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 .
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 . 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.
I will also briefly discuss a set of very recent experiments on ferromagnetic micro-particles sediment on the bottom surface of the flat cell that are energized by a single-axis homogeneous alternating magnetic field applied perpendicular to the surface supporting the particles. Upon application of the field the magnetic torque on each particle is transferred to the mechanical torque giving rise to a rolling motion of the particle. Experiments reveal a rich collective dynamics of magnetic rollers in a certain range of excitation parameters. Flocking and spontaneous formation of steady vortex motions have been observed. The effects are fine-tuned and controlled by the parameters of the driving magnetic field. Formation of the self-organized collective states spontaneously breaking the symmetry of the underlying interactions has been attributed to the interplay of inelastic inter-particle collisions and self-induced hydrodynamic flows in the system.