Momentum Transfer between Gas Molecules and Metallic Surfaces in Free Molecule Flow

Abstract

Measurements of the normal momentum transfer between gases and metallic surfaces were obtained under conditions of free molecule flow with a torsion balance and molecular beam apparatus. Helium, hydrogen, neon, nitrogen, carbon dioxide, and argon were investigated on tungsten, platinum-blackened-tungsten, platinum, and aluminum surfaces which were contaminated to an undetermined degree with oxides and adsorbed gases. A thermal beam of gas molecules was directed against the test surface at normal incidence, and momentum transfer measurements were obtained with the surface at various temperatures over the range 25° to 550° C. The experimental results indicate that the efficiency of the momentum transfer process increases with the molecular weight of the test gas and the roughness of the test surface, but is relatively independent of the substrate material under the present conditions. The momentum transfer rates for helium and hydrogen are significantly less than those measured for the heavier gases at the same surface temperature. The accommodation of the test gases to the surface temperature appears to be incomplete except, possibly, for argon and carbon dioxide. The results cannot be explained successfully by means of the classical model which assumes that the re-emitted gas consists of two components, one experiencing specular reflection and the other complete accommodation at the surface. A modified model and a corresponding momentum accommodation coefficient are introduced and employed in the discussion of the experimental data.