Herman Wijshoff - Academia.edu (original) (raw)

Papers by Herman Wijshoff

Bulletin of the American Physical Society, 2018

The invention relates to a printhead comprising multiple substantially closed ink chambers (13), ... more The invention relates to a printhead comprising multiple substantially closed ink chambers (13), the ink chambers being mutually separated by at least one wall (12), wherein each of the chambers comprises an electro-mechanical converter (15), where actuation of the converter leads to a volume change of the corresponding chamber. The invention also relates to an inkjet printer comprising such a printhea

The Science of Inkjet and Droplets, 2015

Inkjet-Based Micromanufacturing, 2012

The actuation in piezo inkjet is done with a piezoelectric element, which deforms the channels. C... more The actuation in piezo inkjet is done with a piezoelectric element, which deforms the channels. Crosstalk is the effect that also neighboring channels can be influenced by the actuation of a channel. Crosstalk effects via the structure of the printhead are electrical crosstalk, direct crosstalk, and pressure induced crosstalk. The latter crosstalk effect goes also through the ink domain and another ink domain related crosstalk effect is acoustic crosstalk via the ink supply. Furthermore, disturbing resonances in the printhead structure can be excited. Finally, the variations in drop properties can become very large when crosstalk effects interfere with residual vibrations in the ink domain

Surfactants have been widely studied and used in controlling droplet evaporation. In this work, w... more Surfactants have been widely studied and used in controlling droplet evaporation. In this work, we observe and study the crystallization of sodium dodecyl sulfate (SDS) within an evaporating glycerol−water mixture droplet. The crystallization is induced by the preferential evaporation of water, which decreases the solubility of SDS in the mixture. As a consequence, the crystals shield the droplet surface and cease the evaporation. The universality of the evaporation characteristics for a range of droplet sizes is revealed by applying a diffusion model, extended by Raoult's law. To describe the nucleation and growth of the crystals, we employ the 2-dimensional crystallization model of Weinberg [J. Non-Cryst. Solids 1991, 134, 116]. The results of this model compare favorably to our experimental results. Our findings may inspire the community to reconsider the role of high concentration of surfactants in a multicomponent evaporation system.

Journal of Fluid Mechanics, Dec 15, 2020

Bulletin of the American Physical Society, 2020

Droplets with a well-controlled and narrow size distribution are required in many industrial and ... more Droplets with a well-controlled and narrow size distribution are required in many industrial and medical applications. In this work we study droplet formation from a piezo inkjet print head by using ultra high-speed imaging at 10 million frames per second. Also a second technique was applied, in which a single flash of 5ns is used to illuminate the drop formation. The acquired recordings are used to calculate the flow rate and velocity development inside the droplet. The result could then be compared with a 1 dimensional model based on the lubrication approximation. In the model the equations are translated into a second order accurate scheme where the pinch-off singularity is prevented by adding a finite regularisation film.

Arjan Fraters, Maaike Rump, Roger Jeurissen, Marc van den Berg, Youri de Loore, Hans Reinten, Her... more Arjan Fraters, Maaike Rump, Roger Jeurissen, Marc van den Berg, Youri de Loore, Hans Reinten, Herman Wijshoff, 4 Devaraj van der Meer, Detlef Lohse, Michel Versluis, and Tim Segers 1 Physics of Fluids group, Max-Planck Center Twente for Complex Fluid Dynamics, Department of Science and Technology, MESA+ Institute, and J. M. Burgers Centre for Fluid Dynamics, University of Twente, Enschede, Netherlands Department of Applied Physics, Eindhoven University of Technology, Eindhoven, Netherlands Canon Production Printing Netherlands B.V., Venlo, Netherlands Department of Mechanical Engineering, Eindhoven University of Technology, Eindhoven, Netherlands BIOS Lab-on-a-Chip group, Max-Planck Center Twente for Complex Fluid Dynamics, MESA+ Institute for Nanotechnology, University of Twente, Enschede, Netherlands

