Rheology and dynamics of active microtubule suspensions

Biofilm formation, mammalian reproduction, and bacterial infection are ubiquitous and practical examples of suspensions containing self-driven particles. These 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. Here, we systematically explore 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 increase 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.