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The interaction of gas molecules with a surface is important for the study of the flow of gases across surfaces (as in the external flow across flat or curved surfaces) or those surfaces that confine the gas (as in the flow through a pipe). The interaction of gas molecules and surfaces depends on the properties of the gas and the surface. An important criterion when considering the gas flow is the dimensionless Knudsen number Kn, which is the ratio of the mean distance between collisions of two gas molecules λ to a representative length for the surface. The mean free path between collisions, in turn, depends on the density of the gas and its temperature, with temperature serving as a measure of the distribution of energies of the gas molecules. (For gas molecules in the atmosphere, for example, assuming standard temperature and pressure conditions (i.e. 25°C, 1 atm), then λ = 8×10-8 m.)
In the case of highly rarefied gases, where Kn becomes very large, the characteristics describing a gas flowing past a surface (such as drag) depends on the degree of the accommodation of the gas molecules, as influenced the accommodation coefficients. (The drag on satellites in the near vacuum of outer space is such an example.) For such gases that have nearly a Maxwellian energy distribution there are four basic accommodation coefficients for a rotationally symmetric surface. These coefficients consist of the three that account for the interchange of normal momentum, tangential momentum, and energy before and after a gas molecule strikes the surface, as well as the coefficient for the interchange of normal momentum and energy.
Knudsen Flow in a Tube with Arbitrary Gas-Wall Interaction. (with
On the Burnett Theory of Thermal Transpiration with Wall Accommodation. Journal of Chemical Physics,
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Second-Order Effects in Gas-Surface Accommodation. (with J. Možina and
Gas-Surface Accommodation Coefficients from Viscous Slip and Temperature Jump
Coefficients. Physics of Fluids, 17, 107104-1 to 107104-8 (2005) [DOI: 10.1063/1.2111133].