Structure and dynamics of thin liquid films on solid substrates

In the absence of surface tension gradients, and on smooth, chemically homogeneous substrates, wetting liquids build molecularly thin films, conveniently studied at the edge of spreading microdroplets. Spatially resolved ellipsometry is a relevant technique for this study. At the microscopic scale, microdroplets usually take a stepped pyramidal shape due to the layering induced by the solid surface. The thickness and growth dynamics of the successive layers give information on the behaviour of the liquid close to the solid. 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 lengths. For moderate or high friction, the successive layers grow at a comparable rate and the spreading of the droplet is mainly controlled by the mesoscopic scale. Thus, the relevant parameter is the bulk viscosity eta. For low friction, only the first layer grows significantly. Then the relevant parameter is the number N of monomers or the molar mass M of the polymer chain. (C) 1999 Elsevier Science B.V. All rights reserved.