Packed swarms on dirt: two-dimensional incompressible flocks with quenched and annealed disorder

TL;DR

This paper demonstrates that two-dimensional incompressible polar active fluids can maintain an ordered, coherently moving phase despite quenched disorder, revealing a new universality class distinct from equilibrium ferromagnets.

Contribution

It introduces a new universality class for active fluids with quenched disorder and calculates its scaling laws using multiple renormalization group methods.

Findings

Ordered phase persists with quenched disorder in 2D

Scaling exponents are consistent across three RG schemes

Predictions applicable to biological and artificial active systems

Abstract

We show that incompressible polar active fluids can exhibit an ordered, coherently moving phase even in the presence of quenched disorder in two dimensions. Unlike such active fluids with annealed (i.e., time-dependent) disorder only, which behave like equilibrium ferromagnets with long-range interactions, this robustness against quenched disorder is a fundamentally non-equilibrium phenomenon. The ordered state belongs to a new universality class, whose scaling laws we calculate using three different renormalization group schemes, which all give scaling exponents within 0.02 of each other, indicating that our results are quite accurate. Our predictions can be quantitatively tested in readily available artificial active systems, and imply that biological systems such as cell layers can move coherently in vivo, where disorder is inevitable.