The molecular spreading of nonpolar perfluoropolyether films on amorphous carbon surfaces

The spreading mechanism of nonpolar perfluoropolyether films on carbon surf aces is examined in the mesoscopic regime, including both submonolayer and multilayer films. For the submonolayer film, adsorption-desorption is a mai n mechanism for spreading, and the surface diffusion coefficients increase as the film thickness increases. The driving force for the spreading in the submonolayer regime is the gradient of the disjoining pressure, which is d escribed by the two-dimensional virial equation. For the multilayer film re gime, the spreading characteristics are determined by the molecular weight and the disjoining pressure gradient, which is assumed to be purely van der Waals in nature. We adopt a partial slip boundary condition to analyze the multilayer film, which qualitatively explains the dependence of the surfac e diffusion coefficient on film thickness. (C) 2000 Academic Press.