Validity of Determining Pit Pore Sizes in Softwoods By Surface Tension Resistance with Rate of Flow

An apparatus was built to measure pore diameters in wood and other materials by forcing air or a liquid upwards through the bottom of a liquid-saturated specimen. Pressures up to 100 lb./sq. in. could be applied from a compressed air line by using the air to force a liquid through the specimen or air alone. This apparatus was used to test the validity of Jurin’s equation for pore size-pressure relationship, by determining the pressure required to overcome the effect of surface tension in the pores. Measurements were made on Engelmann spruce cross-sections of average thickness 4.88 mm. (less than maximum fiber length), on green eastern hemlock sapwood and outer heartwood varying in thickness 0.25 to 1.75 inches, on white cedar sapwood and Douglas-fir heartwood. Measurements on the 4.88 mm. average thickness spruce by displacing water by air or isobutyl alcohol gave similar average maximum effective radii, 2.89 and 3.01 microns respectively, for the tapered-end lumen openings. Octyl alcohol gave an average value of 3.76 microns, whereas benzene gave a far greater value, namely 10.99 microns, probably due to its solvent effect on resins in the wood resulting in changes in interfacial tensions. Thicker specimens (more than 0.33 cm) of softwoods gave average maximum pore sizes per pit traversed in displacing water or ethyl alcohol by air of 0.2 to 0.4 microns. Experiments were carried out to determine whether or not it is possible to measure the rate of flow of the displacing liquid to determine the distribution of pore sizes. The results were unsatisfactory. In the case of eastern hemlock the flow of isobutyl alcohol through the water-soaked specimens against increasing pressure gave an initial upward curvature followed by a downward curvature, indicating that the Poiseuille equation no longer holds at the higher pressures. It is shown that pore sizes of wood can be determined by applying pressure to overcome the effect of surface tension by using cross-sections or edge grain specimens with the end grain sealed.

You must be logged in to download any documents. Please login (login accounts are free) or learn how to Become a Member