The kinematics of active fluid mixing in micron-scale systems

Chen, Yen-Chen

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The kinematics of active fluid mixing in micron-scale systems

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Microtubule-based active fluid generates turbulences in micron-scale systems by overcoming the constraints from the confinement boundary. However, how the active turbulences facilitate mixing remains poorly understood. I used a combined experimental and modeling approach to demonstrate that the active fluid can enhance the mixing performances, including the vorticity generation, the mixing size, and the stretching rate, in micron-scale systems, when compared to the passive fluid. The experiments first suggested that active vortices originate from the nonuniform spatial distribution of microtubule bundle orientations based on existing active nemato-hydrodynamic equations. In addition, the active fluid’s activity level was shown to enhance the stretching rate, while decreasing the mixing size. To simultaneously improve both parameters, I applied a dynamical boundary condition to drive the fluid confined in square cavities. At sufficiently large driving speeds, a boundary-driven vortex is formed and merges with all local active vortices to become an enlarged cavity-sized vortex, which simultaneously increases the mixing size and the stretching rate. Furthermore, larger boundary moving speeds yield larger advantages for both mixing performances over the passive fluid. If the materials are shear-sensitive, then the interfacial flow coupling can be applied to improve the mixing performances which require the confinement of the active fluid into a cylinder-like water-in-oil droplet. This geometry allows for the active fluid to self-organize from chaotic flows into a dominant intra-droplet vortex, thus increasing the mixing size. Additionally, there exists a millimeter-scale interfacial coupling length, within which configurations in oil can affect the intra-droplet vortex formation, thus controlling the mixing size without making physical contact with the droplet. This allowed me to develop two milli-fluidic devices that can spontaneously control the intra-droplet mixing sizes: one by locally deforming the droplet and the other by tuning the thickness of the surrounding oil layer without touching the droplet. My work highlights the underlying principles of enhancing the mixing performances of the active fluid in micron-scale systems.

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