Colloquium Ehrenfestii 

Abstract:

I shall review the physics of suspensions of self-propelled particles, both synthetic and natural (= motile bacteria). Just as experiments on passive colloids by Perrin and others opened up the field of equilibrium statistical mechanics at the beginning of the 20th century, it is hoped that today's experiments on active colloids may similarly stimulate developments in the many-body physics of very far from equilibrium systems in the 21st century. After reviewing what is known about dilute (non-interacting) suspensions of synthetic active particles, I will show recent experimental results on phase separation and self assembly in suspensions interacting motile bacteria. We find that activity suppresses phase separation and leads to the formation of self-assembled rotors. Simple theory and model simulations provide insights into the underlying physics of these findings.