two-dimensional projection but with disparate three-dimensional shapes. The model I present elucidates and quantifies this difference for the first time. Supported by funding from EPSRC via grant EP/I013377/1.

Combustion of two-component miscible droplets in reduced gravity

NASA Technical Reports Server (NTRS)

Shaw, Benjamin D.; Aharon, Israel; Gage, James W.; Jenkins, Andrew J.; Kahoe, Thomas J.

1995-01-01

This research focuses on the combustion of binary miscible droplets initially in the mm size range. Experiments are performed using the NASA Lewis 2.2 sec drop tower in Cleveland, Ohio, where mixtures of alkanes and/or alcohols are studied. The fuel components are selected to have significantly different volatilities. Initial oxygen mole fractions from about 0.15-0.5 and initial pressures from 0.2-2 atm are employed. Different inerts are used (He, CO2, Ar, N2) to change burning rates and sooting behaviors. Objectives are to observe the following: (1) Transient droplet diameters (including three-staged combustion behaviors and microexplosion; (2) Transient flow behaviors (sudden flame contraction, luminosity, extinction); and (3) Behaviors of observable soot particles. theoretical and computational research in support of this program has also been undertaken. This research includes analytical studies to determine the effects of small but nonzero gravitational levels on droplet gasification, analytical studies of hydrodynamic stability of spherically-symmetrical droplet gasification (to address the question as to whether spherically-symmetrical droplet gasification may be destabilized from capillary, i.e., Marangoni effects), and computational modeling of effects of capillary stresses on droplet gasification.

Dielectrophoretic levitation of droplets and bubbles

NASA Technical Reports Server (NTRS)

Jones, T. B.

1982-01-01

Uncharged droplets and bubbles can be levitated dielectrophoretically in liquids using strong, nonuniform electric fields. The general equations of motion for a droplet or bubble in an axisymmetric, divergence-free electrostatic field allow determination of the conditions necessary and sufficient for stable levitation. The design of dielectrophoretic (DEP) levitation electrode structures is simplified by a Taylor-series expansion of cusped axisymmetric electrostatic fields. Extensive experimental measurements on bubbles in insulating liquids verify the simple dielectrophoretic model. Other have extended dielectrophoretic levitation to very small particles in aqueous media. Applications of DEP levitation to the study of gas bubbles, liquid droplets, and solid particles are discussed. Some of these applications are of special interest in the reduced gravitational field of a spacecraft.

Effects of turbulence on the collision rate of cloud droplets

NASA Astrophysics Data System (ADS)

Ayala, Orlando

This dissertation concerns effects of air turbulence on the collision rate of atmospheric cloud droplets. This research was motivated by the speculation that air turbulence could enhance the collision rate thereby help transform cloud droplets to rain droplets in a short time as observed in nature. The air turbulence within clouds is assumed to be homogeneous and isotropic, and its small-scale motion (1 mm to 10 cm scales) is computationally generated by direct numerical integration of the full Navier-Stokes equations. Typical droplet and turbulence parameters of convective warm clouds are used to determine the Stokes numbers (St) and the nondimensional terminal velocities (Sv) which characterize droplet relative inertia and gravitational settling, respectively. A novel and efficient methodology for conducting direct numerical simulations (DNS) of hydrodynamically-interacting droplets in the context of cloud microphysics has been developed. This numerical approach solves the turbulent flow by the pseudo-spectral method with a large-scale forcing, and utilizes an improved superposition method to embed analytically the local, small-scale (10 mum to 1 mm) disturbance flows induced by the droplets. This hybrid representation of background turbulent air motion and the induced disturbance flows is then used to study the combined effects of hydrodynamic interactions and airflow turbulence on the motion and collisions of cloud droplets. Hybrid DNS results show that turbulence can increase the geometric collision kernel relative to the gravitational geometric kernel by as much as 42% due to enhanced radial relative motion and preferential concentration of droplets. The exact level of enhancements depends on the Taylor-microscale Reynolds number, turbulent dissipation rate, and droplet pair size ratio. One important finding is that turbulence has a relatively dominant effect on the collision process between droplets close in size as the gravitational collision mechanism diminishes. A theory was developed to predict the radial relative velocity between droplets at contact. The theory agrees with our DNS results to within 5% for cloud droplets with strong settling. In addition, an empirical model is developed to quantify the radial distribution function. (Abstract shortened by UMI.)

Studies of oscillatory combustion and fuel vaporization

NASA Technical Reports Server (NTRS)

Borman, G. L.; Myers, P. S.; Uyehara, O. A.

1972-01-01

Research projects involving oscillatory combustion and fuel vaporization are reported. Comparisons of experimental and theoretical droplet vaporization histories under ambient conditions such that the droplet may approach its thermodynamic critical point are presented. Experimental data on instantaneous heat transfer from a gas to a solid surface under conditions of oscillatory pressure with comparisons to an unsteady one-dimensional model are analyzed. Droplet size and velocity distribution in a spray as obtained by use of a double flash fluorescent method were investigated.

An interface tracking model for droplet electrocoalescence.

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

Erickson, Lindsay Crowl

This report describes an Early Career Laboratory Directed Research and Development (LDRD) project to develop an interface tracking model for droplet electrocoalescence. Many fluid-based technologies rely on electrical fields to control the motion of droplets, e.g. microfluidic devices for high-speed droplet sorting, solution separation for chemical detectors, and purification of biodiesel fuel. Precise control over droplets is crucial to these applications. However, electric fields can induce complex and unpredictable fluid dynamics. Recent experiments (Ristenpart et al. 2009) have demonstrated that oppositely charged droplets bounce rather than coalesce in the presence of strong electric fields. A transient aqueous bridge forms betweenmore » approaching drops prior to pinch-off. This observation applies to many types of fluids, but neither theory nor experiments have been able to offer a satisfactory explanation. Analytic hydrodynamic approximations for interfaces become invalid near coalescence, and therefore detailed numerical simulations are necessary. This is a computationally challenging problem that involves tracking a moving interface and solving complex multi-physics and multi-scale dynamics, which are beyond the capabilities of most state-of-the-art simulations. An interface-tracking model for electro-coalescence can provide a new perspective to a variety of applications in which interfacial physics are coupled with electrodynamics, including electro-osmosis, fabrication of microelectronics, fuel atomization, oil dehydration, nuclear waste reprocessing and solution separation for chemical detectors. We present a conformal decomposition finite element (CDFEM) interface-tracking method for the electrohydrodynamics of two-phase flow to demonstrate electro-coalescence. CDFEM is a sharp interface method that decomposes elements along fluid-fluid boundaries and uses a level set function to represent the interface.« less

