Density Fluctuations and Energy Spectra of 3D Bacterial Suspensions

TL;DR

This study experimentally investigates density fluctuations and energy spectra in 3D bacterial suspensions, confirming theoretical predictions and revealing universal correlations in active turbulence across various concentrations and scales.

Contribution

It provides experimental validation of scaling laws in 3D active fluids and explores spectral properties and universal correlations in bacterial suspensions.

Findings

Verification of giant number fluctuation scaling law in 3D

Persistence of scaling behavior at small scales in low concentration suspensions

Identification of a universal density-independent correlation between density fluctuations and kinetic energy

Abstract

We experimentally study density fluctuations and energy spectra of bulk \textit{E. coli} suspensions of different concentrations. Our results verify the predicted scaling law of giant number fluctuations in three-dimensional (3D) wet active fluids. We find that such a scaling behavior persists at small scales even in low concentration suspensions well below the transition concentration to active turbulence. Our experiments also support the prediction on the energy spectra of dilute pusher swimmers and illustrate the spectral properties of the active turbulence of dense bacterial suspensions in the bulk limit. By comparing density fluctuations and kinetic energy in active turbulence, we identified a universal density-independent and scale-invariant correlation between the two quantities across a wide range of length scales.