Diffusion is essentially caused by the random thermal motion of fluid molecules. As an example a mass diffusion process can be induced by bringing into contact two miscible fluids in a configuration where convective motions are avoided, like piling-up a bottom layer of a solution of sugar and water with a top layer of pure water at the same temperature. Here the bottom layer is denser and this prevents the onset of any convective motion. The fluids are initially separated by a sharp horizontal interface which becomes progressively