Sedimentation and levitation of catalytic active colloids

Abstract

Gravitational effects in colloidal suspensions can be easily turned off by matching the density of the solid microparticles with the one of the surrounding fluid. By studying the motion of catalytic microswimmers with tunable buoyant weight, we show that this strategy cannot be adopted for active colloids with asymmetric mass distribution. If the average buoyant weight decreases, pronounced accumulation at the top wall of a sample cell is observed due to a counteralignment of the swimming velocity with the gravitational field. Even when the particles reach a flat wall, gravitational torques still determine the properties of the quasi-two-dimensional active motion. Our results highlight the subtle role of gravity in active systems.