Active magnetic colloids: emergent dynamics and self-assembly

Magnetic colloidal ensembles subject to an external energy injection often develop nontrivial collective dynamics and self-assembled phases. Dispersions of magnetic particles suspended at a liquid-air or liquid-liquid interface and driven far-from-equilibrium by a transversal alternating magnetic field develop nontrivial dynamic self-assembled structures [1-3]. Experiments revealed new types of nontrivially ordered phases ("asters", "magnetic snakes") emerging in such systems in a certain range of excitation parameters. These remarkable magnetic non-equilibrium structures emerge as a result of the competition between magnetic and hydrodynamic forces. Above certain frequency threshold some of the dynamic magnetic structures spontaneously break the symmetry of self-induced surface flows (symmetry breaking instability) and turn into swimmers [3].

Nontrivial active self-assembly have been also observed in seemingly "trivial" geometries: suspended magnetic ensemble driven out of equilibrium by uniaxial alternating magnetic fields applied parallel to the liquid interface [4]. New dynamic self-assembled structures are reported, ranging from gas of rotators to dynamic wires. Transitions between different self-assembled phases with parameters of external driving magnetic field are observed.