Within-tree variations in the surface free energy of wood assessed by contact angle analysis

Although the heterogeneity of wood could preclude assessment of the performances of adhesion, the within-tree variations in its thermodynamic properties still remain unexplored. This study analyzed the surface free energy of wood (γ W ) calculated by the geometric mean approach along the radius and at different sampling heights in black spruce and balsam fir with a statistical model, employing apparent contact angles of three testing liquids. Sampling height and species affected the apparent contact angles, and consequently the polar (γ W P) and dispersed (γ W D) components of γ W, but no significant difference was observed along the radius. On average, γ W was 43.77 mJ m -2 at the stem base and gradually decreased, attaining 27.19 mJ m -2 at 14 m of height. γ W P was markedly higher than γ W D with an opposite pattern along the tree height. These findings demonstrated the huge variability in γ W along the stem, which could be related to the structural or chemical features of wood