Submicron Dropwise Condensation under Superheated and Rarefied Vapor Condition

PubMed Central

Anand, Sushant; Son, Sang Young

2010-01-01

Phase change accompanying conversion of a saturated or superheated vapor in presence of subcooled surfaces is one of the most common occurring phenomena in nature. The mode of phase change which follows such a transformation is dependent upon surface properties like as of contact angle and thermodynamic conditions of the system. In present studies, an experimental approach is used to study the physics behind droplet growth on a partially wetting surface. Superheated vapor at low pressures of 4–5 torr was condensed on subcooled silicon surface with static contact angle as of 60° in absence of non-condensable gases, and the condensation process monitored using Environmental Scanning Electron Microscope (ESEM) with submicroscopic spatial resolution. The condensation process was analyzed in the form of size growth of isolated droplets for before a coalescence event ended the regime of single droplet growth. Droplet growth obtained as a function of time reveals that the rate of growth decreases as the droplet increases in size. This behavior is indicative of an overall droplet growth law existing over larger time scales of which the current observations in their brief time intervals could be fitted in. A theoretical model based on kinetic theory further support the experimental observations indicating a mechanism where growth occurs by interfacial mass transport directly on condensing droplet surface. Evidence was also found which establishes the presence of submicroscopic droplets nucleating and growing in between microscopic droplets for partially wetting case. PMID:20942412

Dynamic Model of Basic Oxygen Steelmaking Process Based on Multizone Reaction Kinetics: Modeling of Decarburization

NASA Astrophysics Data System (ADS)

Rout, Bapin Kumar; Brooks, Geoffrey; Akbar Rhamdhani, M.; Li, Zushu; Schrama, Frank N. H.; Overbosch, Aart

2018-06-01

In a previous study by the authors (Rout et al. in Metall Mater Trans B 49:537-557, 2018), a dynamic model for the BOF, employing the concept of multizone kinetics was developed. In the current study, the kinetics of decarburization reaction is investigated. The jet impact and slag-metal emulsion zones were identified to be primary zones for carbon oxidation. The dynamic parameters in the rate equation of decarburization such as residence time of metal drops in the emulsion, interfacial area evolution, initial size, and the effects of surface-active oxides have been included in the kinetic rate equation of the metal droplet. A modified mass-transfer coefficient based on the ideal Langmuir adsorption equilibrium has been proposed to take into account the surface blockage effects of SiO2 and P2O5 in slag on the decarburization kinetics of a metal droplet in the emulsion. Further, a size distribution function has been included in the rate equation to evaluate the effect of droplet size on reaction kinetics. The mathematical simulation indicates that decarburization of the droplet in the emulsion is a strong function of the initial size and residence time. A modified droplet generation rate proposed previously by the authors has been used to estimate the total decarburization rate by slag-metal emulsion. The model's prediction shows that about 76 pct of total carbon is removed by reactions in the emulsion, and the remaining is removed by reactions at the jet impact zone. The predicted bath carbon by the model has been found to be in good agreement with the industrially measured data.

Dynamic Model of Basic Oxygen Steelmaking Process Based on Multizone Reaction Kinetics: Modeling of Decarburization

NASA Astrophysics Data System (ADS)

Rout, Bapin Kumar; Brooks, Geoffrey; Akbar Rhamdhani, M.; Li, Zushu; Schrama, Frank N. H.; Overbosch, Aart

2018-03-01

In a previous study by the authors (Rout et al. in Metall Mater Trans B 49:537-557, 2018), a dynamic model for the BOF, employing the concept of multizone kinetics was developed. In the current study, the kinetics of decarburization reaction is investigated. The jet impact and slag-metal emulsion zones were identified to be primary zones for carbon oxidation. The dynamic parameters in the rate equation of decarburization such as residence time of metal drops in the emulsion, interfacial area evolution, initial size, and the effects of surface-active oxides have been included in the kinetic rate equation of the metal droplet. A modified mass-transfer coefficient based on the ideal Langmuir adsorption equilibrium has been proposed to take into account the surface blockage effects of SiO2 and P2O5 in slag on the decarburization kinetics of a metal droplet in the emulsion. Further, a size distribution function has been included in the rate equation to evaluate the effect of droplet size on reaction kinetics. The mathematical simulation indicates that decarburization of the droplet in the emulsion is a strong function of the initial size and residence time. A modified droplet generation rate proposed previously by the authors has been used to estimate the total decarburization rate by slag-metal emulsion. The model's prediction shows that about 76 pct of total carbon is removed by reactions in the emulsion, and the remaining is removed by reactions at the jet impact zone. The predicted bath carbon by the model has been found to be in good agreement with the industrially measured data.

