Collective Behavior of Motile Rods in Active Nematics

Using numerical simulation of a vibrated granular system, we investigate the collective dynamics of polar rods immersed in a monolayer of apolar rods. The monolayer of apolar rods behaves as an active nematic medium at high concentration with the presence of topological defects, while remaining isotropic at low concentration. The embedded polar rods introduce local polarity and motility into the system. Our results reveal that the polar rods undergo a phase transition from a homogeneous polar state to a phase-separated flocking regime, characterized by the emergence of elongated flocks moving in uncorrelated, random directions as the concentration of the medium increases, reminiscent of flocking observed in birds, fish schools, or bacterial colonies. Furthermore, when intrinsic angular noise is introduced, the flocks transform into a homogeneous polar phase, which subsequently transitions into a homogeneous disordered state as the noise strength is further increases.