Investigating the effects of wettability and gaseous nanobubbles on roughened wall–fluid interface using molecular dynamics simulation

This paper reports on the use of molecular dynamics (MD) simulation to investigate the coupling effects of wettability, surface roughness and interfacial nanobubbles (INBs) on wall–fluid interfaces. The fluid properties close to the wall–fluid interface, such as potential energy, density, diffusion coefficients of fluid molecules and effective slip length are simulated. In the cases without surface nanobubbles, regions with lower potential energy have a higher probability of hosting water molecules. The local translational and rotational diffusion coefficients of water within the cavities are strongly influenced by wettability but largely unaffected by hydrodynamic effects. In cases where INBs exist, variations in wettability result in distinctly different argon morphologies. Argon nanobubbles form a convex shape on Wenzel-like interfaces but a shallow concave shape on Cassie-like interfaces. The phenomenon of water molecules invading grooves tends to occur on Wenzel-like interfaces; however, this depends largely on the morphology of the grooves. The high mobility and high density of argon molecules indicate that the state of the argon molecules within the grooves may require further investigation. Our results also show that the effective slip length is significantly influenced by wall–fluid wettability as well as the morphology of INBs.     

A molecular dynamics study on the thermal rectification effect at the solid–liquid interfaces between the face-centred cubic (FCC) of gold (Au) with the surfaces of (100), (110) and (111) crystal planes facing the liquid methane (CH4)

A molecular dynamics study on the solid–liquid (S-L) interfaces for solid wall of gold having the face-centred cubic of (100), (110) and (111) crystal planes contacting liquid methane was examined using non-equilibrium molecular dynamics simulations. An investigation on the thermal rectification effect was performed by measuring the thermal boundary conductance (TBC) at the S-L interface. Thermal rectification can be defined as the differences in the TBC at the interface between the two opposite heat flow directions; one is from the liquid to solid and vice versa. The thermal rectifications are up to 13% for (110) crystal plane, followed by 6% and 0.3% for (111) and (100) crystal planes, respectively. It was found that the TBC at the S-L interface was influenced by the magnitude of the adsorption of liquid molecules at the vicinity of the interface. The results show that due to the different temperature distribution, different magnitude of the adsorption of liquid molecules is generated for the two opposite heat flow directions. On the surface of the solid walls for (110) crystal plane, where lattice-scale corrugation exists, it was found that there exists difference in distance between the surface layers of the solid and liquid across the interface between the cases of the two opposite heat flow directions, which affects the TBC at the interface. The present results suggest that the factors that influence the thermal rectification at the S-L interface are the magnitude of the adsorption of liquid molecules and the surface structure of the solid walls that differ significantly among the three types of crystal planes.     

Effect of various surface conditions on nanochannel flows past permeable walls

In this paper, effects of surface conditions on the gas flow in nanochannels with permeable walls have been investigated by molecular dynamics simulations. The hydrodynamic characteristics of the gas flow in nanochannels, including the density distributions, slip length and boundary friction coefficient, have been significantly influenced by the molecular interactions between gas molecules and wall atoms. The density layering phenomena are observed at the fluid–wall interface under a strong fluid–wall interaction (FWI). Particularly, there is a peak of the gas density on the permeable surface where the gas density is increased and about 3 times larger than the bulk one under the strong FWI. It indicates a strong non-continuum density distribution on the permeable surface and the step down in density from nanopores to the bulk region. On the other hand, the nanoscale vortices are produced in the nanopores. Moreover, the mass flux of gas flow in nanopores is reduced, and the hydrodynamic boundary has been shifted above the permeable surface further under the weak FWI. Slip characteristics on the permeable surface under various conditions are explored. It has been found that the slip length on the permeable surface may vary as a logarithmic function with the molecular mean free path. Apparently, the skin friction on the permeable surface is affected by the velocity slip. These results are significant in the understanding of nanoscale hydrodynamics.     

