Spontaneous mirror-symmetry breaking induces inverse energy cascade in 3D active fluids

TL;DR

This paper demonstrates that mirror-symmetry breaking in 3D active fluids can induce an inverse energy cascade, a phenomenon previously thought impossible in 3D turbulence, with implications for biological and engineered systems.

Contribution

It provides the first analytical and numerical evidence of an inverse energy cascade in 3D active fluids caused by spontaneous mirror-symmetry breaking.

Findings

Inverse cascade observed in microbial suspension flows

Mirror-symmetry breaking controls energy transfer direction

Active scale selection leads to Beltrami flows

Abstract

Classical turbulence theory assumes that energy transport in a 3D turbulent flow proceeds through a Richardson cascade whereby larger vortices successively decay into smaller ones. By contrast, an additional inverse cascade characterized by vortex growth exists in 2D fluids and gases, with profound implications for meteorological flows and fluid mixing. The possibility of a helicity-driven inverse cascade in 3D fluids had been rejected in the 1970s based on equilibrium-thermodynamic arguments. Recently, however, it was proposed that certain symmetry-breaking processes could potentially trigger a 3D inverse cascade, but no physical system exhibiting this phenomenon has been identified to date. Here, we present analytical and numerical evidence for the existence of an inverse energy cascade in an experimentally validated 3D active fluid model, describing microbial suspension flows that spontaneously break mirror symmetry. We show analytically that self-organized scale selection, a generic feature of many biological and engineered nonequilibrium fluids, can generate parity-violating Beltrami flows. Our simulations further demonstrate how active scale selection controls mirror-symmetry breaking and the emergence of a 3D inverse cascade.