Momentum diffusion refers to the diffusion, or spread of momentum between particles (atoms or molecules) of matter, usually in the liquid state.

In the case of the laminar flow of a liquid past a solid surface, momentum diffuses across the boundary layer near the surface. The gradient in this case occurs between the liquid in contact with the surface, which does not move at all and has zero momentum, 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.

Diffusion | Fluid dynamics