Ultrathin films in wetting evidenced by x-ray reflectivity

Abstract

The role of thin films in different situations of wetting has been investigated by x-ray reflectivity, a technique that allows an independent determination of their thicknesses, densities, and roughnesses, using a nonvolatile siloxane oil spreading on different kinds of surfaces. The spreading of a microscopic droplet involves at least two stages: the development of a ``tongue'' of molecular thickness controlled by the spreading parameter and the friction on the substrate is followed by a surface diffusion process. The final stages of spreading are shown to be largely dependent on the polymer-substrate interactions. Increasing the chain length of the polymer slows the process down, but does not lead to a qualitatively different behavior. This study has been complemented with capillary rise experiments in order to measure diffusion constants. The results are discussed using a simple model of surface flow.