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

Abstract

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 ( $$ \gamma_{W}^{P} $$ ) and dispersed ( $$ \gamma_{W}^{D} $$ ) 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. $$ \gamma_{W}^{P} $$ was markedly higher than $$ \gamma_{W}^{D} $$ 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.