Higher-order meniscus oscillations driven by flow-focusing leading to bubble pinch off and entrainment in a piezo acoustic inkjet nozzle

TL;DR

This paper investigates how flow-focusing driven by higher-order meniscus oscillations causes bubble pinch-off in piezo acoustic inkjet nozzles, revealing mechanisms to improve printing stability.

Contribution

It uncovers the role of geometrical flow focusing and phase inversion in bubble pinch-off, offering insights to suppress bubble entrainment in inkjet printing.

Findings

Bubble pinch-off is driven by flow focusing and meniscus oscillations.

Phase inversion between oscillation components leads to bubble formation.

Waveform design can suppress high-frequency oscillations to prevent bubbles.

Abstract

The stability of high-end piezo-acoustic 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 acoustically driven 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 higher-order meniscus oscillations on top of the global high-amplitude 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. When the subsequent high-frequency pressure wave hits the now toroidal-shaped meniscus, it accelerates the toroidal ring outward resulting in the formation of an air cavity that can pinch off. 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 acoustic waves through appropriate waveform design. The present work therefore aids the improvement of the stability of inkjet printers through a physical understanding of meniscus instabilities.