Lipid droplets fusion in adipocyte differentiated 3T3-L1 cells: A Monte Carlo simulation

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

Boschi, Federico, E-mail: federico.boschi@univr.it; Department of Computer Science, University of Verona, Strada Le Grazie 15, 37134 Verona; Rizzatti, Vanni

Several human worldwide diseases like obesity, type 2 diabetes, hepatic steatosis, atherosclerosis and other metabolic pathologies are related to the excessive accumulation of lipids in cells. Lipids accumulate in spherical cellular inclusions called lipid droplets (LDs) whose sizes range from fraction to one hundred of micrometers in adipocytes. It has been suggested that LDs can grow in size due to a fusion process by which a larger LD is obtained with spherical shape and volume equal to the sum of the progenitors’ ones. In this study, the size distribution of two populations of LDs was analyzed in immature and maturemore » (5-days differentiated) 3T3-L1 adipocytes (first and second populations, respectively) after Oil Red O staining. A Monte Carlo simulation of interaction between LDs has been developed in order to quantify the size distribution and the number of fusion events needed to obtain the distribution of the second population size starting from the first one. Four models are presented here based on different kinds of interaction: a surface weighted interaction (R2 Model), a volume weighted interaction (R3 Model), a random interaction (Random model) and an interaction related to the place where the LDs are born (Nearest Model). The last two models mimic quite well the behavior found in the experimental data. This work represents a first step in developing numerical simulations of the LDs growth process. Due to the complex phenomena involving LDs (absorption, growth through additional neutral lipid deposition in existing droplets, de novo formation and catabolism) the study focuses on the fusion process. The results suggest that, to obtain the observed size distribution, a number of fusion events comparable with the number of LDs themselves is needed. Moreover the MC approach results a powerful tool for investigating the LDs growth process. Highlights: • We evaluated the role of the fusion process in the synthesis of the lipid droplets. • We compared the size distribution of the lipid droplets in immature and mature cells. • We used the Monte Carlo simulation approach, simulating 10 thousand of fusion events. • Four different interaction models between the lipid droplets were tested. • The best model which mimics the experimental measures was selected.« less

Homogeneous Freezing of Water Droplets and its Dependence on Droplet Size

NASA Astrophysics Data System (ADS)

Schmitt, Thea; Möhler, Ottmar; Höhler, Kristina; Leisner, Thomas

2014-05-01

The formulation and parameterisation of microphysical processes in tropospheric clouds, such as phase transitions, is still a challenge for weather and climate models. This includes the homogeneous freezing of supercooled water droplets, since this is an important process in deep convective systems, where almost pure water droplets may stay liquid until homogeneous freezing occurs at temperatures around 238 K. Though the homogeneous ice nucleation in supercooled water is considered to be well understood, recent laboratory experiments with typical cloud droplet sizes showed one to two orders of magnitude smaller nucleation rate coefficients than previous literature results, including earlier results from experiments with single levitated water droplets and from cloud simulation experiments at the AIDA (Aerosol Interaction and Dynamics in the Atmosphere) facility. This motivated us to re-analyse homogeneous droplet freezing experiments conducted during the previous years at the AIDA cloud chamber. This cloud chamber has a volume of 84m3 and operates under atmospherically relevant conditions within wide ranges of temperature, pressure and humidity, whereby investigations of both tropospheric mixed-phase clouds and cirrus clouds can be realised. By controlled adiabatic expansions, the ascent of an air parcel in the troposphere can be simulated. According to our new results and their comparison to the results from single levitated droplet experiments, the homogeneous freezing of water droplets seems to be a volume-dependent process, at least for droplets as small as a few micrometers in diameter. A contribution of surface induced freezing can be ruled out, in agreement to previous conclusions from the single droplet experiments. The obtained volume nucleation rate coefficients are in good agreement, within error bars, with some previous literature data, including our own results from earlier AIDA experiments, but they do not agree with recently published lower volume nucleation rate coefficients. This contribution will show the results from the re-analysis of AIDA homogeneous freezing experiments with pure water droplets and will discuss the comparison to the literature data.

The potential role of sea spray droplets in facilitating air-sea gas transfer

NASA Astrophysics Data System (ADS)

Andreas, E. L.; Vlahos, P.; Monahan, E. C.

2016-05-01

For over 30 years, air-sea interaction specialists have been evaluating and parameterizing the role of whitecap bubbles in air-sea gas exchange. To our knowledge, no one, however, has studied the mirror image process of whether sea spray droplets can facilitate air-sea gas exchange. We are therefore using theory, data analysis, and numerical modeling to quantify the role of spray on air-sea gas transfer. In this, our first formal work on this subject, we seek the rate-limiting step in spray-mediated gas transfer by evaluating the three time scales that govern the exchange: τ air , which quantifies the rate of transfer between the atmospheric gas reservoir and the surface of the droplet; τ int , which quantifies the exchange rate across the air-droplet interface; and τ aq , which quantifies gas mixing within the aqueous solution droplet.

Droplet depinning in a wake

NASA Astrophysics Data System (ADS)

Hooshanginejad, Alireza; Lee, Sungyon

2017-03-01

Pinning and depinning of a windswept droplet on a surface is familiar yet deceptively complex for it depends on the interaction of the contact line with the microscopic features of the solid substrate. This physical picture is further compounded when wind of the Reynolds number greater than 100 blows over pinned drops, leading to the boundary layer separation and wake generation. In this Rapid Communication, we incorporate the well-developed ideas of the classical boundary layer to study partially wetting droplets in a wake created by a leader object. Depending on its distance from the leader, the droplet is observed to exhibit drafting, upstream motion, and splitting, due to the wake-induced hydrodynamic coupling that is analogous to drafting of moving bodies. We successfully rationalize the onset of the upstream motion regime using a reduced model that computes the droplet shape governed by the pressure field inside the wake.

Investigation of Critical Burning of Fuel Droplets. [of liquid rocket propellant

NASA Technical Reports Server (NTRS)

Chanin, S. P.; Shearer, A. J.; Faeth, G. M.

1976-01-01

An earlier analysis for the combustion response of a liquid monopropellant strand (hydrazine) was extended to consider individual droplets and sprays. While small drops gave low or negative response, large droplets provided response near unity at low frequencies, with the response declining at frequencies greater than the characteristic liquid phase frequency. Temperature gradients in the liquid phase resulted in response peaks greater than unity. A second response peak was found for large drops which corresponded to gas phase transient effects. Spray response was generally reduced from the response of the largest injected droplet, however, even a small percentage of large droplets can yield appreciable response. An apparatus was designed and fabricated to allow observation of bipropellant fuel spray combustion at elevated pressures. A locally homogeneous model was developed to describe this combustion process which allows for high pressure phenomena associated with the thermodynamic critical point.

