Segregation in Drying Binary Colloidal Droplets

Abstract

When a colloidal\nsuspension droplet evaporates from a solid surface,\nit leaves a characteristic deposit in the contact region. These deposits\nare common and important for many applications in printing, coating,\nor washing. By the use of superamphiphobic surfaces as a substrate,\nthe contact area can be reduced so that evaporation is almost radially\nsymmetric. While drying, the droplets maintain a nearly perfect spherical\nshape. Here, we exploit this phenomenon to fabricate supraparticles\nfrom bidisperse colloidal aqueous suspensions. The supraparticles\nhave a core–shell morphology. The outer region is predominantly\noccupied by small colloids, forming a close-packed crystalline structure.\nToward the center, the number of large colloids increases and they\nare packed amorphously. The extent of this stratification decreases\nwith decreasing the evaporation rate. Complementary simulations indicate\nthat evaporation leads to a local increase in density, which, in turn,\nexerts stronger inward forces on the larger colloids. A comparison\nbetween experiments and simulations suggest that hydrodynamic interactions\nbetween the suspended colloids reduce the extent of stratification.\nOur findings are relevant for the fabrication of supraparticles for\napplications in the fields of chromatography, catalysis, drug delivery,\nphotonics, and a better understanding of spray-drying.