The Coupling and Desynchronization of Magnetic Microsphere Dimers Under a Rotating Magnetic Field

Magnetically driven colloids provide a simple platform for studying synchronization and nonlinear dynamics in externally driven many-body systems. Although desynchronization of an individual magnetic particle is well established, a particle pair contains coupled rotational degrees of freedom whose synchronization pathways remain less understood. Here, we experimentally investigate single magnetic microspheres and microsphere dimers in a viscous fluid under a rotating magnetic field across a wide frequency range. As the driving frequency increases, the dimer passes through four characteristic regimes, from synchronous near-contact rotation to asynchronous radial breathing and fully desynchronized motion. The collective revolution of the dimer loses phase locking with the field before the spin of an individual microsphere, revealing a two-level synchronization structure. We evaluate the instantaneous magnetic interaction using the full dipole–dipole force rather than assuming a radial hydrodynamic force. The results indicate that radial breathing arises from phase-dependent dipolar forcing associated with loss of collective phase locking, whereas low-Reynolds-number hydrodynamics primarily contributes viscous resistance and rotational coupling. These findings link particle spin, collective revolution, and radial breathing through their relative phases and provide a mechanistic framework for understanding and controlling magnetically driven colloidal assemblies.