Particle assembly using confined electro-hydrodynamics: Driven versus active assembly*

We give examples of driven colloidal shell assembly on drop interfaces by application of DC-electrohydrodynamics and dielectrophoresis, using standard leaky dielectric carrier fluids for non-polarisable and/or polarizable colloids. We summarize how this can be used to fabricate static or dynamic colloidal shells with patchy structure and functionality.
Secondly, we give examples of active granular bead assembly in suspension by exploiting the electrohydrodynamic Quincke-rotation effect (i.e. electric field induced rotational motion of non-electrically-conducting particles suspended in a leaky-dielectric fluid). We study a system in quasi-2D microfluidic confinement with an applied DC-electric field perpendicular to the 2D-plane, where a monolayer of Quincke rotors are forced to “live” near a flat solid surface, and thus become a monolayer of self-propelled Quincke rollers. A Quincke roller’s direction of motion in the 2D plane is individual and normal to the applied electric field, with any direction possible, i.e. the electric field supplies energy (“food”), but not direction. Unlike animals, the “metabolism” of Quincke-rollers is instantaneous, and their “food” is translated into motion immediately. We have demonstrated several types of assemblies that emerge from populations of such confined granular Quincke-rollers confined, such as crystallites, dynamic clusters, clusters of clusters, vortices, swirls, and polar liquids with uniform direction of motion.