Unraveling interfacial jetting phenomena induced by focused surface acoustic waves

Surface acoustic waves have been known to be able to drive fluid jetting phenomena, though the underlying physical mechanism has remained unclear. In a setup designed to reliably jet small fluid droplets, a pair of focused single-phase unidirectional transducers were placed facing each other along the x-axis of a 128°Y-X lithium niobate substrate. Driving both transducers at the same time, the laterally focused acoustic energy appears beneath a drop placed on the surface, refracting into the drop and causing destabilization of the drop´s free surface. Above a critical Weber number We, an elongated jet forms for drops with dimensions greater than the fluid sound wavelength. Further increases in We leads to single droplet pinch-off and subsequent axisymmetric break-up to form multiple droplets. Derivation of a simple relationship based on the acoustics and fluid physics predicts the jetting behavior across a wide range of Newtonian fluids, droplet sizes, and input powers.