Thermal Creep in Gases

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Abstract
The development of a pressure gradient in a gas‐filled capillary due to the application of a temperature gradient is known as thermal transpiration. In the near‐continuum limit (small Knudsen number) the mechanism for the transpiration effect is a creeping motion of the gas in a thin layer adjacent to the surface. Measurements performed in the near‐continuum limit are compared with a number of theoretical calculations for the thermally induced creep velocity. The experimental data for He, Ne, Ar, Kr, and N2 agree best with the recent theoretical work of Loyalka and indicate little or no dependence on gas type, in contrast with another recent theory which indicates a marked dependence on the thermal accommodation coefficients. In addition, the effect of the thermal creep coefficient upon the evaluation of rotational collision numbers from thermal transpiration measurements is discussed.