This letter reports picoliter liquid droplet generation using an orifice-free acoustic ejector operating at its harmonic frequencies. 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 our design, we do not need to lap the bulk piezoelectric lead zirconate titanate (PZT) substrate or deposit high temperature processing PZT thin film, but we use harmonic frequencies of the bulk form to reduce the wavelength. The fabricated acoustic ejector with a size of 1200 × 1200 μm~2 has been shown to be very effective up to the ninth harmonic (180 MHz), continuously ejecting～10 μm diameter droplets, corresponding to droplet volumes as small as 0.5 pl.
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