No abstract

Molecular dynamics is employed to investigate tracer diffusion in hard sphere fluids. Reduced densities (rho*=rhosigma(3), sigma is the diameter of bath fluid particles) ranging from 0.02 to 0.52 and tracers ranging in diameter from 0.125sigma to 16sigma are considered. Finite-size effects are found to pose a significant problem and can lead to seriously underestimated tracer diffusion constants even in systems that are very large by simulation standards. It is shown that this can be overcome by applying a simple extrapolation formula that is linear in the reciprocal cell length L(-1), allowing us to obtain infinite-volume estimates of the diffusion constant for all tracer sizes. For higher densities, the range of tracer diameters considered spans diffusion behavior from molecular to hydrodynamic regimes. In the hydrodynamic limit our extrapolated results are clearly consistent with the theoretically expected slip boundary conditions, whereas the underestimated values obtained without extrapolation could easily be interpreted as approaching the stick limit. It is shown that simply adding the Enskog and hydrodynamic contributions gives a reasonable qualitative description of the diffusion behavior but tends to overestimate the diffusion constant. We propose another expression that fits the simulation results for all densities and tracer diameters.

We present the theoretical phase diagrams of the classical Heisenberg fluid in an external magnetic field. A consistent account of correlations is carried out by the integral equation method. A nonmonotonic effect of fields on the temperature of the gas-liquid critical point is found. Within the mean spherical approximation this nonmonotonic behavior disappears for short-range enough spin-spin interactions. ͓S1063-651X͑99͒51204-X͔ PACS number͑s͒: 64.70.Fx, 75.50.Mm, 05.70.Jk, 61.20.Gy The behavior of magnetic fluids in an external field has commanded more and more attention in recent years and has some peculiarities. In the presence of an external magnetic field the orientational ͑magnetic͒ phase transition is absent, but there are the first order transitions between ferromagnetic phases of different densities ͑e.g., gaseous and liquid phases͒. Physical properties of anisotropic fluids ͑to these belong also, besides magnetic fluids, nematic liquid crystals͒ are determined by the interplay between orientational and translational degrees of freedom. Therefore, by varying the magnetic field it is possible to effect structural properties of magnetic fluids, in particular, to change the region of the gas-liquid coexistence. Such investigations with a calculation of phase diagrams were carried out for model spin systems within the mean field ͑MF͒ approximation ͓1,2͔. It was found that for fluids of hard spheres carrying Ising spins an external magnetic field decreases the temperature of the gas-liquid critical point. On the other hand, the presence of isotropic van der Waals attractions between molecules can lead to the inverse effect ͓2͔. In Ref. ͓2͔ the fluid of hard spheres with the classical Heisenberg spins and strong isotropic attractions was considered also. It was shown that at weak magnetic fields there can be two first order phase transitions in this model: gas-liquid and liquid-liquid. In strong fields the weak liquid-liquid transition disappears.The need to take into account orientational-translational correlations for the description of physical properties of magnetic fluids stimulated studies of the external field effects by more complex techniques. The effect of an external field on the gas-liquid critical point was studied by the functional integration and Green function methods ͓3͔ for the quantum Heisenberg ferrofluid and by the Monte Carlo and integral equation methods for the classical one ͓4,5͔. The pair potentials of those models consisted of contributions of hard spheres and of the spin-spin interaction ͑the so-called ideal Heisenberg fluid͒. In these works the conclusion was that an external magnetic field favors the phase separation, i.e., the application of the external field increases the gas-liquid critical temperature. Let us note that the results of Refs. ͓4,5͔ are obtained for quite strong fields. In our point of view, it was the effect of small fields that is of special interest. This follows from the fact that at small fields orientational fluctuations are large and the corresponding...

The theoretical phase diagrams of the magnetic (Ising) lattice fluid in an external magnetic field is presented. It is shown that, depending on the strength of the nonmagnetic interaction between particles, various effects of external field on the Ising fluid take place. In particular, at moderate values of the nonmagnetic attraction the field effect on the gas-liquid critical temperature is nonmonotoneous. A justification of such behavior is given. If short-range correlations are taken into account (within a cluster approach), the Curie temperature also depend on the nonmagnetic interaction.PACS numbers 64.70.Fx, 77.80.Bh, 75.50.Mm, 64.60.Kw. Anisotropic liquids are very sensitive matters. Such are nematic liquid crystals and ferrofluids. Many efforts have been made in order to investigate effects of shape and flexibility of the molecules, of long and short-range interactions on the properties of anisotropic liquids. External field effects are still worthier of attention, because the application of an external field allows to change properties of anisotropic liquids dynamically (in contrast to the effects of molecules' shape, etc., which are static).In ferrofluids an external magnetic field removes the magnetic order-disorder transition, nevertheless the first order transitions between ferromagnetic phases of different densities remain. The external field deforms the phase diagram of a magnetic fluid shifting coexistence lines between these phases. Kawasaki studied a magnetic lattice gas [1], which implements one of the ways to model properties of magnetic fluids. On temperature-density phase diagrams in Ref.[1] one can see that the gas-liquid binodal of the magnetic fluid significantly lowers after the application of an external magnetic field. Vakarchuk with coworkers [2] and Lado et al. [3,4] studied another model of magnetic fluids -the fluid of hard spheres with embedded Heisenberg spins. They concluded that in such a system the temperature of the gas-liquid critical point (the top of binodal) increases after the application of an external field. The nature of such a discrepancy can be various, because the types of models (continuum fluids [2][3][4] and the lattice gas [1]) as well as approximations used differ.In this letter results of a more detailed investigation of the magnetic (Ising) lattice gas are reported. The first new point is the inclusion of the nonmagnetic interaction between particles. Another one consists in overcoming of limitations applied by the mean field approximation (MFA). It is known that this approximation is good for very long-range potentials only (and becomes accurate for a family of the infinite-ranged ones, the so-called Kac potentials [5,6]). The MFA can not reproduce some essential features of the systems with the nearest-neighbor interaction (such as the percolation phenomena in the quenched diluted Ising model, differences in magnetic properties of the quenched site-disordered Ising model and the annealed one [7]). Such drawbacks can be overcome with the two-site cluste...

We describe an integral equation method for obtaining the distribution of a nematic fluid near a wall and interacting with a uniform orienting field. Complete density-orientational profiles are calculated for a model nematic with different wall-particle interactions and different orientations of the wall with respect to the field. For orienting walls we identify particular long-range correlations that are responsible for reorientation of the bulk nematic at zero external field. These correlations become stronger as the wall-particle interaction is increased in range; they become longer ranged as the orienting field is weakened. Special attention is focused on systems where the wall-particle interaction favors orientations perpendicular to the surface. The local director orientation can vary discontinuously with the distance from the surface when the orienting influences of the field and the wall are antagonistic. At high densities smectic-like structures appear. Adsorption phenomena are also discussed. For inert hard walls, the ordered fluid avoids the surface, and a surface layer where the particles tend to orient perpendicular to the bulk director appears. Experimentally, this might be seen as wetting of the wall by a less-ordered fluid.