Experimental, water droplet impingement data on two-dimensional airfoils, axisymmetric inlet and Boeing 737-300 engine inlet

NASA Technical Reports Server (NTRS)

Papadakis, M.; Elangovan, E.; Freund, G. A., Jr.; Breer, M. D.

1987-01-01

An experimental method has been developed to determine the droplet impingement characteristics on two- and three-dimensional bodies. The experimental results provide the essential droplet impingement data required to validate particle trajectory codes, used in aircraft icing analyses and engine inlet particle separator analyses. A body whose water droplet impingement characteristics are required is covered at strategic locations by thin strips of moisture absorbing (blotter) paper, and then exposed to an air stream containing a dyed-water spray cloud. Water droplet impingement data are extracted from the dyed blotter strips, by measuring the optical reflectance of the dye deposit on the strips, using an automated reflectometer. Impingement efficiency data obtained for a NACA 65(2)015 airfoil section, a supercritical airfoil section, and Being 737-300 and axisymmetric inlet models are presented in this paper.

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

PubMed

Richard, David; Speck, Thomas

2018-06-14

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

Topological Defects in Liquid Crystals: Studying the Correlation between Defects and Curvature

NASA Astrophysics Data System (ADS)

Melton, Charles

2015-03-01

Topological defects have recently been the subject of many fascinating studies in soft condensed matter physics. In particular, linking the evolution of topological defects to curvature changes has been a focus, leading possible applications in the areas such as cosmetics, pharmaceuticals, and electronics. In this study, defects in nematic liquid crystal droplets are investigated via laboratory and theoretical techniques. Nematic liquid crystal defects are reproduced via Monte Carlo simulations using a modified 2D XY-Model Hamiltonian. The simulation is performed on a curved surface to replicate a nematic droplet and examine possible defect configurations. To complement this theoretical work, we have trapped nematic droplets inside a dual-beam optical trap. This system allows controllable non-contact droplet deformation on a microscope based platform. Future work will focus on using the trap to stretch nematic droplets, correlating the changing topological defects with theoretical predictions.

SU-E-T-489: Quantum versus Classical Trajectory Monte Carlo Simulations of Low Energy Electron Transport.

PubMed

Thomson, R; Kawrakow, I

2012-06-01

Widely-used classical trajectory Monte Carlo simulations of low energy electron transport neglect the quantum nature of electrons; however, at sub-1 keV energies quantum effects have the potential to become significant. This work compares quantum and classical simulations within a simplified model of electron transport in water. Electron transport is modeled in water droplets using quantum mechanical (QM) and classical trajectory Monte Carlo (MC) methods. Water droplets are modeled as collections of point scatterers representing water molecules from which electrons may be isotropically scattered. The role of inelastic scattering is investigated by introducing absorption. QM calculations involve numerically solving a system of coupled equations for the electron wavefield incident on each scatterer. A minimum distance between scatterers is introduced to approximate structured water. The average QM water droplet incoherent cross section is compared with the MC cross section; a relative error (RE) on the MC results is computed. RE varies with electron energy, average and minimum distances between scatterers, and scattering amplitude. The mean free path is generally the relevant length scale for estimating RE. The introduction of a minimum distance between scatterers increases RE substantially (factors of 5 to 10), suggesting that the structure of water must be modeled for accurate simulations. Inelastic scattering does not improve agreement between QM and MC simulations: for the same magnitude of elastic scattering, the introduction of inelastic scattering increases RE. Droplet cross sections are sensitive to droplet size and shape; considerable variations in RE are observed with changing droplet size and shape. At sub-1 keV energies, quantum effects may become non-negligible for electron transport in condensed media. Electron transport is strongly affected by the structure of the medium. Inelastic scatter does not improve agreement between QM and MC simulations of low energy electron transport in condensed media. © 2012 American Association of Physicists in Medicine.

Applications of Electromagnetic Levitation and Development of Mathematical Models: A Review of the Last 15 Years (2000 to 2015)

NASA Astrophysics Data System (ADS)

Gao, Lei; Shi, Zhe; Li, Donghui; Zhang, Guifang; Yang, Yindong; McLean, Alexander; Chattopadhyay, Kinnor

2016-02-01

Electromagnetic levitation (EML) is a contact-less, high-temperature technique which has had extensive application with respect to the investigation of both thermophysical and thermochemical properties of liquid alloy systems. The varying magnetic field generates an induced current inside the metal droplet, and interactions are created which produce both the Lorentz force that provides support against gravity and the Joule heating effect that melts the levitated specimen. Since metal droplets are opaque, transport phenomena inside the droplet cannot be visualized. To address this aspect, several numerical modeling techniques have been developed. The present work reviews the applications of EML techniques as well as the contributions that have been made by the use of mathematical modeling to improve understanding of the inherent processes which are characteristic features of the levitation system.

Experimental Water Droplet Impingement Data on Airfoils, Simulated Ice Shapes, an Engine Inlet and a Finite Wing

NASA Technical Reports Server (NTRS)

Papadakis, M.; Breer, M.; Craig, N.; Liu, X.

1994-01-01

An experimental method has been developed to determine the water droplet impingement characteristics on two- and three-dimensional aircraft surfaces. The experimental water droplet impingement data are used to validate particle trajectory analysis codes that are used in aircraft icing analyses and engine inlet particle separator analyses. The aircraft surface is covered with thin strips of blotter paper in areas of interest. The surface is then exposed to an airstream that contains a dyed-water spray cloud. The water droplet impingement data are extracted from the dyed blotter paper strips by measuring the optical reflectance of each strip with an automated reflectometer. Experimental impingement efficiency data represented for a NLF (1)-0414 airfoil, a swept MS (1)-0317 airfoil, a Boeing 737-300 engine inlet model, two simulated ice shapes and a swept NACA 0012 wingtip. Analytical impingement efficiency data are also presented for the NLF (1)-0414 airfoil and the Boeing 737-300 engine inlet model.

