The Movement of Fluids through Wood

A digression is required to appreciate the complexities of the inside, outside aspects of drying  

Two terms, permeable and porous, can be distinguished. Porosity relates to the proportion of free space in a material. A low-density wood or cellular foam is porous in that it contains a large void volume. A permeable material, on the other hand, is defined in terms of the ease of fluid flow. If the cells are interconnected then air/water can escape when compressed and the material is porous and permeable (a sponge). A material is porous and impermeable where the cells are closed and the 

Poiseuille s law for laminar flow of fluids states that the flow rate is proportional to the radius (R) of the flow passage to the fourth power, indicating that longitudinal flow through pits and lumens is limited by the small openings in the pit membrane  

The moisture content of the heartwood is only a little higher than the fibre saturation point (Table 3.1), so most pits in heartwood are aspirated. These pits are also clogged to some extent with extractives. Heartwood is impermeable. 

The presence of water vapour interfaces, for example a bubble, within a capillary disrupts the process of permeation. Such interfaces significantly influence the drying and preservative treatment of wood. 

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