Exploiting the Interplay of Acoustic Waves and Fluid Motion for Particle Manipulation

Abstract

Acoustofluidics is an emerging research field that combines both acoustics and fluid dynamics. With acoustic tweezers technique being developed for years, it is featured for its contactless, noninvasive, and biocompatibility which makes the method suitable for various applications in the field of biology, material sciences, and chemistry. Especially when handling small objects, e.g., cells, nanoparticles, C. elegans, and zebrafish larvae, the native environment involved is mainly liquid. During the acoustic propagation inside the liquid, fluid motion will also be initiated and will influence the object movement in addition to the acoustic radiation forces. This brings up the multidisciplinary study combining the acoustic wave and fluid motion for object manipulation within liquids. This technical development has revealed huge potential for applying acoustofluidic studies into different applications. However, there are still several technical bottlenecks that must be overcome for acoustofluidic technology to provide maximum impact. For example, cell patterning using standing acoustic waves commonly has the regular grid-like shape and sees the fluid motion as an unwanted side effect without an effective way to minimize it. The current target particle size that can be controlled using acoustics is between ~mm to µm, thus hindering the exploration of nanoscale objects. In this dissertation, I explored the combined effect of acoustics and fluid dynamics, and validated that the interplay of both effects can derive new research insights and can be applied to particles with a smaller size range (i.e., nanometer). Specifically, I studied the synergetic effect of acoustics and flow in three classic fluid systems: bulk fluids, droplets, and continuous flow. For bulk fluids, we designed an acoustofluidic holography platform that can initiate and utilize fluid motion with arbitrary designed acoustic fields. With the design and implementation of the holographic acoustic lens, our method can pattern cells into an arbitrary shape that can potentially benefit tissue engineering or cell mechanics studies. Besides patterning, we also demonstrated that, with the same experimental configuration, we can utilize vortex acoustic streaming to achieve different functions, e.g., particle rotation, concentration, and separation. For droplets, we observed a new physics phenomenon which can drive the spin of a liquid droplet using surface acoustic wave. With this external angular momentum and Stokes drift effect, we found the nanoparticles can be rapidly concentrated or differentially concentrated in one spinning droplet. Furthermore, we demonstrated that the single spinning droplet can serve as one unit that possesses a specific function and we can assemble the units for a more flexible manipulation function. We built a dual droplet acoustofluidic centrifuge system that can achieve nanoparticle separation and transport and utilized the platform for exosome subgroup separation. For continuous flow, as acoustic separation technique has been developed for years, we have explored two directions that may be utilized for small animal blood apheresis study. One direction is the high-throughput platelet separation using a plastic device. This method significantly increased the throughput and moved one step towards clinical usage. Another direction is building the integrated system for plasma separation. Built around the surface acoustic wave separator, we assembled the fluid driving unit, temperature control unit, and separation unit into a prototype-like system. We then performed the proof-of-concept experiment to identify the feasibility of applying the acoustofluidic separation method to small animal models (i.e., mice).