Electric field induced sheeting and breakup of dielectric liquid jets

NASA Astrophysics Data System (ADS)

Khoshnevis, Ahmad; Tsai, Scott S. H.; Esmaeilzadeh, Esmaeil

2014-01-01

We report experimental observations of the controlled deformation of a dielectric liquid jet subjected to a local high-voltage electrostatic field in the direction normal to the jet. The jet deforms to the shape of an elliptic cylinder upon application of a normal electrostatic field. As the applied electric field strength is increased, the elliptic cylindrical jet deforms permanently into a flat sheet, and eventually breaks-up into droplets. We interpret this observation—the stretch of the jet is in the normal direction to the applied electric field—qualitatively using the Taylor-Melcher leaky dielectric theory, and develop a simple scaling model that predicts the critical electric field strength for the jet-to-sheet transition. Our model shows a good agreement with experimental results, and has a form that is consistent with the classical drop deformation criterion in the Taylor-Melcher theory. Finally, we statistically analyze the resultant droplets from sheet breakup, and find that increasing the applied electric field strength improves droplet uniformity and reduces droplet size.

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

Penner, Joyce E.; Zhou, Cheng

Observation-based studies have shown that the aerosol cloud lifetime effect or the increase of cloud liquid water path (LWP) with increased aerosol loading may have been overestimated in climate models. Here, we simulate shallow warm clouds on 05/27/2011 at the Southern Great Plains (SGP) measurement site established by Department of Energy's Atmospheric Radiation Measurement (ARM) Program using a single column version of a global climate model (Community Atmosphere Model or CAM) and a cloud resolving model (CRM). The LWP simulated by CAM increases substantially with aerosol loading while that in the CRM does not. The increase of LWP in CAMmore » is caused by a large decrease of the autoconversion rate when cloud droplet number increases. In the CRM, the autoconversion rate is also reduced, but this is offset or even outweighed by the increased evaporation of cloud droplets near cloud top, resulting in an overall decrease in LWP. Our results suggest that climate models need to include the dependence of cloud top growth and the evaporation/condensation process on cloud droplet number concentrations.« less

Experimental investigations of interaction of an air-droplet-crystal flow with a solid body in the problem of a flyer icing

NASA Astrophysics Data System (ADS)

Kashevarov, Alexey V.; Miller, Alexey B.; Potapov, Yuriy F.; Stasenko, Albert L.; Zhbanov, Vladimir A.

2018-05-01

An experimental facility for modeling of icing processes in various conditions (supercooled droplets, ice crystals and mixed-phase) is described and experimental results are presented. Some methods of icing processes characterization with non-dimensional coefficients are suggested. Theoretical model of a liquid film dynamics, mass and heat transfer during its movement on the model surface is presented. The numerical calculations of liquid film freezing and run-back ice evolution on the surface are performed.

Evaporation kinetics of sessile water droplets on micropillared superhydrophobic surfaces.

PubMed

Xu, Wei; Leeladhar, Rajesh; Kang, Yong Tae; Choi, Chang-Hwan

2013-05-21

Evaporation modes and kinetics of sessile droplets of water on micropillared superhydrophobic surfaces are experimentally investigated. The results show that a constant contact radius (CCR) mode and a constant contact angle (CCA) mode are two dominating evaporation modes during droplet evaporation on the superhydrophobic surfaces. With the decrease in the solid fraction of the superhydrophobic surfaces, the duration of a CCR mode is reduced and that of a CCA mode is increased. Compared to Rowan's kinetic model, which is based on the vapor diffusion across the droplet boundary, the change in a contact angle in a CCR (pinned) mode shows a remarkable deviation, decreasing at a slower rate on the superhydrophobic surfaces with less-solid fractions. In a CCA (receding) mode, the change in a contact radius agrees well with the theoretical expectation, and the receding speed is slower on the superhydrophobic surfaces with lower solid fractions. The discrepancy between experimental results and Rowan's model is attributed to the initial large contact angle of a droplet on superhydrophobic surfaces. The droplet geometry with a large contact angle results in a narrow wedge region of air along the contact boundary, where the liquid-vapor diffusion is significantly restricted. Such an effect becomes minor as the evaporation proceeds with the decrease in a contact angle. In both the CCR and CCA modes, the evaporative mass transfer shows the linear relationship between mass(2/3) and evaporation time. However, the evaporation rate is slower on the superhydrophobic surfaces, which is more significant on the surfaces with lower solid fractions. As a result, the superhydrophobic surfaces slow down the drying process of a sessile droplet on them.

Evaporation of impact water droplets in interception processes: Historical precedence of the hypothesis and a brief literature overview

NASA Astrophysics Data System (ADS)

Dunkerley, David L.

2009-10-01

SummaryIntra-storm evaporation depths exceed post-storm evaporation depths in the interception of rainfall on plant canopies. An important fraction of the intra-storm evaporation may involve the small impact (or splash) droplets produced when raindrops, and perhaps gravity drops (drips released from plant parts), collide with wet plant surfaces. This idea has been presented as a new conception by Murakami [Murakami, S., 2006. A proposal for a new forest canopy interception mechanism: splash droplet evaporation. Journal of Hydrology 319, 72-82; Murakami, S., 2007a. Application of three canopy interception models to a young stand of Japanese cypress and interpretation in terms of interception mechanism. Journal of Hydrology 342, 305-319; Murakami, S., 2007b. A follow-up for the splash droplet evaporation hypothesis of canopy interception and remaining problems: why is humidity unsaturated during rainfall? In: Proceedings of the 20th Annual Conference. Japan Society of Hydrology and Water Resources (in Japanese). < http://www.jstage.jst.go.jp/article/jshwr/20/0/20_62/_article>] but was in fact advanced by Dunin [Dunin, F.X., O'Loughlin, E.M., Reyenga, W., 1988. Interception loss from eucalypt forest: lysimeter determination of hourly rates for long term evaluation. Hydrological Processes 2, 315-329] more than 20 years ago. In addition, Dunin et al. considered that canopy ventilation might be enhanced in intense rain. This note draws attention to the historical precedence of the work of Dunin et al. and also presents a short review of literature on impact droplet production, highlighting areas where data are still required for the full exploration of the role of droplet evaporation in canopy interception. Droplet production needs to be properly parameterised and included in models of interception processes and landsurface-atmosphere interactions.

