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Dynamics of Active Particles

Cécile Cottin-Bizonne Institut Lumière Matière, Université C. Bernard, Lyon

The behavior of self-propelled particles, that are intrinsically out of equilibrium, is rather fascinating and opens new questions. Here we study experimentally the dynamics of active colloids : from the behavior at low density up to crowded conditions. Our system is a monolayer of micron-size gold-platinum Janus particles in water. Upon addition of hydrogen peroxide, the colloids become self-propelled due to the different chemical gradient on both sides. If at low density they behave as perfect hot gas, at intermediate densities we observe a new phenomena : the formation of clusters. Measuring the aggregation and fragmentation rates we have rationalized the cluster size distribution and fluctuations. In the dense regime, we notice an unexpected non-monotonic behavior as the activity level rises up.

In a second part I will present experimental studies on the dynamics of interfacial swimmers : the spontaneous motion of camphor particles at a liquid-gas interface. Those Marangoni swimmers present complex problems starting with the simplest question of their swimming properties when symmetry breaking arises spontaneously rather than being encoded in the particle properties. I will discuss recent investigations we conducted on the dynamics of lonely camphor swimmer as well as on the original collective dynamics emerging from the interactions between many swimmers.