Emergent dynamics and self-assembly of colloids in time-dependent magnetic fields

Strongly interacting colloids subject to an external periodic forcing often exhibit nontrivial collective dynamics and self-organization. Driven magnetic colloids proved to be an excellent model system to explore emergent collective behavior and out-of-equilibrium self-assembly. New self-assembled structures emerging in time-dependent magnetic fields are often not accessible under equilibrium conditions. In this presentation I will demonstrate that dispersions of magnetic particles suspended at a liquid-air, liquid-liquid interfaces or in the bulk and driven out-of-equilibrium by an alternating magnetic field develop nontrivial dynamic self-assembled phases and structures. Experiments reveal new types of nontrivially ordered phases and collective dynamics emerging in such systems in a certain range of excitation parameters. These remarkable non-equilibrium structures emerge as a result of the competition between magnetic and hydrodynamic forces. The dynamic phases are reversible and fine-tuned by the parameters of the driving magnetic field. Above certain frequency threshold or on external perturbation some of the dynamic structures spontaneously break the symmetry of self-induced flows and turn into swimmers, spinners or can be used as robotic manipulators at microscale. Furthermore, collective motion of self-assembled dynamic structures at interfaces often generates chaotic fluid flow reminiscent of two-dimensional turbulence - active turbulence. In case of a rotational magnetic field two-dimensional active spinner materials can be realized.