Colloidal Microworms Propelling via a Cooperative Hydrodynamic Conveyor Belt
We study propulsion arising from microscopic colloidal rotors dynamically assembled and driven in a
viscous fluid upon application of an elliptically polarized rotating magnetic field. Close to a confining
plate, the motion of this self-assembled microscopic worm results from the cooperative flow generated by
the spinning particles which act as a hydrodynamic “conveyor belt”. Chains of rotors propel faster than
individual ones, until reaching a saturation speed at distances where induced-flow additivity vanishes. By
combining experiments and theoretical arguments, we elucidate the mechanism of motion and fully
characterize the propulsion speed in terms of the field parameters.