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Density profiles at a planar liquid‐liquid interface

Molecular dynamics of liquid‐liquid interface in an immiscible binary mixture of simple particles at low temperature and high pressure reveals stable equilibrium oscillatory structures in the density profiles of both components. The layering vanishes albeit slowly with the increase of the surface area but is not affected by an increase of the depths of bulk liquid layers. A simple Gaussian broadening predicts scaling of the oscillatory portion of the density profiles and good agreement is found. The width of the dip in the total density is also examined.

Molecular dynamics calculation of the liquid structure up to a solid surface

The technique of molecular dynamics is used to calculate the properties of a liquid phase in contact with a fcc crystal phase. A system of 1680 particles which interact through a Lennard‐Jones potential is divided into two closed subsystems by imposing artificial boundaries across which the particles can interact, but not pass. One of the subsystems is melted and a state corresponding to coexisting liquid–solid bulk states is obtained. The constraint is removed and it is observed that the state is maintained, which suggests that the system is in thermodynamic equilibrium. The interface shows a gradual decline of order, and the structure in the middle of the fluid system cannot be distinguished from that of a pure liquid with the same temperature and density. The density profile is also calculated for a fluid up to a solid surface without lattice structure where the layering in the interface is seen to be more pronounced.

Fluid alkanes in confined geometries

A comparative study of confined fluid films composed of three different alkanes has been carried out using molecular dynamic simulation techniques. The films were confined in thin slit pores, only a few molecular diameters thick, and the substances studied were n‐butane, n‐decane, and 5‐butyl‐nonane. The properties of the film were obtained in equilibrium conditions and under shear. All the studied films show a strong layering of the distribution of methylene subunits. Chains at the solid boundaries align with the walls and show a tendency to stretch. The diffusion parallel to the solid walls is found to be higher in the proximity of the walls than in the inner part of the pore. The molecular motion normal to the confining walls can be described as noncorrelated molecular transitions between the contact layer and the inner part of the pore. Shear flow was induced in the film by moving the solid walls. The resulting velocity profiles across the pore were computed as well as the viscosity of the films. The ...

Simulating shear flow

In the present article, we have analyzed to which extent the steady states produced in simulations of fluids undergoing shear flow, can truly be representations of experimental steady states. For this purpose, we have performed nonequilibrium molecular dynamics (NEMD) simulations of two different fluid systems undergoing shear flow. One system is a Lennard‐Jones (LJ) fluid where the viscous heat produced by shearing the system is eliminated only in certain regions of the simulation box. The other system is a polymer immersed in an atomic solvent. In this case, the viscous heat was removed by coupling a homogeneous thermostat to different degrees of freedom in the system. The results of these simulations show that at the shear rates commonly produced in simulation, the rate of production of viscous heat is very large. This heat is eliminated by the thermostat at rates that are higher than the rates of transport of heat across the fluid. Moreover, the heat has no time to redistribute into the different degr...

Fluid n‐decane undergoing planar Couette flow

Molecular dynamic simulations of fluid n‐decane undergoing planar Couette flow have been carried out. The n‐decane chains were modeled using an anisotropic version of the united‐atom model with fixed bond lengths. The results of our simulations verify the equivalence between the steady state time averages of the atomic and molecular stress tensors. In the range of shear rates which have been investigated, our results show that the values of the viscosity coefficient depend mainly on the shape of the intermolecular potential and they are insensitive to the shape of the torsion potential. Moreover, the viscosity coefficient as a function of the shear rate shows a region of shear thinning at low shear rates followed by a region of shear thickening. The study of the alignment induced in the fluid by the perturbing field shows that after a region of fast increasing of the degree of alignment with increasing shear rate, this reaches a maximum and slowly begins to decrease. This behavior can be related to the fa...

Spinodal decomposition under shear flow

The spinodal decomposition of a two-dimensional model binary fluid undergoing planar Couette flow has been studied by molecular dynamics simulation. The effect of the strength of the shear field on the growth of the domains was analyzed. The main effect of the shear field is the deformation of the domains which results in anisotropic structure developments. We have characterized these anisotropic structures by measuring the domain size in two different directions, the direction of the flow and the direction of the shear. We find that the dependence of the deformation of the domains on the strain applied to the system shows the same behavior as found in experiments. Moreover, we find that the shear flow can enhance the domain growth in the direction of the flow and it can restrain and even suppress this growth in the direction normal to the flow. The influence of the morphology on rheological properties was also analyzed. We find that viscosity depends on the quench time and the shear field, and is caused by the extension and direction of the interfacial area.

SHEAR FLOW AT LIQUID-LIQUID INTERFACES

Two nonmiscible liquids separated by planar interfaces and undergoing shear flow have been simulated with nonequilibrium molecular dynamics (NEMD) methods. A homogeneous shear scheme was used for imposing shear flow in the system. The homogeneous shear algorithm needs to be combined with a profile‐unbiased thermostat (PUT) in order to assure meaningful results in our nonhomogenous system. Local values of several quantities such as viscosity, local stream velocity, temperature, shear stress, and rate of entropy production were calculated. Planar Couette flow appears in the ‘‘bulk’’ regions of the system with a slip between the two streams of bulk fluid at the interfaces. The shear stress is constant across the system (PUT results) at low strain rates but at high shear rates the shear stress at the interface is lower than in the bulk region. The shear viscosity at the interfaces is lower than in the bulk region showing that the transport of momentum in the former region is less efficient than in the bulk. A...

Equation of State for a Lennard‐Jones Fluid

The influence of higher‐order terms in the Barker–Henderson approximation to the Zwanzig perturbation expansion is examined for a Lennard‐Jones fluid. We find that the error introduced by ignoring these higher‐order terms is negligible compared with the error due to the replacement of the reference system by a hard‐sphere system in which the diameter of the spheres ensures coincidence to only first order between the free energy of the reference system and the free energy of the hard‐sphere system. In calculating the contribution to the pressure due to the perturbation, one often uses the Percus–Yevick solution for the hard‐sphere reference system. The error introduced by this approximation is found to be considerable.

Oscillating chemical reactions and phase separation simulated by molecular dynamics

Molecular dynamics (MD) of stationary chemical kinetics is used to simulate oscillating chemical reactions in a system of N classical mechanical particles with Lotka–Volterra kinetics. The MD includes oscillations in a (closed) system with conserved energy and time reversible dynamics as well as oscillating chemical reactions in an open and driven non-equilibrium system, and with and without a competing phase separation of the different components in the reactions. The approach allows a detailed investigation of the kinetics and demonstrates on a molecular level, the phenomenon oscillating reactions for various chemical and reaction kinetics details. When phase separation takes place during the oscillations the kinetics is no longer simple diffusion driven.