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Imbibition-drying properties of wood controlled by bound water transport
Water transfers (evaporation, humidification, sap flow) are fundamental in the life of natural materials such as plants or trees. Wood also constitutes a material of great interest for construction for its impact on thermal insulation and humidity regulation, but imbibition-drying cycles may induce damages (swelling, fractures) in wooden construction. Water transfers in such materials rely on a subtle hydraulic system : water can be contained in pores and move thanks to capillary effects, but it can also be absorbed and move inside the cell-walls in the form of “bound water”, which is at the origin of wood swelling or shrinkage. The question of the interplay between these two water types during a transfer caused by a change in external conditions remains open. Here we focus on imbibition and drying properties of wood, in particular with the help of Magnetic Resonance Imaging, which allows to distinguish bound and free water transport inside the sample.
First we show that the imbibition dynamics is controlled by water adsorption and diffusion in the walls : a change from poor to good wetting is observed when cell-walls are saturated with bound water, which allows liquid displacement [1]. As a consequence, the dynamics of imbibition is strongly damped (by several orders of magnitude of time) as compared to the standard Washburn prediction, but water can freely climb over significant heights as soon as sufficient water has been adsorbed into cell-walls. This process might contribute to the regulation of water absorption in unsaturated wood, allowing it to store available bound water in progressively higher depths, instead of leaving free water rapidly flow through it.
We then show that drying processes mainly rely on the water transport in the form of bound water. For slow drying the free water is extracted first in depth, so that its distribution remains uniform up to its full disappearance, an effect resulting from its transport as bound water towards the free surface of the sample [2]. On the other side a sufficiently fast drying develops in the form of two-step diffusion process, with only bound water remaining over some distance below the free surface, but still ensuring the transport of free water from the region below towards the free surface [3]. Remarkably, in that case the drying rate is limited by the diffusion of bound water. This allows a longer storage of free water in depth and ensures the reversibility of the process at any time, i.e. the re-humidification of wood in the presence of wetness.
[1] M. Zhou, S. Caré, A. King, D. Courtier-Murias, S. Rodts, G. Gerber, P. Aimedieu, M. Bonnet, M. Bornert, P. Coussot, Liquid uptake governed by water adsorption in hygroscopic plant-like materials, Physical Review Research, 1, 033190 (2019)
[2] H. Penvern, M. Zhou, B. Maillet, D. Courtier-Murias, M. Scheel, J. Perrin, T. Weitkamp, S. Bardet, S. Caré, P. Coussot, How bound water regulates wood drying, Physical Review Applied, 14, 054051 (2020)
[3] M. Cocusse, M. Rosales, B. Maillet, R. Sidi-Boulenouar, E. Julien, S. Caré, P. Coussot, Two-step diffusion in cellular hygroscopic (plant-like) materials, submitted (2021)