Purely viscous acoustic propulsion of bimetallic rods

Synthetic microswimmers offer models for cell motility and their tunability makes them promising candidates for biomedical applications. Here we measure the acoustic propulsion of bimetallic micro-rods that, when trapped at the nodal plane of a MHz acoustic resonator, swim with speeds of up to 300 microns per second. While past acoustic streaming models predict speeds that are more than one order of magnitude smaller than our measurements, we demonstrate that the acoustic locomotion of the rods is driven by a viscous, non-reciprocal mechanism relying on shape anisotropy akin to that used by swimming cells and that reproduces our data with no adjustable parameters.