Romain Bordage completed his PhD thesis entitled “Spontaneous and mechanically-assisted water transport in beech wood: investigation using X-ray tomography and digital volume correlation” under the joint supervision of:
Yoshiharu Nishiyama, co-supervisor – CNRS Research Director – CERMAV,
Laurent Orgéas, co-supervisor – CNRS Research Director – 3SR Lab, and
Sabine Rolland du Roscoat, co-supervisor – Professor, University of Grenoble Alpes – 3SR Lab.
Abstract:
“Filling liquids in wood is a key stage in some wood transformation processes, such as resin impregnation to produce wood composites or delignification treatments with aqueous solutions to densify wood and enhance mechanical properties. Such a filling can occur spontaneously in contact with water (imbibition) and involves several water transport mechanisms: vapor or capillary liquid transport of free water through the vessels and lumens of the cellular architecture of wood, diffusion of bound water through the polymeric wood cell walls, and interface mass exchanges. The multiscale structures of wood, the effect of water content on its physico-chemical properties, and the substantial swelling of cell walls, lead to complex transport mechanisms that remain unclear. To better understand them, we performed a series of imbibition experiments on beech samples with water, coupled with high resolution X-ray microtomography and 3D image analysis, in particular Digital Volume Correlation (DVC).
Firstly, unidirectional imbibition tests were conducted on centimetric beech samples along the three principal wood directions. In parallel, the 3D images acquired at the cell scale unveiled a complex honeycomb-like architecture. Their main structural descriptors were extracted, and the solid diffusion and permeability tensors were estimated using both homogenization-based cell scale simulations and the high-resolution 3D images. The combination of experimental and numerical results suggests that water diffusion in the cell walls and in the pores is the leading transport mechanisms, regardless of the transport direction, including the longitudinal one.
To verify this scenario, a second series of imbibition experiments was carried out using a laboratory X-ray tomograph to obtain 3D in situ observations at the vessel scale. The swelling strain fields were assessed using DVC, thereby quantifying the temporal and spatial distribution of bound water in the samples. Meanwhile, the error between correlated 3D images was used to capture the vessel filling by free liquid water. For the experiments performed along the radial and the tangential directions, the important role of bound water diffusion mechanisms is highlighted. Furthermore, the presence of free liquid water in the pores occurred without clear capillary fronts, suggesting sparse water perspiration mechanisms from the bulk saturated cell walls to their surface and re-condensation of vapor to free liquid water. For longitudinal imbibition experiments, free liquid water transport occurred as discrete, stepwise, and erratic water uptake in vessels with capillary fronts, leading to a wide macroscopic flow front with many discrete channels. However, bound water diffusion can again be regarded as the leading transport mechanism: the kinematics of this front is identical to that of the diffusion one, which is in turn captured by the swelling front.
Lastly, in order to speed up wood imbibition, we investigated an alternate, fast, mechanically-assisted imbibition route. For that purpose, an unsaturated beech sample immersed in water was subjected to cyclic compression at increasing compression strain and small strain rate, while 3D images were acquired in situ with the X-ray microtomograph of the ESRF BM5 beamline. During compression, water progressively fills the pores. After a succession of 4 compression cycles up to a 50% compression Hencky strain, 90% pore filling was observed. Meanwhile, cell walls bent/buckled during loading without exhibiting severe damage. These elastoviscoplastic densification mechanisms are simultaneously coupled with cell wall swelling induced by the diffusion of bound water, which confers the sample an apparent superelastic response.”
Key words:
water transport in wood, imbibition, X-ray micro-tomography, digital volume correlation.