Mixing of two liquids with different diffusivities

FEFLOW simulation of a double-diffusion experiment carried out by Pringle et al. (Transp. Porous Media 47 (2002) 1, 195-214). The Hele-Shaw cell is filled with a sucrose solution over a denser sodium chloride solution. Despite the initially stable density layering, fingers form because of the different diffusivities of the two solutions. The colouring represents the concentration of a dye added in low concentration. The white wiggles at the tips of the fingers are little oscillations (numerical instabilitie