Electrostatic effects in coating and printing processes

Abstract

Coating and printing are interfacial processes that are highly relevant in industry. Precision coatings impart functionalities and boost the performance of products. On the other hand, high-resolution roll-to-roll printing is being increasingly explored for creating dense and flexible printed electronics at high speeds. Electrostatic effects often significantly influence both these processes. However, in industry, much of the current understanding of these effects is empirical and has not received a rigorous treatment. This thesis discusses how electrostatics and hydrodynamics couple in coating and printing applications, and presents different modes of investigation: simplified thin-film models and flow visualization experiments, to understand the underlying physics of these processes. Throughout this work, the electric response of liquids has been described by the perfect (non-conducting) and leaky dielectric (partially conducting) models, which are representative of many liquids used in industry. In coating processes, electrostatic charges are known to accumulate on the substrate due to various upstream operations (e.g. corona treatment, friction in roll-to-roll equipment). This leads to the buildup of an electric field in the subsequently coated film, which in turn causes the formation of defects due to electrostatically driven flows. Thus, in order to obtain high quality coatings, it is desirable to keep them resistant to electrostatic destabilization. We have carried out a systematic study via the construction of electrohydrodynamic lubrication models to understand the influence of charged substrates and charged interfaces on the leveling of liquid coatings. Based on our findings, we develop simple heuristics that can be used to design coatings that are stable to substrate charging and charge contamination. Electric fields are also present in some printing processes. Developed in the late1960s, electrostatic assist (ESA) has been long used to remove printing defects and enhance image quality in gravure printing, a high-resolution roll to-roll process. ESA involves the application of an electric field to pull ink out of cavities and transfer it onto the desired substrate. However, there is limited understanding of how this process works, which hinders its development as a tool for printed electronics. In order to address this issue, we develop a model for electrostatically assisted meniscus deformation near a cavity (this describes the first stage of electrostatic assist). Our calculations show that electric fields pull up the ink meniscus either at the edges or at the center of the cavity, depending on the ink conductivity. This suggests that ink contact with the substrate will be improved during ESA but air entrapment occurs for a certain range of conductivities, which would be detrimental to print quality. Our model also enables us to investigate the effect of cavity shape and spacing on the mode of deformation of the ink surface. In order to validate the findings from our electrohydrodynamic model, we have carried out flow visualization experiments to track the deformation of liquids contained in cavities, and these corroborate the qualitative trends of meniscus deformation predicted by the model.