Momentum diffusion

Momentum diffusion most commonly refers to the diffusion, or spread of momentum between particles (atoms or molecules) of matter, often in the Fluid state. This transport of momentum can occur in any direction of the fluid flow. Momentum diffusion can be attributed to either external Pressure or Shear stress or both.

Diffusion due to pressure

When pressure is applied on an incompressible fluid the velocity of the fluid will change. The fluid accelerates or deccelerates depending on the relative direction of pressure with respect to the flow direction. This is because applying pressure on the fluid has caused momentum diffusion in that direction. Understanding the exact nature of diffusion is a key aspect towards understanding momentum diffusion due to pressure.

Momentum diffusion due to Shear stresses

A fluid flowing along a flat plate will stick to it at the point of contact and this is known as the no-slip condition. This is an outcome of the adhesive forces between the flat plate and the fluid.The presence of the wall has an effect up to a certain distance in the fluid (in the direction perpendicular to the wall area and flow ) and this is known as the boundary layer.