  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. The polar state is stable because
particles achieve a compromise between turning away from left and right
neighbors. Unlike for alignment interactions, the emergence of polar order from
turn-away interactions requires particle repulsion. At high concentration,
repulsion produces flocking Wigner crystals. Whereas repulsion often leads to
motility-induced phase separation of active particles, here it combines with
turn-away torques to produce flocking. Therefore, our findings bridge the
classes of aligning and non-aligning active matter. Our results could help to
reconcile the observations that cells can flock despite turning away from each
other via contact inhibition of locomotion. Overall, our work shows that
flocking is a very robust phenomenon that arises even when the orientational
interactions would seem to prevent it.
