Effect of particle concentration on the persistence of motion in active matter systems

We studied the transition from persistent to diffusive motion when varying particle concentrations in two different 2D active and macroscopic systems. The first one comprises magnetic spherical steel particles under an alternating magnetic field, which turns them into self-propelled stochastic particles. The second system is composed of a set of Hexbug-nano™ robots. Each of those tiny robots has an internal motor that, in combination with its elongated and inclined shape and the flexibility of its legs, provides it with persistent motion. We discovered that both systems similarly behave when varying the particle concentration and the available area for motion. Our results show that particle motions exhibit persistence when the concentration of active particles is low. As the particle concentration increases, until a threshold, the interactions between them grow in number, forcing them to change their direction more frequently and, turning diffusive their behaviour. New emergent phenomena are observed when a high particle concentration regime is explored. The formation of aggregates is observed, and the free particles consequently move in open spaces with less particle concentration recovering their persistent motion. The collective synchronization of particle motion is observed in a range of particle concentrations.