Density-dependent behavior of self-propelled particles at the air-water interface*

We explore the dynamics of active (self-propelled) particles at the millimeter scale using camphor-infused discs that glide at the air-water interface. Self-propulsion is generated by gradients in surface tension and particles are confined within a circular boundary. Novel dynamics emerge due to the long-range interactions between the particles and the boundary. Our macroscopic active system experiences active noise and significant inertial effects. We study the collective motion of the camphor discs using our combined experimental and theoretical approach, e.g., particle tracking, correlation functions, and the Langevin equation of motion. We find that complex dynamics emerge as a function of increasing particle density, including: a transition from ballistic to diffusive to caged motion; an overall shift from coherent vortex-like motion to randomness; and a bursting behavior reminiscent of stochastic synchronization.