Going with the flow: colloidal dynamics at moving immiscible fluid interfaces

A wide array of processes, from membrane defouling to contaminant transport and groundwater remediation, involve interactions between deposited colloidal particles and an immiscible fluid interface. Previous works studying the interactions between individual particles and a moving interface have shown that the interplay between colloidal interactions, hydrodynamics, and capillarity plays a critical role in determining the transport of both colloids and fluid. However, in many cases, particle deposits form dense aggregates, giving rise to new complexities that cannot be described by single-particle models. To address this gap in knowledge, we visualize interactions between multilayer particle deposits and moving immiscible fluid droplets in microchannels. As the fluid interface passes over particles, we observe that they strongly adsorb to it, despite their lack of surface activity under quiescent conditions. We show that this behavior arises due to the influence of capillary forces exerted by the fluid interface as it impinges on particles, forcing them to overcome the barrier to adsorption. Our work thus reveals new ways to help guide the development of more accurate models to describe how deposited particles can be transported by immiscible fluid interfaces in the environment.