Visualization and quantification of liquid water transport in softwood by means of neutron radiography

Liquid water uptake in an orthotropic, cellular, hierarchical and natural material namely wood is investigated using neutron radiography. During water uptake in wood, liquid does not move up as a regular front as uptake rates differ in latewood and earlywood. In addition, moisture is adsorbed by the cell wall, resulting in a swelling that influences the process of moisture transport in wood. The high sensitivity of neutron to hydrogen atoms enables an accurate determination of the change in moisture content in the wood at the growth ring scale. The analysis of the spatial and temporal change of water content distribution shows that liquid water transport has different characteristics, depending on the direction of uptake and initial moisture content state.Our results show that latewood cells play a more significant role in water uptake than earlywood cells and that ray tracheids also contribute to liquid transport. Latewood tracheids possess smaller cell lumens than earlywood cells that make them the preferential pathways for transport along the longitudinal direction. The process of liquid uptake is different in the radial and tangential directions as the path of the liquid is more intricate, involving also the rays and requiring more often traversing pits. In tangential direction, water uptake is occurring first in the latewood with a subsequent radial redistribution towards the earlywood. In radial direction, the growth ring boundary decreases the liquid transport rate, an indication that a significant portion of the rays are interrupted at that location. The moisture uptake rate in initially moist specimens is seen to be higher. Liquid transport leads to sorption and thus swelling of the specimens, which was dealt with by affine registration. Water uptake in wood cellular structure is a three-dimensional process that is controlled by the morphologic and sorption properties of the material at its different scales.