Effect of nanostructure on heat transfer between fluid and copper plate: a molecular dynamics simulation study

A molecular simulation is developed to study the effect of surface nanostructures on nanoscale flows. Based on this method, particles equation of motion is solved through the Verlet algorithm. Meanwhile, a physically sound method is applied to control the momentum and temperature of the simulation box. By adding an external force on the top copper plate according to the velocity difference between on-the-fly and desired velocities, simulations on convection of argon flows between two solid walls are performed. The top wall, which holds a higher temperature, moves at a constant velocity relative to bottom one along with the streamwise direction. These simulation results show that the nanostructures particularly affect fluid density oscillations adjacent to solid wall and nanostructures. In addition, these nanostructures also have significant effects on temperature and velocity distributions in simulation system.     

Molecular dynamics simulation of thermal conductivity of an argon liquid layer confined in nanospace

Molecular dynamics simulations were used to study the thermal conductivity of liquid argon ultra thin films confined between two plates spaced several nanometres apart. The research focused on the dependence of the liquid argon thermal conductivity on the liquid layer thickness and the interaction between liquid and solid. The results show that the thermal conductivity of liquid argon ultra thin films confined between two plates depends on the distance between the two plates and the existence of solid-like liquid layering at the liquid–solid interface and the average migration frequency of all liquid molecules. Stronger interactions between the liquid and the solid resulted in a larger number of atoms in the solid-like liquid layer along the surface and hence smaller thermal resistance between the liquid and the solid. However, as the strength of the interaction with the solid increased, the thermal conductivity was reduced due to fewer atoms near the hot solid boundary and less molecular migration.     

Molecular dynamics simulation of fluid properties by the streamwise oscillation of the solid wall

In this paper, the hydrodynamics of streamwise wall oscillations on a Couette flow are studied using the molecular dynamics method. Firstly, based on the two-dimensional Couette flow model which is made up of copper wall and argon fluid, the characters of the fluid near the streamwise oscillation wall are simulated under the condition of varied oscillating parameters. By scrupulous data processing, some significative results such as the velocity distribution, the density distribution, the potential energy curves of the flow field and the frictional force of the wall are obtained. Secondly, the mechanism how the wall oscillation brings about change to the frictional drag at liquid–solid interface is investigated. And the results indicate that the frictional drag can be reduced significantly by applying appropriate streamwise oscillation to the solid wall. The drag reduction rate mainly depends on the oscillation parameters. In addition, the decrease in the fluid’s density near the wall is another important reason behind the frictional drag reduction.     

Properties of Confined Square-well Fluids using the Gibbs Ensemble Simulation Technique

In this work we have used the extension of the Gibbs ensemble simulation technique to inhomogeneous fluids [Panagiotopoulos, A.Z. (1987) "Adsorption and capillary condensation of fluid in cylindrical pores by Monte Carlo simulation in the Gibbs ensemble", Mol. Phys. , 62 (3), 701-719], which has been applied to adsorption phenomena of confined fluids. Fluid molecules are described by spherical particles interacting via a square-well potential. The fluid is confined in two types of walls: symmetrical (two hard walls) and non-symmetrical (one square-well wall and one hard wall). In order to analyze the behavior of the confined fluid by varying the potential parameters, we evaluated the bulk and confined densities, the internal energies and the density profiles for different supercritical temperatures. A variety of adsorption profiles can be obtained by using this model. The simulation data reported here complements the available simulation data for this system and can be useful in the development of inhomogeneous fluid theories. Since the square-well parameters can be related to real molecules this system can also be used to understand real adsorption systems.     

Molecular dynamics simulations of energy accommodation coefficients for gas flows in nano-channels

The energy accommodation coefficient (EAC) used in thermal boundary condition in micro- and nano-gas flows is reported to be always less than unity and greatly influenced by wall characteristics, for example, the wall temperature. A statistical EAC definition was described to calculate the EAC for thermal conduction in argon gas between two smooth platinum plates from two-dimensional non-equilibrium molecular dynamics simulations. The non-equilibrium EAC at the upper wall was calculated for different upper wall temperatures and a fixed bottom wall temperature. The equilibrium EAC at each temperature can then be extrapolated from a series of non-equilibrium EACs as the temperature difference between the two walls approaches zero. The analyses of the effects of wall temperature for various Knudsen number on non-equilibrium and equilibrium EACs show that, for a given lower bottom wall temperature, the non-equilibrium EAC at a high temperature wall increases with an increase in the wall temperature. For a given wall temperature difference, the non-equilibrium EAC increases with the increase in the wall temperatures. The equilibrium EAC also becomes larger at higher temperatures.     

