The effect of sharp-edge acoustic streaming on mixing in a microchannel

Strong acoustic streaming in a liquid has been previously observed near sharp-edge structures inside a microchannel. Thanks to its non-invasive characteristic, powerful disturbance into a laminar flow and low-cost equipment requirement, such a sound-driven second-order mean flow has promising potential applications in Process Intensification(PI). Unlike MHz-level ultrasonic wave, the wavelength of the audible acoustic wave in the current study ($\lambda$ ~0.5~m ) is much larger than the characteristic dimensions of the microchannel (0.5~mm), which means the vibration of fluids has the same phase state. In this study, a Y-type (Polydimethylsiloxane, PDMS) micromixer with sharp-edges patterns is pre-fabricated and bonded onto a glass slide. A piezoelectric transducer is stuck next to the channel and it provides an acoustic field at a frequency range of 2 to 3 kHz. The streaming is characterized under various acoustic/flow conditions. Based on fluorescence visualisation analysis and Iodide-Iodate method together with Internal Exchange Model (IEM), both macro- and micro-mixing performance with different amplitudes of acoustic field and sharp edges are quantitatively investigated. It is showed that with the streaming flow around an array of sharp edges or even a single one, two separate flows across the width of the channel can quickly merge into each other with strong $v\_\omega$ acoustic vibration in the channel. In terms of micromixing, under acoustic streaming excitation, the segregation index decreases sharply to 0.01 (good mixing) while that without acoustic streaming is 0.06 (bad mixing). Micromixing time based on IEM decreases by a factor of 10: 0.04s with acoustic streaming instead of 0.3s without streaming. Meanwhile, the mixing enhancement by acoustic streaming is very sensitive to the throughput of the channel. Under weak acoustic field and high throughput, the mixing improvement becomes weak or even negligible.