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

本项目旨在对不同类型声表面波（SAW）的极化转换，即垂直极化（VP）和边界极化（BP）SAW之间的转换进行必要的研究。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。通过利用偏振转换，我们计划抵消常规SAW驱动的微流体的主要缺点，即声能寄生泄漏到形成微流体通道和腔室的壁和顶部的聚合物容器材料中。这种常见的微流体致动器装置的叉指换能器（IDT）位于通道外部并产生垂直偏振（VP）SAW。这是由以下事实激发的，即，只有在垂直方向上的机械表面位移引起动量到流体中的传递，即体声波到流体中的预期辐射，这对于声流体致动是必不可少的。然而，在流体容器外部产生的这种VP SAW在通过容器壁时也将声能辐射到容器材料中。这在常用聚合物的情况下尤其重要。除了可用于流体内部的致动目的的减小的振幅之外，这种声能到容器中的辐射还可能由于局部加热而导致容器材料的劣化。此外，容器内的体波可以破坏性地干扰从衬底-流体界面辐射的体波，这然后干扰常规的基于SAW的微流体装置的操作。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。这里的目的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BP SAW外被激发，由于几乎没有阻尼的显著表面法向振动分量的存在，SAW通过聚合物容器。当到达通道内部时，由于波与位于衬底-流体界面处的适当散射体的相互作用，SAW极化从边界类型转换为垂直类型。通过这种受控的偏振转换在通道内产生的VP SAW最终将像在传统器件中那样驱动流体。通过实验和理论研究相结合，我们期望得到偏振转换效应的全面理解。然而，理论部分并不局限于本项目框架内的实验案例。模拟将提供更广泛的偏振转换效应模式，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.