A density functional theory for Yukawa chain fluids in a nanoslit

A weighted density functional theory is developed for Yukawa chain fluids confined in a nanoslit. The excess free-energy functional is separated into repulsive and attractive contributions. A simple Heaviside function is used as the weighting function to calculate the weighted density in both contributions. The excess free-energy functional of repulsive interaction is calculated by the equation of state developed by Liu et al., while the contribution to excess free-energy functional by attractive interaction is calculated using the statistical associating fluids theory for chain molecules with attractive potentials of variable range. For pure fluids, the predicted density profiles near the nanoslit wall are in good agreement with simulations. The effect of cut-off introduced in the weighting function for the attractive part is examined; in addition, the surface excess and partition coefficient are calculated. The density profiles are also predicted for mixtures of two Yukawa chain fluids with different chain lengths, hard-core diameters, fluid–fluid and wall–fluid interactions. This work reveals that it is important to decompose the excess free-energy functional into repulsive and attractive contributions, and a simple weighting function can be used for both contributions.     

Simulation of heat conduction in nanocomposite using energy-conserving dissipative particle dynamics

We report on the simulation of heat conduction in nanocomposite by using a novel mesoscopic particle method, the energy-conserving dissipative particle dynamics (eDPD) method. The original eDPD method is extended to account for the interfacial thermal resistance occurs at the angstrom-wide interface between materials, and we also investigated the choice of time step in eDPD simulations. For nanocomposite with randomly dispersed nanoparticles, the eDPD simulations predict that the thermal conductivity of matrix material can be enhanced by embedding high thermal conductivity nanoparticles, but the effectiveness of such a strategy diminishes as the interfacial thermal resistance between the nanoparticle and matrix material increases. These results are in quantitative agreement with the classical Maxwell–Garnett model. Further simulations indicate that the enhancement of thermal conductivity can be affected by the alignment of nanoparticles with respect to the temperature gradient, which cannot be predicted by the classical models. These simulation results indicate that eDPD method can be a versatile method for studying thermal transport in heterogeneous materials and complex systems.     

A Monte Carlo Study of the Argon Fluid Around a Benzene Molecule

Abstract

Monte Carlo simulations have been carried out for argon fluid containing one benzene molecule at the supercritical region. The purpose in this study is to examine the effect of plate-like molecule on the structure of neighboring fluid composed of simple spherical molecules of the system. In the first neighbor shell of argon from the center of benzene molecule, the average potential energy of argon atoms is confirmed to have a large density dependence. This potential energy is relatively large in the high density region. It is found that the spatial distribution of argon fluid is significantly affected by the molecular shape of benzene and it has little direct connection with the attractive interaction between benzene and argon.     

A grand canonical Monte Carlo simulation study of argon and krypton confined inside weakly attractive slit pores

Grand canonical Monte Carlo simulations are performed to investigate the adsorption of argon and krypton inside weakly attractive slit pores. We examine the effects of confinement on these monoatomic fluids (modelled using the triangle-well potential) in a hard wall slit pore as also when the pore-fluid interactions are uniformly and weakly attractive. The effects of temperature and pressure on the adsorption isotherms of these confined fluids are found to be the same as those reported in literature. The equilibrium density profiles for argon and krypton exhibit both uniform distribution and layering under different conditions. In addition, for krypton, under specific conditions inside the narrow pores, we note the development of frustrated layering.     

