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 μm 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 μs, and the highest ejection rate is estimated to be 222 Hz. When the ejector is driven with a pulse of 81.81 μs 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.