(142m) Pickering Emulsions - a Paradigm Shift

Emulsions stabilized by solid particles are known as Pickering emulsions. Although Pickering emulsions are encountered in various natural and industrial processes such as crude oil recovery, oil separation, cosmetic preparation, and waste water treatment, the underlying phenomena are poorly understood. Recently, there is an increasing interest in Pickering emulsions because of their potential in numerous new applications.

Using confocal laser scanning microscopy, we have studied the assembly of colloidal-sized polystyrene particles in polydimethylsiloxane (oil)-in-water Pickering emulsions. Monodisperse polystyrene particles, when included in the emulsions at low concentrations, were found to form small patches with local hexagonal order, separated by other particle-free domains. Polystyrene particles with different sizes and different wettability could simultaneously segregate to the emulsion interface. In a most intriguing result, even mixtures of hydrophobic and hydrophilic solid particles were found to simultaneously segregate to the same interface.

We have also investigated the multiphase interactions and self-assembly of dodecanethiol-capped silver nanoparticles (1-5 nm) at trichloroethylene-water interface. The nanoparticles formed randomly distributed multilayers at the liquid/liquid interface, in contrast to the monolayer observed for colloidal-sized polystyrene particles.

Finally, we have used Pickering emulsions as a model system to investigate the particle mobility, aggregate structure, and the mechanism of aggregate growth at the two-dimensional level. Remarkably, the rate of diffusion of the colloidal-sized polystyrene particles at the oil (5 cSt.)-water interface is only moderately slower than in the bulk water phase. The diffusion constant of solid particles is significantly reduced by increasing the viscosity of the oil phase and dependent on the size of the cluster and interfacial curvature. Furthermore, we successfully observed the in-situ structural formation of solid particles at the oil-water interface, which may have important implications on the stabilization and subsequent destabilization of the emulsion.