Studies of active matter, from molecular assemblies to animal groups, have revealed two broad classes of behaviour: 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 behaviour: 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 towards high-density regions. Particles thus self-propel towards 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.

对从分子组装到动物群的活性物质的研究揭示了两大类行为：对齐的趋势产生定向顺序和集体运动，而粒子排斥导致自我捕获和运动诱导的相分离。在这里，我们报告了第三类行为：产生主动相分离的定向相互作用。结合自行推进的 Janus 胶体的理论和实验，我们表明，这些粒子的后部比前部更强的排斥力会产生非互易扭矩，从而将粒子运动重新定向到高密度区域。因此，粒子会自行向拥挤区域推进，从而导致相分离。簇保持流动并表现出快速的粒子周转，与通常由自我捕获引起的阻塞簇相比，并且接口足够宽，可以跨越整个集群。总的来说，我们的工作确定了一种基于扭矩的活性流体相分离机制，我们的理论预测这些取向相互作用会产生缺乏内部取向顺序的共存相。