Hydrodynamic assembly and chiral synchronization of self-propelled particles*

Active colloids self-organize to a variety of collective states, ranging from highly motile “molecules” to rather complex 3D structures. The underlying forces, however, are not well characterized, and in particular the role of hydrodynamic interactions as compared to the electric, magnetic or chemical surface forces which drive the non-equilibrium states. Using large-scale simulations, we show that hydrodynamic interactions, together with a gravity-like aligning field, lead to tunable self-assembly of active colloidal spheres near a surface. The observed structures depend on the hydrodynamic characteristics: particles driven at the front, pullers, form small chiral spinners consisting of two or three particles, whereas those driven at the rear, pushers, assemble to large dynamic crystals. The rotational motion of the puller spinners, arises from spontaneous chiral symmetry breaking. The spinners’ flow field mediates chiral transfer to neighbor clusters and may even result in hydrodynamic synchronization of their rotation.