Infrared imaging and acoustic sizing of a bubble inside a MEMS piezo ink channel

Piezo drop-on-demand inkjet printers are used in an increasing number of applications because of their reliable deposition of droplets onto a substrate. Droplets of a few picoliters are ejected from an inkjet nozzle at frequencies of up to 100 kHz. However, the entrapment of an air microbubble in the ink channel can severely impede the productivity and reliability of the printing system. The air bubble disturbs the channel acoustics, resulting in disrupted drop formation or failure of the jetting process. Here we study a micro-electro-mechanical systems-based printhead. By using the actuating piezo transducer in receive mode, the acoustical field inside the channel was monitored, clearly identifying the presence of an air microbubble inside the channel during failure of the jetting process. The infrared visualization technique allowed for the accurate sizing of the bubble, including its dynamics, inside the intact printhead. A model was developed to calculate the mutual interaction between the channel acoustics and the bubble dynamics. The model was validated by simultaneous acoustical and infrared detection of the bubble. The model can predict the presence and size of entrapped air bubbles inside an operating ink channel purely from the acoustic response

Sticky bubbles

We discuss the physical forces that are required to remove an air bubble immersed in a liquid from a corner. This is relevant for inkjet printing technology, as the presence of air bubbles in the channels of a printhead perturbs the jetting of droplets. A simple strategy to remove the bubble is to ush the ink past the bubble by providing a high pressure pulse. In this report we rst compute the viscous drag forces that such a ow exerts on the bubble. Then, we compare this to the \sticking forces" on the bubble, due to the capillary interaction with the wall. From this we can estimate the required ow velocities for bubble removal, as a function of channel geometry, contact angle and ink properties. Finally, we investigate other ways to exert a force on a trapped bubble. In particular we focus on forces induced by electric elds which can alter the contact angle of the drop, or by locally applying thermal gradients. Once again, these forces are compared to the sticking forces to identify the parameters where the bubble can be removed

Free surface flow and acousto-elastic interaction in piezo inkjet

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 possibl

Drop dynamics in the inkjet printing process

The inkjet printing process involves a chain of processes in many physical domains at different length and time scales. The final goal is the deposition of droplets of all kinds of fluids with any desired volume and velocity. To comply with the increasing and diverging requirements for today's inkjet technology, a fundamental understanding of the underlying processes is very important. By combining state of the art experimental and numerical techniques, the physics behind the chain of processes are being explored. The fundamental knowledge gained is crucial for the further development of the inkjet printing technology which became mature in graphical printing applications and plays a key role in many emerging new industrial and medical applications

Printhead health in industrial inkjet printing, in-line and off-line detection of poor drop formation

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 choice as control strategy

The stability of viscous liquid filaments

The stability of liquid filaments is relevant both in industrial applications, such as inkjet printing and atomization, and in nature, where the stability of filaments has a large influence on the final drop size distribution of rain droplets and waterfalls. The liquid filament may either stably collapse into a single droplet, or break up into multiple droplets. Which scenario is realized depends on the viscosity and the aspect ratio of the filament. Here we study the collapse of an axisymmetric liquid filament is analytically and with a numerical model. We find that a long, high viscous filament can only break up due to the Rayleigh-Plateau instability, whereas a low viscous filament can break up due to end-pinching. The theory shows quantitative agreement with recent experimental findings by Castr\'{e}jon-Pita et al.