The porous nature of paper can be regarded as consisting of three components, namely, an external void system, an internal void system and discontinuities. The external void system can be associated with the concept of paper roughness and is totally open in one direction. The internal void system can be regarded as that which can be observed by simple optical microscopy (e.g. paper sections cut at an oblique angle). The discontinuities can be defined as all those phenomena that deprive the voids between fibres of the characteristics which would justify regarding them as capillaries with smooth surfaces. These would include the structural elements of the fibre surface, the fines in paper, the fibrils and the fibril bundles protruding into the interfibre voids, the pits in the fibre surface and pores in the cell walls.
The interactions between the components of the model system proposed above have been investigated using thickness, light transmission and water absorption measurements.
*Shortened version of the original article, prepared by R. W. Hoyland, UMIST, Manchester.