Effective viscosity of actively swimming algae suspensions

Suspensions of actively swimming microorganisms exhibit an effective viscosity which may depend on volume fraction, cell shape, and the nature of locomotion (e.g. ``pushers'' vs. ``pullers''). Here we report experimental measurements of shear viscosity for suspensions of unicellular green algae (\textit{Dunaliella primolecta}, a biflagellated ``puller''). We use a cone-and-plate rheometer to measure the dynamic shear viscosity for both motile and non-motile suspensions of \textit{D. primolecta}. Viscosity increases with concentration for both cases, but the active suspensions of ``pullers'' have a comparatively lower effective viscosity than passive suspensions. This observation contrasts recently proposed theories which predict that ``pullers'' should instead have a higher viscosity than non-motile suspensions. Additionally, we observe shear-induced migration of active suspensions and consider its impact on the resulting effective shear viscosity.