Interface film resistivities for heat and mass transfers-integral relations verified by non-equilibrium molecular dynamics

Integral relations that predict interface film transfer coefficients for evaporation and condensation have recently been derived. According to these relations, all coefficients can be calculated for one-component systems, using the thermal resistivity and the enthalpy profile through the interface. The integral relations were tested in this work using nonequilibrium molecular dynamics simulations for argon-like particles and n-octane molecules. The simulations confirm the integral relations within the accuracy of the calculation for both systems. Evidence is presented for the existence of an excess thermal resistivity on the gas side of the surface, and the fact that this property is decisive for interface heat and mass transfer coefficients. The integral relations were used to predict the mass transfer coefficient for n- octane as a function of surface tension. The findings are important for modeling of one-component phase transitions.     

Resistances for heat and mass transfer through a liquid-vapor interface in a binary mixture

In this paper we calculate the interfacial resistances to heat and mass transfer through a liquid-vapor interface in a binary mixture. We use two methods, the direct calculation from the actual nonequilibrium solution and integral relations, derived earlier. We verify, that integral relations, being a relatively faster and cheaper method, indeed gives the same results as the direct processing of a nonequilibrium solution. Furthermore we compare the absolute values of the interfacial resistances with the ones obtained from kinetic theory. Matching the diagonal resistances for the binary mixture we find that kinetic theory underestimates the cross coefficients. The heat of transfer is, as a consequence, correspondingly larger.     

Equilibrium and nonequilibrium molecular dynamics simulations of the thermal conductivity of molten alkali halides

The thermal conductivity of molten NaCl and KCl was calculated through the Evans-Gillan nonequilibrium molecular dynamics (NEMD) algorithm and Green-Kubo equilibrium molecular dynamics (EMD) simulations. The EMD simulations were performed for a "binary" ionic mixture and the NEMD simulations assumed a pure system for reasons discussed in this work. The cross thermoelectric coefficient obtained from Green-Kubo EMD simulations is discussed in terms of the homogeneous thermoelectric power or Seebeck coefficient of these materials. The thermal conductivity obtained from NEMD simulations is found to be in very good agreement with that obtained through Green-Kubo EMD simulations for a binary ionic mixture. This result points to a possible cancellation between the neglected "partial enthalpy" contribution to the heat flux associated with the interdiffusion of one species through the other and that part of the thermal conductivity related to the coupled fluxes of charge and heat in "binary" ionic mixtures.     

Water under temperature gradients: polarization effects and microscopic mechanisms of heat transfer

We report non-equilibrium molecular dynamics simulations (NEMD) of water under temperature gradients using a modified version of the central force model (MCFM). This model is very accurate in predicting the equation of state of water for a wide range of pressures and temperatures. We investigate the polarization response of water to thermal gradients, an effect that has been recently predicted using Non-Equilibrium Thermodynamics (NET) theory and computer simulations, as a function of the thermal gradient strength. We find that the polarization of the liquid varies linearly with the gradient strength, which indicates that the ratio of phenomenological coefficients regulating the coupling between the polarization response and the heat flux is independent of the gradient strength investigated. This notion supports the NET theoretical predictions. The coupling effect leading to the liquid polarization is fairly strong, leading to polarization fields of ~10(3-6) V m(-1) for gradients of ~10(5-8) K m(-1), hence confirming earlier estimates. Finally we employ our NEMD approach to investigate the microscopic mechanism of heat transfer in water. The image emerging from the computation and analysis of the internal energy fluxes is that the transfer of energy is dominated by intermolecular interactions. For the MCFM model, we find that the contribution from hydrogen and oxygen is different, with the hydrogen contribution being larger than that of oxygen.     

Flux of gases across the air-water interface studied by reversed-flow gas chromatography.

