Institute of Thermodynamics and Fluid Mechanics, Technische Universität Ilmenau, D-98684 Ilmenau, Germany.
Institute of Micro- and Nanotechnologies, Technische Universität Ilmenau, D-98684 Ilmenau, Germany.
Lab Chip. 2022 May 17;22(10):2011-2027. doi: 10.1039/d1lc01113h.
By integrating surface acoustic waves (SAW) into microfluidic devices, microparticle systems can be fractionated precisely in flexible and easily scalable Lab-on-a-Chip platforms. The widely adopted driving mechanism behind this principle is the acoustic radiation force, which depends on the size and acoustic properties of the suspended particles. Superimposed fluid motion caused by the acoustic streaming effect can further manipulate particle trajectories and might have a negative influence on the fractionation result. A characterization of the crucial parameters that affect the pattern and scaling of the acoustically induced flow is thus essential for the design of acoustofluidic separation systems. For the first time, the fluid flow induced by pseudo-standing acoustic wave fields with a wavelength much smaller than the width of the confined microchannel is experimentally revealed in detail, using quantitative three-dimensional measurements of all three velocity components (3D3C). In Part I of this study, we focus on the fluid flow close to the center of the surface acoustic wave field, while in Part II the outer regions with strong acoustic gradients are investigated. By systematic variations of the SAW-wavelength and channel height , a transition from vortex pairs extending over the entire channel width to periodic flows resembling the pseudo-standing wave field is revealed. An adaptation of the electrical power, however, only affects the velocity scaling. Based on the experimental data, a validated numerical model was developed in which critical material parameters and boundary conditions were systematically adjusted. Considering a Navier slip length at the substrate-fluid interface, the simulations provide a strong agreement with the measured velocity data over a large frequency range and enable an energetic consideration of the first and second-order fields. Based on the results of this study, critical parameters were identified for the particle size as well as for channel height and width. Progress for the research on SAW-based separation systems is obtained not only by these findings but also by providing all experimental velocity data to allow for further developments on other sites.
通过将表面声波（SAW）集成到微流控设备中，可以在灵活且易于扩展的芯片实验室平台上精确地对微粒子系统进行分离。这一原理所采用的广泛驱动机制是声辐射力，它取决于悬浮颗粒的大小和声学特性。由声波流效应引起的叠加流体运动可以进一步操纵颗粒轨迹，并可能对分离结果产生负面影响。因此，对影响声诱导流模式和比例的关键参数进行特征描述，对于声流分离系统的设计至关重要。首次使用定量的三维测量（3D3C）详细地实验揭示了具有比受限微通道宽度小得多的波长的准驻声波场诱导的流体流动。在本研究的第一部分，我们专注于表面声波场中心附近的流体流动，而在第二部分，我们研究了具有强声梯度的外部区域。通过对表面声波波长和通道高度的系统变化，可以发现从延伸到整个通道宽度的涡对到类似于准驻波场的周期性流动的转变。然而，电功率的适应性仅影响速度比例。基于实验数据，开发了一个经过验证的数值模型，其中系统地调整了关键材料参数和边界条件。考虑到在基底-流体界面处的纳维滑动长度，模拟与测量的速度数据在较大的频率范围内具有很强的一致性，并能够对一阶和二阶场进行能量考虑。基于本研究的结果，确定了与颗粒尺寸以及通道高度和宽度相关的关键参数。通过这些发现，不仅可以为基于表面声波的分离系统的研究取得进展，还可以提供所有实验速度数据，以便在其他站点进行进一步的开发。
