Shape changes during the drying of droplets of suspensions

During drying of droplets of suspensions, several flow regimes contribute to the radial flow of powder to the periphery to leave a pile-up of powder at the rim. It is shown that the shape of the droplet residues can be controlled both by restricting evaporation and by combining high and low boiling point solvents which modify particle flows and produce a range of droplet residues varying from a concave "doughnut" shape, sometimes with a central hole, to a convex dome shape. Addition of formamide to aqueous suspensions is shown to affect powder deposition by setting up a Marangoni flow rather than by reducing evaporation at the periphery. The results find direct application in thick-film combinatorial printing of ceramics to form small disks by droplet drying.