Numerical Simulation of Coupled Wetting and Transport Phenomena in Inkjet Printing

Inkjet printing is a complex process including coupled wetting, heat and species transport. The print quality depends on accurate positioning of ink droplets and correct dot sizes. Temperature differences between printhead and substrate give rise to heat transfer upon droplet impact. Furthermore, inks are complex fluids containing, among other components, surface-active substances. The aim of this thesis is to develop an improved understanding of the coupled transport processes and to derive recommendations for increased print quality. For that purpose, individual droplets impacting a solid substrate are studied. Furthermore, the off-centered collision of an impacting droplet with a previously applied neighboring droplet is investigated. Droplet impact, spreading, and substrate-sided collision are studied using numerical simulation. The algebraic volume of fluid method is used to describe the two-phase flow. Special focus is put on the evaluation of interfacial forces. The contact line dynamics are described using a subgrid-scale model in combination with localized slip. The model is validated for droplet impact and substrate-sided collision using experimental data. Heat transfer between fluids and solid is taken into account. Furthermore, a novel two-field approach for the description of a soluble surface-active substance is introduced. Using analytical and numerical reference solutions, the method is verified regarding transport of this surfactant with and along the interface, adsorption to the interface, and conservation of the surfactant amount. Simulation results for the impact and spreading of individual droplets show an influence of the ink's viscosity throughout the entire process. Lower viscosity due to increased temperature produces faster spreading. It is found that within the capillary-driven regime the influence of the initial droplet temperature is small compared to the initial substrate temperature. For surfactant-laden droplets spreading over a partially wettable substrate, an additional adsorption-limited spreading regime is observed. Increasing the rate of adsorption to the liquid-gas interface results in faster spreading within this regime. Simulations of the collision of an impacting droplet with a previously applied neighboring droplet on the substrate show an offset of the coalesced droplets towards the first droplet. This offset persists well into the capillary-driven spreading regime. It can be attributed to a flow from the second to the first droplet driven by a larger Laplace pressure jump across the interface in the region of the second droplet. Increasing the impact velocity or decreasing the droplet interval reduces this asymmetry and consequently the offset. In contrast to that, the influence of initial droplet temperatures as well as surfactant concentrations on droplet position after collision is found to be small. Nevertheless, the simulation results for the collision of surfactant-laden droplets show that increasing the surfactant concentration results in further spreading on partially wetting substrates, similar to what is also observed for individual droplets