Shape-induced pairing of active colloids propelled by induced-charged electrophoresis*

Metallo-dielectric Janus particles can self-propel in liquid solution when exposed to AC electric fields via induced charge electrophoresis (ICEP). In recent experiments conducted in the Snezhko group, it was observed that ICEP Janus particles with discoidal shape can spontaneously form stationary bound pairs separated by a liquid-filled gap. Paired particles exhibit “head-on” alignment, i.e., the particles’ axes of symmetry are aligned with the center-to-center vector, and the gap width decreases with increasing frequency of the AC field. Strikingly, this pairing is not observed for spherical particles. In order to rationalize these observations, we develop a theoretical model for collisions between two ICEP particles interacting via self-induced hydrodynamic and electric fields. We recover the experimental observations, finding that hydrodynamic interactions dominate the pairing; effectively, the particles behave as field-tunable “squirmers.” In particular, we show that “head-on” alignment is driven by a coupling between the oblate shape of the particles and hydrodynamic flows generated by swimming activity. In the framework of our model, we explore general implications for shape-induced pairing of active colloids. Our findings suggest that self-organization of ICEP particles could be “programmed” via particle shape and surface patterning.