  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.
