The effects of substrate roughness on ultrathin water films

Abstract

Real surfaces are rough and chemically heterogeneous on many length scales and wetting phenomena on these surfaces are affected by this inhomogeneous nature. We have used x-ray reflectivity to examine the structure of static precursing films which precede the macroscopic meniscus of water on glass. We have examined the thickness of the film, the roughness of the interfaces bounding the film, and the conformality or correlations between the roughness features of these bounding interfaces. The wetting film consists of three basic components: the bulk meniscus, a thin (≤100 Å) precursing wetting film which has a sudden termination, and beyond that, a much thinner film characteristic of the glass surface in contact with water vapor only. The roughness and chemical heterogeneity of the solid manifest themselves in the roughness of the water/vapor interface of the film, in the irregularity of the terminating boundary of the precursor wetting film, and in the hysteretic pinning of that boundary. The roughness of the glass surface is observed to impose additional roughness to the water/vapor interface beyond that which would exist on a bulk interface. These results are discussed in terms of the disjoining pressure which governs the behavior of such films.