Water penetration in the microstructure of hardwood revealed by NMR relaxometry

The imbibition of water in the microstructure of cubic hardwood (oak and poplar) samples was followed by NMR, with sample faces either open to air or coated with paint, and along the different wood directions. Dynamic NMR relaxometry allows to clearly distinguish and quantify the amount of water appearing in fibers, in vessels, or as bound water, over time. It appears that the water penetrates first in the form of bound water in the wood sample. Subsequently, fibers are infiltrated at a slower rate, followed by vessels, which exhibit the slowest rate of invasion. For poplar, vessels even start to be invaded only after all the fibers have been filled. Furthermore, the invasion dynamics of the different phases are qualitatively similar when all open faces of the wood sample are coated with paint, preventing any air extraction by these faces. A simple capillary imbibition model fails to fully describe these processes, indicating that the wetting properties vary depending on the presence of bound water in the cell walls, and subsequently, on the presence of water in fibers. Finally, given that bound water penetrates prior to free water, the diffusion coefficient of bound water can be estimated from the data across different directions (L, R, T), which enables the characterization of moisture exchange between construction materials and ambient air under hygroscopic conditions.