Flocking in Polymerization-Driven Active Matter

Active materials are dense systems composed of motile building blocks that consume energy on a small system scale, often leading to emergent pattern formation on a larger scale. Cells are active, out-of-equilibrium environments that have served as inspiration for the design of various model active systems. While most in vitro reconstituted system use molecular motors as force producers, cells also polymerize biopolymers to generate forces needed to move and divide. Here, we utilize the force-generating capabilities of actin networks to construct a polymerization-driven active material that exhibits rich emergent phenomenology. We mimic the behavior of Listeria – a pathogen that hijacks the cytoskeletal actin of a host cell – to produce the motility of polystyrene microspheres using a set of purified proteins. Beads propel themselves through solution and interact with one another, giving rise to persistent flocks of many beads. We provide evidence that interactions are mediated by the reaction-diffusion dynamics of biochemical components in the system through a combination of experimental techniques and theoretical simulations. From studying these phenomena we glean insight into the link between reaction-diffusion and collective behavior as well as the reorganization and dynamics of branched actin networks.