Bacteria hinder large-scale transport and enhance small-scale mixing in time-periodic flows

TL;DR

This study investigates how bacteria in active fluids influence scalar mixing and transport, revealing that bacteria hinder large-scale transport but can enhance small-scale mixing, with implications for natural and industrial processes.

Contribution

It provides experimental and simulation evidence that bacterial activity reduces large-scale transport and flow chaoticity while increasing small-scale mixing in time-periodic flows.

Findings

Bacteria hinder large-scale transport and reduce overall mixing rate.

Bacterial activity attenuates fluid stretching and lowers flow chaoticity.

At small scales, bacterial activity enhances local mixing.

Abstract

Understanding mixing and transport of passive scalars in active fluids is important to many natural (e.g. algal blooms) and industrial (e.g. biofuel, vaccine production) processes. Here, we study the mixing of a passive scalar (dye) in dilute suspensions of swimming Escherichia coli in experiments using a two-dimensional (2D) time-periodic flow and in a simple simulation. Results show that the presence of bacteria hinders large scale transport and reduce overall mixing rate. Stretching fields, calculated from experimentally measured velocity fields, show that bacterial activity attenuates fluid stretching and lowers flow chaoticity. Simulations suggest that this attenuation may be attributed to a transient accumulation of bacteria along regions of high stretching. Spatial power spectra and correlation functions of dye concentration fields show that the transport of scalar variance across scales is also hindered by bacterial activity, resulting in an increase in average size and lifetime of structures. On the other hand, at small scales, activity seems to enhance local mixing. One piece of evidence is that the probability distribution of the spatial concentration gradients is nearly symmetric with a vanishing skewness. Overall, our results show that the coupling between activity and flow can lead to nontrivial effects on mixing and transport.