Collective behavior of photoactive macroscopic particles: an experimental study of cluster dynamics

We present a system of photoactive macroscopic particles whose self-propulsion energy comes from a light source. By using a programmable LED panel, we have external control over the activity of the agents in space and time. This is common in microscopic systems, but novel in macroscopic ones. The macroscopic size of the agents and their interaction through collisions allows us to precisely adjust the experimental parameters, which is not easily guaranteed in cases with hydrodynamic and/or social interactions. As a result, our experiment can serve as a genuine limit case for more complex active systems.

We present the clustering behavior of these agents: by varying the particle activity and the population size, we observe power-law-like distributions of cluster properties and identify a transition from unstable to stable clustering. Lower particle activities and higher populations lead to stable clusters that persist over time, a transition that can be captured by a simple kinetic model.