Molecular dynamics study on explosive boiling of ultra-thin liquid over solid substrate: considering interface wettability of Argon/MoS2

A molecular dynamics (MD) study is carried out to reveal the phenomenon about the normal and explosive boiling of ultra-thin liquid argon film absorbed on MoS₂ surface with different wetting conditions. The three-phase molecular system is composed of a solid MoS₂ wall, a liquid argon film and a vapour argon region. Initially, the three-phase simulated system is thermally equilibrated at a low temperature. Then the MoS₂ heat source is suddenly heated up to two different high temperatures those far above critical point of liquid argon, and the argon experienced a phase transition process in the NVE ensemble. The simulation results show that the good wetting properties and high heat source temperature dramatically enhance phase transition efficiency, accelerating the heat transfer rate, shortening the boiling time, and increasing the evaporation rate, and they have remarkable effects on temperature and pressure histories, density distribution during whole boiling process. Explosive boiling is more likely to occur at high superheated degree, but evaporation occurs at low superheated degree. In addition, at a high superheated degree, it can be conclude from the simulation results that the better wetting properties of the solid-liquid interface is, the shorter time of the explosive boiling is needed.     

The Effects of Thermal Radiation on an Unsteady MHD Axisymmetric Stagnation-Point Flow over a Shrinking Sheet in Presence of Temperature Dependent Thermal Conductivity with Navier Slip

In this paper, the magnetohydrodynamic (MHD) axisymmetric stagnation-point flow of an unsteady and electrically conducting incompressible viscous fluid in with temperature dependent thermal conductivity, thermal radiation and Navier slip is investigated. The flow is due to a shrinking surface that is shrunk axisymmetrically in its own plane with a linear velocity. The magnetic field is imposed normally to the sheet. The model equations that describe this fluid flow are solved by using the spectral relaxation method. Here, heat transfer processes are discussed for two different types of wall heating; (a) a prescribed surface temperature and (b) a prescribed surface heat flux. We discuss and evaluate how the various parameters affect the fluid flow, heat transfer and the temperature field with the aid of different graphical presentations and tabulated results.     

The phase diagram of the Lennard-Jones fluid using temperature dependent interaction parameters

The forces of interaction between argon atoms can be described by the Lennard-Jones potential model. It is hypothesised that the use of temperature dependent interaction parameters, instead of using temperature independent interaction parameters, may lead to improvement in the prediction of the vapour–liquid coexistence curve. Published second virial coefficient data were used to fit a simple two-parameter temperature dependent model for the collision diameter and well depth. Vapour–liquid coexistence curve for argon was simulated in the NVT Gibbs ensemble Monte Carlo technique. The simulations were carried out using each of the temperature independent and temperature dependent parameters in the temperature range: 110–148 K. The critical temperature and density were determined using the Ising-scaling model. The results using temperature dependent parameters produce, overall, a more accurate phase diagram compared to the diagram generated using temperature independent interaction parameters. The root mean square deviation is reduced by 42.1% using temperature dependent interaction parameters. Also, there was no significant difference between the results obtained using temperature dependent interaction parameters and the highly accurate and computationally demanding phase diagrams based on three body contributions.     

Molecular dynamics simulation of helium–argon gas mixture under various wall conditions

As computational capabilities increase, molecular dynamics (MD) simulations become important tools of simulating reality. These simulations are especially useful for compressible gas mixture problems. In this study, binary diffusion of helium and argon was examined using a hard-sphere MD simulation method. For the sake of computational speed, low spacing ratios were chosen. Binary mass diffusion of gases in two equally sized halves of a box was simulated for identical initial kinetic energies and number densities. It has been noted that a purely mass diffusion mechanism of different gases is not physically possible. The resultant gas mixtures of several diffusion simulations were used as initial conditions for combined heat transfer – Couette flow, and heating and cooling experiments. The results showed the interesting behaviour of the mixture, which was subjected to various wall conditions. Energy of heavier molecules is found to be more sensitive to the wall velocities and less sensitive to the wall temperatures than lighter molecules. Diffusion, heat transfer, viscosity and heat capacity coefficients are deduced as well.     

