Emergence of Local Ordering and Mesoscale Giant Number Fluctuations in Active Turbulence

TL;DR

This paper investigates how increasing activity in dense active suspensions leads to local ordering and giant number fluctuations, revealing a transition to a mixed state with vortices and chaotic regions.

Contribution

It introduces a generalized hydrodynamic model capturing the emergence of mesoscale structures and giant fluctuations in active turbulence, unifying these phenomena with an energy-based order parameter.

Findings

Formation of intense vortices at high activity

Presence of giant number fluctuations at mesoscale

Development of a bimodal velocity distribution

Abstract

We study spatiotemporal chaos in two-dimensional dense active suspensions using a generalized hydrodynamic model. Increasing activity induces a structural transition marked by the formation of intense vortices and giant number fluctuations at the mesoscale. The flow self-organizes into locally polar-ordered regions coexisting with chaotic domains, producing a bimodal velocity distribution and enhanced correlations. This mixed-state morphology underlies the universal statistical behavior observed beyond a critical activity threshold. Reducing the instability timescale yields similar transitions, showing that both activity and instability act as control parameters for pattern formation. An energy-based order parameter derived from the system's budget quantifies and unifies these structural transitions across the phase space of activity and instability timescales.