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

本项目旨在研究不同类型表面声波的偏振转换，即垂直极化（VP）和边界极化（BP）表面声波之间的转换。我们的方法结合了实验和理论工作，特别是针对saw驱动的微流体。通过利用极化转换，我们计划抵消传统saw驱动微流体的一个主要缺点，即声能量寄生泄漏到形成微流体通道和腔室壁和顶部的聚合物容器材料中。这类常用的微流体驱动装置的数字间换能器（IDT）位于通道外，产生垂直极化（VP）声呐。这是因为只有在垂直方向上的机械表面位移才会引起动量向流体的传递，即预期的体声波向流体的辐射，这对于声流体驱动是必不可少的。然而，这种在流体容器外产生的VP SAW在通过容器壁时也会将声能辐射到容器材料中。这在常用聚合物的情况下尤为重要。除了流体内部可用于驱动目的的振幅降低外，这种向容器内辐射的声能也会由于局部加热而导致容器材料的降解。此外，容器内的体波可以破坏性地叠加从基底-流体界面辐射的体波，从而干扰传统的基于saw的微流控装置的运行。为了克服这些缺点，我们的目标是在声微流控器件中部署偏振转换。这里的目标是通过应用初始边界极化（BP）表面波来改进SAW驱动装置：一旦在微通道外激发BP SAW，由于缺乏显着的表面法向振动成分，几乎没有阻尼，因此SAW通过聚合物容器。当到达通道内部时，由于波与位于基材-流体界面的适当散射体的相互作用，声表面波的极化由边界型转变为垂直型。通过这种受控的极化转换，在通道内产生的VP SAW最终将像传统设备一样驱动流体。通过实验和理论研究相结合，我们希望对极化转换效应有一个全面的认识。然而，理论部分并不局限于本项目框架内的实验案例。模拟将提供极化转换效应的更广泛的模式，预测它可以提供的优势，从而为进一步的研究活动铺平道路。

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.