We all know wood is an important material for many every-day applications such as construction and carpentry. Interaction with water is vital for growing trees but it is also important to understand how objects made of wood behave as a function of humidity; how they behave when they dry or get wet.
We studied freshly fallen spruce and birch wood that were dried in a controlled manner, either in ambient condition (room-dried), in an oven, or freeze dried. We are experts with synchrotron methods so we had the opportunity to study how the drying affects the water uptake of the dried wood material in either direction parallel, or perpendicular to the wood fibres. A custom-made sample environment allowed us to study the water uptake process as function of time since first contact with liquid water (rewetting). The sample was placed over a reservoir of water that was slowly filled until the water made contact with the sample after which water transport in the sample and the associated nanostructural changes were studied as function of time and distance from the water surface.
X-ray scattering can be used to study the internal structure of materials at different length scales. Time-resolved X-ray experiments are often conducted at synchrotrons, which are large experimental facilities built solely to produce X-ray radiation with well-defined and optimized properties. ForMAX is a beamline of the MAX IV synchrotron in Lund, Sweden focusing on research on soft matter, such as forest-based materials. There we studied this rewetting process with both good spatial and temporal resolution (with the scale of 1 mm and 1 s), something that is not possible conventional X-ray sources. The simultaneous small- and wide-angle X-ray scattering reveals us information on how the water inside the cell walls affect the structure of wood at different length scales, mainly the nanoscale.
We were able to see differences in water interactions between the two species, the different drying methods, and the orientation of the wood specimen. We obtained scattering results from 76 samples as a function of both time and location (effectively height from the water surface). To understand and effectively analyse the massive data set, both principal component analysis and clustering (a classification tool) were used to complement the more traditional X-ray scattering analysis. We saw the water contributing to the scattering patterns directly. It also altered the nanoscale structure of wood as witnessed by the changes in the cellulose peak positions and the shape of the small-angle scattering region.
Keywords: wood-water interaction, synchrotron X-ray scattering, spruce, birch, rewetting
Authors
Patrik Ahvenainen
Aalto University, Department of Bioproducts and Biosystems, Espoo, Finland
Aleksi Zitting
Université de Lorraine,LERMAB, France
Enriqueta Noriega Benitez
Aalto University, Department of Bioproducts and Biosystems, Espoo, Finland
Ville Liljeström
Aalto University, Department of Applied Physics, Espoo, Finland
Ryan Trevorah
University of Helsinki, Department of Physics, Helsinki, Finland
Paavo Penttilä
Aalto University, Department of Bioproducts and Biosystems, Espoo, Finland
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