Self-stabilising water-in-oil droplets through controlled spontaneous emulsification

The application of emulsion droplets as vessels for confined reactions, encapsulation, and material assembly is limited by the difficulty in achieving strong stabilisation against coalescence, particularly for water-in-oil (W/O) emulsions. Here, we investigate the time-dependent influence of spontaneous emulsification on macroemulsion stability, which remains poorly understood. We hypothesise that this non-equilibrium process can be harnessed as a practical stability-enhancing strategy and characterise the factors influencing this process in a variety of systems.

Daughter droplet behaviour in W/O macroemulsions predominantly containing the non-ionic surfactant Span 80 was studied through time-resolved optical microscopy, dynamic light scattering, and rheology. The effects of surfactant composition and viscosity on spontaneous emulsification behaviour and stability were investigated in mixed oil phases of isoparaffin and either oleic acid or mineral oil.

Our study provides mechanistic insights and design guidelines for formulating emulsions which self-stabilise through daughter droplets assembling into elastic interfacial structures. This strategy does not require expensive components and is not specific to emulsion composition. Importantly, we also show that spontaneous emulsification does not always enhance stability. Effective stabilisation requires robust interfacial films between daughter and parent droplets and controlled continuous phase viscosity: if the viscosity is low, interfacial structures do not assemble, whereas a very high viscosity leads to buckling and deformation.