Theory 25/Knudsen Number/Membrane separation
Selectivity Is a Knudsen Effect
The claim
Gas separation in nanoporous membranes is set by the Knudsen number, so a membrane designed for one pressure regime can invert its selectivity in another.
01Mechanism
When the mean free path exceeds the pore size, molecules diffuse independently and separation follows molecular weight; at lower Knudsen, other mechanisms dominate.
02Prediction
Selectivity should invert across a Knudsen threshold.
03Refutation
Selectivity fixed by pore size would remove the pressure dependence.