Self-organization of colloidal nanospheres inside emulsion droplets: Higher-order clusters, supraparticles, and supraballs

Monodisperse cross-linked polystyrene nanospheres were self-organized to obtain colloidal aggregates from toluene-in-water emulsion droplets employed as confining geometries. During evaporation of toluene droplets by heating, the polystyrene nanospheres were packed into higher-order clusters, supraparticles with intermediate ordering or supraballs with hexagonally ordered structures, depending on the size of the emulsion droplets, and the number of constituent polystyrene particles inside the droplets. The shrinkage of toluene droplets induced inward capillary pressure, leading to particle self-assembly into ordered spherical supraballs when a large number of polystyrene particles were contained inside the droplets. Ink-jet technology was adopted to generate monodisperse toluene droplets containing polystyrene particles, which were transformed into monodisperse supraballs after evaporation-driven self-assembly of the polystyrene nanospheres. As an alternative approach, water-in-toluene droplets were also adopted as confining geometries for the generation of supraballs.