Knudsen diffusion
![Schematic drawing of a molecule in a cylindrical pore in the case of Ashley Knudsen diffusion; are indicated the pore diameter (d) and the free path of the particle (l).](/Images/godic/202501/22/Knudsen_diffusion.svg0701.png")
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 example in a long pore with a narrow diameter (2–50 nm) because molecules frequently collide with the pore wall.