In the present work the reversed-flow gas chromatographic technique was applied for the study of flux of gases across the air-water interface. The model system was vinyl chloride-water, which is of great significance in food and environmental chemistry. Using suitable mathematical analysis, equations were derived by means of which the following physicochemical quantities were calculated: diffusion coefficient of vinyl chloride (VC) into water, partition coefficient of VC between the water (at the interface and the bulk) and the carrier gas nitrogen, overall mass transfer coefficients of VC in the gas (nitrogen) and the liquid (water), gas and liquid film transfer coefficients of VC, gas and liquid phase resistances for the transfer of VC into the water, and finally the thickness of the stagnant film in the liquid phase, according to the two-film theory of Whitman. From the variation of the above parameters with temperature, as well as the volume and the free surface area of the water, useful conclusions concerning the mechanism for the transfer of VC into water were extracted. These are discussed in comparison with the same parameters calculated from empirical equations or determined experimentally by other techniques.     

Slip length of water on graphene: limitations of non-equilibrium molecular dynamics simulations

Data for the flow rate of water in carbon nanopores is widely scattered, both in experiments and simulations. In this work, we aim at precisely quantifying the characteristic large slip length and flow rate of water flowing in a planar graphene nanochannel. First, we quantify the slip length using the intrinsic interfacial friction coefficient between water and graphene, which is found from equilibrium molecular dynamics (EMD) simulations. We then calculate the flow rate and the slip length from the streaming velocity profiles obtained using non-equilibrium molecular dynamics (NEMD) simulations and compare with the predictions from the EMD simulations. The slip length calculated from NEMD simulations is found to be extremely sensitive to the curvature of the velocity profile and it possesses large statistical errors. We therefore pose the question: Can a micrometer range slip length be reliably determined using velocity profiles obtained from NEMD simulations? Our answer is "not practical, if not impossible" based on the analysis given as the results. In the case of high slip systems such as water in carbon nanochannels, the EMD method results are more reliable, accurate, and computationally more efficient compared to the direct NEMD method for predicting the nanofluidic flow rate and hydrodynamic boundary condition.     

Characterization of Simultaneous Heat and Mass Transfer Phenomena for Water Vapour Condensation on a Solid Surface in an Abiotic Environment—Application to Bioprocesses

The phenomenon of heat and mass transfer by condensation of water vapour from humid air involves several key concepts in aerobic bioreactors. The high performance of bioreactors results from optimised interactions between biological processes and multiphase heat and mass transfer. Indeed in various processes such as submerged fermenters and solid-state fermenters, gas/liquid transfer need to be well controlled, as it is involved at the microorganism interface and for the control of the global process. For the theoretical prediction of such phenomena, mathematical models require heat and mass transfer coefficients. To date, very few data have been validated concerning mass transfer coefficients from humid air inflows relevant to those bioprocesses. Our study focussed on the condensation process of water vapour and developed an experimental set-up and protocol to study the velocity profiles and the mass flux on a small size horizontal flat plate in controlled environmental conditions. A closed circuit wind tunnel facility was used to control the temperature, hygrometry and hydrodynamics of the flow. The temperature of the active surface was controlled and kept isothermal below the dew point to induce condensation, by the use of thermoelectricity. The experiments were performed at ambient temperature for a relative humidity between 35?C65% and for a velocity of 1.0?ms^?1. The obtained data are analysed and compared to available theoretical calculations on condensation mass flux.     

Determination of Bulk and Surface Diffusion Coefficients from Experimental Data for Thin Liquid Film Drainage

This note presents a method for the determination of the surface diffusion coefficient and surface diffusion flux. The theoretical considerations are based on the Onsager linear theory for the definition of the surface diffusion flux and on the Einstein theorem for the definition of the surface diffusion parameter. In this interpretation the surface diffusion coefficient differs from the one commonly defined in the literature. It does not depend on the surfactant concentration and it is a function only of the type of surfactant and the liquid/liquid interface. The theoretical calculations indicate that the effect of the surface diffusion on the film drainage is stronger than that predicted by previous theoretical studies. The experimental data for thin liquid film drainage in the case of low surfactant concentration in the continuous phase could be used for the calculation of the bulk and surface diffusion coefficients. In the present study we utilized the experimental data for the drainage of nitrobenzene films stabilized by different concentrations of dodecanol. Copyright 2000 Academic Press.     

