The three dimensional structure of paper materials plays a critical role in the paper manufacturing process especially via its impact on the transport properties for fluids. Dewatering of the wet web, pressing and drying will benefit from knowledge of the relationships between the web structure and its transport coefficients. Among transport, moisture diffusion in paper is central to the understanding and optimal design of paper products for their performance in different environmental conditions. Our recent research of moisture sorption in paper has indicated that diffusion of water vapor through the pore space is an important mechanism for transport [1,2]. The effect of the three dimensional structure of the paper sheet on the diffusion of moisture is significant.
The structure of the pore space within a paper sheet is imaged in serial sections using x-ray microtomography. The three dimensional structure is reconstructed from these sections using digital image processing techniques. The structure is then analyzed by measuring traditional descriptors for the pore space such as specific surface area and porosity. In addition, morphometric and quantitative stereological techniques are used to characterize the structure. Techniques of mathematical morphology  used include erosion, dilation, closing, opening and binarization with subsequent skeletonization.
A sequence of microtomographs was imaged at approximately 2 μm intervals and the three-dimensional pore-fiber structure was reconstructed. The pore size distributions for both in-plane as
well as transverse pores were measured. Significant differences in the in-plane (X-Y) and the transverse directions in pore characteristics are found and may help partly explain the different liquid and vapor transport properties in the in-plane and transverse directions. The results from the mathematical morphological study show that the pore space and the fiber space are bicontinuous. Some network measures of both these spaces are the network nodal density and bond co-ordination number distribution, both of which are determined. Significant transport properties for the pore space include the saturated water permeability and water vapor diffusivity. Due to the anisotropic nature of the structure, these are three-dimensional tensors in general.