Dynamics and Manipulation of Emergent Magnetic Roller Vortices

Active colloids display complex collective behavior. We report the emergence of unconfined macroscopic vortices in the system of ferromagnetic colloidal rollers energized by a vertical alternating magnetic field. We demonstrate that observed spontaneous self-organization is not a consequence of a geometrical confinement or finite system size. By introduction of inert scatterers we elucidate the complex nature of interactions of the roller-vortex with the inert particles. The capability of certain inert particles to effectively pin the vortex and manipulate its dynamics is investigated. Furthermore, we demonstrate the potential of a magnetic roller vortex to effectively capture and transport inert particles. Our work provides new insights into behavior of a broad class of active systems where collective motion is caused by a fine interplay between long-range and short-range interactions.