Cluster Dynamics in Macroscopic Photoactive Particles

We present an experimental study on the collective behavior of macroscopic self-propelled particles that are externally excited by light. This property allows for testing the system response to the excitation intensity in a very versatile manner. We discover that, for low excitation intensities, clustering at the boundaries is always present, even when this is prevented by implementing flower-shaped confining walls. For high excitation intensities, however, clusters are dissolved more or less easily depending on their size. Then, a thorough analysis of the cluster dynamics allows us to depict a phase diagram depending on the number of agents in the arena and the excitation intensity. To explain this, we introduce a simple kinetic model where cluster evolution is governed by a balance between adsorption and desorption processes. Interestingly, this simple model is able to reproduce the phase space observed experimentally.