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. 
Fernando Martinez-Pedrero, Antonio Ortiz-Ambriz, Ignacio   Pagonabarraga, and Pietro Tierno
cond-mat.soft