Orthotropic wetting dynamics of wood: trade-off between surface spreading and penetration

Wetting is an important and effective factor to evaluate the gluing and painting performance of wood and its products. In this study, the dynamic wetting behaviors of four coniferous wood species (Chinese fir, radiata pine, Masson pine and Jack pine) were tested. All the samples were extracted prior to the wetting test to eliminate the influence of surface extractives. The sessile drop method was used, and distilled water was selected as the testing liquid. Time-dependent wettability, including changes in contact angle (CA) and liquid penetration volume, was evaluated on different planes (transverse, radial and tangential). The wettability showed obvious directional characteristics. In the transverse plane, water spread significantly fast, with no equilibrium CA observed. In the radial and tangential planes, CA reached equilibrium after 120 s of wetting, while penetration still occurred. The whole wetting process was divided into three stages, as the penetration rate exhibited a "slow-fast-slow" trend. The variation in penetration among the radial and tangential planes showed species dependency, and the influences of anatomical factors were simulated. It was concluded that surface spreading and penetration remained in a trade-off balance. Overall, the results of this study showed the orthotropic wetting characteristics of extracted wood and revealed the dynamics of spreading and penetration. These findings may also be helpful for explaining wood-water interactions.