Flow-induced phase inversion of emulsions in tapered microchannels†
Phase inversion emulsification (PIE) is a process of generating emulsions by inverting the continuous and dispersed phases of pre-existing emulsions. Although PIE is conventionally performed in batch processes, flowing emulsions through precisely engineered channels (i.e. flow-induced phase inversion emulsification (FIPIE)) can induce PIE and potentially enable continuous processing. In this study, we demonstrate flow-induced phase inversion of oil-in-water (O/W) emulsions using microfluidic channels with gradual constriction. We investigate the importance of wetting properties and geometric characteristics of microfluidic channels on FIPIE. We show that two dimensionless groups, Ca and the ratio of droplet-size to channel dimensions determine the outcome of the process. In situ observation of individual droplets undergoing FIPIE reveals that the rupture of films of the continuous (water) phase between oil droplets and a wetting oil layer on the surface of microchannels is the most crucial step for phase inversion. Finally, we compare our experimental observations with a scaling relationship that is based on the force balance between disjoining pressure and Laplace pressure, which provides insights into the underlying physical phenomena responsible for the rupture of the aqueous film and the occurrence of FIPIE. We believe our work provides critical insights and parameters for designing channels and pores that can be used for continuous PIE.