Optimization of process parameters in piezo- and electrohydrodynamic inkjet printing

Abstract

Stable drop jettability is mandatory for a successful, technical scale inkjet printing, and accordingly, this aspect has attracted much attention in fundamental and applied research. In piezo inkjet printing (PIJ), drops are ejected by reverse piezo-electric effect. Previous studies were mainly focused on Newtonian fluids or polymer solutions. Here, we have investigated the drop jetting for zinc oxide (ZnO) particulate suspensions. Generally, the inverse Ohnesorge number Z = Oh-1, which relates viscous forces to inertia and surface tension, is sufficient to predict the jettability of single phase fluids. For the inkjet printer setup used here, jetting was possible for Newtonian fluids with 2.5 < Z < 26, but in the identical Z-range, nonjetting and nozzle clogging occurred for certain suspensions. A so-called ring-slit device, which allows for simultaneous formation and detection of aggregates in strongly converging flow fields, and single particle detecting techniques, which allow for an accurate determination of the number and size of micrometer-sized aggregates in suspensions of nanoparticles, were used to study this phenomenon. Nozzle clogging is induced by heterocoagulation of micrometer-sized aggregates and ZnO nanoparticles in the elongational flow field at the nozzle exit. Clogging may occur even if the size of these aggregates is well below the nozzle diameter and their concentration is on the order of only a few hundred parts per million (ppm). Accordingly, increased colloidal stability of nanoparticles and reduced aggregate concentration result in better drop jettability. Also, a nozzle design resulting in a shorter exposure time of the ink to elongational flow and an increased flow velocity helps to avoid nozzle clogging. In electrohydrodynamic (EHD) inkjet printing where droplet/jet is generated by electrostatic force, physical as well as electrical properties of the fluid should be taken into account to achieve desired performance. In this study, a systematic approach was suggested to find the processing windows of EHD inkjet printing. Six dimensionless parameters were organized and applied to the printing system of ethanol/terpineol mixtures. Based on the correlation of dimensionless voltage and charge relaxation length, the jet diameter of cone-jet mode was characterized, and the semi-cone angle was compared with the theoretical Taylor angle. In addition, the ratio of electric normal force and electric tangential force on the charged surface of Taylor cone was recommended as a parameter judging the degree of cone-jet stability. The smaller the ratio, the more stable the cone-jet was. This approach was systematic and effective to obtain Taylor cone of cone-jet mode and to evaluate the jetting stability. The control of inks with optimized experimental parameters by this method will improve the jetting performance in EHD inkjet printing. This study is expected to present processing protocols for designing experiments in piezo- and electrohydrodynamic inkjet printing by understanding the processing characteristics and issues, and contribute to the progress of inkjet technology to produce on-demand droplet/jet.