Acoustic streaming flow driven about an array of sharp-edged obstacles

Acoustic streaming is a process that is used as a flow control mechanism for mixing, sorting, and enhanced transport phenomena.  In this work, we present experimental results examining the superposition of acoustic streaming and bulk flow in a microchannel that incorporates an array of sharp-edge obstacles with fixed porosity of 90%.  In the absence of bulk flow, we perform experiments over a parameter space consisting of obstacle morphology (circle, square, triangle, cross) and input voltage (4 V – 12 V) with a fixed frequency of 5.8 kHz.  Microscopic particle image velocimetry (microPIV) measurements yield a velocity range from 37 μm/s to 674 μm/s. In all shapes, an overall clockwise rotation was found at the right side of the PZT and anticlockwise rotation at the left side of PZT. Although the peak acoustic streaming velocities are different for each shape, we find that the velocity scales nearly quadratically as a function of applied voltage (U_o∼V²), which is consistent with scaling analyses of acoustic streaming in microfluidic systems.  A bulk flow of ~185 μm/s is imposed on the microchannel at the same time as a 10 V, 5.8 kHz signal.  We find that the resulting flow field can be reconstructed by adding the bulk flow field without streaming to the acoustic streaming flow field without bulk flow. This observation will lead to simplified models of oscillating microstreaming under conditions where the streaming component can be considered steady, because the pressure-driven flow and acoustic driven flow can be modelled separately and then simply superimposed, eliminating the need to model both phenomena simultaneously.  Furthermore, this work may find applications in a variety of fields, including, but not limited to, microfluidic flow control, heat transfer, and sample mixing