title: Flocking ferromagnetic colloids.
abstract: Assemblages of microscopic colloidal particles exhibit fascinating collective motion when energized by electric or magnetic fields. The behaviors range from coherent vortical motion to phase separation and dynamic self-assembly. Although colloidal systems are relatively simple, understanding their collective response, especially under out-of-equilibrium conditions, remains elusive. We report on the emergence of flocking and global rotation in the system of rolling ferromagnetic microparticles energized by a vertical alternating magnetic field. By combing experiments and discrete particle simulations, we have identified primary physical mechanisms, leading to the emergence of large-scale collective motion: spontaneous symmetry breaking of the clockwise/counterclockwise particle rotation, collisional alignment of particle velocities, and random particle reorientations due to shape imperfections. We have also shown that hydrodynamic interactions between the particles do not have a qualitative effect on the collective dynamics. Our findings shed light on the onset of spatial and temporal coherence in a large class of active systems, both synthetic (colloids, swarms of robots, and biopolymers) and living (suspensions of bacteria, cell colonies, and bird flocks).
authors.affiliation: Materials Science Division, Argonne National Laboratory, 9700 South Cass Avenue, IL 60439, USA.
authors.forename: Andreas
authors.initials: A
authors.lastname: Kaiser
authors.affiliation: Materials Science Division, Argonne National Laboratory, 9700 South Cass Avenue, IL 60439, USA.
authors.forename: Alexey
authors.initials: A
authors.lastname: Snezhko
authors.affiliation: Materials Science Division, Argonne National Laboratory, 9700 South Cass Avenue, IL 60439, USA.; Department of Engineering Sciences and Applied Mathematics, Northwestern University, 2145 Sheridan Road, Evanston, IL 60202, USA.
authors.forename: Igor S
authors.initials: IS
authors.lastname: Aranson
keywords: Collective Behavior; Synchronization; active matter; colloids; flocking; self-assembly