KNUDSEN DIFFUSION IN RANDOM AND CORRELATED NETWORKS OF CONSTRICTED PORES

Diffusion of gases in pores with converging-diverging geometry is studied and Knudsen effective diffusivities in networks of constricted pores are computed. It is shown analytically that asymmetric (unequal mouth size) pores and, in general, pore structures that are asymmetric about their midsection are isotropic with respect to the sense of diffusion only if the diffuse law for the surface reflections is valid. Other reflection laws may lead to largely different diffusivities in opposite directions. Monte Carlo simulation of diffusion in individual pores showed that constrictions cause drastic reduction of the tube diffusivity, while periodic cavities yield increased diffusivity values compared to those in straight tubes of the same mouth size. Novel spatial correlation schemes are developed for mouth and constriction sizes and their effects on the value of the Knudsen effective diffusivity are studied. It was found that mouth-to-mouth and mouth-to-constriction size correlations is constricted-pore networks result in enhanced diffusion rates through the generation of clusters and paths of highly conducting pores; however, correlation of constriction sizes may increase or decrease the diffusion coefficient, depending on the distribution function of the network conductances.