Thermodynamics and kinetics of binary nucleation in ideal-gas mixtures.

PubMed

Alekseechkin, Nikolay V

2015-08-07

The nonisothermal single-component theory of droplet nucleation [N. V. Alekseechkin, Physica A 412, 186 (2014)] is extended to binary case; the droplet volume V, composition x, and temperature T are the variables of the theory. An approach based on macroscopic kinetics (in contrast to the standard microscopic model of nucleation operating with the probabilities of monomer attachment and detachment) is developed for the droplet evolution and results in the derived droplet motion equations in the space (V, x, T)—equations for V̇≡dV/dt, ẋ, and Ṫ. The work W(V, x, T) of the droplet formation is obtained in the vicinity of the saddle point as a quadratic form with diagonal matrix. Also, the problem of generalizing the single-component Kelvin equation for the equilibrium vapor pressure to binary case is solved; it is presented here as a problem of integrability of a Pfaffian equation. The equation for Ṫ is shown to be the first law of thermodynamics for the droplet, which is a consequence of Onsager's reciprocal relations and the linked-fluxes concept. As an example of ideal solution for demonstrative numerical calculations, the o-xylene-m-xylene system is employed. Both nonisothermal and enrichment effects are shown to exist; the mean steady-state overheat of droplets and their mean steady-state enrichment are calculated with the help of the 3D distribution function. Some qualitative peculiarities of the nucleation thermodynamics and kinetics in the water-sulfuric acid system are considered in the model of regular solution. It is shown that there is a small kinetic parameter in the theory due to the small amount of the acid in the vapor and, as a consequence, the nucleation process is isothermal.

Phase Transformation of Droplets into Particles and Nucleation in Atmospheric Pressure Discharges

NASA Astrophysics Data System (ADS)

Iqbal, M. M.; Stallard, C. P.; Dowling, D. P.; Turner, M. M.

2013-09-01

We investigate the mechanism of phase transformation of liquid precursor droplets into nano-particulates in an atmospheric pressure discharge (APD). This phase transformation is possible when the solid to a liquid mass ratio of slurry droplet reaches a threshold value. The behaviour of phase transformation of a single slurry droplet of HMDSO is described by developing a numerical model under the saturation condition of evaporation. It is observed from the temporal evolution of inner radius (Ri) of a single slurry droplet that its value approaches zero before the entire shifting of a liquid phase and which explains with an expansion in the crust thickness (Ro - Ri) . The solid traces of nano-particles are observed experimentally on the surface coating depositions because the time for transferring the slurry droplet of HMDSO into solid state is amplified with an increment in the radii of droplets and the entire phase transition occurs within residence time for the nano-sized liquid droplets. The GDE coupled with discharge plasma is numerically solved to describe the mechanism of nucleation of nano-sized particles in APD plasma under similar conditions of the experiment. The growth of nucleation in APD plasma depends on the type of liquid precursor, such as HMDSO, TEOS and water, which is verified with a sharp peak in the nucleation rate and saturation ratio. Science Foundation Ireland under Grant No. 08/SRC/I1411.

Coupling liquid chromatography/mass spectrometry detection with microfluidic droplet array for label-free enzyme inhibition assay.

PubMed

Wang, Xiu-Li; Zhu, Ying; Fang, Qun

2014-01-07

In this work, the combination of droplet-based microfluidics with liquid chromatography/mass spectrometry (LC/MS) was achieved, for providing a fast separation and high-information-content detection method for the analysis of nanoliter-scale droplets with complex compositions. A novel interface method was developed using an oil-covered droplet array chip to couple with an LC/MS system via a capillary sampling probe and a 4 nL injection valve without the need of a droplet extraction device. The present system can perform multistep operations including parallel enzyme inhibition reactions in nanoliter droplets, 4 nL sample injection, fast separation with capillary LC, and label-free detection with ESI-MS, and has significant flexibility in the accurate addressing and sampling of droplets of interest on demand. The system performance was evaluated using angiotensin I and angiotensin II as model samples, and the repeatabilities of peak area for angiotensin I and angiotensin II were 2.7% and 7.5% (RSD, n = 4), respectively. The present system was further applied to the screening for inhibitors of cytochrome P450 (CYP1A2) and measurement of the IC50 value of the inhibitor. The sample consumption for each droplet assay was 100 nL, which is reduced 10-100 times compared with conventional 384-multi-well plate systems usually used in high-throughput drug screening.

pH-Sensitive self-propelled motion of oil droplets in the presence of cationic surfactants containing hydrolyzable ester linkages.

