Microfluidic handling of particles and cells plays a critical role in fundamental biological analyses and biomedical diagnostics applications. The field of acoustofluidics has garnered recent interest by merit of being contactless, label-free, pollution-free, low power (compared to typical optical tweezers), biocompatible, and widely applicable. Traditionally, acoustic tweezers have been based on single-use surface acoustic wave (SAW) devices where the SAW generated on the piezoelectric SAW substrate interacts directly with the sample under manipulation. However, the price of piezoelectric SAW substrates (e.g., LiNbO₃) is still rather costly, which poses a significant challenge to lowering costs while also avoiding biological cross-contamination by disposing of part of the device. Therefore, there has been an interest to develop two-chip acoustofluidic platforms comprising of a reusable SAW substrate and a disposable superstrate that is in contact with the sample to be manipulated. This thesis describes the application of acoustofluidic techniques in a two-chip format for two different functions: (1) acoustic micro-centrifugation for particle concentration and separation in a sessile droplet and (2) patterning and moving particles and cells in two orthogonal directions within a closed microchamber.

The concentration of micro to nano bioparticles is an indispensable step for sample preparation in many microfluidic devices. This thesis advances the state of art currently based on simple plain superstrates by investigating the performance of droplet micro-centrifugation on a complex surface micromachined silicon (SMS) superstrate comprising multiple deposited and patterned thin film layers that are meant to mimic a generic microfabricated sensing device. To advance methods to shape waves more efficiently and reliably on a chip to promote micro-centrifugation, we describe the use of low-cost (compared to microfabrication) laser-cut patterned superstrates that enable particle concentration over a wide range of operating frequencies to cover a wide range of particle sizes (several microns to sub-microns). The practical issue of finding a suitable coupling agent was considered by examining four kinds of coupling agents for transmission efficiency and long-term stability.

For particle patterning and translation (i.e., acoustic manipulation), we propose a polymer (i.e., Polydimethylsiloxane (PDMS)) thin film for the coupling agent to balance between acoustic coupling efficiency, stability over time, and reusability. As with the case for micro-centrifugation, we studied the feasibility of acoustic manipulation on an SMS superstrate and reported notable alterations in the particle separation distances compared to a plain silicon superstrate, which we attribute to a change of wave type from Lamb wave to Rayleigh wave on SMS superstrate.

To demonstrate the biocompatibility of these acoustic techniques, we verified that there was high cell viability after SAW treatment in both the cases of particle concentration in a sessile droplet and particle manipulation in a closed microchamber.

The two-chip setup was further explored to demonstrate the integration of two different techniques: acoustic localization of sparse particle and miniaturized mass sensing on a single chip for enhanced detection and minimization of blank measurements. The SAW was coupled into a microelectromechanical systems (MEMS) chip fabricated with a piezoelectric resonant sensor to localize scattered microparticles within a limited detection area, with the added mass resulting in a clear shift in the resonance frequency of the MEMS device, and the mass sensitivity is consistent in different measurements.