Colloidal fingering in miscible liquids

We report a new "Colloidal fingering" instability that form spontaneously when a colloidal suspension is placed above a miscible liquid of higher mass density in an isothermal situation. A similar in appearance, though of a different origin, "salt fingers" develop when a warm salty solution is placed above colder fresh water of a higher mass density. Eliminating other possibilities, we argue that the instability is due to double mass-diffusive convection resulting from the large difference be­tween the mass diffusivity of the miscible liquids and the colloidal-mass diffusivity in the mixed medium. The principle is general and this kind of fingering should be observable in a variety of colloidal systems and biofilms, including where it can rise upwards.