Formation of a colloidal band via pH-dependent electrokinetics

Electroosmosis on non-uniformly charged surfaces often induces intriguing flow behaviors, which can be utilized in applications such as mixing processes and designing micromotors. Here, we demonstrate non-uniform electroosmosis induced by electrochemical reactions. Water electrolysis creates pH gradients near the electrodes that cause a spatiotemporal change in the wall zeta potential, leading to non-uniform electroosmosis. Such non-uniform electroosmotic flows induce multiple vortices, which promote continuous accumulation of particles that subsequently form a colloidal band. The band develops vertically into a “wall” of particles that spans from the bottom to the top surface of the chamber. Such a flow-driven colloidal band can be potentially used in colloidal self-assembly and separation processes irrespective of the particle surface properties. For instance, we demonstrate these vortices can promote rapid segregation of soft colloids such as oil droplets and fat globules.