Dynamics and rupture of doped motility induced phase separation

Adding a small amount of passive (Brownian) particles to a two-dimensional dense suspension of repulsive active Brownian particles does not affect the appearance of a motility-induced phase separation into a dense and a dilute phase, caused by the persistence of the active particles' direction of motion. Unlike a purely active suspension, the dense slab formed in an elongated system of a passive-active mixture may show, over long periods of time, a stable and well-defined propagation of the interfaces, because of the symmetry breaking caused by the depletion of passive particles on one side of the slab. We investigate these dynamical structures via average density profile calculations, revealing an asymmetry between the two interfaces, and enabling a kinetic analysis of the slab movement. The apparent movement of the dense slab is not a pure source/sink effect, nor a rigid displacement of all the particles, but a self-sustained combination of both effects. Furthermore, we analyse the specific fluctuations that produce, cancel and abruptly reverse the slab motion.