Modelling the imbibition of an ink-like fluid into the thin coating layer of paper

Abstract

To achieve good results with inkjet printing, it is important to know how an ink droplet will spread over and into a coated paper. In this thesis, a pore-scale and a continuum-scale model are developed to study the infiltration of small droplets into the thin coating layer of paper and to describe the effect of ink properties on this infiltration. With the use of CFD software OpenFOAM, the volume of fluid method is applied for direct pore-scale modelling. The computational domain is constructed with the use of FIB-SEM images of a real paper coating layer. The results are comparable with experimental data. The final ink imprint inside a coating layer is influenced by the droplet position on the coating and the contact angle. Changing the viscosity, density and surface tension has very little effect on the final ink distribution. However, an increase in viscosity and a decrease in surface tension result in a decreased rate of spreading and infiltration. An increase in density results in slightly slower imbibition, while the rate of spreading remains the same. When the droplet arrives at the coating with a velocity of 5 m/s, the rate of spreading and infiltration slightly increases compared to a sessile droplet. Moreover, the wetted area at the paper surface increases slightly in size. HYDRUS is used for continuum-scale modelling. Average hydraulic properties are used to describe the fibrous and coating layer, which means that local differences in pore structure are not incorporated. The results show a large wetted area, containing a large unsaturated region. The results are not comparable with experimental data and the results of OpenFOAM, so pore-scale modelling is a much better option to describe this process.