Self-organized band state of driven hard needles*

Systems of driven particles display new nonequilibrium phases with self-organized structures. We study an active-matter model of self-propelled hard needles in two dimensions, containing interacting anisotropic Brownian particles with a non-conservative driving force along each rod's polar director. We present a dynamic density functional theory (DDFT) for this system derived from microscopic Langevin equations of driven rods with steric interactions. This model facilitates a full numerical solution of the equations, without requiring the approximations of hydrodynamic expansion. Comparison to Brownian dynamics particle simulations allows us to test the predictions of the DDFT model. We find that driving leads to a spatially inhomogeneous nematic band state that, in contrast to previous work, is stabilized by spatially varying polar order perpendicular to the band. This result challenges the assumption made in previous work that a self-propelled system with apolar interactions directly maps onto an active nematic. We discuss the implications for the microscopic mechanism of band stabilization.