Emergent coherent states and flow rectification in active magnetic colloidal monolayers

We report on an emergent coherent behavior of a ferromagnetic colloidal monolayer suspended at a liquid-air interface and powered by a single-axis alternating in-plane magnetic field. Large-scale coherent states are manifested by formation of dynamic vortex arrays within the colloidal monolayer. Emergence of the dynamic coherence and self-organized patterns in this non-equilibrium colloidal system is attributed to a spontaneous symmetry breaking due to subtle interplay between magnetic, steric, and hydrodynamic forces acting on individual colloidal particles. Collective response of the magnetic colloidal monolayer induces in-plane rectified hydrodynamic surface flows which in turn entrains the particles, resulting in a variety of coherent states.