  Studies of active matter, from molecular assemblies to animal groups, have
revealed two broad classes of behavior: a tendency to align yields
orientational order and collective motion, whereas particle repulsion leads to
self-trapping and motility-induced phase separation. Here, we report a third
class of behavior: orientational interactions that produce active phase
separation. Combining theory and experiments on self-propelled Janus colloids,
we show that stronger repulsion on the rear than on the front of these
particles produces non-reciprocal torques that reorient particle motion toward
high-density regions. Particles thus self-propel toward crowded areas, which
leads to phase separation. Clusters remain fluid and exhibit fast particle
turnover, in contrast to the jammed clusters that typically arise from
self-trapping, and interfaces are sufficiently wide that they span entire
clusters. Overall, our work identifies a torque-based mechanism for phase
separation in active fluids, and our theory predicts that these orientational
interactions yield coexisting phases that lack internal orientational order.
