Pair aligning improved motility of Quincke rollers

Density-dependent speed is studied in a two-dimensional active colloid in which the colloidal particles are propelled by an external electric field via a Quincke rotation. Above the critcal electric field, dense dynamic clusters form spotaneously, in which the particles are highly aligned in velocity and move much faster than isolated units. Detailed observations on pair collision reveal that the alignment of velocity is induced by the long-ranged hydrodynamic interactions and the improvement of speed in the clusters arises from pair aligning in which two particles are closely paired and rotate synchronically. In the aligning state, the short-range in-plane dipole-dipole attraction enhances the rotation torque and gives rises to a larger rolling speed. The pair aligning becomes difficult and unstable at high electric field where the normal dipole-dipole repulsion becomes dominant. As a consequence, the dependence of speed on density becomes weak increasingly upon the increase of the electric field. This result offers an interpretation for the discrepancy between our and previous observations on Quincke rollers.