Instability and surface diffusion of ultrathin liquid perfluoropolyether films on amorphous carbon surfaces

Abstract

A study is performed on the instability and surface diffusion of ultrathin liquid films of perfluoropolyethers (PFPE) on carbon-coated discs. Several carbon-coated discs have been used from the same lot, surface roughness has been measured by atomic force microscope, and average surface energy has been measured by contact angle measurements. Then, some discs are partially dip coated, producing a spreading edge of the lubricant film, using Zdol of molecular weights of about 4000 and 2000 g/mol, and used in the measurement of the spreading profiles with time. Some other discs are fully dip coated, scratched by a friction tester, and then replenishment of grooves with time has been measured by ellipsometer (calibrated verbatim by X-ray photoelectron spectroscopy (XPS)). Mono-disperse PFPE lubricant films of film thicknesses of about 0.9–3 nm have been used. Diffusion coefficients have been extracted from the spreading profiles using the Matano interface method and used in the diffusion equation to simulate the replenishment profiles of the grooves for the same initial profiles for both simulation and experiment. The diffusion equation has been observed to simulate the replenishment profiles up to the instability plane, height which creates non-linearity in the experimental replenishment profiles above it and fails to model any further.