Unexpected experimental results for capillary suction in wood: Analysis on the fibre level

Abstract

A series of simple experiments on capillary water uptake in rectangular sticks of Scots Pine (Pinus sylvestris L.) gave some unexpected results. Sticks of sapwood and heartwood with lengths of 100, 200 and 300 mm, with the four long sides sealed, were used in the experiments. The samples were allowed to suck water, one group for 1000 h and one group for 1900 h. After exposure, as anticipated, the heartwood samples had a moisture content (MC) distribution with high values at the bottom and steeply decreasing values towards the top. The sapwood samples, in contrast, had moisture distributions with high MC at the bottom and the top, and low MC in the middle. Additional experiments to study flow paths and timescales of the process were performed using dyed water. A Fickian approach is discussed in some detail. It is clear that the process represented by the experimental results cannot be explained within a Fickian framework. The analysis has to be done on the fibre level, where an explanation is proposed. The influence from bordered pits is of significant importance and their resistance dominates the timescales. A key factor in the proposed explanation is the high lateral capillary conductivity within the saw-damaged top surface. However, some questions remain unanswered.