Effect of Gas Species on Gas–Monolayer Interactions: Tangential Momentum Accommodation

The tangential momentum accommodation coefficient (TMAC) of five gas species—He, N 2, Ar, CO 2, and SF 6 —was measured on an octadecyltrichlorosilane-coated glass surfaces at 1.0 atm and 24 ± 2 °C. The TMAC was determined from measurement of the damping on a glass sphere near a glass plate when the separation between the sphere and plate was less than 1500 nm so that the flow was at or near the slip-flow regime. The results show that the damping depends on the gas species and that the fitted accommodation coefficient for these gases decreases as the molar mass increases. This trend can be explained using a simple physical model of the collision between two spheres, consisting of an incoming sphere to model the gas and an initially stationary sphere to model part of the monolayer adsorbed on the solid. In this simple model, heavier incoming molecules conserve more average tangential momentum than do lighter molecules. We also consider the possibility of separating a mixture of gases based on differences in TMACs.