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 128ÃÂðY-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.
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