Physical Effects Controlling Flock Formation in Systems of Self-Propelled Rods

Abstract

Collective motion occurs in a wide range of biological settings, including flocks of birds, swimming bacteria, and cytoskeletal filaments. Motivated by swimming microorganisms and filament motion in a motility assay, we study a simple model of self-propelled rods using Brownian dynamics simulations and analytic theory. A flock is a group of neighboring rods that move collectively. Because the physical interactions between the rods are purely repulsive, the formation of flocks requires nonequilibrium driving. We characterize the physical interactions controlling flock stability, internal flock structure, and the distribution of flock sizes. Quantifying the dynamic phases of the flocking system through measurement of correlation functions, order parameters, and stress tensor gives insight into the origin of flocking in a simplified model system.