New mechanism of motility-induced phase separation in active colloids*

At thermodynamic equilibrium, phase separation arises from attractive interparticle interactions. However, self-propelled particles can phase separate even if they have purely repulsive interactions. This phenomenon, called motility-induced phase separation (MIPS), has become a landmark of active matter physics. The conventional mechanism of MIPS is the decrease of particle speed due to repulsive interactions in high-density regions, which leads to further accumulation of particles. In this talk, I will demonstrate a new mechanism of MIPS, which instead of relying on a slowdown of particle motion with density, is based on interaction torques that reorient particles toward high-density regions. We show that such torques take place in suspensions of Janus colloids driven by an electric field. From the electrostatic interactions between the particles, we derive hydrodynamic equations that show how MIPS arises from orientational interactions in this system. Furthermore, we predict that, in contrast to the repulsion-based MIPS scenario, the phase diagram of torque-based MIPS exhibits reentrance, with the system reentering the uniform phase at high self-propulsion speed.