Mocha Diffusion: viscous fingering during drop spreading on miscible fluids

Mocha diffusion is a well-known technique for generating beautiful, flower-like patterns on ceramic surfaces. A thin coating of wet, clay "slip" is first applied to the surface, followed by drops of dark ink with additives such as ethanol or vinegar. The result is a complex fingering pattern resembling viscous fingering in a Hele Shaw cell. In the laboratory, we use shear-thinning solutions of sodium alginate or clay and water as the sub-fluid, and drops of food coloring containing water and propylene glycol as the spreading fluid. Crucially, the spreading fluid must be a mixture of two fluids with different surface tensions to help drive the flow through Marangoni forces. Of equal importance is the rheology of the subfluid. A shear thinning fluid promotes fingering through less resistance to fast moving fingers. Surprisingly though, we also find fingering patterns on Newtonian fluids such as thick, sucrose solutions. In this case, the sucrose concentration is so high that the addition of a spreading water layer can drastically lower the local viscosity, enhancing the fingering instability.