Metallic microswimmers driven up the wall by gravity*

As a natural and functional behavior, various microorganisms exhibit gravitaxis by orienting and swimming upwards against gravity. We study the swimming of autophoretic nanomotors, which are bimetallic and rod-shaped particles, and find that when moving near inclined walls, these tail-heavy rods preferentially orient upwards and swim up along the wall. Through experiment and theory, we identify two mechanisms that contribute to their gravitactic behavior. First, a buoyancy or gravitational torque acts on these rods to align them upwards. Further, hydrodynamic interactions of the rod with the inclined wall induce a fore-aft drag asymmetry on the rods that reinforces their orientation bias and promotes their upward motion.