(17h) Harnessing Surface-Driven Instabilities at Oil-Water Interfaces for Extractions

We report on pH-driven surface instabilities at oil-water interfaces decorated with the surfactant Di(2-ethylhexyl) phosphoric acid (DEHPA). We observe spontaneous active motion of an oil droplet (with DEHPA dissolved within) when deposited onto an aqueous interface at pH above 7. We explain the pH sensitive ballistic propulsion of the droplet via the deprotonation of the surfactant and the resulting surface activity gradients on the surface of the oil droplet which lead to Marangoni flows. Remarkably, upon introduction of an oppositely charged species, such a metal salt solution at the aqueous interface, we observed a transition from active ballistic propulsion to a spinning motion of the droplet. In conjunction with this transition, we also observe a striking flow of the metal cation species into oil droplet. We provide a simple model of a gradient-field in the metal cation distribution to explain the transition from the translation to the spinning motion of the droplet. Overall, we present here the foundational knowledge for a model active-matter system that can potentially be deployed as a novel extraction technique.