Ultrasound jetting and atomization of sessile and impacting droplets

We present the results of our experimental and theoretical study of the jetting and atomization of a sessile and an impacting droplet subject to an ultrasonic surface vibration with a frequency of 2.5 MHz and piezoceramic transducer (PZT) surface velocity of 0.2-1.5 ms(-1). Such high-frequency vibrations result in the formation of a liquid jet shooting outwardly from the surface of the droplet, which eventually breaks up into droplets. Part of the liquid remains on the surface, which eventually goes through ultrasonic atomization. We have divided this process into three categories of (i) jetting, (ii) jet breakup and liquid layer formation, and (iii) liquid layer atomization, and characterized jetting and jet breakup separately. For impacting droplets, jetting occurs immediately and at relatively low surface vibration velocities as compared to those for sessile droplets. We have developed a model to predict the PZT surface velocity for the onset of jetting, the jet diameter, jet velocity, and spreading diameter based on momentum, force, and energy balances. The liquid jet may break up into droplets due to Rayleigh-Plateau breakup (ejecting droplets on the order of jet diameter, lambda(a)) and acoustic breakup of the jet (ejecting both few hundred micrometer and few micrometer droplets).