Understanding demulsification in pickering emulsions

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Pickering emulsions are dispersions of two immiscible fluids of any type either oil-in-water (o/w), water-in-oil (w/o) stabilized by solid particles instead of molecular surfactants. They possess ultrastability due to strong particle adsorption at the fluid-fluid interface with high desorption energies. These emulsions can be stabilized by microgel particles, that could be either made of synthetic polymers like poly-(N-isopropylacrylamide) (pNIPAM) or biocompatible microgels made by the physical crosslinking of various animal and plant proteins. Because of their ultra-stability, often these particles are difficult to desorb unless external stimuli or forces are applied. Nevertheless, demulsification is important to allow release of actives or hydrophobic nutrients from the internal (emulsified) droplets, which has attracted relatively little attention in literature.The aim of this review is to understand the factors affecting demulsification in Pickering emulsions, particularly fabricated using synthetic polymeric particles to inform future demulsification in emulsions stabilized by biocompatible particles. From the detachment energy theories, the variation of the force with displacement is largely governed by the interfacial tensions between the fluids, the wettability and the particle size [1]. Preliminary findings suggest that even though the adhesion of particles to a surface can be strong and appear irreversible in many situations, it is possible to cause desorption or detachment of these particles through the application of a suitable external field. A range of physical conditions (pH, temperature, heat, shearing) or mixing with additives (surfactants, enzymes, salts, bacteria) have been used in literature to demulsify Pickering emulsions. There is significant research in this area on synthetic particles, however, there are still uncertainties regarding the mechanisms that trigger demulsification. In contrast, there is very limited research on demulsification of biocompatible particles.Having a better understanding of the factors that influence demulsification can help unlock the full potential of Pickering emulsions and its many applications such as the release of active ingredients in the chemical, food, and pharmaceutical industries.