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Water transport in wood plays a central role in many industrial processes, yet the mechanisms governing imbibition still remain difficult to characterise (and thus to understand) due to the anisotropic and multiscale structure of wood and to the intricate coexistence of bound and free water during imbimtion. In this work, water imbibition in European beech (Fagus sylvatica) is investigated along the longitudinal, radial, and tangential directions using in situ 4D X-ray microtomography combined with digital volume correlation (DVC). The time-resolved tomographic images are analysed to quantify both the wood swelling induced by bound water uptake and the presence of free liquid water:
• The swelling strain field, derived from Hencky strain tensor field, is used as an indicator of bound water content in the cell walls, assuming a proportional relationship between swelling and bound water concentration: the time evolution of the swelling strain is used to analyze the cell wall diffusion of bound water. Effective apparent diffusion coefficients of the order of 10⁻⁹ m² s⁻¹ are obtained, with a marked anisotropy: diffusion is faster along the longitudinal direction than in the radial and tangential ones. These values are consistent with recently reported diffusion coefficients for bound water in hardwoods.
• In addition, the residuals of the DVC analysis reveal the presence of free liquid water in the vessels. For longitudinal imbibition, a discrete water front is observed, characterised by localised and abrupt jumps separated by periods of stagnation. The average kinetics of this front is significantly slower than that predicted by classical capillary models, indicating that capillary rise alone cannot control liquid water transport at the sample scale. A comparison between the evolution of the free water front and the swelling kinetics shows that the advance of free water is governed by the diffusion of bound water in the cell walls, while capillary effects operate locally once sufficient wetting and connectivity conditions are met.
• In contrast, no distinct liquid front is observed during radial and tangential imbibition, where pore filling appears progressive and spatially diffuse, as a possible recondensation process of bound water after full saturation of cell walls.
Inline with some recent literature works, these results tends to prove that bound water diffusion acts as a major water transport mechanism controlling water imbibition in beech wood and provide a unified experimental framework to analyse coupled diffusion–capillarity processes in biosrouced materials using 4D imaging.
| Country | France |
|---|---|
| Green Housing & Porous Media Focused Abstracts | This abstract is related to Green Housing |
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