Factors influencing contact angle measurements on wood particles by column wicking

The present work focuses on a capillary rise technique, referred to here as column wicking, for determining contact angles on wood particles. The liquid front rise versus time for different probe liquids has been measured for extracted and non-extracted spruce wood particles packed into glass columns. Wood is a porous, heterogeneous, and hygroscopic material. The sorption process of certain polar liquids in the wood substance, i.e. bulk sorption, is exothermic and causes swelling. This bulk sorption process and the resulting release of heat are observed as a distinct temperature increase within the columns during the wicking of water, formamide, and methanol. No temperature increase is observed for ethylene glycol, diiodomethane, and hexane. In some cases, the increase in temperature is observed in advance of the moving visible liquid front line. This may indicate that vapor is moving in advance of the liquid front, resulting in bulk sorption and the corresponding release of heat. An apparent non-linearity is observed when the square of the capillary rise is plotted versus time, mainly for water, formamide, and methanol. This non-linearity is strongly dependent on the probe liquid used and the variation in wood particle size. For the wicking of water, the bulk sorption, and hence the swelling of the wood particles, seems to appear instantaneously at the wetting front line, but for formamide and methanol a time delay is observed. The bulk sorption and resulting swelling of the wood particles strongly influence the determination of the effective interstitial pore radius between the particles, and thus the determination of contact angles by use of the Washburn equation.