Engineering micro-vortex streaming via acoustofluidics
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Acoustofluidic technologies, the integration of acoustics into microfluidics, offer rich possibilities for particle manipulation in life sciences. One promising aspect of these technologies is acoustic micro-vortex steaming, resulting from the energy dissipation of acoustics into fluids. There are two opposing directions for the development of micro-vortex steaming: the first one is increasing rotational flow to enhance microscale fluid motion for laminar fluid mixing and capture of biological particles; the other is suppressing rotational flow to create stable acoustic pressure fields for particle patterning, deflection and separation. Although these developments have demonstrated success in microfluidic mixing and cell separation, their ability to realize nanoparticle separation and precisely control fluid mixing, particularly for viscous samples, multi-fluids, and sequential fluids, remains limited. In this dissertation, we target at expanding micro-vortex streaming based acoustofluidic technologies by addressing the existed technological hurdles and introducing new physical concept of topological insulator. To this end, we first developed a sharp edge based acoustofluidic micromixer that enables robust and strong mixing. Robust, efficient, and strong mixing in microfluidics is essential to viscous biological sample preparation. Inspired by the concept that the energy band of phononic crystals depends strongly on their structure height and substrate thickness, we maximized micro-vortex streaming via rational design of the microchamber and glass substrate thickness. The device is able to not only mix fluids across a wide range of flow rates up to 150 µL min-1, but also process fluids with viscosities to 95.9 mPa.s. Using this strong micro-vortex streaming, we were able to realize on-chip liquefaction for human stool samples. This device provides a promising platform to be integrated with portable stool diagnostics. Next, we developed a sharp edge based acoustofluidic micromixer capable of achieving rapid, multi-fluid (≥2) and multi-step (≥2) mixing, which is difficult to realize in hydrodynamic fluid focusing method. Rapid, multi-step and multi-fluid mixing is critical to nanomaterial synthesis for drug delivery. These novel capabilities are realized simply by varying the strength and sites of micro-vortex streaming. With this platform, we synthesized homogeneous poly(lactide-co-glycolide)-block-poly (ethylene glycol) (PLGA-PEG) nanoparticles by rapid mixing, high-molecular PLGA-PEG nanoparticles by strong mixing, PLGA-lipid core-shell nanoparticles by two-step mixing and chitosan nanoparticles by three-fluid mixing. When combined with varying flow rates and reagent concentrations, the acoustofluidic platform allows for nanoparticle synthesis with unprecedented control of nanoparticle size and structure. We have also developed a surface acoustic wave (SAW)-based, disposable acoustofluidic platform for bacteria separation by suppressing micro-vortex streaming and enhancing acoustic pressure field. SAW induced micro-vortex streaming is generated by the viscous attenuation of SAW propagation. In SAW-based devices, the acoustic steaming competes with acoustic radiation force. To overcome SAW induced steaming, we generated standing SAW to form time-averaged momentum flux in opposite direction and then cancelling it out. To increase acoustic radiation force, we designed unidirectional transducers that enable SAW to propagate primarily in one direction, thus tremendously increasing acoustic energy intensity. Using this device, we were able to pattern 400 nm polystyrene particles within the disposable microchannel, as well as separate 600 nm silicon dioxide and 200 nm silver nanoparticles from 1 µm polystyrene particles. Additionally, our disposable device achieved high-purity separation of bacteria from human red blood cells (RBCs). This method of unidirectional transducer design provides a way of enhancing acoustic radiation force to suppress acoustic streaming. Finally, we developed a valley hall based topological acoustofluidic device with the characteristic of chiral micro-vortex streaming by introducing the concept of topological insulator. Topological insulators, which originate from condensed matter physics, have recently been exploited for unconventional wave propagation. One of the prominent features of valley-hall based topological insulators is chiral vortex feature of energy flux. By electroplating copper micropillars on a lithium niobate substrate, we established hexagonal latticed copper pillars with valley hall effect in microfluidics, where SAW was utilized as excitation source. We numerically and experimentally demonstrated clockwise and anticlockwise of vortex streaming by tracing 200 nm fluorescent polystyrene particles. To our knowledge, this is the first visualization of chiral vortex feature in topological insulators. Within the microfluidic community, this allows for novel functionalities including unidirectional particle rotation and back-movement immune particle transport. In the topological physics space, the liquid domain within microfluidic devices enables a new technology for characterizing topological spin textures, which is difficult to be realized in solid or air domain. Furthermore, multiphysics nature of the system enriches the physics of topological insulators. Therefore, the topological acoustofluidics developed here expands not only the field of microfluidics but also the field of topological insulators. In summary, this dissertation serves to further the knowledge of micro-vortex streaming along three planes: enhancing micro-vortex streaming, suppressing micro-vortex streaming, and introducing topological physics to generate chiral micro-vortex streaming. Finally, I will provide my perspective for the next-generation development of micro-vortex streaming based technologies and possible emerging applications. I hope that eventually the microfluidic and physical communities can benefit from each other.
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