School of Engineering, University of Glasgow, Glasgow, UK.
Ultrasonics. 2014 Jan;54(1):268-74. doi: 10.1016/j.ultras.2013.04.019. Epub 2013 May 15.
Acoustic radiation force has been demonstrated as a method for manipulating micron-scale particles, but is frequently affected by unwanted streaming. In this paper the streaming in a multi-transducer quasi-standing wave acoustic particle manipulation device is assessed, and found to be dominated by a form of Eckart streaming. The experimentally observed streaming takes the form of two main vortices that have their highest velocity in the region where the standing wave is established. A finite element model is developed that agrees well with experimental results, and shows that the Reynolds stresses that give rise to the fluid motion are strongest in the high velocity region. A technical solution to reduce the streaming is explored that entails the introduction of a biocompatible agar gel layer at the bottom of the chamber so as to reduce the fluid depth and volume. By this means, we reduce the region of fluid that experiences the Reynolds stresses; the viscous drag per unit volume of fluid is also increased. Particle Image Velocimetry data is used to observe the streaming as a function of agar-modified cavity depth. It was found that, in an optimised structure, Eckart streaming could be reduced to negligible levels so that we could make a sonotweezers device with a large working area of up to 13 mm × 13 mm.
声辐射力已被证明是一种操纵微米级颗粒的方法，但经常受到不需要的流场的影响。本文评估了多换能器准驻波声粒子操纵装置中的流场，发现其主要由埃克特流形式主导。实验观察到的流场呈现出两个主要的漩涡，它们的速度在驻波形成的区域达到最高。开发了一个与实验结果吻合良好的有限元模型，表明产生流体运动的雷诺应力在高速区域最强。探索了一种技术解决方案来减少流场，即在腔室的底部引入一层生物相容性琼脂凝胶，以减小流体深度和体积。通过这种方式，我们减少了经历雷诺应力的流体区域；单位流体体积的粘性阻力也增加了。粒子图像测速数据用于观察琼脂改性腔深对流场的影响。结果发现，在优化的结构中，可以将埃克特流减小到可忽略的水平，从而可以制造出工作面积达 13mm×13mm 的声镊器件。
