Microparticle Separation Using Bulk Acoustic Waves in a PMMA Prism with a Tilted-Angle Channel
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Separation of microparticles without tagging or relying on charge, polarization or magnetic properties is important in many applications including biological and chemical analysis. Piezoelectric transducers can be used to create a standing acoustic wave field in a microfluidic channel, and these acoustic fields have been used to separate particles by size. In a standing acoustic wave field, particles experience a force towards the acoustic nodes which is proportional to the particle volume. This force is opposed by the drag force on the particle from the fluid. The drag force is proportional to particle radius. Consequently, larger particles move faster towards acoustic nodes than smaller particles. By applying these forces at an angle it is possible to separate particles by size in a continuous flow device. The tilted angle design enables particle separation of greater than a single wavelength. The overarching objective of the research was to sort particles by size. I developed standing surface acoustic wave (SSAW) devices on lithium niobate, and used PDMS (polydimethylsiloxane) as a microfluidic channel material. I used these devices to demonstrate particle concentration and understand the acoustic behavior of the devices and particle behavior in this acoustic environment. PDMS has the drawbacks of high attenuation, and difficulty in fabrication for large scale applications, so it is advantageous to use PMMA (Poly(methyl methacrylate)) as a channel material because of its mechanical robustness, its lower attenuation, and its suitability for manufacturing. Consequently, I developed SSAW devices with PMMA channels to understand this material. v I explored the idea of using an acoustic field, tilted (rotated, yawed) with respect to microfluidic flow, to separate particles by size in a SSAW device. Though effective, this separation scheme has limits on channel height (which reduces throughput), and limits the size range of particles that can be separated. To address the limitations of SAW (surface acoustic wave) tilted angle particle separation devices, I created a PMMA prism and excited bulk acoustic waves (BAW) using PZT (lead zirconate titanate) transducers through the PMMA into a tilted angle channel in the prism. I used microfluidic flow in this acoustic field to separate particles by size. The PMMA prism tilted angle channel design is easier to fabricate and more mechanically robust than a tilted angle SAW device. By analyzing particle trajectories using simulated and experimental results, I have determined how changing design and operation parameters of the separation device influences how particles move. Understanding the particle trajectories of different particle sizes allows us to choose design and operational parameters to enable particle separation. Particle separation in PMMA prism devices works, but a particle separation device should be optimized for a specific separation application for the most effective particle separation. My key contributions include: the tilted angle configuration, the PMMA prism and BAW configuration, and combining simulated and experimental results to generate phase diagrams characterizing particle trajectories.