PhD on inkjet printing:
How do meniscus shape instabilities lead to air entrapment in piezo-acoustic inkjet printing? (experiments, numeric, theory)

In piezo-acoustic ink-jet devices a voltage pulse applied to a piezo-electric element causes the ink-filled channel to deform, thereby creating a pressure wave in the ink, ultimately leading to the jetting of droplets out of the nozzle. A notorious problem in these systems is the entrainment of tiny air bubbles during operation. Once entrained, the bubbles grow by rectified diffusion, consequently disrupt the pressure waves and finally seriously disturb the droplet formation process. Though in the last years we have made considerable progress in detecting bubbles and monitoring their growth, the exact mechanism and the exact conditions of the entrainment process at the nozzle remain elusive. The same holds for the underlying meniscus instabilities.

The objective of this project is to understand these instabilities and to be able to predict under what conditions bubble entrainment will take place. The working hypothesis is that either a Rayleigh-Taylor or a capillary instability of the meniscus is responsible for the entrainment. The probability of air entrapment is strongly related to the extreme displacement of the meniscus relative to the nozzle edge and, related to this, the filling with ink of the nozzle.

On the experimental side, we will use high-speed imaging techniques in order to visualize the entrainment. On the numerical side we will use two models developed by Océ based on commercial CFD codes (Ansys, Fluent, Flow3D), an in-house boundary integral code, and an in-house level set code to describe the movement of the accelerated meniscus and its interaction with the nozzle wall and the surrounding air.

The available control parameters are the driving pulse (shape and amplitude), the driving pressure, the driving frequency, the nozzle shape, size and wettability properties, and the material properties of the ink such as viscosity and surface tension.

Research

The Physics of Fluids group works on a variety of aspects in fluid mechanics, in particular on those related to bubbles. The focus of our work is the fundamental understanding the phenomena of the physics of fluids, bubbles and jets, which we undertake by experimental, numerical and theoretical means. Within the group both fundamental and applied work, in close collaboration with industrial partners, is done. Our research is embedded in the MESA+ and MIRA Institutes in Twente and the J.M. Burgers Research Center for fluid mechanics (JMBC). The group receives external research funds from NWO-I, ERC, EU, STW, NWO, SenterNovem, and several industrial partners.

We seek highly talented, enthusiastic, and exceptionally motivated candidates with an MSc. degree in Physics or engineering. The candidate must have strong communication skills, including fluency in written and spoken English.

Fixed-term contract: four years.

When fulfilling a PhD position at the NWO-I, the Institutes Organisation of NWO, you will get the status of junior scientist. You will have an employee status and can participate in all the employee benefits the NWO Institutes Organisation (NWO-I) offers. You will get a contract for four years. Your salary will be up to a maximum of 2,834 euro gross per month. The salary is supplemented with a holiday allowance of 8 percent and an end-of-year bonus of 8.33 percent. 
You are supposed to have a thesis finished at the end of your four year term with NWO-I. 

A training programme is part of the agreement. You and your supervisor will make up a plan for the additional education and supervising that you specifically need. This plan also defines which teaching activities you will be responsible (up to a maximum of ten percent of your time). The conditions of employment of NWO-I are laid down in the Collective Labour Agreement for Research Centres (Cao-Onderzoekinstellingen), more exclusive information is available at this website under Personeelsinformatie (in Dutch) or under Personnel (in English). General information about working at NWO-I can be found in the English part of this website under Personnel. The 'Recruitment code' applies to this position.

University of Twente

University of Twente
Faculty of Science and Technology
Physics of Fluids
Building MEANDER (27)
Enschede, the Netherlands