Synchronized Circular Motion of Optically Confined Marangoni Microswimmers

Understanding the collective actuation of microscopic structures driven by external fields can lead to the development of next-generation autonomous machines. With this goal in mind, we investigated the light-induced collective motion of thermocapillary microswimmers at the air–water interface. We found that Marangoni forces, which lead to long-range repulsive interparticle interactions, can cause microswimmers to synchronize their circular motion in a collective chase mode that resembles predator–prey behavior, which is often observed in nature. We examined different degrees of confinement in small systems containing 2–6 particles of different individual swimming velocities and shapes. Thanks to the strong repulsive interactions between particles, a sustained chasing mode was observed for particle packing fractions above a critical value of 0.25. At lower packing fractions, swimmers transition between chasing, bouncing, and intermittent pausing, likely due to time-varying activity levels. Additionally, we report that a new synchronized mode can be introduced by incorporating chirality into particle shapes, where the microswimmers collectively reverse the direction of their circular motion periodically. Our results point to a simple but powerful mechanism of obtaining collective synchronization in synthetic confined systems, where particles are designed with different shapes and activity levels.