  The swimming direction of biological or artificial microscale swimmers tends
to be randomised over long time-scales by thermal fluctuations. Bacteria use
various strategies to bias swimming behaviour and achieve directed motion
against a flow, maintain alignment with gravity or travel up a chemical
gradient. Herein, we explore a purely geometric means of biasing the motion of
artificial nanorod swimmers. These artificial swimmers are bimetallic rods,
powered by a chemical fuel, which swim on a substrate printed with
teardrop-shaped posts. The artificial swimmers are hydrodynamically attracted
to the posts, swimming alongside the post perimeter for long times before
leaving. The rods experience a higher rate of departure from the higher
curvature end of the teardrop shape, thereby introducing a bias into their
motion. This bias increases with swimming speed and can be translated into a
macroscopic directional motion over long times by using arrays of
teardrop-shaped posts aligned along a single direction. This method provides a
protocol for concentrating swimmers, sorting swimmers according to different
speeds, and could enable artificial swimmers to transport cargo to desired
locations.
