Driving and assembling magnetic particles with incoherent fields

Magnetic particles in suspension can chain under static applied magnetic fields or be transported to target sites with static magnetic field gradients. This is clear and has led to innovations from magnetorheological fluids to magnetic targeting of tumors to magnetic delivery of drugs. But how do magnetic particles behave in more complicated fields and field gradients? Can we use these complications to our advantage? Can we understand the physics? There has been significant research toward these questions for coherent (simple rotating) fields by many research groups including ours. Dynamically tuning the direction (but not magnitude) of the field will disrupt the simple chaining of particles. For two isolated particles, this disruption is found to be an elegant systematic dance between particle pairs. For large populations of particles, fascinating group dynamics occur. We have looked at these dynamics for chains as well as for multipolar ring formations. In addition, we have been investigating the behavior of magnetic particles under incoherent fields, in which there is a mismatch in the frequency domain for x, y, and z field functions. Surprisingly, we find that incoherent fields can generate coherent assemblies! We apply our findings to the realms of magnetic assembly, magnetic drug delivery and magnetic rheology in an attempt to enhance current practices through complicating the applied fields.