Synchronized Circular Motion of Optically Confined Marangoni Microswimmers

TL;DR

This study demonstrates how optically confined microswimmers at the air-water interface can synchronize their circular motion through Marangoni forces, revealing new collective behaviors influenced by particle interactions, shape, and confinement.

Contribution

We reveal a novel mechanism for collective synchronization of microswimmers driven by Marangoni forces, including a chiral mode that reverses direction periodically.

Findings

Synchronization occurs above a critical packing fraction of 0.25.

Chiral particle shapes induce periodic reversal of collective motion.

Different behaviors observed at varying confinement and activity levels.

Abstract

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 light-induced collective motion of thermocapillary microswimmers at the air-water interface. We found that Marangoni forces, which lead to long-ranged repulsive interparticle interactions, can cause microswimmers to synchronize their circular motion in a collective chase mode that resembles predator-prey behavior 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 in 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.