Impedance characterization of a coupled piezo-tube-fluid system for micro droplet generation

Abstract

Piezoelectric drop-on-demand (DoD) devices are able to produce micro droplets with diameters even smaller than the nozzle. This extended operation mode of DoD is investigated based on natural resonances of a coupled acoustic-mechanical system which consists on a cylindrical piezoceramic actuator surrounding a metal tube with fluid inside. The oscillatory response of the system is characterized by its electrical impedance which is measured using a vector network analyzer to get the resonance behavior at a large frequency range. Because the aspect ratio of tube diameter to tube length is small, the first axial natural resonances along the tube occurs at much lower frequencies than the first radial natural resonances and, therefore, a separated treatment of radial and axial resonances is performed. However, resonance effects in either case can be observed only in specific frequency ranges of the self-resonances of the piezo-tube vibrating system. The effect of the liquids inside the tube on the resonance proportions is emphasized by relating the measured impedances of the liquids to measurements with air for suppression of the vibrations due to the coupled solid components piezo and tube. Because the highest observed resonance frequencies are clearly in a range above 1 MHz, it is derived here that Bessel shape wave forms are a possible explanation for the generation mechanism of droplets at sizes smaller than the nozzle diameter.