Diffusion & convection
Mass transfer plays a role in many situations. In small, confined volumes or close to surfaces diffusion is the mode of transfer. An example where diffusion is important is to when molecules passes a porous membrane or filter. Another example is transport in biological cell systems. Diffusion in various geometries can be modelled using the Langevin equation or Random walk methodology.
Diffusion is very slow on long distances. But in bigger volumes convection is often present and is the dominating mode of transfer. The diffusion-convection equation can be used where both diffusion and convection contributes significantly to the transport.