Motility-driven segregation, self-trapping and interfaces in active granular matter

We report unexpected states of organization in experiments and simulations on mixtures of motile fore-aft asymmetric rods and spherical beads. When the sum of the packing fractions of beads and rods exceeds about 0.7 the system spontaneously demixes into rod-rich and bead-rich regions. Crucially, the polar rods first flock without demixing, and then the aligned rods segregate from the beads. We present a coarse-grained theory of the instability leading to phase separation, through the interplay of the active forcing of the beads by the rods and the asymmetry of the steric interaction of the rod with the beads. We show further in experiments and simulations that in a suitably engineered channel geometry a small minority of motile rods can sequester and immobilize a bead-rich domain, and we present a theory of such self-jamming. In fully two-dimensional experiments at high total area fraction we show that the interface between bead-rich and bead-poor regions is decorated by rods pointing towards high bead density, and we measure the spatial power spectrum of interface height fluctuations.