OSCILLATORY BEHAVIOR OF DENSITY PROFILES - RELEVANCE FOR FLUID INTERFACIAL PHENOMENA

For a given short-ranged interatomic potential the density profile of an inhomogeneous fluid decays into bulk in the same fashion as the bulk radial distribution function g (r). General statistical mechanical arguments predict damped oscillatory decay of the liquid-vapour density profile into the saturated liquid for temperatures T removed from the critical value T(c). Explicit density functional calculations for a square-well model fluid near its triple point yield an oscillatory profile whose amplitude is about 2% of the bulk liquid density. The amplitude decreases for higher T and for T greater-than-or-equal-to 0.9 T(c) only pure exponential decay is found. Capillary-wave fluctuations reduce the amplitude of the oscillations but do not change their period and decay length. The existence of density oscillations gives rise to an oscillatory binding potential for an adsorbed liquid film. This has significant repercussions for wetting transitions and allows the possibility of a sequence of layering transitions between adsorbed liquid layers. The same length scales which determine the decay of g(r) determine the asymptotic decay of the solvation force for a fluid confined between two planar walls. In the case of long-ranged interatomic potentials, density profiles exhibit power-law decay at longest range but, for liquid densities, their intermediate range damped oscillatory decay is governed by the same leading-order pole structure that describes short-ranged forces. Recent work on the decay of correlations in liquid mixtures is also described.