Jettable fluid space and jetting characteristics of a microprint head

The influence of fluid droplet properties on the droplet-on-demand jetting of a Newtonian model fluid (water-isopropanol-ethylene glycol ternary system) has been studied. The composition of the fluid was adjusted to investigate how the Ohnesorge number (Oh) influences droplet formation (morphology and speed) by a microfabricated short-channel shear-mode piezoelectric transducer. The fluid space for satellite-free single droplet formation was indeed found to be bound by upper and lower Oh limits, but these shift approximately linearly with the piezo pulse voltage amplitude V-o, which has a stronger influence on jetting characteristics than pulse length. Therefore the jettable fluid space can be depicted on a V-0-Oh diagram. Satellite-free droplets of the model fluid can be jetted over a wide Oh range, at least 0.025 to 0.5 (corresponding to Z = Oh(-1) of 40 to 2), by adjusting V-o appropriately. Air drag was found to dominate droplet flight, as may be expected. This can be accurately modelled to yield droplet formation time, which turned out to be 20-30 us under a wide range of jetting conditions. The corresponding initial droplet speed was found to vary linearly with V-o, with a fluid-dependent threshold but a fluid-independent slope, and a minimum speed of about 2 m s(-1). This suggests the existence of iso-velocity lines that run substantially parallel to the lower jetting boundary in the V-0-Oh diagram.