Relationship of capillary pressure and water saturation in wood

Abstract

An understanding of free water movement in the gross capillary structures of wood requires a knowledge of capillary pressure. A method is described whereby the relationship between capillary pressure and water saturation can be determined under decreasing saturation. Water-saturated core samples of wood were centrifuged at increasing rates, and the average saturation was measured at each rate. The results indicate that the degree of saturation, and therefore the moisture content, at a given capillary pressure varied greatly with different woods. Theoretically, all free water that is held in the macroscopic capillaries can be removed by the action of centrifugal force. However, in some woods, the capillary pressure curve becomes asymptotically vertical at moisture contents above the fiber saturation point (30 percent MC), indicating that free water still remained in the wood. It is postulated that the transport of free water is affected not only by capillary pressure considerations but also by the rate of drainage as influenced by surface roughness along the lumen walls.