Steering toward the crowd: Orientational interactions induce active phase separation

Motile agents can condense even if they do not attract each other. This phenomenon, known as motility-induced phase separation (MIPS), has been seen to arise from central repulsive forces: self-propelled particles collide with one another, slow down, and self-trap in dense regions. In this talk, I will present a new mechanism for MIPS, which is based not on central forces but on torques. I will show that self-propelled Janus particles interact via non-reciprocal torques that reorient their motion toward high-density regions. Hence, particles self-propel up their own density gradient – a behavior dubbed autotaxis – which drives phase separation. Because this torque-based aggregation requires no self-trapping, particles in clusters retain substantial speed, and clusters remain dilute and fluid. Overall, our work establishes that orientational interactions (torques) can yield coexisting phases with no internal orientational order.