Emergence of multi-vortex states in swarms of active magnetic rollers*

Active matter represents an emergent class of out-of-equilibrium dissipative systems demonstrating complex self-organization, collective behavior, and tunable functionalities. In both artificial and biological active systems, the vortex phase is of great interest because of its spontaneous emergence from a translational motion of elementary units and its ability to maintain the emergent dynamic structures. However, the generation of long-lived multi-vortex states without geometrical confinement has been challenging. We report that flocking magnetic rollers driven by a vertical a.c. magnetic field [1,2] can form long-lived multi-vortex states in an unconfined environment. We reveal that the absence of confining boundaries allows the emergence of multiple vortices with a wide distribution of vortex size and angular speed. In addition, these vortices exhibit equal probability distribution of clockwise or counterclockwise rotation with characteristic opposite chiralities of nearest neighboring vortices.

1. Snezhko et al., Sci. Adv. 3, e1601469 (2017)
2. Kokot and Snezhko. Nat. Commun. 9, 2344 (2018)