Colloidal fingering in miscible liquids

When a liquid is forced through a second liquid with a differing density or viscosity, the interface can become unstable resulting in finger-like patterns that grow and bifurcate. 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. Here, we report the discovery of colloidal fingers that form spontaneously when a colloidal suspension is placed above a miscible liquid of higher mass density in an isothermal situation. 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.

Automated summary

When a liquid is forced through a second liquid with different density or viscosity, the interface can become unstable, resulting in finger-like patterns. Specifically, colloidal fingers form when a colloidal suspension is placed above a miscible liquid of higher mass density.