Dentry Michael B, Yeo Leslie Y, Friend James R
Monash University, Clayton, VIC 3800, Australia.
Micro/Nanophysics Research Laboratory, RMIT University, Melbourne, VIC 3000, Australia.
Phys Rev E Stat Nonlin Soft Matter Phys. 2014 Jan;89(1):013203. doi: 10.1103/PhysRevE.89.013203. Epub 2014 Jan 21.
Acoustic streaming underpins an exciting range of fluid manipulation phenomena of rapidly growing significance in microfluidics, where the streaming often assumes the form of a steady, laminar jet emanating from the device surface, driven by the attenuation of acoustic energy within the beam of sound propagating through the liquid. The frequencies used to drive such phenomena are often chosen ad hoc to accommodate fabrication and material issues. In this work, we seek a better understanding of the effects of sound frequency and power on acoustic streaming. We present and, using surface acoustic waves, experimentally verify a laminar jet model that is based on the turbulent jet model of Lighthill, which is appropriate for acoustic streaming seen at micro- to nanoscales, between 20 and 936 MHz and over a broad range of input power. Our model eliminates the critically problematic acoustic source singularity present in Lighthill's model, replacing it with a finite emission area and enabling determination of the streaming velocity close to the source. At high acoustic power P (and hence high jet Reynolds numbers ReJ associated with fast streaming), the laminar jet model predicts a one-half power dependence (U∼P1/2∼ ReJ) similar to the turbulent jet model. However, the laminar model may also be applied to jets produced at low powers-and hence low jet Reynolds numbers ReJ-where a linear relationship between the beam power and streaming velocity exists: U∼P∼ReJ2. The ability of the laminar jet model to predict the acoustic streaming behavior across a broad range of frequencies and power provides a useful tool in the analysis of microfluidics devices, explaining peculiar observations made by several researchers in the literature. In particular, by elucidating the effects of frequency on the scale of acoustically driven flows, we show that the choice of frequency is a vitally important consideration in the design of small-scale devices employing acoustic streaming for microfluidics.
声流是微流控中一系列令人兴奋的流体操纵现象的基础，这些现象的重要性正在迅速增长。在微流控中，声流通常呈现为从器件表面发出的稳定层流射流的形式，它是由在液体中传播的声束内声能的衰减驱动的。用于驱动此类现象的频率通常是临时选择的，以适应制造和材料问题。在这项工作中，我们试图更好地理解声频和声功率对声流的影响。我们提出并利用表面声波通过实验验证了一个层流射流模型，该模型基于莱特希尔的湍流射流模型，适用于20至936MHz以及广泛输入功率范围内的微米到纳米尺度的声流。我们的模型消除了莱特希尔模型中存在的严重问题声源奇点，用有限的发射面积取而代之，并能够确定靠近声源处的声流速度。在高声功率P（因此与快速声流相关的高射流雷诺数ReJ）下，层流射流模型预测出与湍流射流模型类似的二分之一次幂依赖性（U∼P1/2∼ReJ）。然而，层流模型也可应用于低功率产生的射流——因此射流雷诺数ReJ较低——此时声束功率和声流速度之间存在线性关系：U∼P∼ReJ2。层流射流模型能够预测广泛频率和功率范围内的声流行为，这为微流控器件的分析提供了一个有用的工具，解释了文献中几位研究人员所做的奇特观察。特别是，通过阐明频率对声驱动流尺度的影响，我们表明频率的选择在设计采用声流进行微流控的小型器件时是一个至关重要的考虑因素。
