Féderation de recherche Fabri de Peiresc

How capillary effects govern drying of porous media, and how adsorption governs wood wetting and imbibition

Orateur : Philippe Coussot
Laboratoire Navier, École des Ponts - CNRS-IFSTTAR, Paris

Résumé : Imbibition-drying cycles play a major role in building or soil materials as it is at the origin of humidity transfers, pollutant transport and deposition, clogging induced by particle transport and accumulation, ion transport inducing chemical attacks, shrinkage, fracture, etc. Here we focus on two different aspects : drying in porous media and wood imbibition. During ideal isothermal, convective drying of a porous medium (evaporation of a pure liquid from fixed solid structure) two regimes are observed : the Constant Rate Period (CRP) and the Falling-Rate Period. During the CRP the evaporation rate is set by external conditions and capillary effects drive the liquid from the interior to the sample’s outer surface. This regime plays a major role in various situations as during this period about 90% of the liquid can be withdrawn from the sample at a constant rate, and the elements possibly suspended in the liquid are transported towards the sample free surface. Through Magnetic Resonance Imaging (MRI) allowing to get the liquid distribution in time throughout the sample we show various possible consequences of this effect : drying of a porous medium yet covered by a wet layer, “3D coffee-ring effect”, homogeneous shrinkage of colloidal gels. Then we focus on imbibition properties in hardwoods, which exhibit a relatively simple hydraulic structure (long open vessels). From Synchrotron and MRI observation we show that when a wood sample is put in contact with water, capillary imbibition dynamics is dramatically damped (velocity decreased by several orders of magnitude), but the liquid can still climb over significant heights as soon as sufficient water amount has been adsorbed (as bound water) in the solid structure. Actually, the liquid-air interfaces in the capillary vessels remain planar, but significantly advance along the vessels.

The generality of the process for hygroscopic systems is demonstrated with a model material, i.e. a hydrogel (see Figure). This may be described by a model of diffusion with moving boundaries, so that imbibition in such systems appears analogous to the propagation of a front of solidification in a liquid.

Date et Lieu : le Vendredi 15 février 2019 à 11h00, salle de séminaire IRPHE