Effective viscosity of microswimmer suspensions

TL;DR

This paper investigates how the collective motility of swimming microalgae affects the suspension's viscosity, revealing shear thinning behavior and linking microscopic orientation to macroscopic rheology.

Contribution

It provides experimental measurements of effective viscosity in live microalgae suspensions and relates these to cell orientation under flow, advancing understanding of active suspension rheology.

Findings

Live microalgae suspensions have higher viscosity than dead cells at same volume fraction.

Suspensions exhibit shear thinning behavior under flow.

Cell orientation influences the macroscopic rheological properties.

Abstract

The measurement of a quantitative and macroscopic parameter to estimate the global motility of a large population of swimming biological cells is a challenge Experiments on the rheology of active suspensions have been performed. Effective viscosity of sheared suspensions of live unicellular motile micro-algae (\textit{Chlamydomonas Reinhardtii}) is far greater than for suspensions containing the same volume fraction of dead cells and suspensions show shear thinning behaviour. We relate these macroscopic measurements to the orientation of individual swimming cells under flow and discuss our results in the light of several existing models.