A molecular dynamics study to determine the solid-liquid interfacial tension using test area simulation method (TASM)

Molecular dynamics (MD) studies on heat transfer from a heated nanoparticle into the surrounding fluid have indicated that the fluid next to a spherical nanoparticle can get heated well above its boiling point without observing a phase change, while a contradicting behavior was observed for a flat surface-fluid interface. Another interesting observation is that the critical heat flux was found to increase with increase in the wetting characteristics of solid. Thus, the interfacial tension or free energy of solid-liquid interface could play a pivotal role in the mechanism of heat transfer. A recent study by Gloor et al. [J. Chem. Phys. 123, 134703 (2005)] has proposed test area simulation method (TASM) for the determination of interfacial tension. The present study involves the determination and the comparison of solid-liquid interfacial tension for planar and spherical interfaces using MD based on TASM and analyze the results. A higher interfacial tension value is observed for spherical nanoparticle fluid interface compared to flat surface fluid interface. The results also indicate that the solid-liquid interfacial tension is a size and temperature dependent property. The results from this study are also expected to give better insights into the possible reasons for the observed differences in the thermal transport for spherical nanoparticle-liquid interface compared to planar-liquid interface.     

Molecular dynamics simulation of the equilibrium liquid–vapor interphase with solidification

The equilibrium structure of the finite, interphase interfacial region that exists between a liquid film and a bulk vapor is resolved by molecular dynamics simulation. Argon systems are considered for a temperature range that extends below the melting point. Physically consistent procedures are developed to define the boundaries between the interphase and the liquid and vapor phases. The procedures involve counting of neighboring molecules and comparing the results with boundary criteria that permit the boundaries to be precisely established. Two-dimensional radial distribution functions at the liquid and vapor boundaries and within the interphase region demonstrate the physical consistency of the boundary criteria and the state of transition within the region. The method developed for interphase boundary definitions can be extended to nonequilibrium systems. Spatial profiles of macroscopic properties across the interphase region are presented. A number of interfacial thermodynamic properties and profile curve-fit parameters are tabulated, including evaporation/condensation coefficients determined from molecular flux statistics. The evaporation/condensation coefficients away from the melting point compare more favorably with transition state theory than those of previous simulations. Near the melting point, transition theory approximations are less valid and the present results differ from the theory. The effects of film substrate wetting on evaporation/condensation coefficients are also presented.     

Hydroxyl radical at the air-water interface

Interaction of the hydroxyl radical with the liquid water surface was studied using classical molecular dynamics computer simulations. From a series of scattering trajectories, the thermal and mass accommodation coefficients of OH on liquid water at 300 K were determined to be 0.95 and 0.83, respectively. The calculated free energy profile for transfer of OH across the air-water interface at 300 K exhibits a minimum in the interfacial region, with the free energy of adsorbtion (DeltaGa) being about 1 kcal/mol more negative than the hydration free energy (DeltaGs). The propensity of the hydroxyl radical for the air-water interface manifests itself in partitioning of OH radicals between the bulk water and the surface. The enhancement of the surface concentration of OH relative to its concentration in the aqueous phase suggests that important OH chemistry may be occurring in the interfacial layer of water droplets, aqueous aerosol particles, and thin water films adsorbed on solid surfaces. This has profound consequences for modeling heterogeneous atmospheric chemical processes.     

Dynamics and kinetics of heat transfer at the interface of model diamond [111] nanosurfaces

A molecular dynamics simulation was performed to study the effect of an applied force on heat transfer at the interface of model diamond [111] nanosurfaces. The force was applied to a small, hot nanosurface at 800, 1000, or 1200 K brought into contact with a larger, colder nanosurface at 300 K. The relaxation of the initial nonequilibrium interfacial force occurs on a subpicosecond time scale, much shorter than that required for heat transfer. Heat transfer occurs with exponential kinetics and a rate constant that increases linearly with the interfacial force according to 7 x 10(-4) ps(-1)/nN. This rate constant only increases by at most 10% as the temperature of the hot surface is increased from 800 to 1200 K. Replacing the interfacial H-atoms on both surfaces by D atoms also has a very small effect on the heat transfer. However, if one nanosurface has H atoms on its interface and the other nanosurface's interface has D atoms, then there is a marked 25% decrease in the rate constant for heat transfer. Increasing the size of the hot surface, and, thus, the interfacial contact area, increases the rate of heat transfer but not the rate constant. For the same interfacial force, different anharmonic models for the nanosurfaces' potential energy function give the same heat transfer rate constant. The possibility of quantum effects for heat transfer across the diamond interface is considered.     

