Acoustic control of Rayleigh-regime liquid-jet breakup using a through-hole MHz transducer

Abstract

This experimental study demonstrates MHz-order acoustic control of Rayleigh-regime liquid-jet breakup using a compact through-hole lithium niobate thickness-mode transducer integrated at the nozzle exit and driven near its 6.7 MHz resonance. Jet dynamics were measured by high-speed shadowgraphy at 50,000 fps, and breakup lengths were obtained from instantaneous jet-length records using an autocorrelation-based method that identifies statistically separated breakup events. Acoustic actuation produced visible interfacial corrugations and reduced the normalized breakup length, $L_b/d$, for both water and glycerol--water jets. In water, the response was threshold-like at low Weber number and weakened as the jet velocity increased to We ~ 50. A reduced-order threshold correlation, fit to the water amplitude and velocity sweeps, captured this abrupt shortening and the subsequent loss of actuation efficacy. More viscous glycerol--water mixtures shortened immediately at low forcing, indicating a distinct acoustic-coupling regime rather than a universal collapse of the water response. A wetting-control experiment showed that liquid spreading at the nozzle base can also shorten the jet, so the measured response reflects coupled radiation-pressure forcing, capillary-wave feedback, and wetting-mediated disturbance injection. Resolved droplet tracking further showed that MHz excitation changes the visible gate-crossing droplet population, although sub-resolution mist and micron-scale atomization were not measured. These results establish through-hole MHz actuation as a compact route for modifying free-surface jet breakup and provide a repeatable image-analysis protocol for experimental-fluid-mechanics measurements.