Thermal Considerations in Bulk Acoustic Wave Devices towards Optimised Acoustofluidics

The main goal of my studies was obtaining bulk acoustofluidic devices able to achieve high energy density inside a water-filled cavity. This led us to explore the interplay of sound and thermal effects more in details. Hence, this thesis covers both thermoacoustic effects in Bulk Acoustic Wave devices and a revisited approach for designing them, focusing on achieving good performance. Thermoacoustic effects arise every time an acoustic field interacts with a temperature gradient. Its working mechanism relies on the change in physical properties that heat induces in every substance. By changing the temperature of a fluid, its interaction with sound will also change, hence creating acoustic forces that depend on the temperature distribution. We studied this phenomenon in various configurations, by inducing differential heating either by conduction at the channel boundaries or light absorption within the fluid itself. We employed tracing particles to characterize the resulting thermoacoustic streaming in three-dimensions for each configuration. Comparing our measurements with simulations performed with a finite-element method software, we could probe the limits of our understanding of thermoacoustics.I also studied how to generate strong acoustic fields inside a water-filled cavity, mainly using so-called mechanical interfaces. With this term, I refer to any material that we interposed between the actuator and resonator, namely in between the piezoelectric element(s) and the microfluidic chip. In this thesis, I present two approaches: using a bulky double-parabolic structure made in aluminium to input strong vibration and using a thin sheet of copper to shield the microfluidic chip from unwanted heat from the actuator. With this latter setup, we could achieve temperature-controlled acoustic focusing in a microchannel with a relatively simple design, opening up to robust and reliable acoustophoresis at various flow rates and input electrical power. As another approach, we employed a double-parabolic reflector through which we could couple a small microfluidic chip with two large piezoelectric elements. With this design, we could achieve very strong acoustic energy density in the water cavity, as we reported the highest measured energy density in literature as of today. However, our investigation showed that this design is not optimized for focusing a clean mode of sound waves. This means that there is still much more research to be done, which would potentially lead to even higher acoustic energy inside the microchannel.
This thesis contributes to the development of acoustofluidics by investigating the interplay of sound and heat and presenting few novel approaches to couple actuators and resonators. Employing mechanical interfaces allows for good sound transmission into the microfluidic channel, whilst preventing unwanted heat form reaching the analytes.