Self-Stabilisation of Water-in-Oil Droplets Through Controlled Spontaneous Emulsification

The application of emulsion droplets as confining vessels for confined chemical reactions, reagent encapsulation, and material assembly has been revolutionized by controlled emulsification techniques such as microfluidics, which enable precision control over droplet size, structure and composition. However, without robust stabilisation against thermodynamically favourable coalescence, droplets merge and size distributions broaden over time, restricting the benefits offered by these emulsification tools. Here we present a novel, inexpensive and highly effective strategy to enhance resistance against coalescence through controlled surfactant-assisted spontaneous emulsification at the interface of water-in-oil emulsion droplets. This non-equilibrium process results in the formation of a viscoelastic barrier consisting of micro-droplets that self-organise to surround droplet surfaces. The evolved barrier occurs within minutes of emulsification and can suppress coalescence even for large, close-packed droplets under mechanical agitation, or elevated temperature. We characterize this dynamic self-stabilisation mechanism by microscopy and rheological measurements, which add valuable insights to the current understanding of spontaneous emulsification behaviour. Importantly, we show that this stabilisation method is not compositionally restricted, and therefore presents an exciting, versatile strategy for improving the utility of emulsions without the need for expensive surfactant or oil systems.