PubMed

Banno, Taisuke; Kuroha, Rie; Toyota, Taro

2012-01-17

Self-propelled oil droplets in a nonequilibrium system have drawn much attention as both a primitive type of inanimate chemical machinery and a dynamic model of the origin of life. Here, to create the pH-sensitive self-propelled motion of oil droplets, we synthesized cationic surfactants containing hydrolyzable ester linkages. We found that n-heptyloxybenzaldehyde oil droplets were self-propelled in the presence of ester-containing cationic surfactant. In basic solution prepared with sodium hydroxide, oil droplets moved as molecular aggregates formed on their surface. Moreover, the self-propelled motion in the presence of the hydrolyzable cationic surfactant lasted longer than that in the presence of nonhydrolyzable cationic surfactant. This is probably due to the production of a fatty acid by the hydrolysis of the ester-containing cationic surfactant and the subsequent neutralization of the fatty acid with sodium hydroxide. A complex surfactant was formed in the aqueous solution because of the cation and anion combination. Because such complex formation can induce both a decrease in the interfacial tension of the oil droplet and self-assembly with n-heptyloxybenzaldehyde and lauric acid in the aqueous dispersion, the prolonged movement of the oil droplet may be explained by the increase in heterogeneity of the interfacial tension of the oil droplet triggered by the hydrolysis of the ester-containing surfactant. © 2011 American Chemical Society

Dehydration induced phase transitions in a microfluidic droplet array for the separation of biomolecules

NASA Astrophysics Data System (ADS)

Nelson, Chris; Anna, Shelley

2013-11-01

Droplet-based strategies for fluid manipulation have seen significant application in microfluidics due to their ability to compartmentalize solutions and facilitate highly parallelized reactions. Functioning as micro-scale reaction vessels, droplets have been used to study protein crystallization, enzyme kinetics, and to encapsulate whole cells. Recently, the mass transport out of droplets has been used to concentrate solutions and induce phase transitions. Here, we show that droplets trapped in a microfluidic array will spontaneously dehydrate over the course of several hours. By loading these devices with an initially dilute aqueous polymer solution, we use this slow dehydration to observe phase transitions and the evolution of droplet morphology in hundreds of droplets simultaneously. As an example, we trap and dehydrate droplets of a model aqueous two-phase system consisting of polyethylene glycol and dextran. Initially the drops are homogenous, then after some time the polymer concentration reaches a critical point and two phases form. As water continues to leave the system, the drops transition from a microemulsion of DEX in PEG to a core-shell configuration. Eventually, changes in interfacial tension, driven by dehydration, cause the DEX core to completely de-wet from the PEG shell. Since aqueous two phase systems are able to selectively separate a variety of biomolecules, this core shedding behavior has the potential to provide selective, on-chip separation and concentration.

Bütschli dynamic droplet system.

PubMed

Armstrong, Rachel; Hanczyc, Martin

2013-01-01

Dynamical oil-water systems such as droplets display lifelike properties and may lend themselves to chemical programming to perform useful work, specifically with respect to the built environment. We present Bütschli water-in-oil droplets as a model for further investigation into the development of a technology with living properties. Otto Bütschli first described the system in 1898, when he used alkaline water droplets in olive oil to initiate a saponification reaction. This simple recipe produced structures that moved and exhibited characteristics that resembled, at least superficially, the amoeba. We reconstructed the Bütschli system and observed its life span under a light microscope, observing chemical patterns and droplet behaviors in nearly three hundred replicate experiments. Self-organizing patterns were observed, and during this dynamic, embodied phase the droplets provided a means of introducing temporal and spatial order in the system with the potential for chemical programmability. The authors propose that the discrete formation of dynamic droplets, characterized by their lifelike behavior patterns, during a variable window of time (from 30 s to 30 min after the addition of alkaline water to the oil phase), qualify this system as an example of living technology. The analysis of the Bütschli droplets suggests that a set of conditions may precede the emergence of lifelike characteristics and exemplifies the richness of this rudimentary chemical system, not only for artificial life investigations but also for possible real-world applications in architectural practice.

Numerical study on morphology and solidification characteristics of successive droplet depositions on a substrate

NASA Astrophysics Data System (ADS)

Adaikalanathan, Vimalan

Successive droplet impingement finds extensive applications in additive manufacturing technologies such as 3D printing, Liquid Metal Jetting and Net Form Manufacturing. Deposition, deformation and solidification of droplets are the constitutive stages in the process which determine the final outcome. Detailed knowledge about the flow behaviour, phase transformation and free surface deformation is required to have a complete understanding and optimization of the process parameters. Experimental research in this field is only limited to imaging techniques and post solidification analysis which only provide superficial information while overlooking most of the governing phenomenon. Knowledge of the physics governing the fluid and thermal behaviours can be applied to study the process with real time data pertaining to flow field, temperature profiles and solidification. However, free surface tracking, surface tension modelling, non-isothermal solidification and convection dominant heat transfer pose mathematical challenges in the solution of the governing equations. Moreover, deposition of droplets on pre-solidified splats or non-flat surfaces requires accurate special attention. The objective of the present work is to model the successive droplet impacts and simultaneous solidification and deformation. The highly non-linear flow field governed by the Navier Stokes equation is solved using a Two Step Projection method. The surface tension effects are accounted for through a Continuum Surface Force technique. One of the crucial elements in the study is the interface tracking algorithm. A Coupled Level Set Volume of Fluid (CLSVOF) method is formulated to give an accurate orientation of the drastically deforming interface and also facilitates generation of multiple droplets in a fixed domain at a user defined frequency, thereby conserving computational resources. The phase change is modelled using an enthalpy formulation of the energy equation with an implicit source term accounting for the latent heat. It is coupled with the flow solver through an Enthalpy-Porosity technique. A modified boundary condition which incorporates the contact resistance has also been implemented. The case of multiple eutectic solder droplet depositions has been simulated to study the various aspects of splat morphology and solidification characteristics. Effects of impact conditions on single as well as successive droplet depositions have been examined. The role of convection terms in the energy equation has been emphasized and quantitatively analysed. The effect of impact velocity is manifested as surface curvature of the pre-solidified splat and in turn, affects morphology of the subsequent droplets. Initial droplet temperature influences the solidification time of both single and multiple droplets. Under certain conditions, remelting of pre-solidified splat has been observed and its causes have been discussed. Contact resistance has been reported in the literature and has been found to have a strong influence not only on the heat transfer but also the spreading behaviour. Frequency of successive impingements is also an important factor affecting the metallurgical bonding properties.

