Rheology of bacterial suspensions under confinement

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.

Swimming Suppresses Correlations in Dilute Suspensions of Pusher Microorganisms

“…Together with the definition of EðkÞ [Eq. (70)], this result implies that EðkÞ ∼ k 0 even for a single microswimmer; i.e., the dipolar field has a constant energy content at every scale.…”

“…Below the onset of collective motion, the mean-field kinetic theory predicts that the suspension is homogeneous and isotropic, as featureless as a suspension of noninteracting microswimmers. These assumptions are widely used when describing rheological properties of very dilute suspensions [60][61][62][63][64][65][66][67][68][69][70][71][72] and enhanced diffusivity of tracer particles [35,[73][74][75][76][77][78][79][80][81][82][83][84][85][86][87][88][89][90]. However, recent large-scale lattice-Boltzmann simulations of dipolar swimmers [42,45] reveal the presence of very strong correlations below the onset of collective motion.…”

Pressure-driven changes to spontaneous flow in active nematic liquid crystals

“…A number of authors have considered models of dilute suspension of active swimmers in a pressure gradient [25][26][27] and dense continuum models of channel flow, albeit in a tumbling nematic regime [28]. Others have reported experimental studies of similar systems [29], while there are also general reviews of the rheology of active fluids in [13,30,31]. In this paper we add to these previous investigations, considering a dense continuum model of an active fluid subjected to an external pressure gradient along a channel.…”

A combined rheometry and imaging study of viscosity reduction in bacterial suspensions

“…Symmetry again holds the key: Pullers are not predicted to show NVI, and, indeed, a positive viscosity increment was found in Chlamydomonas suspensions by using cone-plate rheometry (9). For pushers, NVI was inferred in Bacillus subtilis in nonrheometric geometries (10) and directly measured for E. coli in a microfluidic rheometer (11,12) and in a cylindrical Couette geometry (13). Throughout this work, viscosity refers to a global rheological measure of a sample's properties (sometimes known as the "apparent viscosity"); the local viscosity experienced by a bacterium is equal to the viscosity of the surrounding aqueous medium under all conditions: The drag from this unchanged local viscosity remains the source of flagellar propulsion.…”