Soft active (or living) matter is a particular class of a nonequilibrium systems. Active systems are driven out of equilibium on a microscopic scale and often gives rise to unexpected emergent behavior and unusual material properties. Examples are suspensions of motile microorganisms or synthetic microswimmers, motility assays or cell tissues. The simplest model for an active system is a suspension of active Brownian particles with short-range repulsive interactions. Strikingly, such systems exhibit clustering and motility-induced phase separation even in the absence of attraction and aligning interactions.

Even more interesting are mixtures of active and passive particles, which provide, for example, an novel route for switchable self-assembly. We focus on the collective dynamics of such mixtures over a wide composition range. A novel steady-state of well-defined propagating interfaces is observed, where the interface between the dense and the dilute phase propagates and the bulk of both phases is (nearly) at rest. Two kind of interfaces, advancing and receding, are formed by spontaneous symmetry breaking, induced by an instability of a planar interface due to the formation of localized vortices. The propagation arises due to flux imbalance at the interface, resembling the growth behavior of rough surfaces far from equilibrium.