Phase transition to large scale coherent structures in 2d active matter turbulence

TL;DR

This paper investigates how microswimmer suspensions in 2D lead to a phase transition resulting in large-scale vortex structures, bridging bacterial turbulence and hydrodynamic turbulence through a subcritical transition.

Contribution

It demonstrates a sharp phase transition to large-scale vortex formation in 2D active matter turbulence using a modified Navier-Stokes model.

Findings

Sharp transition to large-scale vortices

Separation of bacterial and hydrodynamic turbulence

Inverse energy cascade driven by enstrophy conservation

Abstract

The collective motion of microswimmers in suspensions induce patterns of vortices on scales that are much larger than the characteristic size of a microswimmer, attaining a state called bacterial turbulence. Hydrodynamic turbulence acts on even larger scales and is dominated by inertial transport of energy. Using an established modification of the Navier-Stokes equation that accounts for the small scale forcing of hydrodynamic flow by microswimmers, we study the properties of a dense supensions of microswimmers in two dimensions, where the conservation of enstrophy can drive an inverse cascade through which energy is accumulated on the largest scales. We find that the dynamical and statistical properties of the flow show a sharp transition to the formation of vortices at the largest length scale. The results show that 2d bacterial and hydrodynamic turbulence are separated by a subcritical phase transition.