20114
J. Phys. Chem. C 2010, 114, 20114–20119
Effect of Molecularly-Thin Films on Lubrication Forces and Accommodation Coefficients in Air Christopher D. F. Honig†,‡ and William A. Ducker*,† Department of Chemical Engineering, Virginia Tech, Blacksburg 24061, USA, and Department of Chemical and Biomolecular Engineering, UniVersity of Melbourne, Melbourne 3010, Australia ReceiVed: July 29, 2010; ReVised Manuscript ReceiVed: October 9, 2010
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 ∼25 °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 ∼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. Introduction In this article, we examine the lubrication force acting on a particle as it approaches a flat plate in air. This process affects a number of interesting applications such as the fouling of surfaces, thermal spraying, and the delivery of inhalants. A similar process, the lubrication of a particle approaching a wall in liquid has recently been the subject of intense interest.1,2 That interest has centered on discovering whether the no-slip boundary condition is obeyed exactly at a solid-liquid interface. In addition to answering a fundamental physical question, the creation of partial slip at solid-liquid interfaces through the application of thin organic films could potentially find practical use in reducing the amount of energy expended in the pumping of liquids. The verdict on this question is pretty much decided: both experiment and theory show that solid-liquid interfaces Very accurately obey the no slip boundary condition for films in the continuum limit at normal shear rates (
