Gravity induced structures of microswimmers on a surface*

Microswimmers can self-organize to a variety of collective states. The underlying forces, however, are not well characterized. Besides the electric, magnetic or chemical surface forces, hydrodynamic interactions are believed to play important role on the collective motion of active colloids. In typical experiments of artificial self-propelled colloids, the particles are observed to sediment. We study microswimmer suspensions by lattice Boltzmann simulations, using a squirmer model. We show that hydrodynamic interactions, together with a gravity-like field, lead to tunable collective behaviors of self-propelled spheres near a surface. For example chiral spinners, swarming clusters and living crystals are observed. We rationalize the formation of these structures, in detail, based on hydrodynamic interactions between a pair of swimmers. These interactions depend crucially of the swimming mechanism, pusher or puller, and can be tuned such that they are either directional, leading to the formation of small chiral spinners or mutually attractive, creating large hydrodynamically bound motile aggregates.