A continuum model for emergent self-propulsion and flocking through polymerization.

Intracellular actin polymerization is known to drive propulsion of some infectious bacteriae like Listeria. Such propulsion has also been reconstituted in-vitro, using soft oil-based droplets or polyestyrene beads coated by pVCA, which triggers actin polymerization, creating a polymer tail behind the propelling bead. We have now developed an experimental system to investigate the collective behavior of actin-propelled beads. We observe novel forms of emergent behaviors, including linear flocking of the beads. I will present a model to investigate the hypothesis that flocking results in part due to an effective attractive interaction between beads created by the asymmetric polymerization of actin on bead surfaces. We describe a simple reaction-diffusion model for this mechanism that reproduces an astonishing amount of the experimental phenomenology; surface polymerization, a polymer tail, bead self-propulsion, and finally linear flocking. The model predicts that the flocking can be enhanced or eliminated by tuning the length scale and depth of the monomer depletion zone, which is qualitatively consistent with the experimental results.