LAUSR

In this study, uncoated paper was characterized. Three-dimensional structure of the layer was reconstructed using imaging results of micro-CT scanning with a relatively high resolution $$(0.9~\upmu \hbox {m})$$(0.9μm). Image analysis provided the pore space of the layer, which was used to determine its porosity and pore size distribution. Representative elementary volume (REV) size was determined by calculating values of porosity and permeability values for varying domain sizes. We found that those values remained unchanged for domain sizes of $$400\times 400\times 150\,\upmu \hbox {m}^{3}$$400×400×150μm3 and larger; this was chosen as the REV size. The determined REV size was verified by determining capillary pressure–saturation imbibition curves for various domain sizes. We studied the directional dependence of curves by simulating water penetration into the layer from various directions. We did not find any significant difference between curves in different directions. We studied the effect of compression of paper on curves. We found that up to 30% compression of the paper layer had very small effect on the curve. Relative permeability as a function of saturation was also calculated. Water penetration into paper was visualized using confocal laser scanning microscopy. Dynamic visualization of water flow in the paper showed that water moves along the fibers first and then fills the pores between them.