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Capillary imbibition governed by water adsorption in hygroscopic plant-like structure- Philippe Coussot

Laboratoire Navier (ENPC-Univ Gustave Eiffel-CNRS), Champs sur Marne, France

Plant matter is being used increasingly in construction and in various other applications thanks to its remarkable porous and mechanical properties, but water transfers play a critical role on these properties and their possible alteration. Water in plants may be either in a “free” liquid state in capillaries, or in a “bound” state after adsorption in cell-walls, associated with significant deformation of the structure. Here we show that the coexistence of these two effects strongly affect the transport properties.

We demonstrate this from Synchrotron and MRI observation in hardwoods, which exhibit a relatively simple hydraulic structure. Capillary imbibition dynamics appears to be dramatically damped (velocity decreased by several orders of magnitude), but the liquid can still climb over significant heights (in contradiction with its dynamics) as soon as sufficient bound water has been adsorbed. This contradiction is confirmed by 3D Synchrotron images of the internal structure obtained during imbibition, which show that the liquid-air interfaces in the capillary vessels remain planar, which implies negligible Laplace pressure, but significantly advance along the vessels, again unexpectedly.

From MRI measurements allowing to distinguish bound and free water, but also direct measurements of the induced macroscopic deformation distribution in time, we then show that this contradiction is explained by the adsorption of a slight amount of bound water in the capillary walls. This adsorption governs the process : it momentarily damps wetting and then allows further advance later when the walls are saturated with bound water. The generality of the process for hygroscopic systems is demonstrated with a model material, i.e. hydrogel, from which both the position and shape evolution of liquid-air interface and the adsorption and propagation of bound water may be directly observed (see below). This suggests the development of bio-inspired porous materials able to absorb liquid with a tunable timing, for pharmaceutical or chemical engineering applications.

We finally discuss the opposite process, i.e. liquid transfers in hardwood structures during drying, as observed from MRI and Synchrotron imaging, and in particular show the essential role of bound water.


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