Intrinsic viscosity of actively swimming microalgae 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''). Although several dilute-regime theories have been offered for active suspensions, no experimental study to our knowledge has been able to resolve the dilute-regime intrinsic viscosity of actively swimming microorganism suspensions. Here we use a cone-and-plate rheometer to experimentally measure the dynamic shear viscosity for motile and non-motile suspensions of unicellular green algae (Dunaliella primolecta, a biflagellated ``puller''). The low viscosity biological samples require careful experimental protocols to avoid settling and flow-induced migration, and to minimize precision error. With these protocols in place we can distinguish the intrinsic viscosity which we show is higher for the motile ``puller'' swimmers compared to the immobilized counterparts. This observation is consistent with recently proposed dilute-regime theories which predict that ``pullers'' should have a higher viscosity than non-motile suspensions.