Thermal Fluctuations of Thin Wetting Films on Disordered Solids

We consider thin liquid layers which completely wet nonideal (either rough or heterogeneous in composition) solid surfaces. Two different sources of fluctuations of the liquid surface are identified, and the competition between them is investigated. For thick enough films (but still in the submicrometer range), the liquid surface fluctuations are dominated by thermally induced capillary waves. For thin films, capillary waves are strongly damped and the liquid surface fluctuations are correlated with the disorder of the underlying solid surface. The fluctuation spectrum is explicitly calculated for a van der Waals liquid and for several different types of solid disorder. Our findings are in agreement with recent X-ray specular reflection and diffuse scattering experiments.