Collective Dynamics of Deformable Self-Propelled Particles with   Repulsive Interaction
  We investigate dynamics of deformable self-propelled particles with a repulsive interaction whose magnitude depends on the relative direction of elongation of a pair of particles. A collective motion of the particles appears in two dimensions. However this ordered state becomes unstable when the particle density exceeds a certain critical threshold and the dynamics becomes disorder. We show by a mean field analysis that this novel transition characteristic to deformability occurs due to a saddle-node bifurcation. 
Yu Itino, Takahiro Ohkuma and Takao Ohta
cond-mat.soft cond-mat.stat-mech