Nonequilibrium surfactant partitioning into microdroplets generates local phase inversion conditions and interfacial instability

Abstract

Droplets far from equilibrium experience different compositions and local environments compared with bulk oil and water phases at equilibrium. Understanding the pathways involved in emulsion progression towards equilibrium is valuable for designing complex fluids for many purposes including coatings, food, chemical separations, active matter, and enhanced oil recovery. Here we report how microscale oil droplets, which partition nonionic surfactants and also solubilize, can follow an unexpected pathway wherein a spherical droplet transitions through an interfacial instability and dissociates. This process depends on the oil hydrophobicity, the concentration and ethylene oxide number of the surfactant, the initial droplet diameter, and the presence of neighboring droplets. We propose a mechanism based on local phase inversion that explains both the visual appearance of the droplet dissociation behavior as well as the trends in its counterintuitive dependence on specific conditions like oil and surfactant chemical structure and surfactant concentration.