Guang-Wen Wu

Closed-loop liquid-liquid equilibria and the global phase behaviour of binary mixtures involving hard-sphere + van der Waals interactions

Calculations of the critical properties of binary mixtures of components of equal size are reported using the Guggenheim equation of state. The calculations are used to determine the global phase diagram of binary mixtures. Type VI phase behaviour is predicted successfully indicating that closed-loop liquid-liquid equilibria can be obtained from hard-sphere + van der Waals interactions. Closed-loop liquid-liquid equilibria occur in the region of the global phase diagram characterized by moderately strong unlike interactions and components with very dissimilar critical temperatures. The Guggenheim equation can predict all experimentally known phase behaviour types. In addition, other hypothetical phase behaviour types are also predicted.

Equilibrium and nonequilibrium molecular dynamics methods for determining solid–liquid phase coexistence at equilibrium

The solid–liquid equilibrium phase transition of a one-component Lennard-Jones system is determined by equilibrium and nonequilibrium molecular dynamics simulation methods. One method uses the observation that the scaling exponent of the pressure or energy of a shearing Lennard-Jones liquid is approximately 1 at the solid phase. This enables us to locate the density of the coexisting solid phase. The coexisting liquid phase density is then obtained by constructing a tie line between the coexisting solid phase point and the liquid phase curve. Alternatively, the coexisting liquid phase density can be efficiently obtained by observing the change in pressure as a function of strain rate and density. The coexisting solid phase density can be then obtained from a tie line from the liquid curve to the solid curve. These calculations are the first reported use of combined equilibrium and nonequilibrium molecular dynamics methods for phase coexistence at equilibrium. Our results are in very good agreement with th...

Molecular simulation of the high-pressure phase equilibria of binary atomic fluid mixtures using the exponential-6 intermolecular potential

Abstract Molecular simulation results using the exponential-6 intermolecular potential are reported for the phase behaviour of the atomic binary mixtures of neon+xenon, helium+neon, helium+argon and helium+xenon. These binary mixtures exhibit both vapour–liquid and liquid–liquid phase equilibria up to very high pressures. Comparison with experiment indicates good overall agreement. The results indicate that the exponential-6 intermolecular potential is a useful generic potential for molecular simulation.

Liquid-crystal behavior of hard ellipsoid dimers

Liquid crystals exhibit orientation-dependent phases ranging from a disordered (isotropic) phase to a highly ordered crystalline phase. In between these extremes, increasing order can result in nematic and smectic phases. Typically, molecular simulation studies of liquid-crystal behavior use a nonspherical hard-body monomer. In this work, molecular simulation is used to study dimers of hard prolate ellipsoids. The results indicate that dimers of hard prolate ellipsoids exhibit a rich diversity of liquid-crystal behavior including smectic phases. In some cases, the dimer model may be a more realistic alternative to the conventional monomer model for liquid-crystal behavior.

New phase for one-component hard spheres

A completely new phase for one-component hard spheres is reported in an unexpected region of the phase diagram. The new phase is observed at compressibility factors intermediate between the solid and the metastable branches. It can be obtained from either Monte Carlo simulations alone or a combination of Monte Carlo and molecular dynamics calculations. An analysis of the intermediate scattering function data shows that the new phase is in a stable equilibrium. Radial distribution function data, configurational snapshots, bond order parameters, and translational order parameters obtained from molecular simulations indicate that the new phase is significantly different from the isotropic liquid, metastable, or crystalline phases traditionally observed in hard sphere systems. This result significantly changes our previous understanding of the behavior of hard spheres.

Molecular simulation of liquid-crystal transitions in hard prolate ellipsoid monomers and dimers

Abstract Molecular simulation results are reported for the liquid-crystal phase transitions of both monomers and dimers of hard prolate ellipsoids. The effect of different elongations (γ) is studied. In contrast to earlier work, evidence is found for the onset of isotropic-ordered phase transitions for monomers of relatively small elongations, although the transition is not stable. Dimers of prolate hard ellipsoids behave very differently to ellipsoid monomer of equal size. In the dimer case, a clear isotropic-ordered phase transition is observed whereas such a clear transition is absent in monomers.

Response to “Comment on: ‘New phase for one-component hard spheres’ ” [J. Chem. Phys. 120, 11686 (2004)]

The nature of the phases observed in a hard-sphere system is considered. Simulation data, obtained via both Monte Carlo simulation and molecular dynamics support the existence of a new phase for hard spheres. The data include configurational snapshots, compressibility factors, radial distribution functions, order parameters, and self-intermediate scattering functions. To facilitate further investigation of the new phase, the relevant configuration files are available on the internet.