Steady rotation and wall-mediated dynamics of magnetic Janus particles in oscillating fields

Active colloids are synthetic particles inspired by the self-propulsion and adaptability of microorganisms. These particles hold promise to perform intelligent microscale tasks, yet their behavior near boundaries and substrates remains poorly understood. Here we identify how the particle-substrate separation, and applied field frequency dictate the dynamics of magnetic Janus microparticles actuated by an oscillating magnetic field. First, we show that the continuous rotation of the non-inertial Janus particle arises from the coupling between its permanently aligned and field-induced magnetic moments. Second, by varying the particle's height above the substrate, we identify distinct motion regimes, namely, rolling, hybrid, and rotational, where each is defined by characteristic changes in the in-plane rotation, translational dynamics, and trajectory. We reveal the emergence of quasi-steady states at intermediate heights and dynamic in- and out-of-plane rotations that give rise to field frequency-dependent linear and alternating arcs and loops i.e. trochoidal trajectories. The insights gained here provide a framework for designing surface microrollers with programmable kinematics driven by time-varying magnetic fields.