Characterizing the hydraulic properties of a paper coating layer using FIB-SEM tomography and 3D pore-scale modeling

Paper used in the printing industry generally contains a relatively thin porous coating covering a thicker fibrous base layer. The three-dimensional pore structure of coatings has a major effect on fluid flow patterns inside the paper medium. Understanding and quantifying the flow properties of thin coating layers is hence crucial. Pore spaces within the coating have an average size of about 180 nm. We used scanning electron microscopy combined with focused ion beam (FIB-SEM) to visualize the nano-scale pore structure of the paper coating layer. Post-processing of the FIB-SEM images allowed us to reconstruct the three-dimensional pore space of the coating. The 3D FIB-SEM images were analyzed in detail to obtain pore size distribution and porosity value. The permeability was estimated using the Geo-Dict software, based on solutions of the Stokes equation. By determining the porosity and the permeability for increasingly larger domain sizes, we estimated the size of a representative elementary volume (REV) for the coating layer to be 60 µm3, which is well within the volume analyzed by FIB-SEM. The estimated porosity and permeability of the REV domain were 0.34 and 0.09 mDarcy, respectively, relatively close to previous literature values for coatings ((Patrick A. Gane et al., 1996), (Alam et al., 2009)). Using the pore morphology method, the capillary pressure-saturation (Pc-S) and relative permeability curves of the REV domain could be constructed next. The Pc-S curves showed that the coating has a high air entry suction, which is very favorable for printing in that an ink will invade the coating as soon as it applied to the coating. Our results are essential for macroscale modelling of ink penetration into a coating layer during inkjet printing. Macroscopic models can be valuable tools for optimization of the penetration depth and the spreading of ink on and within paper substrates.