Momentum diffusion

Momentum diffusion refers to the diffusion, or spread of momentum between particles (atoms or molecules) of matter, usually in the liquid state. In fluids, this is what causes viscosity.

In the case of the laminar flow of a liquid past a solid surface, momentum diffuses across the boundary layer which forms at the boundary where the solid meets the liquid. The gradient in this case occurs between the liquid in contact with the surface, which does not move at all and has zero momentum (see no-slip condition), and the liquid far away from the wall, which has momentum proportional to the speed at which it is flowing. The rate of transport is governed by the viscosity of the fluid and the momentum gradient.