Abstract
Three different physical methods for studying the capillary structure of porous materials are developed and applied to the study of the fine effective continuous capillary structure of wood. The three characteristic capillary properties measured are, (1) the ratio of effective capillary cross section to the effective length, (2) the average effective capillary radius, and (3) the maximum effective capillary radius. Some of the important characteristics of the porous structure of wood are briefly presented in order to show that the major part of the void structure, the fiber cavities, have very little to do with the permeability of wood. The pores in the pit membranes which connect the adjacent fiber cavities are shown to be the structure effective in controlling permeability of wood. The pores in the pit membranes in general, are below microscopic visibility, thus necessitating other means of study. The ratio of the effective capillary cross section to the effective capillary length is determined from electrical resistance measurements of strong salt solutions filling the void structure and from the specific resistance of the salt solutions in bulk. Strong salt solutions are used in order to make the surface conductivity negligible as well as the conductivity of the swollen cell wall. The average effective capillary radius is determined by combining the results of the preceeding measurements with results obtained from hydrostatic flow studies of a continuous water phase extending through the section. Poiseuille's law is used for the calculations. A differential pressure drop apparatus was devised for these measurements in which the flow of liquid through a standard capillary tube and the test specimen connected in series is determined by measuring the pressure drop through each. The maximum effective capillary radius is determined by measuring the gas pressure required to overcome the effect of the surface tension of water in the capillary systems. The data show the large differences between the fine capillary structure of heartwood and of sapwood.
Published Version
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