Knudsen flow describes the movement of fluids with a high Knudsen number, that is, where the characteristic dimension of the flow space is of the same or smaller order of magnitude as the mean free path.
For a gas passing through small holes in a thin wall in the Knudsen Flow regime, the number of molecules that pass through a hole is proportional to the pressure of the gas and inversely proportional to its molecular weight. It is therefore possible to effect a partial separation of a mixture of gases if the components have different molecular weights. The technique is used to separate isotopic mixtures such as uranium using porous membranes.[1] It has also been successfully demonstrated for use in hydrogen production, as a technique for separating hydrogen from the gaseous product mixture created when water is heated at high temperatures using solar or other energy sources.[2]