  Active fluids have applications in micromixing, but little is known about the
mixing kinematics of systems with spatiotemporally-varying activity. To
investigate, UV-activated caged ATP was used to activate controlled regions of
microtubule-kinesin active fluid and the mixing process was observed with
fluorescent tracers and molecular dyes. At low P\'eclet numbers (diffusive
transport), the active-inactive interface progressed toward the inactive area
in a diffusion-like manner that was described by a simple model combining
diffusion with Michaelis-Menten kinetics. At high P\'eclet numbers (convective
transport), the active-inactive interface progressed in a superdiffusion-like
manner that was qualitatively captured by an active-fluid hydrodynamic model
coupled to ATP transport. Results showed that active fluid mixing involves
complex coupling between distribution of active stress and active transport of
ATP and reduces mixing time for suspended components with decreased impact of
initial component distribution. This work will inform application of active
fluids to promote micromixing in microfluidic devices.
