The stability of inkjet printers is a major requirement for high-quality-printing. However, in piezo-driven inkjet printheads, air entrapment can lead to malfunctioning of the jet formation. The piezoactuator is employed to actively monitor the channel acoustics and to identify distortions at an early stage. Modifications of the response of the piezoactuator indicate entrapped air bubbles and these allow us to investigate them. When we employ the signal as a trigger for high-speed imaging, we can visualize the consequences of the entrained bubbles on the droplet formation. Two mechanisms are found to cause air entrapment: First, a distorted droplet formation caused by small particles, and, second, an accumulation of ink on the nozzle plate, which favors void formation once the meniscus is pulled back.

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.

Document VersionPublisher's PDF, also known as Version of Record (includes final page, issue and volume numbers)Please check the document version of this publication:• A submitted manuscript is the author's version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Air bubbles entrapped in the ink channel are a major problem in piezo-driven inkjet printing. They grow by rectified diffusion and eventually counteract the pressure buildup at the nozzle, leading to a breakdown of the jetting process. Experimental results on the droplet velocity u drop as a function of the equilibrium radius R 0 of the entrained bubble are presented. Surprisingly, u drop ͑R 0 ͒ shows a pronounced maximum around R 0 =17 m before it sharply drops to zero around R 0 =19 m. A simple one-dimensional model is introduced to describe this counterintuitive behavior which turns out to be a resonance effect of the entrained bubble.

Flow patterns of ink layers on the nozzle plate of a piezo-driven printhead are investigated. Two different flow types are identified. First, a jet of droplets induces a radial airflow in the direction of the jet, which in turn causes a liquid flow towards the nozzle. Second, the movement of the meniscus in the nozzle causes an equally strong flow, but completely different flow patterns. The results are presented in a phase diagram with pulse amplitude and firing frequency as parameters.

In piezo inkjet printing, nozzle failures are often caused by an ink layer on the nozzle plate. It is experimentally shown that the ink layer at the nozzle is formed through streamers of ink, emanating from a central ink band on the nozzle plate. The streamers propagate over a wetting nanofilm of 13nm thickness, directed toward the actuated nozzles. The motion of the front end of the streamers follows a power law in time with an exponent 12. The observations are consistent with a surface tension gradient driven flow. The origin of the Marangoni flow is an effective lower surfactant concentration of the ink around the nozzle.