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Emergence of self-organized multivortex states in flocks of active rollers

Koohee Han,G. Kokot,O. Tovkach,A. Glatz,I. Aranson,A. Snezhko

Published 2020 in Proceedings of the National Academy of Sciences of the United States of America

ABSTRACT

Significance Active materials maintained out of equilibrium due to external energy consumption promise access to tunable structures capable of swarming, shape shifting, and self-healing—properties known mostly throughout the biological world. Developing fundamental understanding and controls of these synthetic systems remains challenging. Active colloidal dispersion of rollers energized by an alternating magnetic field reveals complex emergent dynamics of self-organized vortex structures. Reminiscent of the collective behavior observed in living systems, the emergent vortices are well localized in space and unbound from the geometrical constraints. Our findings provide fundamental insight into the emergence of macroscopic behavior in active roller dispersions and suggest strategies for self-assembled dynamic materials and particulate manipulation at the microscale. 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. Here, we show by experiments and computational modeling that concentrated magnetic rollers self-organize into multivortex states in an unconfined 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. These results may stimulate design strategies for self-assembled dynamic materials and microrobotics.

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