Directional anisotropy of wood surface wettability: morphological dynamics of droplet behavior observed from different directions

Wetting plays a crucial role in evaluating the bonding and coating performance of wood and its products. This study investigates the directional dynamic wettability of Douglas fir ( Pseudotsuga menziesii ) wood, focusing on changes in droplet morphology both in parallel and perpendicular directions to the grain. Wettability was assessed using a contact angle measurement system via the sessile drop method. Contact angle, penetration ratio and surface free energy were analyzed, considering the effects of wood tissue type (earlywood, EW; transition wood, TW; and latewood, LW), droplet volume (9, 12 and 15 μL), and grain orientation. The results demonstrated a significant anisotropic spreading of droplets, with greater spreading observed parallel to the grain. The contact angle was consistently lower in the perpendicular direction, with a difference of approximately 10°. After 60 s, the penetration for a 15 μL droplet was 65.4 %, 48.4 % and 41.5 %, respectively for EW, TW and LW. Surface free energy was found to be 17–24 % higher in the parallel direction compared to the perpendicular direction. Anatomical characterization confirmed that variations in cell wall ratio and surface roughness significantly affected wettability. These findings highlight the directional and heterogeneous wettability of wood, providing valuable insights for improving wood coating, bonding, and related processes.