Harmonic Operation of Acoustic Transducer for Droplet Ejection Application

This paper reports the use of harmonic frequencies for acoustic ejection in order to eject micron-sized droplets. For an acoustic ejector working at the thickness-mode resonance, the droplet size is primarily determined by the acoustic wavelength, which is proportional to the piezoelectric substrate thickness. In the newly invented design, we do not lap the bulk piezoelectric PZT substrate or use PZT thin films, but use harmonic frequencies of the bulk form to reduce the wavelength. The acoustic ejectors utilizing harmonic frequencies have been fabricated and shown to be very effective up to the 9th harmonic (~180 MHz), ejecting droplets as small as 10 μm in diameter.