Self-organization and mixing of microtubule-kinesin active fluid in an activity gradient

Active fluid, composed of kinesin-driven extensile bundles of microtubules, consumes ATP locally to create a self-mixing flow. Mean speed of microtubule-kinesin active fluid was shown to be tunable by varying its components’ concentrations. Such tunability demonstrated the controllability of active fluid with uniform activity. However, how active fluid self-organizes when its activity is non-uniform remains poorly understood. Here, we characterized active fluid behavior and its associated mixing performance in an activity gradient. The activity gradient was created by imposing a temperature gradient because our previous work showed that microtubule-kinesin active fluid exhibited an Arrhenius response to temperature: Increasing temperature sped up active fluid flow, and thus, along a temperature gradient, active fluid flowed faster on one side and slower on the other, forming an activity gradient. We characterized how such a gradient influenced the mixing performance of active fluid in terms of mixing efficiency, stretching rate, and mean squared displacement, comparing with an activity-uniform sample. Our work suggests that applying an activity gradient can serve as a new in-situ method for controlling self-organization and mixing performance of microtubule-kinesin active fluid.