Using surface acoustic waves (SAW) for the agitation and manipulation of fluids and immersed particles or cells in lab-on-a-chip systems has been state of the art for several years. Basic tasks comprise fluid mixing, atomization of liquids as well as sorting and separation (or trapping) of particles and cells, e.g. in so-called acoustic tweezers. Even though the fundamental principles governing SAW excitation and propagation on anisotropic, piezoelectric substrates are well-investigated, the complexity of wave field effects including SAW diffraction, refraction and interference cannot be comprehensively simulated at this point of time with sufficient accuracy. However, the design of microfluidic actuators relies on a profound knowledge of SAW propagation, including superposition of multiple SAWs, to achieve the predestined functionality of the devices. Here, we present extensive experimental results of high-resolution analysis of the lateral distribution of the complex displacement amplitude, i.e. the wave field, alongside with the electrical S-parameters of the generating transducers. These measurements were carried out and are compared in setups utilizing travelling SAW (tSAW) excited by single interdigital transducer (IDT), standing SAW generated between two IDTs (1DsSAW, 1D acoustic tweezers) and between two pairs of IDTs (2DsSAW, 2D acoustic tweezers) with different angular alignment in respect to pure Rayleigh mode propagation directions and other practically relevant orientations. For these basic configurations, typically used to drive SAW-based microfluidics, the influence of common SAW phenomena including beam steering, coupling coefficient dispersion and diffraction on the resultant wave field is investigated. The results show how tailoring of the acoustic conditions, based on profound knowledge of the physical effects, can be achieved to finally realize a desired behavior of a SAW-based microacoustic-fluidic system.

中文翻译：

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用于微流体应用的表面声波场的复杂性

使用表面声波 (SAW) 来搅动和操纵芯片实验室系统中的流体和浸入粒子或细胞已成为数年来的最新技术。基本任务包括流体混合、液体雾化以及颗粒和细胞的分类和分离（或捕获），例如在所谓的声学镊子中。尽管在各向异性压电基板上控制 SAW 激发和传播的基本原理已经得到充分研究，但目前还不能以足够的精度全面模拟包括 SAW 衍射、折射和干涉在内的波场效应的复杂性。然而，微流体执行器的设计依赖于对 SAW 传播的深入了解，包括多个 SAW 的叠加，以实现设备的预定功能。在这里，我们展示了复杂位移幅度横向分布的高分辨率分析的广泛实验结果，即波场，以及生成换能器的电 S 参数。这些测量是在利用由单个叉指式换能器 (IDT) 激发的行进 SAW (tSAW)、在两个 IDT（1DsSAW、1D 声学镊子）和两对 IDT（2DsSAW、2D 声学镊子）之间生成的立式 SAW 的设置中进行的和比较的) 相对于纯瑞利模式传播方向和其他实际相关的方向具有不同的角度对齐。对于这些基本配置，通常用于驱动基于 SAW 的微流体，常见 SAW 现象的影响，包括光束转向，研究了合成波场上的耦合系数色散和衍射。结果表明，如何根据对物理效应的深入了解来定制声学条件，以最终实现基于 SAW 的微声流体系统的理想行为。