Vorticity-induced flow-focusing leads to bubble entrainment in an inkjet printhead: synchrotron X-ray and volume-of-fluid visualizations

TL;DR

This study combines advanced imaging and simulations to reveal how vorticity and baroclinic torque cause bubble entrainment in inkjet printheads, impacting print stability.

Contribution

It uncovers the flow-focusing mechanism driven by vorticity and baroclinic torque responsible for bubble formation, validated through experiments and numerical simulations.

Findings

Good agreement between experiments and simulations.

Identification of flow-focusing caused by baroclinic torque.

Multiple bubble pinch-off classes observed and captured numerically.

Abstract

The oscillatory flows present in an inkjet printhead can lead to strong deformations of the air-liquid interface at the nozzle exit. Such deformations may lead to an inward directed air jet with bubble pinch-off and the subsequent entrainment of an air bubble, which is highly detrimental to the stability of inkjet printing. Understanding the mechanisms of bubble entrainment is therefore crucial in improving print stability. In the present work, we use ultrafast X-ray phase-contrast imaging and direct numerical simulations based on the Volume-of-Fluid method to study the mechanisms underlying the bubble entrainment in a piezo-acoustic printhead. We first demonstrate good agreement between experiments and numerics. We then show the different classes of bubble pinch-off obtained in experiments, and that those were also captured numerically. The numerical results are then used to show that the baroclinic torque, which is generated at the gas-liquid interface due to the misalignment of density and pressure gradients, results in a flow-focusing effect that drives the formation of the air jet from which a bubble can pinch-off.