Knudsen Diffusion

Synonyms

Knudsen transport modeling

Whenever a species is diffusing inside a confined space, such as cylindrical pore, an interaction between the diffusive molecules and the pore wall is to be expected. One possibility is given by the pore wall physically interacting with the diffusive molecules and hindering, as a consequence, their movement. The phenomenon, called Knudsen diffusion, is relevant whenever the distance between molecular collisions is greater than the pore diameter. This interaction can be the governing factor of the diffusion rates.

Liquid have a relatively small mean-free path, few angstroms, so that Knudsen diffusion in not important in practical cases, but on the other hand, for gas this parameter can assume values larger than 1,000 Å and therefore be significant.

The diffusion coefficient, D _Kn , in the Knudsen regime can be estimated from gas kinetic theory, and it is given by

$$ {D}_{Kn}=4850d\sqrt{\frac{T}{M}} $$

where d is the pore diameter in cm, Tthe...