Diffusion

Diffusion, gradual mixing of different substances placed in mutual contact, due to the random thermal motion of their constituent particles. Most rapid with gases and liquids, it also occurs with solids. Diffusion rates increase with increasing temperature; the rates at which gases diffuse through a porous membrane vary as the inverse of the square root of their molecular weight (gases of low molecular weight diffuse more quickly than those of high molecular weight). Gaseous diffusion is used to separate fissile uranium-235 from nonfissile uranium-238, the gas used being uranium hexafluoride (UF₆).