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.