Emergent Dynamics of Paramagnetic Suspensions under Toggled Magnetic Fields

Suspensions of superparamagnetic colloidal particles are subjected to toggled magnetic fields by switching on and off the field repeatedly. At sufficiently high field strengths, the microstructure of the suspension begins in a solid, gel-like state and anneals to droplet-like domains governed by time-averaged bulk and surface energies. Here, we study the effect of the toggled-field duty cycle. At frequencies range investigated in this study (0.33-5Hz) and high duty cycle ratio (0.8), the suspensions form column-like microstructures that do not evolve into energetically favorable states. In this case, the period the field is off is shorter than the particle characteristic diffusion time scale. As duty cycle decreases, however, microstructures grow to ellipsoidal-shape as a result of the sufficient time for particle diffusion. Under a small range of conditions (1.5-3.5 Hz, 0.2 duty cycle ratio), we observe a new class of wavy-shaped microstructures with dynamics that exhibit a strong and continuous process of rotation, coalescence, and breakup. We attempt to understand these dynamics by considering the hydrodynamic forces exerted by a collection of thermal ratchets composed of particles in an asymmetric process of ballistic approach and diffusive relaxation.