Viscoelastic Particle-Laden Interface Inhibits Coffee-Ring Formation.

We investigate the evaporation-driven pattern formation in drying drops containing mixtures of polystyrene and soft microgel particles. The well-known coffee-rings that form when drops containing polystyrene particles are dried can be completely undone in the presence of a small quantity of soft colloids. The addition of soft colloids facilitates the adsorption of polystyrene particles to the water-vapor interface leading to a steep increase in their concentration and also imparts viscoelasticity to the interface. Time-resolved video microscopy is used to conclusively show the formation of a gel-like particle-laden interface. The mean square displacement of the polystyrene particles adsorbed to the interface confirms their immobile nature at the interface. This viscoelastic interface almost prevents the bulk flow-assisted migration of polystyrene particles toward the drop edge, leading to the suppression of coffee-ring effect and the formation of uniform particulate deposits.