Mass transfer model of triethylamine across the n-decane/water interface derived from dynamic interfacial tension experiments

This publication presents a detailed experimental and theoretical study of mass transfer of triethylamine (TEA) across the n-decane/water interface. In preliminary investigations, the partition of TEA between n-decane and water is determined. Based on the experimental finding that the dissociation of TEA takes place in the aqueous and in the organic phase, we assume that the interfacial mass transfer is mainly affected by adsorption and desorption of ionized TEA molecules at the liquid/liquid interface. Due to the amphiphilic structure of the dissociated TEA molecules, a dynamic interfacial tension measurement technique can be used to experimentally determine the interfacial mass transport. A model-based approach, which accounts for diffusive mass transport in the finite liquid bulk phases and for adsorption and desorption of ionized TEA molecules at the interface, is employed to analyze the experimental data. In the equilibrium state, the interfacial tension of dissociated TEA at the n-decane/water interface can be adequately described by the Langmuir isotherm. The comparison between the theoretical and the experimental dynamic interfacial tension data reveals that an additional activation energy barrier for adsorption and desorption at the interface has to be regarded to accurately describe the mass transport of TEA from the n-decane phase into the aqueous phase. Corresponding adsorption rate constants can be obtained by fitting the theoretical predictions to the experimental data. Interfacial tension measurements of mass transfer from the aqueous into the organic phase are characterized by interfacial instabilities caused by Marangoni convection, which result in an enhancement of the transfer rate across the interface.     

Slip flow in graphene nanochannels

We investigate the hydrodynamic boundary condition for simple nanofluidic systems such as argon and methane flowing in graphene nanochannels using equilibrium molecular dynamics simulations (EMD) in conjunction with our recently proposed method [J. S. Hansen, B. D. Todd, and P. J. Daivis, Phys. Rev. E 84, 016313 (2011)]. We first calculate the fluid-graphene interfacial friction coefficient, from which we can predict the slip length and the average velocity of the first fluid layer close to the wall (referred to as the slip velocity). Using direct nonequilibrium molecular dynamics simulations (NEMD) we then calculate the slip length and slip velocity from the streaming velocity profiles in Poiseuille and Couette flows. The slip lengths and slip velocities from the NEMD simulations are found to be in excellent agreement with our EMD predictions. Our EMD method therefore enables one to directly calculate this intrinsic friction coefficient between fluid and solid and the slip length for a given fluid and solid, which is otherwise tedious to calculate using direct NEMD simulations at low pressure gradients or shear rates. The advantages of the EMD method over the NEMD method to calculate the slip lengths/flow rates for nanofluidic systems are discussed, and we finally examine the dynamic behaviour of slip due to an externally applied field and shear rate.     

Evaluation method for heat transfer coefficient of a contact interface across a microscale thin liquid polymer film

The research presented here evaluates the heat transfer coefficient of the contact interface of a thin liquid polymer film between a pair of columnar aluminum bodies with an initial temperature difference of approximately 160 K. We measured the unsteady temperature changes inside the columns. The heat transfer test was performed with three types of liquid polymers: squalane, oleic acid, and silicone oil. The heat transfer coefficient of the polymer films as a fitting parameter was obtained by ensuring the numerically computed time evolution of the columns’ temperature corresponded with the experimentally measured data. The interfacial heat transfer coefficients of the thin polymer films (mean thickness: 60 μm) for all three polymers used were 1.75 kW/m²·K, 2.75 kW/m²·K, and 4.10 kW/m²·K. The present estimating method for determining interfacial heat transfer coefficients was suitable for a material-polymer film-material contact model. The time evolution of the temperature at the contact surfaces was also effectively evaluated using the numerical simulation.     

