Rheology of bacterial suspensions under confinement

TL;DR

This study investigates how confinement affects the rheological properties of dilute bacterial suspensions, revealing a significant decrease in viscosity and elucidating microscopic dynamics through experiments and modeling.

Contribution

It provides the first systematic experimental analysis of bacterial suspension rheology under confinement and introduces a boundary layer model to explain observed behaviors.

Findings

Viscosity decreases with confinement scale smaller than bacterial run length.

Boundary layer of upstream swimming bacteria influences flow behavior.

Experimental results serve as benchmarks for active fluid rheology models.

Abstract

As a paradigmatic model of active fluids, bacterial suspensions show intriguing rheological responses drastically different from their counterpart colloidal suspensions. Although the flow of bulk bacterial suspensions has been extensively studied, the rheology of bacterial suspensions under confinement has not been experimentally explored. Here, using a microfluidic viscometer, we systematically measure the rheology of dilute E. coli suspensions under different degrees of confinement. Our study reveals a strong confinement effect: the viscosity of bacterial suspensions decreases substantially when the confinement scale is comparable or smaller than the run length of bacteria. Moreover, we also investigate the microscopic dynamics of bacterial suspensions including velocity profiles, bacterial density distributions and single bacterial dynamics in shear flows. These measurements allow us to construct a simple heuristic model based on the boundary layer of upstream swimming bacteria near confining walls, which qualitatively explains our experimental observations. Our study sheds light on the influence of the boundary layer of collective bacterial motions on the flow of confined bacterial suspensions. Our results provide a benchmark for testing different rheological models of active fluids and are useful for understanding the transport of microorganisms in confined geometries.