Wetting enhanced by water adsorption in hygroscopic plantlike materials

Water inside hygroscopic porous media such as plantlike systems can be found either freely penetrating in capillaries or absorbed into the solid phase (bound water). Here we demonstrate that the wetting properties (contact angle) of liquid along cell walls significantly depend on the amount of bound water absorbed: a change from poor to good wetting is observed when cell walls are saturated with bound water, which allows liquid displacement. We further show that this process is operative in hydrogels, suggesting that this might be a general property of porous hygroscopic systems. As a consequence, imbibition dynamics is controlled by water adsorption and diffusion in the walls, and even if the dynamics of capillary imbibition is strongly damped (by several orders of magnitude), water can freely climb over significant heights as long as sufficient water has been adsorbed into cell walls or in other hygroscopic walls. Under these conditions, the imbibition process in such systems is not described by the standard model but is analogous to the propagation of a front of solidification in a liquid. This process might contribute to the regulation of water absorption in unsaturated wood, allowing it to store available bound water in progressively higher depths, instead of leaving free water rapidly flow through it. Such a mechanism may be explored to design porous materials with tunable liquid adsorption timings for pharmaceutical or chemical engineering applications.