Thermal fluctuations of thin liquid films

A surprising lowering of the surface energy of liquid surfaces was recently found in x-ray scattering experiments which enhances thermal fluctuations of fluid interfaces at microscopic scales and calls for a re-examination of small-scale interfacial processes. This reduction was predicted for microscopic undulations below a few nm by density functional theory taking into account the long-range attraction of molecular interaction potentials. Here, a self-consistent theory is proposed for the fluctuation of fluid interfaces in arbitrary potentials which can significantly alter thermodynamical and structural properties of liquid drops, thin films, or membranes near a substrate. The substrate-induced hindrance of thermally excited capillary waves increases considerably the thickness of thin liquid films, which cannot be neglected in the analysis of adsorption data. An explicit expression for adsorption isotherms is given depending on temperature, Hamaker constant A, and surface tension gamma which takes into account the influence of capillary waves on the thickness of the fluid film and removes reported discrepancies with the Lifshitz theory of van der Waals forces. Also the steric repulsion potential of a membrane at distance D from a hard wall can be calculated self-consistently in excellent agreement with Monte Carlo simulations.