Active phase separation by turning towards regions of higher density

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