Suppression of acoustic streaming by creating specific acoustic resonances in shaped channels

When a fluid is exposed to acoustic actuations or harmonic boundary vibrations, a steady flow known as acoustic streaming is superimposed on the oscillatory motion. In resonating acoustofluidic devices, the manipulation of nanoparticles by acoustic radiation forces is often hindered by the presence of acoustic streaming. In this study, we demonstrate, both theoretically and numerically, that microscale acoustic streaming can be significantly reduced or even completely eliminated by creating specific acoustic resonances within well-designed fluid cavities. By suppressing acoustic streaming and the corresponding drag force it induces, we demonstrate the potential to use acoustic radiation forces for manipulating nanoparticles, regardless of their size. Additionally, building upon the theoretical findings, we present the experimental realisation of acoustophoretic patterning of polystyrene nanoparticles with diameters ranging from 100 nm to 1 $\unicode {x03BC}$ m in a resonating wavelength-scale acoustofluidic device that operates at sub- or low-MHz frequencies.