Polarization conversion of surface acoustic waves in solid structures - a new driving mechanism for acoustomicrofluidics

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This project is aimed at an essential investigation of the polarization conversion of different types of surface acoustic waves (SAW), namely, the conversion between vertical polarized (VP) and boundary polarized (BP) SAW. Our approach combines both experimental and theoretical work and is especially directed on SAW-driven microfluidics. By taking advantage of the polarization conversion, we plan to counteract a major drawback of conventional SAW-driven microfluidics, namely the parasitic leakage of acoustic energy into the polymeric container material forming walls and roof of microfluidic channels and chambers. The interdigital transducers (IDT) of such usual microfluidic actuator devices are located outside the channel and generate vertical polarized (VP) SAW. This is motivated by the fact, that only the mechanical surface displacement in vertical direction provokes the transfer of momentum into the fluid, i.e. the intended radiation of bulk acoustic waves into the fluid, what is indispensable for acoustofluidic actuation. However, such VP SAW generated outside the fluid container radiate acoustic energy also into the container material when passing the container walls. This is especially significant in case of the commonly used polymers. Beside the decreased amplitude available for actuation purposes inside the fluid, such radiation of acoustic energy into the container also can cause degradation of the container material due to local heating. Moreover, the bulk waves inside the container can destructively superimpose the bulk waves radiated from the substrate-fluid interface what then disturbs the operation of conventional SAW-based microfluidic devices. To overcome these drawbacks we aim at the deployment of polarization conversion in acoustomicrofluidic devices. The objective here is an improvement of SAW-driven devices by the application of initially boundary polarized (BP) surface waves: Once excited outside the microchannel BP SAW pass the polymer container due to the absence of a significant surface-normal vibration component almost without damping. When reaching the channel inside the SAW polarization is converted from boundary to vertical type due to the interaction of the wave with an appropriate scatterer located at the substrate-fluid interface. The VP SAW created inside the channel by this controlled polarization conversion will finally actuate the fluid like in conventional devices.By combining experimental and theoretical investigations we aspire to get a comprehensive understanding of the polarization conversion effect. However, the theoretical part is not restricted to the cases experimentally within the frame of this project. Simulations will provide a broader pattern of the effect of polarization conversion, predicting advantages it can offer and paving thereby the way for further research activity.