
The generation of ink droplets in piezoelectric drop-on-demand printheads is governed by a tightly coupled multiphysics process involving electromechanical actuation, fluid dynamics and free-surface interactions. Conventional simulation approaches often treat these subsystems separately, limiting predictive fidelity for design optimization. In this work, we present a fully coupled three-dimensional fluid structure interaction (FSI) framework that integrates computational structural mechanics (CSM) that incorporates the piezoelectric effect with compressible multiphase computational fluid dynamics (CFD). The framework employs a partitioned solution strategy with strong interface coupling, enabling accurate capture of the dynamic interplay between actuator deformation and droplet ejection. The method is applied to recirculating inkjet printhead designs, utilizing volume-of-fluid (VOF) based free-surface modelling to resolve ink-air interactions and validate performance against benchmark structural and fluidic cases of the Seiko RC1536 printhead. Results demonstrate accurate prediction of droplet formation dynamics, capturing the influence of actuation waveforms, chamber geometry and surface tension effects. By combining high-fidelity electromechanical and fluidic models, this work advances predictive capabilities for inkjet printhead design and operation. The methodology lays the foundation for optimizing printhead architectures, tailoring actuation strategies and accelerating innovation in industrial and functional inkjet applications.

The wood-graining effect is a print defect arising from complex aerodynamic interactions within the narrow gap between printhead and substrate. Unsteady vortical structures, substrate-induced Couette flows and drop-induced entrainment destabilize jetting trajectories, leading to wavering and consolidation of printed tracks. Conventional RANS methods fail to capture these transient dynamics, necessitating high-fidelity turbulence modelling. In this study, large-eddy and detached-eddy simulations are combined with a two-way Lagrangian particle tracking framework to resolve the mutual interactions between jetted drops and surrounding gas flows. This coupling accounts for shielding, wake effects and drop-induced airflow modifications that drive cross-stream displacement. To connect droplet flight dynamics with final print quality, a wall-film model simulates spreading, coalescence and redistribution upon impact, directly linking flow-driven instabilities to visible wood-grain artifacts. The resulting CFD-Lagrangian-film framework provides a predictive description of wood-graining under realistic conditions, enabling systematic exploration of nozzle spacing, print gap and substrate speed. Demonstrated in 600 npi printheads, the study highlights how high nozzle density magnifies the impact of small aerodynamic disturbances. By establishing a physics-based understanding of wood-graining onset and evolution, the methodology supports the design of next-generation printheads and airflow control strategies to mitigate aerodynamic instabilities.

Modern displays utilize color conversion layers to convert blue backlight into colored sub-pixels. Perovskite quantum dots (PQDs) are a very efficient material for this conversion and are currently applied as one continuous film combined with a color filter layer. Inkjet printing could enable the application of PQDs directly into colored sub-pixels, eliminating the filter layer and improving efficiency of the display by design. In this work, PQDs were modified for application by inkjet. UV-curable resin and PQD dispersion were modified to reduce viscoelasticity down into inkjetable range, which was characterized using the TriPAV high frequency rheometer. Printability of PQDs was further shown by dropwatching and manufacturing an inkjet-printed pixel-array demonstrator. The highly loaded ink is UV-curable and solvent-free. It can be applied by inkjet into precise sub-pixel arrays with a pixel thickness of 10µm in one pass. At that thickness, color conversion efficiency and optical density of the PQD sub-pixels fulfill specifications needed for application in modern displays.

