Structure and dynamics of thin liquid films on solid substrates

In the absence of surface tension gradients, and on smooth, chemically homo geneous substrates, wetting liquids build molecularly thin films, convenien tly studied at the edge of spreading microdroplets. Spatially resolved elli psometry is a relevant technique for this study. At the microscopic scale, microdroplets usually take a stepped pyramidal shape due to the layering in duced by the solid surface. The thickness and growth dynamics of the succes sive layers give information on the behaviour of the liquid close to the so lid. In particular, the friction process between the first molecular layer and the substrate is still only partially understood. We report studies of the spreading rate of polydimethylsiloxanes (PDMS) with different chain len gths. For moderate or high friction, the successive layers grow at a compar able rate and the spreading of the droplet is mainly controlled by the meso scopic scale. Thus, the relevant parameter is the bulk viscosity eta. For l ow friction, only the first layer grows significantly. Then the relevant pa rameter is the number N of monomers or the molar mass M of the polymer chai n. (C) 1999 Elsevier Science B.V. All rights reserved.