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KNUDSEN DIFFUSION

1986; Lippincott Williams & Wilkins; Volume: 141; Issue: 4 Linguagem: Inglês

10.1097/00010694-198604000-00006

1538-9243

Stephen M. Clifford, Daniel Hillel,

Soil and Unsaturated Flow

When the mean free path of diffusing gas molecules exceeds the size of the soil pores through which they pass, gaseous transport is no longer dominated by intermolecular collisions, but by collisions with the pore walls, a process known as Knudsen diffusion. Although Knudsen diffusion has been discussed extensively with regard to flow through porous media, it has received comparatively little attention in the soils literature. However, recent theoretical studies of gas phase transport in the soils of Mars, and certain other bodies in the solar system, have revealed that the role played by Knudsen diffusion is often of considerable importance. The results of these studies suggest that a greater awareness of this diffusive process may be of benefit to terrestrial soil scientists as well. In this review, an expression for calculating the Knudsen diffusion coefficient, for pores possessing a nonreentrant cross-sectional shape, is derived. An expression is also presented that defines an effective diffusion coefficient for pore sizes that fall in the transition region, where both Knudsen and bulk diffusion contribute to the transport process. Finally, the parallel pore model of gaseous diffusion is applied to three model pore-size distributions. These examples clearly illustrate that when soil pore sizes fall below a value comparable to the mean free path of the diffusing molecules, the efficiency of gaseous transport is greatly reduced.