The structure and dynamics of microparticles at pickering emulsion interfaces.

We have employed a laser scanning confocal microscope (LSCM) to study the structure and dynamics of microparticles at Pickering emulsion interfaces. The microparticles can have rich morphology at the emulsion interfaces, ranging from an aggregated structure to colloidal lattices, with a possibility of involving heterogeneous particles. With a specific interest in colloidal lattices, we find that although the enhanced electrostatic repulsion explains the formation of colloidal lattices by sulfate-treated polystyrene (S-PS) particles, it fails to interpret the unsuccessfulness of assembling lattices containing single-species carboxylate-treated polystyrene (C-PS) particles. A small percentage of C-PS particles in the colloidal mixture does not disturb the formation of lattices made of S-PS particles. The LSCM also provides a meaningful way to probe dynamic information. The diffusion of single particles at the emulsion interfaces depends strongly on the oil phase viscosity, particle size, and particle wettability. A highly curved emulsion interface slows the motion of microparticles at oil-water interfaces but the interface curvature effect decreases with increasing oil phase viscosity. Although the confocal microscope was originally used as an imaging tool, we find that the thermodynamic equilibrium of colloidal lattices can be disturbed and even destroyed when increasing the output laser intensity.