Capillary Imbibition Damped by Adsorption in Wood Cell Walls

Wood is a highly hygroscopic porous material in which water can exist in three forms: bound water adsorbed within cell walls, liquid water and water vapor in pores. The physical understanding of water transfers in wood remains poor. Imbibition tests with hardwood samples and interpreted with the Lucas-Washburn’s law show a strong contradiction: the very slow dynamics of water imbibition in wood implies a poor wetting, but the final height reached by water over long terms corresponds to an excellent wetting between water and wood cell walls. To clarify this phenomenon, we observed the imbibition process with high-resolution 3D synchrotron images and we found that, although the liquid-air interfaces in wood channels show a planar shape implying a negligible Laplace pressure, they still progress, erratically, throughout the sample. Besides, we carried out new MRI observations allowing to distinguish bound and free water distribution in time, and which show that bound water is absorbed beyond these interfaces. We finally explain these results through a model showing that the adsorption and diffusion of bound water in cell walls modify the liquid-air meniscus and finally damp the capillary imbibition.