On the interface between a fluid and a planar wall

Abstract

The statistical mechanics of inhomogeneous systems is usefully discussed from at least two complementary approaches: (i) the virial route, in which calculations of the structure and the thermodynamic properties are based on knowledge of the intermolecular forces in conjunction with the distribution function hierarchy, and (ii) the fluctuation theory route, based on formal results linking the equilibrium structure and properties of a system to its response to a change in external field. This paper makes use of both of these approaches to discuss the properties of a fluid in the presence of a strong localized external field. Particular attention is paid to the limiting case when the external field acts as a hard wall. Fluctuation theory yields a unified approach to interfaces stabilized by one body external fields of arbitrary strength, the strength determining the stability of the interfacial boundary with respect to wave-like fluctuations; with the weak field limit identified as a free fluid-fluid interface.

We include extensive results of molecular dynamics simulations of hard sphere fluid bounded by a pair of planar hard walls, with bulk densities spanning the dense fluid region. Density profiles, pressure tensor component profiles, surface tension and surface adsorption are discussed. Also calculated is the pair distribution function for two particles on the wall, g_w, since fluctuation theory confers particular importance on the pair correlations at the surface of a hard wall. As expected, the behaviour of the reduced transverse structure factor contrasts with the strong z-dependence found in simulations of liquid-vapour interfaces. An integral equation approach to g _w is used for comparison and further elucidation.