Constrictivity must be distinguished from the effects of Knudsen diffusion.

However there may be instances where both Knudsen diffusion and molecular diffusion are important.

This process is known as Knudsen flow or Knudsen diffusion.

Flow through mesopores is described by Knudsen diffusion.

A Knudsen number much greater than one indicates Knudsen diffusion is important.

See also Knudsen diffusion and constrictivity.

The Knudsen number is a good measure of the relative importance of Knudsen diffusion.

The diffusivity for Knudsen diffusion is obtained from the self-diffusion coefficient derived from the kinetic theory of gases.

In the Knudsen diffusion regime, the molecules do not interact with one another, so that they move in straight lines between points on the pore channel surface.

Knudsen diffusion occurs when the particle interacts with the pore walls more than it does with other particles due to the large free path and narrow pores.

In practice, Knudsen diffusion applies only to gases because the mean free path for molecules in the liquid state is very small, typically near the diameter of the molecule itself.

Knudsen diffusion is a means of diffusion that occurs when the scale length of a system is comparable to or smaller than the mean free path of the particles involved.

For Knudsen diffusion, path length λ is replaced with pore diameter , as species A is now more likely to collide with the pore wall as opposed with another molecule.