Molecular dynamic simulations of fluid n-pentane and n-decane have been performed in order to analyze the self-diffusion. An isotropic united-atom (UA) model as well as anisotropic united-atom (AUA) models have been used to represent the molecular interactions. Self-diffusion coefficients have been calculated. The sensitivity of the self-diffusion coefficient to the shape of the intermolecular potential as well as the torsion potential has been analyzed. The simulation results for the diffusion coefficient are in excellent agreement with experimental data when the molecular interaction is represented by an anisotropic United-Atom model and the internal rotation is governed by a torsion potential proposed in this work.

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 viscosities of the confined fluids in the three cases appear to be the same as those of the bulk, within the uncertainty of the results. No significant influence of the shear flow on the inter- or intramolecular was found.

The isotropic-nematic phase transition in a fluid of moderately long molecules interacting via a hard Gaussian overlap potential is studied using the decoupling approximation and computer simulation. Molecules of length-to-breadth ratios equal to 3 and 5, thought to set the relevant range of molecular elongations in real nematic liquid crystals, are considered. The results of the theory ͑pressure, order parameter, and location of the phase transition͒ and of several of its extensions, are compared with those from computer simulation, and their relative accuracy assessed. We first study the standard decoupling approximation, a resummed Onsager virial expansion where only the ͑exact͒ second virial coefficient, B 2 , is retained, and consider two different mappings to perform the resummation: a fluid of equivalent hard spheres and the isotropic phase of the hard Gaussian overlap fluid. Whereas the former mapping predicts a phase transition already in qualitative agreement with simulation, the mapping to the isotropic phase predicts a transition in closer agreement with the simulation result, shifting the location of the transition to lower pressures. However, the transition is overestimated in both cases, which seems to indicate a poor representation of angular correlations. In order to incorporate higher-order correlations, an approximate method is proposed to evaluate the B 3 and B 4 virial coefficients in the nematic phase numerically. This new information allows us to address two points: ͑i͒ the convergence of the virial series for short molecules, and ͑ii͒ the performance of extended decoupling approximation theories, incorporating the third and the fourth virial coefficients. As expected, inclusion of the high-order virial coefficients improves the results of the corresponding truncated virial expansion for the largest elongation considered, and provides quantitative agreement with the simulations, indicating a fast convergence of the virial series. The standard decoupling approximation provides results of similar accuracy. Also, the extended decoupling approximation including B 3 improves these results, though the extension to B 4 degrades the coexistence data slightly, which might indicate that the latter misrepresents to some extent the importance of angular correlations. In contrast, for molecules with a length-to-breadth ratio of 3, the truncated virial expansion is still inaccurate, whereas the extended decoupling approximation theories perform better, providing almost quantitative agreement with the simulations. As a result of our findings, we conclude that in order to improve the standard decoupling approximation for fluids of short molecules, it is essential to resum the virial series using knowledge of the B 3 virial coefficient and also the B 4 coefficient for the shortest molecules forming nematic phases. © 1997 American Institute of Physics. ͓S0021-9606͑97͒50524-1͔

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.