Monte Carlo computer simulations and electron microscopy of colloidal cluster formation via emulsion droplet evaporation

摘要

We consider a theoretical model for a binary mixture of colloidal particlesand spherical emulsion droplets. The hard sphere colloids interact viaadditional short-ranged attraction and long-ranged repulsion. Thedroplet-colloid interaction is an attractive well at the droplet surface, whichinduces the Pickering effect. The droplet-droplet interaction is a hard-coreinteraction. The droplets shrink in time, which models the evaporation of thedispersed (oil) phase, and we use Monte Carlo simulations for the dynamics. Inthe experiments, polystyrene particles were assembled using toluene droplets astemplates. The arrangement of the particles on the surface of the droplets wasanalyzed with cryogenic field emission scanning electron microscopy. Beforeevaporation of the oil, the particle distribution on the droplet surface wasfound to be disordered in experiments, and the simulations reproduce thiseffect. After complete evaporation, ordered colloidal clusters are formed thatare stable against thermal fluctuations. Both in the simulations and with fieldemission scanning electron microscopy, we find stable packings that range fromdoublets, triplets, and tetrahedra to complex polyhedra of colloids. Thesimulated cluster structures and size distribution agree well with theexperimental results. We also simulate hierarchical assembly in a mixture oftetrahedral clusters and droplets, and find supercluster structures withmorphologies that are more complex than those of clusters of single particles.