Field synchronized bidirectional current in confined driven colloids

Abstract

We investigate the collective colloidal current that emerges when strongly confined magnetic microspheres are subjected to a biased, but spatially uniform, precessing magnetic field. We observe a net bidirectional current composed of colloidal particles which periodically meet assembling into rotating dimers, and exchange their positions in a characteristic, 'ceilidh'-like dance. We develop a theoretical model which explains the physics of the observed phenomena as dimer rupture and onset of current, showing agreement with Brownian dynamic simulations. By varying the tilt angle and the frequency of the applied field, we discover two separate transport mechanisms based on different ways the dimers break up during particle transport. Our results demonstrate an effective technique to drive microscale matter by using a combination of confinement and homogeneous field modulations, not based on any gradient of the applied field.