Viscosity and velocity slip coefficients for gas mixtures: Measurements with a spinning rotor gauge

A selection of experimental measurements for He-Ar, He-N-2, and He-Ne binar y gas mixtures which were made with a spinning rotor gauge (SRG) are report ed. All of the experiments were conducted in the slip regime. Theoretical r esults from a previous paper on the SRG are used to extract values of the v iscosity and the velocity slip coefficient from the experimentally obtained data for each of the gas mixtures. The measured viscosities are in excelle nt agreement with existing literature values. Slip coefficients for binary gas mixtures have not previously been reported. An important issue here is whether the velocity slip coefficients for binary gas mixtures can be predi cted accurately using separately measured tangential momentum accommodation coefficients. The calculation of slip coefficients from theory requires a knowledge of the accommodation coefficients of each constituent of the mixt ure. The dependence of these coefficients on the gas composition is not kno wn and the simplest assumption is to regard them as being independent of th e composition. The slip coefficients computed in conjunction with this work use this simple assumption in a theoretical expression for the slips. Thes e computations led to reasonable agreement of the theoretical results with our data for the He-Ne mixture. Agreement in the cases of the He-Ar and He- N2 mixtures, however, was less than satisfactory. (C) 1999 American Vacuum Society. [S0734-2101(99)02501-4].