Magnetically rotated Janus particles near a substrate

Electric and magnetic fields can drive colloids out-of-equilibrium by encoding non-Brownian dynamics in the particles leading to a variety of transient collective states. Controlling and tuning these collective behavior relies on our understanding of dynamics of active colloids at a single particle level and its dependence on particle, field and surrounding environmental characteristics. To achieve persistent motion of a colloidal particle in viscous environments, it is necessary to devise a mechanism that breaks the time-reversal symmetry in motion and sustains asymmetry in the fluid flow around the particle. Here, we investigate the rotational and translational dynamics of a magnetic patched Janus particle in time varying magnetic field. Specifically, we identify the effects of a nearby stationary substrate on the dynamics of the rotating Janus particle. We observed that Janus particles exhibit rolling motion at the substrate, tumbling motion at intermediate heights leading to trochoidal trajectory, and rotating motion further away from the substrate without any translation. At intermediate heights, the particle exhibits sporadic fluctuations across all modes of motion over short time periods, which we term ‘tumbling’. Using experimental results, particle image velocimetry, and a theoretical model, we demonstrate that the mechanism responsible for curvilinear trajectories at intermediate heights arises from the interplay between magnetic torque, gravitational torque, and the fluid resistance experienced by the particle during motion.