Characterizing chaotic mixing in a biological active nematic*

Active fluids represent an emerging field of soft matter in which the fluid’s constituent particles are not in equilibrium, instead they consume energy and move collectively with unusual dynamics to produce spontaneous chaotic mixing. We study a biological active fluid composed of semi-flexible biopolymers (microtubules), and clusters of molecular motors (kinesin). In this system, the microtubules are bundled together and crosslinked by the kinesin clusters. As the kinesin motors walk along the filaments, the bundles extend from each other and will bend, buckle, and fracture. When confined in 2D at an oil-water interface, the active network behaves as an extensile active nematic. We use fluid dynamic concepts to quantify the mixing efficiency in this active fluid.  Beads are directly coupled to the microtubule bundles and we track their motion during mixing. Bead trajectories are used to measure the local rate of stretching in the fluid and extract the (local) Lyapunov exponent. The rate of local extension can be varied by changing ATP concentration and observing the effect on the Lyapunov exponents.