Effect of Molecularly-Thin Films on Lubrication Forces and Accommodation Coefficients in Air

We show that a thin organic film has a significant effect on the lubrication force (damping) acting on a smooth sphere approaching a smooth flat plate in a gaseous environment. The lubrication forces were determined by the analysis of the width of a power spectrum density of the vibrations of an atomic force microscope cantilever that is attached to the sphere and immersed in the gas at thermal equilibrium. Because the lubrication force is determined by the collisions of gas molecules with both the sphere and the plate, the lubrication force was used to determine the thermal accommodation coefficient of the gas on the solids. We find that clean glass surfaces in ambient air at similar to 25 degrees C exhibit a slip length of 630 +/- 90 nm per surface and a concomitant accommodation coefficient of 0.19, whereas a glass plate with a similar to 1 nm organic film of trimethylsilane exhibits a slip length of 270 +/- 90 nm and an accommodation coefficient of 0.43. If left in air for an extended period of time, the slip length on clean glass surfaces falls, which is consistent with the spontaneous adsorption of airborne contaminants. Thus, our analysis can be used as a simple, nondestructive method for detecting the presence of contaminants or other films on solids.