Phase separation in ordered polar active fluids: A new Universality class

2024/01/11 by Miller, Maxx, Toner, John
#FOS: Physical sciences #Soft Condensed Matter (cond-mat.soft)

paper · doi:10.48550/arxiv.2401.05996

Abstract

We show that phase separation in ordered polar active fluids belongs to a new universality class. This describes large collections of self-propelled entities (``flocks"), all spontaneously moving in the same direction, in which attractive interactions (which can be caused by, e.g., autochemotaxis) cause phase separation: the system spontaneously separates into a high density band and a low density band, moving parallel to each other, and to the direction of mean flock motion, at different speeds. The upper critical dimension for this transition is d_c=5, in contrast to the well-known d_c=4 of equilibrium phase separation. We obtain the large-distance, long-time scaling laws of the velocity and density fluctuations, which are characterized by universal critical correlation length and order parameter exponents ν_⊥, ν_∥ and β respectively. We calculate these to O (ε) in a d=5-ε expansion.

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