Mocha Diffusion: The Art of Spreading Miscible Liquids

Mocha diffusion is a technique in ceramic surface decoration known to produce radial, fern-like patterns. The veined designs are made by dripping a dark dye solution onto a miscible wet clay slip covering the pottery. These dye solutions commonly include additives like acidic tobacco tea, alkaline stale urine, or surfactants. We investigate the underlying mechanisms of this pattern formation by exploring various additives for dye solutions, such as propylene glycol, ammonia, citric acid, and isopropanol, as well as various sub-fluids. In our experiments, dyes are dropped onto a variety of viscous sub-fluids, including sodium alginate, corn syrup, acrylic paint, and sugar water solutions. We observe a large variety of dendritic patterns, meaning there are many ways to design mocha diffusion art. We find that a highly non-linear relationship between water concentration and shear viscosity in the sub-fluid is a key feature needed for mocha diffusion. Contrary to prior conclusions from our group, sub-fluids do not need to be shear-thinning. Our data suggests that partial diffusion of the dye solution into the sub-fluid lowers the local viscosity, creating a channelizing effect. When combined with surface tension gradients (Marangoni flow) in the dye solution that drive the spreading process, dendritic patterns form only when the dye solution has sufficient time for diffusion. This influences the initial formation of fingering patterns and distinguishes mocha diffusion from conventional fluid spreading.