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 Vₒ , which has a stronger influence on jetting characteristics than pulse length. Therefore the jettable fluid space can be depicted on a Vₒ 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 Vₒ 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~μ s under a wide range of jetting conditions. The corresponding initial droplet speed was found to vary linearly with Vₒ , 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 Vₒ Oh diagram.