The use of drop-on-demand (DOD) inkjet technologies is becoming increasingly widespread in many industrial applications ranging from gene chip production to separations to paper printing. Since the development of the first DOD inkjet devices, great advances in inkjet technologies have made ink-jets economical and versatile. As popularity of ink-jets grows so does the need to understand the factors which contribute to drop quality (e.g. drop speed, accuracy, and uniformity). Additionally, gene chip arraying devices have the special requirement that they should be capable to dispensing many different types of liquids using a given nozzle, where a typical ink-jet printer may dispense only a single ink formulation per nozzle.
The study of liquid jets and drops has a long history. In 1879, Lord Rayleigh showed that long cylindrical columns of fluid are unstable as a result of naturally occurring undulations on their surfaces. The driving force behind such instability is surface tension, which drives fluid from locally thin regions to locally thick regions, a runaway process that inevitably causes the jet to break up into drops. Almost a century later, this phenomenon was exploited by Sweet, with the invention of continuous ink-jet (CIJ) printing and the first microelectromechanical device, the ink-jet print head. In CIJ printing, and the currently more popular and cheaper method of ink-jet printing known as drop-on-demand (DOD), the principle goal has been to produce ever-smaller drops. However, doing so has required the manufacture of ever-smaller nozzles. As small nozzles are fraught with problems of clogging, breakage and increased flow resistance, current technology limits us to nozzle and drop sizes of 5-10 μm in printing, and 25-100 μm in the production of DNA or protein microarrays and polymer beads (for use in ion-exchange systems and as spacers in LCD flat screen displays) for which modified ink-jet printers are commonly used.
Some DOD dispensing systems currently in use utilize electrical control signals with particular characteristics in order to achieve the desired drop qualities. For example, some existing systems use a control signal that consists of a waveform with a single polarity, such as half of a square wave. Yet other existing systems use an electrical control signal consisting of two portions, one portion being of a first polarity and the other portion being of a second and opposite polarity, such as a single, full square wave. In some cases, the timed durations of the two portions are identical. Many of these systems provide an electrical control signal that grossly produces one or more large drops, the large drops being created by a fluid meniscus which takes on a generally convex shape on the exterior of ejecting orifice. The large drop is formed when the edges of the meniscus in contact with the orifice separate from the orifice. These systems produce drops of a diameter equal to or greater than the diameter of the orifice. Yet other systems produce drops by resonating the meniscus. Such systems do not generally move the meniscus either toward the exterior of the dispenser, or toward the internal passage of the dispenser, but simply create oscillatory conditions on the meniscus. The drop quality of such oscillatory dispensing methods are likely to be subject to manufacturing imperfections near the orifice, or deposits of material near the orifice, such as dried ink.
Rieer and Wriedt have experimentally studied drop generation process using freely adjustable drive signals. A drop of 8 μm from a nozzle of 40 μm is successfully generated by applying a very carefully designed staircase signal. They have found that the conditions required for small drop formation is very strict, with only a few out of many applied drive pulses leading to small droplets. Chen and Basaran have investigated the small water/glycerin drop formation from a PZT nozzle by applying a succession of three square pulses (negative, positive and negative). A drop of 16 μm is made from a nozzle of 35 μm. Their experiments have shown that the key to generating a small drop is the extrusion of a small tongue from primary drop formed by the positive pulse and the detachment of the tongue during the second negative pulse. They have discussed the effects of control parameters, such as process time tp and the Ohnesorge number Oh, on the ejection of small drops. Small droplets are only observed for intermediate values of tp and Oh. The range of Oh for the tongue to arise is between 0:1 to 0:2 under their experimental conditions. Goghari and Chandra build a pneumatic DOD apparatus which consists a nozzle filled with water/glycerin mixture, a gas cylinder with a solenoid valve and a venting valve connected directly with the nozzle. Opening the valves subsequentially creates alternating negative and positive pressure pulses and produce droplets from the nozzle. A 55˜90 wt % glycerin drop of 150 μm is made from a nozzle of 204 μm in 0:8 ms, instead of tens of μs in Chen and Basaran's experiments.
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