Observation of Moon-like Synchronous Revolution and Rotation of Janus Microparticles Trapped in an Annular Optical Trap

The synchronous revolution–rotation motion of the Moon against the Earth is eye-catching and is universally ascribed to the Moon–Earth tidal lock-in effect. Such a unique Moon-like motion is common in our celestial universe but is rarely encountered and disclosed in the microscopic world. In this article, we report the experimental observation and theoretical analysis of a stable and ceaseless Moon-like revolution–rotation locked-in motion of a Janus particle that is trapped within an annular optical trap (AOT) formed by a 1064 nm infrared laser beam. The Janus particle rotates on its axis with a synodic period that matches its synodic period of revolution around the optical axis. A systematic electromagnetic and Newtonian numerical analysis indicates that this distinctive orientation locking of Janus microparticles in the AOT can be ascribed to the collective and fine action of the optical force and thermophoresis force and their torques to exactly overcome the Stokes drag force and torque. Moreover, the forces and torques exerted on the Janus particle are highly coupled with its position and orientation so that the Janus particle relies on its relative position and velocity feedback to automatically update its orientation for seeking a dynamic equilibrium state where the revolution and rotation angular speed are equal to each other. Such a synchronous lock-in revolution–rotation motion of the Janus particle in the microcosm would significantly deepen the understanding of interaction mechanisms between geometry–engineering composite particles and structured laser beam and help to lay the foundation for building and assembling self-propelled, self-adapting, and biocompatible cellular micromotors.