Spreading of aqueous liquids in unsized papers is by film flow

We use cryo-scanning electron microscopy to visualize the penetration of a wetting fluid into various bleached softwood kraft papers. The results indicate that the fluid movement is due primarily to the advance of the wetting fluid in the form of bulk liquid films along channels formed by fibre overlaps. This is in contrast to the common description of fluid penetration, where the primary flow mechanism is based on the bulk filling of pores. The channels formed by fibre overlaps are shown to form a highly interconnected dense network of flow paths which efficiently transport the wetting fluid. We calculate the flow rates associated with penetration along a number of potential flow paths within the fibre web. The experimentally observed penetration rate is consistent with a film flow process through interfibre channels which is significantly slower than a penetration process dominated by meniscus flow through pores.