Wall Torque Controls Propulsion of Curved Microstructures in Bacterial Baths

The persistent dynamics of active particles makes them explore extended portions of an obstacle's boundary during collisions. From impact to escape, the net applied forces depend on the curvature of the wall and increase in the presence of concave features. Here we systematically investigate the forces exerted by swimming bacteria on microfabricated structures, where the radii of curvature can be varied parametrically. We find that these microsails are propelled with a speed that scales linearly with curvature and is directed from concave to convex side along the axis of symmetry. By solving the collision problem for microswimmers subject to aligning torques at the wall, we show that, despite bacteria rapidly aligning with the surface plane, the resulting force law reduces to that of spherical active particles, with the wall-induced reorientation rate effectively replacing rotational diffusion.