Looking into the rheology active microtubule suspensions

Active suspensions are inherently out-of-equilibrium and can possess anomalous bulk rheological properties. Previous experimental and numerical studies suggest organisms with extensile swimming behavior (e.g. Escherichia coli) can decrease the apparent viscosity of a fluid, while those with contractile swimming behavior (e.g. Chlamydomonas reinhardtii) can increase the apparent viscosity of a fluid. In this talk we will present combined experimental results on the rheology and dynamics of an active suspension of microtubules and kinesin motors driven by ATP. We use a custom-built confocal rheometer to provide simultaneous macroscale rheological measurements and fluorescent imaging of local microtubule dynamics. We find increasing ATP concentration, and therefore increasing activity, yields a significant decrease in the apparent viscosity of the suspension. Simultaneously, using velocimetry techniques, we find significant increases in local velocity fluctuations and deformation rates, suggesting underlying microscale mechanisms for the observed macroscale rheology. We will present a simple model that captures the connection between the local mechanics and the global viscoelasticity.