KNUDSEN DIFFUSION AND EXCITATION TRANSFER IN RANDOM POROUS-MEDIA

Knudsen diffusion inside a disordered porous medium is a transport process strongly dependent on the interfacial geometry and the connectivity of the pore network. In the Knudsen regime, the molecular diffusion is dominated by collisions with the pore walls. The ''random'' walk of a gas molecule is directly related to the statistical properties of a set of consecutive chords of the pore network. In the present paper, we discuss some examples of molecular diffusion and excitation transfer involving directly the chord distribution function of the pore network. In a first part and following the seminal work of Derjaguin, we critically examine how the self-diffusion coefficient can be related to the two first moments of the pore chord distribution. A direct comparison with experimental results and numerical simulations is presented in the case of a model porous medium: the random packing of hard spheres. In a second part, we analyze a quenching or a trapping reaction where an excited gas molecule, diffusing in the Knudsen regime, relaxes primarily by wall effects. We show bow the chord distribution of the pore network permits computation of the survival probability of the tagged molecules. Two situations are more closely analyzed: the strong and the very weak wall quenching efficiency.