Title: Emergence of Self-Organized Multi-Vortex States in Active Magnetic Roller Liquids.
Abstract: Active matter, both synthetic and biological, demonstrates complex spatiotemporal self-organization and the emergence of collective behavior. A coherent rotational motion, the vortex phase, is of great interest because of its ability to orchestrate well-organized motion of self-propelled particles over large distances. However, its generation without geometrical confinement has been a challenge. Flocking magnetic rollers constitute rich and well-controlled experimental realization of active matter [1, 2]. We demonstrate by experiments and computational modeling that concentrated magnetic roller liquids energized by external uniaxial alternating magnetic field self-organize into a state with multiple long-lived vortices in an unconfined environment [3, 4]. We show that the dynamic local roller densifications trigger the formation of multiple vortices in an open environment. We find that the neighboring vortices more likely occur with the opposite sense of rotation. Our studies provide insights into the mechanism for the emergence of coherent collective motion on the macroscale from the coupling between microscale rotation and translation of individual active elements. Experimental techniques and theoretical concepts developed in the course of this work may yield strategies for controlling and manipulation of collective states in active colloids. Potential applications range from mixing and transport at the microscale to micro-robotic functionality.
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