Off-axis gyration induces large-area circular motion of anisotropic microparticles in a dynamic magnetic trap

Abstract: Magnetic tweezers are crucial for single-molecule and atomic characterization, and biomedical isolation of microparticle carriers. The trapping component of magnetic tweezing can be relying on a magnetic potential well that can confine the relevant species to a localized region. Here, we report that magnetic microparticles with tailored anisotropy can transition from localized off-axis gyration to large-area locomotion in a rotating magnetic trap. The microparticles, consisting of assemblies of magnetic cores, are observed to either rotate about its structural geometric center or gyrate about one of the magnetic cores, the switching of which can be modulated by the external field. Raising the magnetic field strength above a threshold, the particles can go beyond the traditional synchronous-rotation and asynchronous-oscillation modes, and into a scenario of large-area circular motion. This results in peculiar retrograde locomotion related to the magnetization maxima of the microparticle. Our finding suggests the important role of the microparticle's magnetic morphology in the controlled transport of microparticles and developing smart micro-actuators and micro-robot devices.