Magnetically driven confined colloids: From enhanced diffusion to bidirectional transport

Inspired by previous experimental results, we use numerical simulations to investigate the collective dynamics of paramagnetic colloidal particles confined between two plates closer than twice the particle diameter and driven by an external precessing magnetic field. We show that, when the field is spatially isotropic there is no net particle current and the colloids display enhanced diffusive dynamics with an effective diffusion coefficient which raises up to 60 times that of the undriven case. In contrast, when the field is spatially anisotropic due to a small tilt angle delta, the particles organize into a robust bidirectional current, flowing along two parallel planes by periodically exchanging their positions. In this regime, we also analyze how the presence of small impurities which can be described as "magnetic holes"affect the particle current breaking the bidirectional flow. Our system provides a general method to transport magnetic colloids in a viscous fluid, without using any field gradient, but based on the fine balance between confinement and magnetic dipolar interactions.