Parallel study of thermal resistance and permeability barrier stability of Enterococcus faecalis as affected by salt composition,growth temperature and pre-incubation temperature

In this study Enterococcus faecalis cells were grown to stationary phase in various conditions resulting in strong but similar variations in both cellular thermoresistance and permeability barrier stability (the temperature T_m that induced rapid dissipation of the ion concentration gradient during constant heating). Cells grown at 17–22°C were heat sensitive and barrier labile whilst cells grown at 10–13°C and 42–47°C were heat resistant and barrier stable. The thermal resistance and barrier stability in heat-sensitive cells, compared to the same parameters in heat-resistant cells, remained low after an additional culture at 43–47°C, indicating a persistent effect of culture at 17–22°C. In cells grown at 10–13°C, these parameters were as low as they were in the heat-sensitive cells, provided the growth media contained an ammonium salt (1%) which thus abolished the cold acclimation. Both parameters were reduced in cells growth at increased salinity (1–3% Na and K salts) and the reduction was more pronounced during growth at 17–22°C. Moreover, cells pre-cultured at 21°C with increased salinity (3% NaCl) displayed strong phenotypic effect during subsequent culturing which reflected in a 6°C decrease in both the optimal temperature and maximal temperature of growth. Compared to other bacterial strains, only a part of the change in membrane stability could be related to the variations in fatty acid composition. The index of unsaturation changed in accordance with the barrier stability and survival of cells. These findings support the conclusion that stability of permeability barrier as affected by the growth temperature, presence of ammonium and cultural conditions of progenitor cells was involved in thermal sensitivity and temperature-acclimation of E. faecalis.     

Lipid bilayers: thermodynamics, structure, fluctuations, and interactions

This article, adapted from our acceptance speech of the Avanti Award in Lipids at the 47th Biophysical Society meeting in San Antonio, 2003, summarizes over 30 years of research in the area of lipid bilayers. Beginning with a theoretical model of the phase transition (J.F.N.), we have proceeded experimentally using dilatometry and density centrifugation to study volume, differential scanning calorimetry to study heat capacity, and X-ray scattering techniques to study structure of lipid bilayers as a function of temperature. Electron density profiles of the gel and ripple phases have been obtained as well as profiles from several fluid phase lipids, which lead to many structural results that compliment molecular dynamics simulations from other groups. Using the theory of liquid crystallography plus oriented lipid samples, we are the first group to obtain both material parameters (KC and B) associated with the fluctuations in fluid phase lipids. This allows us to use fully hydrated lipid samples, as in vivo, to obtain the structure.     

Structure of hydroxylated galactocerebrosides from myelin at the air-water interface.

Hydroxy-galactocerebrosides (mixed chain length, constituent of myelin membranes) from bovine brain are investigated as monolayers at the air-water interface with isotherms, fluorescence microscopy, x-ray reflectivity and grazing incidence diffraction. With grazing incidence diffraction a monoclinic tilted chain lattice is found in the condensed phase. According to x-ray reflectivity, the longest chains protrude above the chain lattice and roughen the lipid/air interface. On compressing the chain lattice, the correlation length increases by approximately 65%; obviously, the sugar headgroups are flexible enough to allow for lattice deformation. With fluorescence experiments, small coexisting fluid and ordered domains are observed, and there is lipid dissolution into the subphase as well. The dissolved hydroxy-galactocerebroside molecules reenter on monolayer expansion. The electron density profiles derived from x-ray reflectometry (coherent superposition) show that the chain-ordering transition causes the molecules to grow into the subphase.     

Molecular Dynamics Study of Nitrogen in Slit Micropores

Abstract

We present results of a computer simulation study of fluid nitrogen in model slit micropores. The model used for the micropore allows for the permeability of the pore wall to the confined fluid to be precisely controlled, while maintaining the atomic nature of the wall. Density and orientation profiles, wall permeabilities and diffusion coefficients have been obtained for systems with pore walls ranging from the almost impermeable to the completely permeable. Both the density and orientation profiles exhibit nonuniform behavior, while we observe anisotropy in the diffusion coefficients.