Atomization regimes in liquid jets due to pressure driven flow in a cylindrical nozzle with MHz frequency acoustic vibration*

In acoustofluidic atomization, high-frequency oscillation accelerates and destabilizes liquid in contact with a vibrating piezoelectric substrate. This method has an advantage over alternatives that rely on nozzles and high pressure-driven flow. However, these methods may be combined to produce atomization superior to either alone. Nozzles may be machined directly in piezoelectric media, producing predictable Rayleigh jetting at moderate pressure-driven flow rates in the absence of acoustic excitation. Upon application of MHz-order acoustic vibration above a threshold amplitude, however, Taylor-mode-like atomization results in monodisperse, micron-scale droplets. High-speed microvideography indicates that it occurs directly from the nozzle without spreading onto the adjacent substrate. Acoustic streaming is not a factor. Instead, we suggest that the acoustic oscillation induces characteristic structure within the nozzle. We model this flow and perform stability analysis to reveal the effects of system parameters on capillary wave growth. We predict the atomization threshold and the resulting droplet sizes. We then compare these predictions with experiments for a wide range of fluids, flow rates, and acoustic amplitudes and frequencies.