Selective evaporation at the nozzle exit in piezoacoustic inkjet printing

TL;DR

This study investigates how selective evaporation at the nozzle exit affects ink composition and print quality in inkjet printing, using experiments, modeling, and simulations to understand the dynamics and improve process stability.

Contribution

It introduces an analytical and numerical framework to model and analyze the effects of evaporation on ink composition and printhead stability in inkjet printing.

Findings

Concentration gradients stabilize due to steady water flux.

Number of drops before returning to initial composition shows a stronger than exponential trend.

Analytical model validated by fluid mechanical simulations.

Abstract

In practical applications of inkjet printing the nozzles in a printhead have intermittent idle periods, during which ink can evaporate from the nozzle exit. Inks are usually multicomponent where each component has its own characteristic evaporation rate resulting in concentration gradients within the ink. These gradients may directly and indirectly (via Marangoni flows) alter the jetting process and thereby its reproducibility and the resulting print quality. In the present work, we study selective evaporation from an inkjet nozzle for water-glycerol mixtures. Through experiments, analytical modeling, and numerical simulations, we investigate changes in mixture composition with drying time. By monitoring the acoustics within the printhead, and subsequently modeling the system as a mass-spring-damper system, the composition of the mixture can be obtained as a function of drying time. The results from the analytical model are validated using numerical simulations of the full fluid mechanical equations governing the printhead flows and pressure fields. Furthermore, the numerical simulations reveal that the time independent concentration gradient we observe in the experiments is due to the steady state of water flux through the printhead. Finally, we measure the number of drop formation events required in this system before the mixture concentration within the nozzle attains the initial (pre-drying) value, and find a stronger than exponential trend in the number of drop formations required. These results shed light on the complex physiochemical hydrodynamics associated with the drying of ink at a printhead nozzle, and help in increasing the stability and reproducibility of inkjet printing.