Inkjet printing has been applied in manufacturing structural and functional materials for decades. There are two kind of methods known as continuous inkjet (CIJ) printing and drop-on-demand (DOD) inkjet printing.[1] In DOD inkjet printing, drops are generated only when a drop needed by producing a pressure pulse in a chamber filled with inks. And before drop generation, the inkjet printing head will be moved to the desired location to locate the drop in a precise position. DOD inkjet printing is a method that directly places materials on demand, which saves the required raw materials and reduces the printing steps. As a consequence, DOD inkjet printing saves more time with lower waste consumption during production than CIJ printing and the equipment has a smaller footprint.DOD inkjet printing can be divided into two methods by which the pressure pulse is generated followed by drop ejection: thermal DOD inkjet printing and piezoelectric DOD inkjet printing. In piezoelectric DOD inkjet printing, the pressure pulse is produced by the mechanical actuation of the chamber walls.[2] When a voltage is applied, the piezoelectric material changes shape, which generates a pressure pulse in the fluid forcing a droplet of ink from the nozzle. It is important to know the inkjet printable range to generate accurate and repeatable drops. Fromm had defined ink printability in drop on demand (DOD) printing using a dimensionless Z number which related to the physical properties of the inks.[3] However, it is still not agreed whether there is a precise Z number range for inkjet printability and not known whether the range varies using different actuating pulses.The goal of our study is to find out the detail relationship between the ink properties and Drop-on-Demand inkjet printing printability and explore whether the printable Z number range change with actuating pulses and different kind of printheads.Here we investigate the influence of Z number and pulse voltage on printability using two inkjet printheads (10 pl Dimatix and 80 μm MicroFab). We have used 10 model inks made from solvent mixtures of ethylene glycol, diethylene glycol and distilled water. A range of actuating pulse voltages has also been studied. We found that the printable Z number range changes with the pulse voltages applied on inkjet printing. When increasing pulse voltage to print the same ink, it becomes printable under low pulse voltage and flying slow in the air and then printing well until at a certain voltage satellites forms and more satellites form when further increasing the pulse voltage. We also found that the printable voltage range is slightly different among inks with Z < 8. Under higher pulse voltages, it is possible to get single droplets with Z < 4, but inks with Z < 4 are printed out with some satellites. However, accurate and stable drops without satellites could be formed using inks of Z < 4 under lower voltages and it is not printable for inks of Z < 4 when printed under lower pulse voltages. These results could give an explanation of the different Z number range shown in different researches published when they using different printheads and pulse voltages.

Optical waveguides transmit light in a controlled manner,1 making its propagation possible over long distances, with only little and predictable loss. This gives a great advantage for optoelectronics and in wavelength sensitive biological systems,2 allowing fast information transfer without electromagnetic interference.3 Typically, optical elements (e.g. printed optical circuit boards) are not required to be flexible or stretchable. However, elasticity and stretchability expand the field of waveguide application. Recent efforts focus on creating fully elastic systems, able to withstand complex deformation (stretching, twisting, bending).4,5The goal of the research was to develop a system that is compatible with an inkjet printing process and allows the fabrication of elastomeric waveguides, employing commercially available products. For that purpose the system has the following requirements:1. The ink should compromise good printing properties and high substrate wettability. It should be chemically stable and photocurable.2. The ink should have good elastomeric properties after curing, and the mechanical properties of the ink should match the ones of the cladding, to avoid delamination at the waveguide/cladding interface at higher strains.3. The cladding should have a lower refractive index than the cured ink, and this difference should be possibly big.4. The printed channel should be homogenous, with stable contact lines, stable cross section along the long axis and possibly big contact angle. Bulge formation has to be avoided.6The first three requirements are met by optimising the composition of polyurethane acrylate inks, and using PDMS as a substrate and the cladding material. Requirement 4 poses a difficult problem to overcome, since the stability of printed channels decreases for high contact angles.6 The contact angle can be adjusted by modifying the surface energy and viscosity of both the ink and the substrate. A method which allowed us to obtain homogenous lines with a high contact angle (see Fig. 1) is described. All structures were fabricated with a standard laboratory printer (Dimatix DMP-2831), with an attached LED lamp (Omni Cure LED, Series 1000). <fig position="float" id="s48_f.1"> <label>Figure 1</label> <caption>LEFT: A typical cross section of the printed waveguides; RIGHT: the top view of the printed waveguides</caption> <graphic mime-subtype="tif" xlink:href="Images\s48_f01.tif" xmlns:xlink="http://www.w3.org/1999/xlink"/> </fig>