On-Demand, Heatless Ejection Of Sub-Mm-Sized Liquid Droplets

This paper reports a nozzle-less, heatless droplet ejector capable of ejecting large liquid droplets of 285 mu m in diameter with a volume of 12.1 nL. The ejector is built on a 1 mm thick lead zirconate titanate (PZT) sheet covered by Fresnel acoustic lens formed with annular rings of air cavities. With 450 V-pp pulsed 2.2 MHz sinusoidal drive, the ejector produces a focused high-intensity acoustic beam at the top surface of liquid contained in a reservoir, and is able to eject liquid droplets from the liquid surface located 5 mm away from the ejector. The ejector consistently ejects one droplet per pulse with a driving pulse width of 18.18 mu s, and the highest ejection rate is estimated to be 222 Hz. When the ejector is driven with a pulse of 81.81 mu s width, the ejected droplet travels 30 cm upwards before falling down due to gravity and the corresponding initial velocity of the ejected droplet is calculated to be 2.42 m/s.