Flow Characterization in A Piezo-Acoustic Inkjet Printhead: Vortex Formation and The Role of Nozzle Tapering

Abstract Piezo acoustic drop-on-demand inkjet printing enables highly controlled droplet deposition with broad industrial applications. However, performance is often compromised by air bubble entrainment, triggered by particles trapped in the flow above the nozzle. Understanding the internal flow within the ink channel is therefore a critical first step toward preventing particle trapping and bubble nucleation. In this study, we use fluorescence microparticle tracking velocimetry (μPTV) to investigate flow inside both straight and tapered inkjet nozzles. These measurements are used to (i) validate direct numerical flow simulations, (ii) understand steady and unsteady flow behavior, and (iii) assess how flow structure depends on driving parameters and nozzle geometry. The simulations are in agreement with experiments: unsteady flow features are qualitatively reproduced, and time-averaged velocity fields show quantitative correspondence. Our results reveal significantly higher peak in-plane vorticity and larger vortex ring structures in the straight nozzle compared to the tapered one. In both geometries, the vortex ring proves robust, and its size is primarily determined by the taper angle, with only a minor influence from the driving amplitude. This improved understanding of ink channel flow provides a foundation for nozzle design strategies that reduce particle trapping and enhance the stability and reliability of inkjet printing.