Chaotic mixing in a biological active fluid

We study an active self-mixing fluid composed of biopolymers (microtubules) and molecular motors (kinesin). The kinesin motors are clustered together and crosslink bundles of microtubules. As the motors hydrolyze ATP, they walk along the microtubule bundles, forcing the bundles to extend away from each other. When confined in 2D at an oil-water interface, the network forms an active nematic with defects that are continuously created and annihilated. We consider these defects to be virtual stirring rods and the microtubule/kinesin system to be the the fluid. We use fluid dynamic concepts to characterize this new self-mixing fluid by measuring the mixing efficiency, or topological entropy, by coupling beads directly onto the microtubule bundles and tracking their motion as they are mixed. The separation between beads in the material is exponential. We use these trajectories to measure the rate of separation in the material and thereby to calculate the topological entropy. In addition, we change the local stretching rate by varying the ATP concentration to study how changing the energy input on the microscale changes the global mixing efficiency.