It is important to make the droplet formation in the printable area for a design of an inkjet device. One of the important variables for the inkjet is an ejecting velocity which can be controlled by the inkjet device. When the drop-on-demand inkjet nozzle is used, unless the ejecting velocity exceeds the minimum velocity, the droplet may not be ejected or the nozzle surface can be wet, thus the next droplet formation may fail. On the other hand, if the droplet velocity is too fast, splashing may happen when the drop impacts on a substrate, which makes the undesirable printing error. The falling velocity of the inkjet droplet changes by the drag force of air along the falling distance. Therefore, it is required to estimate the exact ejecting, falling and terminal velocity of the inkjet droplet. We measured the inkjet droplet velocity ejecting from the nozzle of a piezoelectric drop-on-demand inkjet device with a push mode. The nozzle diameter was 70 μm and polyethylene glycol aqueous solution was used as ink. The inkjet droplets were generated within a range of 500 to 10,000 Hz of frequencies at a reference piezoelectric input voltage. The successive inkjet droplets were captured by the high-speed camera with 100,000 fps, then the inkjet droplet velocities were analyzed visually at each falling location. The initial to terminal velocity changes of the droplets were calculated theoretically to be compared with the experimental results according to the diameter of the droplet, the drag coefficient, and the Reynolds number. The general terminal velocity of the droplet is determined by the density and diameter of the droplet. However, this study confirmed that the terminal velocity of the continuously ejected droplets by the drop-on-demand inkjet was determined by the interval of the droplets which depended on the frequency. The inkjet droplet in the low frequency has the long interval that is enough to consider as an independent droplet so that it can follow the terminal velocity curve of the general droplet. However, with increasing the frequency, the inkjet droplets are generated frequently and its interval gets shorter, and then its terminal velocity is relatively high not following the general terminal velocity curve. It is considered that each droplet gets into the wake field of the former droplet so that its drag force can decrease and the velocity can increase. We analyzed the velocity change of the inkjet droplet according to its diameter and frequency based on the experimental and numerical results.

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