(522b) Going with the Flow: Colloidal Dynamics at Moving Immiscible Fluid Interfaces

A wide array of processes, from contaminant transport and membrane defouling to enhanced oil recovery and groundwater remediation, involve the interactions between colloidal particles deposited on a solid matrix and an immiscible fluid interface moving over it. 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 are dense, multi-particle aggregates, giving rise to new complexities that cannot be described by single-particle models. To address this fundamental gap in knowledge, we use confocal microscopy to directly visualize the interactions between multilayer colloidal particle deposits and moving immiscible fluid droplets in microchannels.

As the immiscible fluid interface passes over particles, we observe that they strongly adsorb to it, despite the fact that the particles are not surface active under quiescent conditions. We show that this surprising behavior arises due to the influence of capillary forces exerted by the fluid interface as it impinges on the particles, forcing them to overcome the electrostatic energy barrier to adsorption. Thus, the surface coverage of the interface by particles increases with time as the fluid droplet traverses the channel. Eventually, the fluid interface becomes saturated with adsorbed particles—triggering an abrupt fluid-solid transition in the rheology of the interface that alters subsequent flow. As a consequence, the interface has a finite “carrying capacity”, continually sloughing off particles when it is sufficiently jammed. While injection of immiscible fluid interfaces (e.g., sparging) has been explored for its potential to remove deposited particles from solid surfaces, our study reveals a limitation of this process, indicating that fluid interfaces can rapidly become saturated by particles. Our results show that this limitation can be overcome by increasing fluid interfacial area, suggesting a new approach to anti-fouling using dispersed droplets. They also help guide the development of more accurate models that describe how deposited particles, bacteria, and viruses can be transported by immiscible fluid interfaces (e.g., wetting/drying cycles) in the environment.