A type of diffusion applied to gases that diffuse through the narrow pores and capillaries of a porous material in which the collisions between the gas molecules and the walls of the pores are appreciable, and consequently influence the rate of diffusion. The movement or motion of gas at low pressure is the result of individual molecular free path motion. Knudsen flow occurs where the mean free path is greater than the radius of the aperture by about a factor of ten. It is named after Danish physicist Martin Hans Christian Knudsen (1871–1949).