Droplet generation from pulsed micro-jets

This paper presents an experimental study on the fluid dynamics of pulsed micro-jets (orifice diameter 20 mu m), including jet breakup and droplet formation. The pulsed micro-jets were generated using a piezoelectric inkjet printhead with varied working voltages and ejection frequencies. For low ejection frequencies, meniscus had time to relax before the next pulse was applied. For every single pulse, fluid was ejected in the form of free liquid ligaments, which eventually contracted into droplets. The maximum length of ligaments increased with the working voltage. Satellite droplets were observed when high working voltage was applied. For high ejection frequencies, a periodically perturbed jet was formed, and droplets were generated from the jet through pinch-off. After pinch-off, liquid threads between droplets contracted into satellite droplets, which coalesced with main droplets. The jet length was longer than the maximum ligament length of the low-frequency ejection. The jetting process with high ejection frequency was found to consist of an undesired startup stage followed by a repeatable stage. Droplets with larger size and higher velocity were generated in the startup stage, and the startup stage lasted longer for a higher working voltage. (C) 2008 Elsevier Inc. All rights reserved.