The Journal of the Acoustical Society of America, 2006

Applied Physics Letters, 2007

Journal of Colloid and Interface Science, 2020

Hypothesis: There are two different sharp-interface models for moving contact lines: slip models ... more Hypothesis: There are two different sharp-interface models for moving contact lines: slip models and precursor film models. While both models predict a mostly constant contact angle during the evaporation of pure droplets, it is expected that they behave differently when surfactants are present, because of the inherent dissimilarities in their respective interface definitions. Simulations: Both contact line models are numerically implemented using lubrication theory to analyze evaporating droplets. A convection-diffusion equation is implemented for insoluble surfactants. For pure droplets the models are compared with experiments performed by Nguyen et al. (2012). Findings: The two contact line models show results comparable to the experiments with pure droplets. If insoluble surfactants are present, the slip model increasingly shows pinning-like behavior as the initial surfactant concentration is increased. This 'quasi-pinning' is found to be consistent with experimental results in literature. The precursor film model, in contrast, shows no significant change when surfactants are added. This lack of change is a result of surfactant flowing from the droplet into the precursor film and vice versa. While suggesting potential solutions to this unphysical behavior, it is concluded that in the context of surfactants, slip models are preferable over precursor film models given the current state of the art.

Bulletin of the American Physical Society, 2019

For most inkjet applications, it is very important that exactly one drop is generated with the de... more For most inkjet applications, it is very important that exactly one drop is generated with the desired speed, volume, shape, and direction as a result of a single actuation action of a printhead. The printheads are the heart of every printer and their condition is crucial for the printing performance. Rayleigh-Taylor instability occurs when inertial forces overcome the restoring surface tension forces, which occurs above a certain critical acceleration of the meniscus. There are several mechanisms that result in satellite drops or misting. A built-in measurement method of the meniscus position, which also allows measurements in the jetting situation, is offered by a sensing nozzle plate with a capacitive layer inside the nozzle. In most cases, the term control is associated with feedback control. Feedback control aims primarily at stabilization and disturbance rejection. Given the highly repetitive character of the jetting process, iterative learning control (ILC) is a logical choic...

Inkjet printing deposits droplets with a well-controlled narrow size distribution. This paper aim... more Inkjet printing deposits droplets with a well-controlled narrow size distribution. This paper aims at improving experimental and numerical methods for the optimization of drop formation. We introduce a method to extract the one-dimensional velocity profile inside a single droplet during drop formation. We use a novel experimental approach to capture two detailed images of the very same droplet with a small time delay. The one-dimensional velocity within the droplet is resolved by accurately determining the volume distribution of the droplet. We compare the obtained velocity profiles to a numerical simulation based on the slender jet approximation of the Navier-Stokes equation and we find very good agreement.

Modeling plays an essential role in our research on new inkjet technologies. Structural modeling ... more Modeling plays an essential role in our research on new inkjet technologies. Structural modeling with Ansys includes piezo-electricity. Acoustic modeling in Ansys and Matlab involves fluid-structure interaction. CFD modeling with Flow3D includes wall-flexibility and free surface flow with surface tension. Added to our measurements this reveals the phenomena involved in our main goal: firing droplets of ink at a very high rate with any desired shape, velocity, dimension and a reliability as high as possible.

Journal of Fluid Mechanics

Long and slender liquid filaments are produced during inkjet printing, which can subsequently eit... more Long and slender liquid filaments are produced during inkjet printing, which can subsequently either retract to form a single droplet, or break up to form a primary droplet and one or more satellite droplets. These satellite droplets are undesirable since they degrade the quality and reproducibility of the print, and lead to contamination within the enclosure of the print device. Existing strategies for the suppression of satellite droplet formation include, among others, adding viscoelasticity to the ink. In the present work, we aim to improve the understanding of the role of viscoelasticity in suppressing satellite droplets in inkjet printing. We demonstrate that very dilute viscoelastic aqueous solutions ( textconcentrationssim0.003,\text {concentrations} \sim 0.003\,\%textconcentrationssim0.003, wt. polyethylene oxide, corresponding to nozzle Deborah number Densim3De_{n}\sim 3Densim3 ) can suppress satellite droplet formation. Furthermore, we show that, for a given driving condition, upper and lower bounds of polymer concentration exist, wi...