  As a natural and functional behavior, various microorganisms exhibit
gravitaxis by orienting and swimming upwards against gravity. Swimming
autophoretic nanomotors described herein, comprising bimetallic nanorods,
preferentially orient upwards and swim up along a wall, when tail-heavy (i.e.
when the density of one of the metals is larger than the other). Through
experiment and theory, two mechanisms were identified that contribute to this
gravitactic behavior. First, a buoyancy or gravitational torque acts on these
rods to align them upwards. Second, 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.
