  The emergent dynamics in phase-separated mixtures of isometric active and
passive Brownian particles is studied numerically in two dimensions. A novel
steady-state of well-defined traveling fronts 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, strongly resembling traveling fronts in
reaction-diffusion systems. Above a threshold, the interface velocity decreases
linearly with increasing fraction of active particles.
