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 ( $\mathit{Oh}$ ) 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 $\mathit{Oh}$ 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 ${V}_{o} {{\ndash}}\mathit{Oh}$ diagram. Satellite-free droplets of the model fluid can be jetted over a wide $\mathit{Oh}$ range, at least 0.025 to 0.5 (corresponding to $Z= {\mathit{Oh}}^{\ensuremath{-} 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\text{{\ndash}} 30~\lrm{\ensuremath{\mu}} \mathrm{s} $ 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~\mathrm{m} ~{\mathrm{s} }^{\ensuremath{-} 1} $ . This suggests the existence of iso-velocity lines that run substantially parallel to the lower jetting boundary in the ${V}_{o} {{\ndash}}\mathit{Oh}$ diagram.