Numerical simulations of the full ink-jet printing processes: From jetting to evaporation

Ink-jet printing requires to perfectly control both the jetting of droplets and the subsequent droplet evaporation and absorption dynamics. Considerable complexity arises due to the fact that ink is constituted of a mixture of different liquids, surfactants and pigments. Using a sharp-interface ALE finite element method, we numerically investigate the main aspects of ink-jet printing, both on the jetting side and on the drying side. We show how a short pause in jetting can result in clogged nozzles due to solvent evaporation and discuss approaches how to prevent this undesired phenomenon. Once the droplets have been jetted on paper and is evaporating, the print quality can be deteriorated by the well-known coffee-stain effect, i.e. the preferential deposition of particles near the rim of the droplet. This can be prevented in several ways, e.g. employing controlled Marangoni flow via surfactants or co-solvents or printing on a primer layer jetted in beforehand, thus creating a homogeneous deposition pattern for a perfect final printout

Inkjet Nozzle Failure by Heterogeneous Nucleation: Bubble Entrainment, Cavitation, and Diffusive Growth

Piezoacoustic drop-on-demand (DOD) inkjet printing is widely applied in high-end digital printing due to its unprecedented precision and reproducibility. Micron-sized droplets of a wide range of chemical compositions can be deposited; however, the stability of piezoacoustic DOD inkjet printing can sometimes be compromised through the stochastic entrainment of bubbles within the ink channel. Here, bubble nucleation, translation, and growth are studied in an experimental silicon-based printhead with a glass nozzle plate using high-speed imaging that is triggered by changes in the ink-channel acoustics. It is found that impurities in the ink can trigger bubble nucleation upon their interaction with the oscillating meniscus. Cavitation inception on a dirt particle during the rarefaction pressure wave is identified as a second mechanism for bubble formation. The acoustic driving pressure within the ink channel, and its change upon bubble nucleation, are obtained from a fit of a Rayleigh-Plesset-type bubble-dynamics equation to the measured time-resolved radial dynamics of the bubble. The measured decrease in channel resonance frequency after bubble entrainment results in a 24% increased ink-jet length. The nucleated bubbles translate toward the ink-channel walls due to acoustic radiation forces and ink streaming. The convective ink flow is characterized using high-speed particle-tracking velocimetry. The vortical flow near the oscillating meniscus is shown to trap the impurities, thereby increasing the particle-to-meniscus interaction probability and, correspondingly, the bubble-entrainment probability

Inkjet Nozzle Failure by Heterogeneous Nucleation:Bubble Entrainment, Cavitation, and Diffusive Growth

\u3cp\u3ePiezoacoustic drop-on-demand (DOD) inkjet printing is widely applied in high-end digital printing due to its unprecedented precision and reproducibility. Micron-sized droplets of a wide range of chemical compositions can be deposited; however, the stability of piezoacoustic DOD inkjet printing can sometimes be compromised through the stochastic entrainment of bubbles within the ink channel. Here, bubble nucleation, translation, and growth are studied in an experimental silicon-based printhead with a glass nozzle plate using high-speed imaging that is triggered by changes in the ink-channel acoustics. It is found that impurities in the ink can trigger bubble nucleation upon their interaction with the oscillating meniscus. Cavitation inception on a dirt particle during the rarefaction pressure wave is identified as a second mechanism for bubble formation. The acoustic driving pressure within the ink channel, and its change upon bubble nucleation, are obtained from a fit of a Rayleigh-Plesset-type bubble-dynamics equation to the measured time-resolved radial dynamics of the bubble. The measured decrease in channel resonance frequency after bubble entrainment results in a 24% increased ink-jet length. The nucleated bubbles translate toward the ink-channel walls due to acoustic radiation forces and ink streaming. The convective ink flow is characterized using high-speed particle-tracking velocimetry. The vortical flow near the oscillating meniscus is shown to trap the impurities, thereby increasing the particle-to-meniscus interaction probability and, correspondingly, the bubble-entrainment probability.\u3c/p\u3

Nozzle flow characterization and motion of entrained bubble in industrial inkjet printer

Piezo-acoustic inkjet printing allows highly controlled deposition of droplets at picoliter volumes. However, sometimes jet stability is compromised by the entrainment of a bubble, which has been shown to occur in conjunction with dirt particle trapping in the printhead, in the vortex ring above the oscillating meniscus (Fraters et al. Phys. Rev. Appl. 12(6) 2019). In this experimental and numerical study, we explore the destabilizing conditions of the flow inside the ink channel that lead to the diffusive growth of the entrained bubble and thereby to complete nozzle failure. We model the unsteady flow inside the channel using a Helmholtz oscillator model for the driving channel acoustics coupled with Navier-Stokes equations for the flow which we validate through time-resolved fluorescent particle tracking velocimetry measurements. Furthermore, bubble dynamics and translation are modeled using the Rayleigh-Plesset equation coupled to a point-particle force balance. We study the flow, particle trapping, and bubble motion for different nozzle geometries and driving conditions, revealing pathways of bubble entrainment and growth, thereby enabling identification and quantification of parameters that ultimately influence the inherent stability of the jetting process

Meniscus Oscillations Driven by Flow Focusing Lead to Bubble Pinch-Off and Entrainment in a Piezoacoustic Inkjet Nozzle

The stability of high-end piezoacoustic drop-on-demand (DOD) inkjet printing is sometimes compromised by the entrainment of an air bubble inside the ink channel. Here, bubble pinch-off from an oscillating meniscus is studied in an optically transparent DOD printhead as a function of the driving waveform. We show that bubble pinch-off follows from low-amplitude high-frequency meniscus oscillations on top of the global high-amplitude low-frequency meniscus motion that drives droplet formation. In a certain window of control parameters, phase inversion between the low- and high-frequency components leads to the enclosure of an air cavity and bubble pinch-off. Although phenomenologically similar, bubble pinch-off is not a result of capillary-wave interaction such as observed in drop impact on a liquid pool. Instead, we reveal geometrical-flow focusing as the mechanism through which, at first, an outward jet is formed on the retracted concave meniscus. The subsequent high-frequency velocity oscillation acts on the now toroidal-shaped meniscus and it accelerates the toroidal ring outward, resulting in the formation of an air cavity that can pinch off. Through incompressible boundary-integral simulations, we reveal that bubble pinch-off requires an unbalance between the capillary and inertial time scales and that it does not require acoustics. The critical control parameters for pinch-off are the pulse timing and amplitude. To cure the bubble entrainment problem, the threshold for bubble pinch-off can be increased by suppressing the high-frequency driving through appropriate waveform design. The present work therefore aids the improvement of the stability of inkjet printers through a physical understanding of meniscus instabilities