Two modes of phase inversion in a drying emulsion

Abstract

We report two different modes of phase inversion in surfactant-stabilized oil-in-water emulsions subjected to a unidirectional drying stress. Coalescence occurs either through a nucleation-and-growth mechanism, where coalesced pockets form and grow randomly throughout the sample, or through a coalescence front that propagates into the sample from the drying end. This drying-induced coalescence results from the development of a pressure gradient from the drying front into the bulk of the sample, established by limited water transport through the Plateau borders. Depending on the steepness of this pressure profile, coalescence occurs throughout the sample or only at the drying front. Moreover, we find that surfactant concentration plays a significant role through its effect on the critical disjoining pressure at which coalescence occurs. Very stable emulsions, characterized by a high critical pressure, tend to develop steep pressure profiles, which favours front-dominated coalescence, while unstable emulsions with low critical pressures develop shallow pressure gradients, favouring nucleation-and-growth dominated coalescence. These results offer new insights into the microscopic mechanisms governing the complex and poorly understood macroscopic phenomena during phase inversion of drying emulsions.