Anomalous fluctuations in homogeneous fluid phase of active Brownian particles

TL;DR

This study reveals giant number fluctuations in the homogeneous fluid phase of active Brownian particles without polar order, driven by velocity correlations, and explores their theoretical and numerical aspects.

Contribution

It demonstrates the presence of giant number fluctuations in the disordered fluid phase of active Brownian particles and introduces a linearized hydrodynamic theory alongside numerical scaling analysis.

Findings

GNF occur in the fluid phase of ABP without polar order

Velocity correlations extend over a large but finite length

Numerical scaling relation for density correlation function

Abstract

Giant number fluctuations (GNF) are an anomaly universally observed in active fluids with polar or nematic order. In this paper, we show that GNF arise in the fluid phase of active Brownian particles (ABP), where the polar order is absent. GNF in ABP extends over a large but finite length which characterizes the growing velocity correlations. To suppress unwanted phase separation and allow ones to explore the disordered fluid phase at large activities, we impart the inertia, or the mass, to the ABP. A linearized hydrodynamic theory captures our findings, but only qualitatively. We find numerically a nontrivial scaling relation for the density correlation function, which the linearized theory cannot explain. The results suggest ubiquitousness of the anomalous fluctuations even in the disordered homogeneous fluid phase in the absence of the directional order.