Relevant Influencing Factors on Droplet Characteristics for a Piezoelectrically Driven Drop-on-Demand Printhead

Abstract

Dispensing minute amounts of fluid is used in many industries, such as in life science, bioengineering, 3D printing, or in electronics manufacturing. Each application for drop-on-demand (DoD) printheads requires different drop volumes and drop velocities. Furthermore, it is necessary to eject droplets made of fluids with different fluid properties, like viscosity, surface tension, or density. Due to this wide range of different applications and demands on printheads it is important to investigate the influence of relevant factors on the droplet formation process. Therefore, the influence of the fluid properties, the printhead geometry, and the electrical excitation form on the droplet formation process are described in this project. In detail, the influence of the surface tension as well as the viscosity of the fluid, the nozzle length and its width, and the amplitude of the applied voltage at different pulse widths on the droplet characteristics are investigated. The used printhead consists of a silicon chip, which includes the fluidic components, and of a bimorph piezoelectric actuator. The printhead is manufactured with rapid manufacturing techniques, such as laser micromachining. The advantage of this method is that the printhead is adaptable to new boundary conditions in a time- and cost-saving manner. In this project, the nozzles have a square shape with a sidelength between 50 and 100 μm and the nozzle length varies between 50 and 200 μm. A fluid mixture is provided which can be varied in its fluid properties. Therefore, the possibility for the independent adjustment of its viscosity and its surface tension is given. The mixture consists of glycerin, distilled water, and isopropanol. An analytical description for each amount of its substances enables to provide a fluid with defined properties. Three kinds of experiments are carried out in order to determine the influence of the fluid properties, the printhead geometry, and the electrical excitation on the droplet formation process. The determination of the minimum excitation voltage needed for droplet ejection and the determination of the droplet volume and its velocity. The main results are: The higher the surface tension, viscosity, and nozzle length, the higher is the minimum excitation voltage. Furthermore, the droplet velocity decreases for an increased surface tension, viscosity, and nozzle length. On the other hand, the droplet velocity increases with an enlarged amplitude of the voltage and pulse width. The droplet volume increases for an increased surface tension, nozzle width, pulse width, and amplitude of the voltage. In general, the reasons for these correlations are the interaction between the strength of the pressure pulse, friction forces, fluidic resistances, and fluid properties. Overall, the possibility to achieve microdroplets made of different fluids and with a specific velocity and volume is described. Furthermore, a fluid mixture, which can be varied in its fluid properties, is presented.

Development of an Inkjet Setup for Printing and Monitoring Microdroplets

“…Pulse amplitude defines how much energy is used to deform the printhead. It has been observed experimentally that the volume and the velocity of the ejected droplets are proportional to the applied voltage [6,7]. Nevertheless, too much energy may lead to ligament formation and the appearance of unwanted satellites.…”

“…Although easy to customize by tailoring the driving waveform, within some limits, the resulting drop characteristics highly depend on the used printhead geometry as well as the fluid rheology, mainly surface tension and viscosity [7,16,17].…”

“…Therefore, manipulating the droplet characteristics entails manipulating the driving waveform. For piezoelectric printheads, the impact of the driving waveform on the ejected droplet during fly has been widely studied [6][7][8], whereas less focus has been put on the direct relationship between the size of the resulting printed droplet. Reference [9] presents a model that relates the amplitude of the input signal of a solenoid valve that actuates the printhead directly with the diameter of the printed droplets, thus allowing a feedback control of the millimeter width of the printed line in terms of the input signal.…”

Current Status of Liquid Metal Printing

“…When voltage is applied to the piezoelectric, the bimorph bends due to mechanical deflection of the piezoelectric. The primary applications for this bimorph (piezoelectric film or thin plate attached to a substrate which amplifies bending) bending actuator includes microelectromechanical systems (MEMS), microlens arrays, and low-temperature solder deposition [117][118][119][120][121]. Shear mode piezoelectric actuators have been developed for inkjet applications [122]; however, to this author's knowledge shear mode printheads have not been used for the jetting of molten metal.…”

Dissolvable microneedle fabrication using piezoelectric dispensing technology

“…The ability to modify and optimise drop formation via adjustment of the fluid properties has been previously reported (Kagerer et al, 2014). Piezoelectric dispensing units generally have an operating viscosity range of 0-20 cP with an optimal range of 4-8 cP.…”