| An imaging system has been developed to visualize Drop-on-demand (DOD) inkjet drop formation and drop impaction on substrates for drop sizes and impaction speeds of the magnitudes encountered in applications. Using a pulsed laser, a low-speed charge-coupled-device (CCD) camera, and signal generators, the imaging system based on flash photography is shown to be able to obtain sharp images with a temporal resolution of 200 ns and a spatial resolution of 0.81 micron/pixel. First, the dynamics of drop-on-demand (DOD) drop formation was studied experimentally. The effects of the driving signal, which controls the piezoelectric transducer that produces the pressure pulse to drive the liquid from the reservoir through the orifice, have been examined along with those of liquid properties. The main stages of DOD drop formation, including ejection and stretching of liquid, pinch-off of liquid thread from the nozzle exit, contraction of liquid thread, breakup of liquid thread into primary drop and satellites, and recombination of primary drop and satellites, are analyzed based on the experimental results. A necessary condition for the recombination of the primary drop and satellite and the limit for liquid thread length without breakup during contraction are proposed. Second, using the visualization system coupled with a motorized stage, microndrop impaction on a smooth substrate was investigated over a wide range of We and Oh typical regime for inkjet printing applications. The results indicate that scaling of micron-drop impaction from millimeter-drop impaction, based on three dimensionless numbers (Oh, We and costheta), is valid. The predictions of maximum spreading ratio by six existing models agree well with experimental values for high-We impaction, but not for low-We and low-contact-angle impactions; however, the model of Part et al. predicts well for high- and low-We impaction due to its inclusion of spontaneous spreading dissipation. Fingering and splashing do not occur in the micron drop impaction on either dry solid substrates or a pre-existing liquid layer. The drying time of a micron drop deposited on a substrate is less than one second and increases as the contact angle of the drop on the substrate increases. |
