Enhanced Diffusion and Magnetophoresis of Paramagnetic Colloidal Particles in Rotating Magnetic Fields

Dispersions of paramagnetic colloids can be manipulated with external magnetic fields to assemble structures and control transport. For fields held steady in time, the structure and dynamics are coupled, which becomes problematic for processes where aggregation competes against particle transport. Rotating the field direction in time drives dispersions out of equilibrium, allowing the structure and dynamics to be tuned independently to enhance transport. Fundamental transport properties, like the diffusivity and magnetophoretic mobility, dictate a suspension’s nonequilibrium response and are crucial to understand to design processes utilizing rotating fields. Here, we investigate the transport properties of paramagnetic colloids in rotating magnetic fields using dynamic simulations. We find that self-diffusion is enhanced in rotating fields compared to steady fields, and that the self-diffusivity in the plane of rotation reaches a maximum value at intermediate rotation frequencies that is larger than the Stokes-Einstein diffusivity of an isolated particle. While the magnetophoretic velocity through bulk fluid decreases with increasing rotation frequency, enhanced in-plane diffusion allows for faster magnetophoretic transport through porous materials in rotating fields.