An experimental study of drop-on-demand drop formation

The dynamics of drop-on-demand (DOD) drop formation have been studied experimentally using an imaging system with an interframe time of 1μs and a spatial resolution of 0.81μm∕pixel. Using a piezoelectrical actuated inkjet printhead with the nozzle orifice diameter of 53μm, experiments were conducted over a range of viscosities (1.0–5.0cP) and surface tensions (35–73mN∕m). 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. The breakup time of liquid threads was found to be dependent mainly on the capillary time based on the length scale of the nozzle orifice and the growth rate of the most unstable disturbance normalized by the inverse of the capillary time. However, a well-designed waveform of driving signal can initiate an abrupt pinch-off of the liquid thread from the nozzle exit. During the contraction of the liquid thread after it has pinched off from the nozzle, two modes of breakup were observed: end-pinching where the liquid thread pinches off from an almost spherical head, and multiple breakup due to capillary waves. The effects of liquid and system parameters on the formation and recombination of the primary drop and satellites were investigated. Based on experimental observations, 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. The primary drop size increases slightly with increasing surface tension and/or decreasing viscosity. The driving voltage to the piezoelectric transducer mainly determines whether satellite formation will occur and the size of satellites, and it has insignificant effect on primary drop size.