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.

An apparatus developed for visualizing drop-on-demand (DOD) drop formation and impaction on substrates is described. Using a pulsed laser, a low-speed charge coupled device camera, and signal generators, an imaging system based on flash photography is shown to be able to obtain sharp images with a temporal resolution of 200ns and a spatial resolution of 0.81μm∕pixel. Several steps are taken to minimize the “first drop problem” so that excellent reproducibility is achieved; drop formation is reproducible with a positional variation of 1μm. The visualization system coupled with a motorized stage allows imaging of the impaction of micron-size drops on surfaces. A wave form generator and an amplifier are used to produce the required wave form for a given printhead and ejected liquid. Demonstration of the system for study of DOD micron-size drop formation and impaction on a substrate is presented. The effects of signal wave form on DOD drop formation are demonstrated using a Trident printhead driven by four different signal wave forms. The effect of surface wettability on micron-size drop impaction is shown using hydrophilic and hydrophobic surfaces.

in Wiley InterScience (www.interscience.wiley.com).Impaction on smooth solid substrates of drops formed by the drop-on-demand (DOD) method was investigated over a wide range of impaction speeds (U 0 5 2.21-12.2 m/s), surface contact angles (h 5 6-107 ), and drop diameters (D 0 5 40.8-50.5 lm). The experimental results were compared with several existing equations for predicting maximum spreading. The dimensionless time to reach maximum spreading ratio, scaled by D 0 /U 0 , ranged from 0.6 to 2.99, depending on Weber number and contact angle. Micron and millimeter drop impactions were compared, and the results indicate that scaling based on three dimensionless numbers (We, Re or Oh, and cos h) is valid, but spreading ratios of millimeter drops are usually slightly larger during the whole process. The difference is ascribed mainly to the effect of gravity. Figure 10. Comparison of (a) 50.5-lm drop, and (b) 2.23-mm drop impaction sequences at We 5 103-105, Oh 5 0.0146-0.0148 and h 5 98-99 , t l 5 4.14 ls and t m 5 1.34 ms.

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A study has been conducted to determine the frictional characteristics of several fabrics used in apparel. The objective was to generate fabric frictional data to help in developing automated assemblies for fabrics. The results revealed that the coefficients of static and dynamic friction between fabrics decreased as the normal force per unit area increased. Wilson's model relating frictional force to normal force and the apparent area of contact closely correlated the data.