Dynamical Self-regulation in Self-propelled Particle Flows
  We study a continuum model of overdamped self-propelled particles with an aligning interaction in two dimensions. By combining analytical and numerical work, we map out the phase diagram for generic parameters. We find that the system self-organizes into two robust structures in different regions of parameter space: solitary waves of ordered swarms moving through a low density disordered background, and stationary asters. The self-regulating nature of the flow yields phase separation, ubiquitous in this class of systems, and controls the formation of solitary waves. Self-propulsion and the associated active convection play a crucial role in aster formation. A new result of our work is a phase diagram that displays these different regimes in a unified manner. 
Arvind Gopinath, Michael F. Hagan, M. Cristina Marchetti and Aparna   Baskaran
physics.bio-ph cond-mat.soft