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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.
The phrase "momentum diffusion" can also refer to the diffusion of the probability for a single particles to have a particular momentum. In this case, it is the probability distribution function that diffuses in momentum space, rather than the (conserved) quantity of momentum that diffuses among many particles.
- K. Berg-Sorenson et al. (1992). "Momentum diffusion of atoms moving in laser fields". Journal of Physics B: Atomic, Molecular and Optical Physics 25: 4195.
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