Active Turbulence in Shear Thinning Fluid

TL;DR

The paper investigates how shear-thinning rheology in non-Newtonian fluids affects dense bacterial turbulence by comparing Newtonian Ficoll and shear-thinning Methocel environments in E. coli suspensions. It combines optical-flow–based flow characterization with a modified Resistive Force Theory to interpret energy transfer and viscosity anisotropy, revealing that shear-thinning effects are largely suppressed at high cell density due to inter-bacterial interactions disrupting polymer networks. A key finding is the nonmonotonic dependence of turbulence energy on Methocel content, and lower-density experiments validate the proposed density-competition mechanism. The study advances understanding of microbial dynamics in physiologically relevant complex fluids and offers a quantitative framework linking microscopic flagellar activity, anisotropic viscosity, and collective dynamics.

Abstract

The study of active matter system has critical importance in revealing the physical essence of biological collective behavior. Dense bacterial suspension - a typical biological active matter, exhibits a wide range of phenomenons, among which bacterial turbulence has received extensive interest in recent years. This seemingly chaotic motion is widely studied in Newtonian fluid. However, studies based on complex fluids have predominantly focused on viscoelastic effects, leaving the role of shear-thinning viscosity largely unexplored despite its prevalence in natural bacterial environments like mucus and gastric fluids. Here, we experimentally employed Ficoll and Methocel polymers to study the impacts of various viscosities by Newtonian fluid and shear-thinning effects by Non-Newtonian fluids on bacterial turbulence. We analyzed various physical properties, including energy, enstrophy, etc., and observed that the shear-thinning effect is significantly suppressed in high-concentration bacterial suspensions. While the ordered arrangement of polymer chains under shear flow leads to the microscopic anisotropic viscosity, the suppression is largely attributed to the disruption of polymer chains caused by strong inter bacterial interactions in dense suspensions. To validate this hypothesis, we conducted experiments at a lower bacterial concentration and verified the findings using theoretical calculations based on the modified Resistive Force Theory.

Figures (5)

• Figure 1: A) A brief schematic diagram of the experimental device and a detailed test fixture. The position of the objective is fixed throughout the experiments. B) Velocity flow field of the bacterial turbulence in pure buffer at a certain frame. The scale bar (white block) is $10 ~\mathrm{\mu m}$. C) Corresponding streamline and vorticity field of the velocity field in B).
• Figure 2: A) Normalized average speed of single swimmers in different polymer solutions. $\bar{v_b} = 18.1575 ~ \pm ~ 0.3364 ~\mathrm{\mu m /s}$. B) Trajectory of swimmers in Buffer, $7.5$% Ficoll solution and $0.25$% Methocel solution. The rougher trajectory of the bacteria is observed in Newtonian solution C) Averaged in-plane kinetic energy of bacterial turbulence normalized by the bacteria activity D) Energy of bacterial turbulence normalized by considering the bacteria averaged speed in polymer solution. All the error bars in this paper represent standard deviation.
• Figure 3: A) Shear rate magnitude $|\gamma|$ of bacterial turbulence in different polymer solutions. B), C) Ratio Length and Correlation time of bacterial turbulence in different polymer solutions. D) Normalized correlation time of bacterial turbulence in different polymer solutions. It is normalized by the energy and the turbulence structure size.
• Figure 4: The energy flux of the bacterial turbulence in different polymer solutions. The negative value of the energy flux indicates the energy is transferred from the small scale to the large scale. A) Ficoll solution. B) Methocel solution.
• Figure 5: A) Normalized energy at different bacteria densities under buffer and $0.25$% Methocel solution. B) Theoretical results of the bacteria swimming speed calculated by modified RFT. The light-colored points are the corresponding experimental results which are transferred to velocity units and normalized to start at $1$.