Convective drying of a porous medium with a paste cover

Abstract

The convective drying of a composite system made of a porous medium covered with a paste is a situation often encountered with soils, roads, building and cultural heritage materials. Here we discuss the basic mechanisms at work during the drying of model composite system made of a homogeneous paste covering a simple granular packing. We start by reviewing the rather well-known case of the convective drying of a simple granular packing (i.e. without paste cover), which serves as a reference for physical interpretations. We show that a simple model assuming homogeneous desaturation then progressive development of a dry front from the sample free surface is in agreement with observations of the internal liquid distribution variations in time. In particular, this model is able to reproduce the saturation vs time curves of various simple granular systems, which supports our understanding of physical mechanisms at work. Then we show the detailed characteristics of drying of initially saturated model composite systems (with kaolin or cellulose paste) with the help of MRI measurements providing the liquid distribution in the sample at different times during the process up to the very last stages of drying. It appears that the granular medium is unaffected (i.e. remains saturated) during an initial period during which the paste shrinks and finally forms a sufficiently rigid porous structure which will not any more shrink later on. Then the drying process is governed by capillary effects down to very low saturation. Over a wide range of saturations both media desaturate homogeneously (within each medium) at different rates which depend on the specific porous structure of the media, so as to maintain capillary equilibrium throughout the sample. During these different stages the drying rate of the whole system remains constant. For sufficiently low saturation in the paste a dry front can develop, both in the paste and the porous medium below, and the drying rate now decreases. These results show that in a drying composite system liquid extraction can occur more or less simultaneously in the different parts of the material up to the very last stages of drying. The corresponding evolution of the distributions of liquid in the different parts of the sample also provides key information for the prediction of ion or particle transport and accumulation in the different parts of a composite system.

How Bound Water Regulates Wood Drying

“…More precisely, we can consider that most vapor forms around the highest free water-air interfaces in the sample, and then diffuses along the upper region (which does not contain free water). Such a process is similar to that observed in granular packings in the so-called "Falling Rate Period" [46,62], and can be described with the help of the above approach providing the drying rate as…”

Drying of a Compressible Biporous Material

“…When the sample is made of two regions with different pore sizes, the region of largest pore size starts emptying first, whatever its location in the system. This leads to the spectacular emptying of a large-pore medium covered by a small-pore medium, while the latter remains apparently saturated (and the only one submitted to an air flux) [13,14]: the air is conveyed through the small-pore medium by transient thin paths, which immediately close when they have reached the large-pore medium.…”

“…On the other hand, some solid structures cannot support the capillary pressure induced by air penetration, and tend to fracture or shrink. This is, for example, the case of gels [15], colloidal pastes [16,17], or clays [14]. This, for example, leads to wood collapse [18], creasing of elastomeric solids [19], or sponge shrinkage [20].…”

Towards the end of drying of granular materials: enhanced evaporation and drying-induced collapse

“…The nuclear magnetic resonance imaging (MRI), as a non-invasive and non-destructive method, has proven effective in probing the evolution of saturation profile during the evaporation process [8,31,32]. Here, the drying experiments were also conducted to obtain the evolution of 1D saturation profile using MRI (Bruker Minispec mq20 equipped with a field gradient) under similar experimental conditions as a supplement to provide qualitative characterization of the drying process.…”