Microwheels on microroads: Enhanced translation on topographic surfaces

Microbot locomotion is challenging because of the reversible nature of microscale fluid flow, a limitation that can be overcome by breaking flowfield symmetry with a nearby surface. We have used this strategy with rotating wheel-shaped microbots, microwheels (mu wheels), that roll on surfaces leading to enhanced propulsion and fast translation speeds. Despite this, studies to date on flat surfaces show that mu wheels roll inefficiently with substantial slip. Taking inspiration from the mathematics of roads and wheels, we demonstrate that mu wheel velocities can be significantly enhanced by changing microroad topography. Here, we observe that periodic bumps in the road can be used to enhance the traction between mu wheels and nearby walls. Whereas continuous mu wheel rotation with slip is observed on flat surfaces, a combination of rotation with slip and nonslip flip occurs when mu wheels roll on surfaces with periodic features, resulting in up to fourfold enhancement in translation velocity. The unexpectedly fast rolling speed of mu wheels on bumpy roads can be attributed to the hydrodynamic coupling between mu wheels and road surface features, allowing nonslip rotation of entire wheels along one of their stationary edges. This road-wheel coupling can also be used to enhance mu wheel sorting and separation where the gravitational potential energy barrier induced by topographic surfaces can lead to motion in only one direction and to different rolling speeds between isomeric wheels, allowing one to separate them not based on size but on symmetry.