Film formation from aqueous polyurethane dispersions of reactive hydrophobic and hydrophilic components; spectroscopic studies and Monte Carlo simulations

Film formation of waterborne two-component polyurethanes is exceedingly complex due to the heterogeneous nature along with simultaneous progression of several parallel physicochemical processes which include water evaporation, cross-linking reactions, phase separation, and droplet coalescence, to name a few. While internal reflection infrared imaging (IRIRI) spectroscopy clearly facilitates analysis of chemical changes resulting from film formation, the complexity of processes leading to formation of specific surface/interfacial entities is a major experimental challenge. For this reason, we combined a spectrum of surface/interfacial analytical approaches including IRIRI, atomic force microscopy, and attenuated total reflectance Fourier transform infrared spectroscopy with Monte Carlo computer simulations to advance the limited knowledge of how temperature, stoichiometry, concentration levels, and reactivities of individual components affect the development of surface morphologies and compositional gradients across the film thickness. These studies show that in heterogeneous systems having both hydrophobic and hydrophilic components stratification of individual components to the film-air (F-A) interface is ultimately responsible for formation of rough surface topographies. These studies show that simultaneous stratification of hydrophobic components along with water evaporation to the F-A interface results in metastable interfacial layers, leading to surface dewetting. Subsequently, surface roughness is enhanced by higher concentrations of water in the cross-linking film.     

Marangoni flow of Ag nanoparticles from the fluid-fluid interface

Fluid flow is observed when a volume of passivated Ag nanoparticles suspended in chloroform is mixed with a water/ethanol (v/v) mixture containing acidified 11-mercaptoundecanoic acid. Following mechanical agitation, Ag nanoparticles embedded in a film are driven from the organic-aqueous interface. A reddish-brown colored film, verified by transmission electron microscopy to contain uniformly dispersed Ag nanoparticles, is observed to spontaneously climb the interior surface of an ordinary, laboratory glass vial. This phenomenon is recorded by a digital video recorder, and a measurement of the distance traveled by the film front versus time is extracted. Surface (interfacial) tension gradients due to surfactant concentration, temperature, and electrostatic potential across immiscible fluids are known to drive interface motion; this well-known phenomenon is termed Marangoni flow or the Marangoni effect. Experimental results are presented that show the observed mass transfer is dependent on an acid surfactant concentration and on the volume fraction of water in the aqueous phase, consistent with fluid flow induced by interfacial tension gradients. In addition, an effective desorption rate constant for the Marangoni flow is measured in the range of approximately 0.01 to approximately 1 s(-1) from a fit to the relative film front distance traveled versus time data. The fit is based on a time-dependent expression for the surface (interface) excess for desorption kinetics. Such flow suggests that purposeful creation of interfacial tension gradients may aid in the transfer of 2- and 3-dimensional assemblies, made with nanostructures at the liquid-liquid interface, to solid surfaces.     

Evaporation of Lennard-Jones fluids

Evaporation and condensation at a liquid/vapor interface are ubiquitous interphase mass and energy transfer phenomena that are still not well understood. We have carried out large scale molecular dynamics simulations of Lennard-Jones (LJ) fluids composed of monomers, dimers, or trimers to investigate these processes with molecular detail. For LJ monomers in contact with a vacuum, the evaporation rate is found to be very high with significant evaporative cooling and an accompanying density gradient in the liquid domain near the liquid/vapor interface. Increasing the chain length to just dimers significantly reduces the evaporation rate. We confirm that mechanical equilibrium plays a key role in determining the evaporation rate and the density and temperature profiles across the liquid/vapor interface. The velocity distributions of evaporated molecules and the evaporation and condensation coefficients are measured and compared to the predictions of an existing model based on kinetic theory of gases. Our results indicate that for both monatomic and polyatomic molecules, the evaporation and condensation coefficients are equal when systems are not far from equilibrium and smaller than one, and decrease with increasing temperature. For the same reduced temperature T/T(c), where T(c) is the critical temperature, these two coefficients are higher for LJ dimers and trimers than for monomers, in contrast to the traditional viewpoint that they are close to unity for monatomic molecules and decrease for polyatomic molecules. Furthermore, data for the two coefficients collapse onto a master curve when plotted against a translational length ratio between the liquid and vapor phase.