Soft meets hard -- how does freeze-thaw cycling affect the microstructure of particle-stabilised emulsions?

Abstract

The freeze-thaw cycling of particle-stabilised emulsions can alter the emulsion structure and stability. This could have significant consequences for using particle stabilisation in industrial applications where increased stability is generally desirable. It is therefore important to characterise the behaviour and stability of these composites under the influence of freeze-thaw cycles. Water-in-oil Pickering emulsions stabilised by poly(methyl methacrylate) particles were subjected to freeze-thaw cycles of the continuous phase under two different conditions - uniform and non-uniform freezing. Confocal microscopy was used to study the emulsion behaviour and structure during these processes. The effect of droplet size and cooling rate on uniformly frozen emulsions was also considered. The final structure of the emulsion after a single freeze-thaw cycle is strongly dependent on the freezing method. Uniformly frozen emulsions show crumpled droplet structures, while non-uniformly frozen emulsions have a non-uniform structure containing foam-like regions not observed in uniform freezing. Droplet size has little effect on the final structure of uniformly frozen emulsions, which we attribute to the Laplace pressure in the droplets being orders of magnitude smaller than the pressure exerted on the droplets by the growing oil crystals. Cooling rate also has little effect as droplets become surrounded and trapped by oil crystals rapidly after samples reach the oil freezing temperature, irrespective of the speed at which they reached that temperature. When compared to surfactant-stabilised emulsions undergoing the same process, we find emulsion structure is recoverable in the surfactant case, whereas particle-stabilised emulsions are irreversibly altered.