From Phase to Microphase Separation in Flocking Models: The Essential Role of Nonequilibrium Fluctuations

Alexandre P. Solon (1) , Hugues Chaté (2, 3, 4) , Julien Tailleur (1)

1 MSC - Matière et Systèmes Complexes
2 LPTMC - Laboratoire de Physique Théorique de la Matière Condensée
3 CSRC - Beijing Computational Science Research Center [Beijing]
4 SPEC - UMR3680 - Service de physique de l'état condensé

We show that the flocking transition in the Vicsek model is best understood as a liquid-gas transition, rather than an order-disorder one. The full phase separation observed in flocking models with Z 2 rotational symmetry is, however, replaced by a microphase separation leading to a smectic arrangement of traveling ordered bands. Remarkably, continuous deterministic descriptions do not account for this difference, which is only recovered at the fluctuating hydrodynamics level. Scalar and vectorial order parameters indeed produce different types of number fluctuations, which we show to be essential in selecting the inhomogeneous patterns. This highlights an unexpected role of fluctuations in the selection of flock shapes.

Domaines: Physique [physics]

Alexandre P. Solon, Hugues Chaté, Julien Tailleur. From Phase to Microphase Separation in Flocking Models: The Essential Role of Nonequilibrium Fluctuations. Physical Review Letters, 2015, 114 (6), pp.068101. ⟨10.1103/PhysRevLett.114.068101⟩. ⟨cea-01367258⟩