Title: Flocking by turning away

Speaker: Dr. Suchismita Das, (MPI-PKS Dresden)

Abstract: Flocking, as paradigmatically exemplified by birds, is the coherent collective motion of active agents. As originally conceived, flocking emerges through alignment interactions between the agents. Here, we report that flocking can also emerge through interactions that turn agents away from each other. Combining simulations, kinetic theory, and experiments, we demonstrate this mechanism of flocking in self-propelled Janus colloids with stronger repulsion on the front than on the rear. Unlike for alignment interactions, the emergence of polar order from turn-away interactions requires particle repulsion. The polar state is stable because particles achieve a compromise between turning away from left and right neighbors. At high particle concentration, the flocks form active Wigner crystals based on particle repulsion. These findings could help to reconcile the observations that cells can flock despite turning away from each other via contact inhibition of locomotion. Moreover, our results provide a robust strategy to design robots that can flock through disordered environments while avoiding collisions. Overall, our work shows that flocking is a very robust behavior that arises even when the orientational interactions would seem to prevent it.