Abstract
We present an experimental study of drop-on-demand inkjet behavior, with particular emphasis on the thresholds for drop generation and formation of satellite drops, using inks covering a range of fluid properties. Drop behavior can be represented as a “phase diagram” in a parameter space bound by the dimensionless number Z (the inverse of the Ohnesorge number) and the Weber number of the fluid jet prior to drop formation, Wej. Stable drop generation is found to be bounded by a parallelogram with minimum and maximum values of 2 < Wej < 25. The lower bound indicates where capillary forces prevent drop ejection, and the upper bound indicates the onset of satellite drop formation. For Z < 50, the critical Wej for drop ejection increases with decreasing Z because of the contribution of viscous dissipation during drop formation. This requires an increase in the voltage required to drive the piezoelectric actuator until at Z ≈ 0.3 no drop ejection is possible. With Z > 4, the value of Wej at which satellite drops form decreases with increasing Z until at very large values of Z single drops can no longer form at any Wej. However, despite the large range of fluid properties over which stable drops can form, the need for a large range of both Z and Wej limits the region of practical ink design to the approximate range of 2 < Z < 20. These results are shown to be compatible with current models of the drop formation process reported in the literature.
Highlights
Inkjet printing has applications beyond the graphic arts in diverse areas such as printed electronics,1,2 printed ceramics,3–5 and biomaterials.6 These all require the formation and deposition of droplets in a controlled manner, with droplets traveling at a stable velocity and with a precise and defined volume, typically in the range of 1–100 pl (10−15–10−13 m3)
The influence of the dimensionless number Z on drop behavior is presented in Fig. 2, which shows a sequence of stroboscopic images of drop formation using inks 1–9, spanning a range of Z from 2 to 37
We identified three drop ejection regimes as the drop Weber number increases: (i) An isolated stable drop. (ii) The drop accompanied by a single satellite, which may have a greater velocity than the leading drop and after a brief time interval coalesces to form a single drop or a velocity equal to or lower than the leading drop and remain as a satellite to the leading drop. (iii) The drop accompanied by multiple satellites
Summary
Inkjet printing has applications beyond the graphic arts in diverse areas such as printed electronics, printed ceramics, and biomaterials. These all require the formation and deposition of droplets in a controlled manner, with droplets traveling at a stable velocity and with a precise and defined volume, typically in the range of 1–100 pl (10−15–10−13 m3). Inkjet printing has applications beyond the graphic arts in diverse areas such as printed electronics, printed ceramics, and biomaterials.. Inkjet printing has applications beyond the graphic arts in diverse areas such as printed electronics, printed ceramics, and biomaterials.6 These all require the formation and deposition of droplets in a controlled manner, with droplets traveling at a stable velocity and with a precise and defined volume, typically in the range of 1–100 pl (10−15–10−13 m3). Most applications outside marking, coding, and low-resolution graphics use piezoelectric actuated drop-on-demand (DOD) inkjet printing, and this study is limited to the formation of droplets from inks delivered by this method. Dijksman and Fromm carried out the earliest work focused on understanding the influence of fluid physical properties on the mechanisms of drop generation during inkjet printing.. Dijksman and Fromm carried out the earliest work focused on understanding the influence of fluid physical properties on the mechanisms of drop generation during inkjet printing. Fromm used a velocity independent dimensionless number Z (the inverse of the Ohnesorge number) in his calculations, Z=
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