Challenges in measuring the water vapour adsorption of wood using long-wavelength infrared thermal imaging

The elusive wood-water relations have been studied for over a century and are among the most researched topics in wood material science. Wood-water relations originate from the polar hydroxyl groups of wood interacting with the polar water molecules and the cell wall spaces available for these molecules. The equilibrium moisture content is the indicator for understanding the moisture behaviour of wood under varying environmental conditions. The literature claims that wood subjected to compression has a lower equilibrium moisture content than wood subjected to tension. The adsorption phenomenon relevant to this work involves the wood polymer surface functional groups known as hydroxyl groups, as the water vapour is initially taken up by these functionalities when dry wood is subjected to a high relative humidity (RH).
We must enhance our understanding of the water vapour adsorption behaviour of wood under a mechanical load, such as compression (restricted swelling), by comparing it to an unloaded reference, as we need to improve the safety factors of building with wood. Thus, there is a need to design a laboratory-scale method based on infrared imaging to visualise the water vapour adsorption under mechanical loading conditions spatially. We are currently attempting to use long-wavelength infrared thermal imaging to quantify the temperature changes in wood as it adsorbs the ambient water vapour. The water vapour is in the form of a high and rapid change in RH. The wavelength is ca. 7.5 to 13.0 μm with the used thermal camera (FLIR E60, Teledyne FLIR LLC, Wilsonville, OR, USA). The wood will also be subject to a compressive load due to swelling restraints that stop it from expanding over time. The observable temperature changes on the wood surface are due to the formation of hydrogen bonds between the polar hydroxyl groups (partial charges δ+ on the H and δ− on the O atoms) of the wood and the polar water molecules (partial charges δ+ on the H and δ− on the O atoms) of the water vapour. We are also weighing the wood in parallel while subjected to thermal imaging to measure the moisture content and hopefully calculate the non-equilibrium heat of adsorption since we do not let the wood equilibrate with its surrounding RH in our used setup.
Regarding my role as the first author of this abstract and lead scientist of the project that this work belongs to, I am interested in learning more about wood-water relations, and I find the thermal imaging approach to studying these relations very interesting. My goal is to be able to use this research in my doctoral thesis. We have faced many challenges in this work, which sometimes demotivate me, but I try to maintain a problem-solving attitude no matter how hard things get. The work reported here is mostly about method development, but we need to obtain sensible results as well. In this context, we report that the moisture content of the wood becomes more stable if we do not let too much draft be subjected to the weighing scale we are using. A more unstable RH value and wave-like thermal behaviour also show a clear relationship. Overall, we need system stability.