  Suspensions of motile cells are model systems for understanding the unique
mechanical properties of living materials which often consist of ensembles of
self-propelled particles. We present here a quantitative comparison of theory
against experiment for the rheology of such suspensions. The influence of
motility on viscosities of cell suspensions is studied using a novel
acoustically-driven microfluidic capillary-breakup extensional rheometer.
Motility increases the extensional viscosity of suspensions of algal pullers,
but decreases it in the case of bacterial or sperm pushers. A recent model
[Saintillan, Phys. Rev. E, 2010, 81:56307] for dilute active suspensions is
extended to obtain predictions for higher concentrations, after independently
obtaining parameters such as swimming speeds and diffusivities. We show that
details of body and flagellar shape can significantly determine macroscale
rheological behaviour.
