Use the Knudsen Diffusion command to model dilute gases or diffusion in very small pores, i.e. situations where no molecule-molecule interactions take place and the diffusion process is solely driven by molecule-wall interactions.

This command models the diffusion of particles by a random-walk algorithm. Only molecule-wall collisions are modeled. Einstein's formula is used to derive the diffusivity from the computed average particle displacement. The resulting dimensionless relative diffusivity depends only on the pore structure of the material. As a post-processing step, you may input the mean thermal velocity of the gas species to to derive a dimensional effective Knudsen diffusivity.