4 Resistance to Diffusion

It is well known that the resistance to viscous flow in porous media primarily arises from viscous shear as the fluid makes its way through small, tortuous channels left open among the collection of solid particles. In contrast, diffusion occurs without loss of momentum by viscous shear. Nevertheless, diffusing species undergo a momentum change through collisions with other species and with solid particles. Resistance to diffusion arises from these collisions and the consequent change in momentum.