Disjoining Pressure, Healing Distance and Film Height Dependent Surface Tension of Thin Wetting Films

Abstract

In this work we simulate the adsorption of wetting liquid argon films on a model substrate. We calculate the disjoining pressure isotherm and show that it is completely dominated by the long range van der Waals interactions. Thick films exhibit the expected Hamaker power law decay, but a quantitative description of thin films requires consideration of the detailed structure of the adsorbed layer. The spectrum of film height fluctuations is calculated, and shown to provide reliable estimates of the disjoining pressure for all films studied. However, it is observed that the full spectrum can only be reproduced provided that we account for a film height dependent surface tension proportional to the derivative of the disjoining pressure. A simple theory is worked out that describes well the observed film height dependence. Having at hand both the surface tension and the disjoining pressure, we calculate the healing distance of the liquid films, which differs from the classical expectation by a constant of the same order of magnitude as the bulk correlation length. We show these findings have important implications on the behavior of adsorbed liquids and determine corrections to the augmented Young-Laplace equation at the subnanometer length scale.