Knudsen Flow:

Knudsen flow or knudsen diffusion is one of the probable mechanisms cited to explain the gas separation by membranes. While the mean free path of the gas molecules is small compared to the pore size of the membranes, the permeation of gas molecules is governed through viscous flow or hydrodynamic flow. The flow rate of the gas molecules within viscous flow is straight proportional to the pressure gradient causing the flow. Inside such condition, the collision frequency among gas molecules exceeds greatly as compared to the collision frequency of gas molecules along with the pore wall of the membranes. No specific separation of a particular gas component may be expected, since of complete molecular chaos. Therefore, while the mean free path of the gas molecules is much larger than the membrane pore diameter, the collision among gas molecules becomes much fewer than the collision among gas molecules and the wall. The condition is known as rarefied gas and the flow under like conditions is commonly referred to as "free molecular diffusion" or "Knudsen Flow". Within Knudsen flows the element gases flow by the membrane pores independently of every other. Therefore, a separation takes place because of the differences in the molecular weight. This is the principle included in the separation of U-235 isotope through means of gaseous diffusion process.