Gregarious versus individualistic behavior in Vicsek swarms and the onset of first-order phase transitions

•We extend the standard Vicsek model to allow particles with individualistic behavior.•A relatively small probability of individualistic motion is sufficient to drive the system from order to disorder.•The new phase transition is discontinuous, as opposed to the Vicsek model’s noise-driven continuous phase transition.

The standard Vicsek model (SVM) is a minimal non-equilibrium model of self-propelled particles that appears to capture the essential ingredients of critical flocking phenomena. In the SVM, particles tend to align with each other and form ordered flocks of collective motion; however, perturbations controlled by a noise term lead to a noise-driven continuous order–disorder phase transition. In this work, we extend the SVM by introducing a parameter αα that allows particles to be individualistic instead of gregarious, i.e. to choose a direction of motion independently of their neighbors. By focusing on the small-noise regime, we show that a relatively small probability of individualistic motion (around 10%10%) is sufficient to drive the system from a Vicsek-like ordered phase to a disordered phase. Despite the fact that the αα-extended model preserves the O(n)O(n) symmetry and the interaction range, as well as the dimensionality of the underlying SVM, this novel phase transition is found to be discontinuous (first order), an intriguing manifestation of the richness of the non-equilibrium flocking/swarming phenomenon.