Acoustofluidics along inclined surfaces based on AlN/Si Rayleigh surface acoustic waves

Conventional acoustofluidics are restricted to manipulation of droplets on a flat surface, and there is an increasing demand for acoustofluidic devices to be performed at inclined surfaces to facilitate multilayered microfluidic device design and enhance system compactness. This paper reports theoretical and experimental studies of acoustofluidic behaviors (including transportation/pumping and jetting) along inclined surfaces using AlN/Si Rayleigh surface acoustic waves (SAWs). It has been demonstrated that for droplets with volume smaller than 3 μL, they could be efficiently transported on arbitrary inclined surfaces. The gravity effect would play a more and more important role in uphill climbing with the increased inclination angle. When the inclination angle was increased up to 90º, a higher threshold power was needed to transport the droplet and the maximum droplet volume which can be pumped also reached its minimum value. Effects of surface inclination angle on droplet jetting angles could be neglected for their volumes less than 2 μL. Moreover, microfluidic and acoustic heating performances of AlN/Si SAWs were further studied and compared with those conventional ZnO/Si SAWs with the same electrode configurations.