Symmetric and Asymmetric Time-Varying Magnetic Fields to Control and Drive Paramagnetic Colloidal Assemblies*

Micron-sized superparamagnetic colloids placed in time-varying magnetic fields can provide us with rich dynamic behavior depending on the magnetic field applied and the particle concentration. In symmetric magnetic fields, a long-range attractive potential is induced resulting in the formation of colloidal clusters, which assemble into circular shapes that exhibit fluid-like behavior at relatively low fields, while becoming crystallized and highly ordered at higher field strengths. These systems exhibit interesting coarsening and spinodal decomposition structural transitions for larger particle systems.

In asymmetric time-varying fields, the colloids assemble into directed collective motion. The simplest case of two bodies by which a smaller arm particle can be driven in an asymmetric orbit in the proximity of a larger torso particle using an eccentric rotating magnetic field (ERM), resulting in both bodies swimming in a directed motion. Furthermore, adding additional particles the arm leads to overlapping orbits and arm synchronization, resulting in faster swimming speeds. This new methodology for being able to achieve nonreciprocal motion at low Re offers insight into directing the motion of collective active systems.