Global Distribution of Cloud Droplet Number Concentration, Autoconversion Rate, and Aerosol Indirect Effect Under Diabatic Droplet Activation

NASA Technical Reports Server (NTRS)

Barahona, Donifan; Sotiropoulou, Rafaella; Nenes, Athanasios

2011-01-01

This study presents a global assessment of the sensitivity of droplet number to diabatic activation (i.e., including effects from entrainment of dry air) and its first-order tendency on indirect forcing and autoconversion. Simulations were carried out with the NASA Global Modeling Initiative (GMI) atmospheric and transport model using climatological metereorological fields derived from the former NASA Data Assimilation Office (DAO), the NASA Finite volume GCM (FVGCM) and the Goddard Institute for Space Studies version II (GISS) GCM. Cloud droplet number concentration (CDNC) is calculated using a physically based prognostic parameterization that explicitly includes entrainment effects on droplet formation. Diabatic activation results in lower CDNC, compared to adiabatic treatment of the process. The largest decrease in CDNC (by up to 75 percent) was found in the tropics and in zones of moderate CCN concentration. This leads to a global mean effective radius increase between 0.2-0.5 micrometers (up to 3.5 micrometers over the tropics), a global mean autoconversion rate increase by a factor of 1.1 to 1.7 (up to a factor of 4 in the tropics), and a 0.2-0.4 W m(exp -2) decrease in indirect forcing. The spatial patterns of entrainment effects on droplet activation tend to reduce biases in effective radius (particularly in the tropics) when compared to satellite retrievals. Considering the diabatic nature of ambient clouds, entrainment effects on CDNC need to be considered in GCM studies of the aerosol indirect effect.

Growth and melting of droplets in cold vapors.

PubMed

L'Hermite, Jean-Marc

2009-11-01

A model has been developed to investigate the growth of droplets in a supersaturated cold vapor taking into account their possible solid-liquid phase transition. It is shown that the solid-liquid phase transition is nontrivially coupled, through the energy released in attachment, to the nucleation process. The model is based on the one developed by J. Feder, K. C. Russell, J. Lothe, and G. M. Pound [Adv. Phys. 15, 111 (1966)], where the nucleation process is described as a thermal diffusion motion in a two-dimensional field of force given by the derivatives of a free-energy surface. The additional dimension accounts for droplets internal energy. The solid-liquid phase transition is introduced through a bimodal internal energy distribution in a Gaussian approximation derived from small clusters physics. The coupling between nucleation and melting results in specific nonequilibrium thermodynamical properties, exemplified in the case of water droplets. Analyzing the free-energy landscapes gives an insight into the nucleation dynamics. This landscape can be complex but generally exhibits two paths: the first one can generally be ascribed to the solid state, while the other to the liquid state. Especially at high supersaturation, the growth in the liquid state is often favored, which is not unexpected since in a supersaturated vapor the droplets can stand higher internal energy than at equilibrium. From a given critical temperature that is noticeably lower than the bulk melting temperature, nucleation may end in very large liquid droplets. These features can be qualitatively generalized to systems other than water.

Single-droplet evaporation kinetics and particle formation in an acoustic levitator. Part 1: evaporation of water microdroplets assessed using boundary-layer and acoustic levitation theories.

PubMed

Schiffter, Heiko; Lee, Geoffrey

2007-09-01

The suitability of a single droplet drying acoustic levitator as a model for the spray drying of aqueous, pharmaceutically-relevant solutes used to produce protein-loaded particles has been examined. The acoustic levitator was initially evaluated by measuring the drying rates of droplets of pure water in dependence of drying-air temperature and flow rate. The measured drying rates were higher than those predicted by boundary layer theory because of the effects of primary acoustic streaming. Sherwood numbers of 2.6, 3.6, and 4.4 at drying-air temperatures of 25 degrees C, 40 degrees C, and 60 degrees C were determined, respectively. Acoustic levitation theory could predict the measured drying rates and Sherwood numbers only when a forced-convection drying-air stream was used to neuralize the retarding effect of secondary acoustic streaming on evaporation rate. At still higher drying-air flow rates, the Ranz-Marshall correlation accurately predicts Sherwood number, provided a stable droplet position in the standing acoustic wave is maintained. The measured Sherwood numbers and droplet Reynolds numbers show that experiments performed in the levitator in still air are taking place effectively under conditions of substantial forced convection. The similitude of these values to those occurring in spray dryers is fortuitous for the suitability of the acoustic levitator as a droplet evaporation model for spray drying. (c) 2007 Wiley-Liss, Inc. and the American Pharmacists Association.

Artificial intelligence exploration of unstable protocells leads to predictable properties and discovery of collective behavior.

PubMed

Points, Laurie J; Taylor, James Ward; Grizou, Jonathan; Donkers, Kevin; Cronin, Leroy

2018-01-30

Protocell models are used to investigate how cells might have first assembled on Earth. Some, like oil-in-water droplets, can be seemingly simple models, while able to exhibit complex and unpredictable behaviors. How such simple oil-in-water systems can come together to yield complex and life-like behaviors remains a key question. Herein, we illustrate how the combination of automated experimentation and image processing, physicochemical analysis, and machine learning allows significant advances to be made in understanding the driving forces behind oil-in-water droplet behaviors. Utilizing >7,000 experiments collected using an autonomous robotic platform, we illustrate how smart automation cannot only help with exploration, optimization, and discovery of new behaviors, but can also be core to developing fundamental understanding of such systems. Using this process, we were able to relate droplet formulation to behavior via predicted physical properties, and to identify and predict more occurrences of a rare collective droplet behavior, droplet swarming. Proton NMR spectroscopic and qualitative pH methods enabled us to better understand oil dissolution, chemical change, phase transitions, and droplet and aqueous phase flows, illustrating the utility of the combination of smart-automation and traditional analytical chemistry techniques. We further extended our study for the simultaneous exploration of both the oil and aqueous phases using a robotic platform. Overall, this work shows that the combination of chemistry, robotics, and artificial intelligence enables discovery, prediction, and mechanistic understanding in ways that no one approach could achieve alone.