Drying kinetics of deformable and cracking nano-porous gels

Abstract

The desiccation of porous materials encompasses a wide range of technological and industrial processes and is acutely sensitive to the hierarchical structure of the porous materials resulting in complex dynamics which are challenging to unravel. Macroscopic observations of the surface and geometry of model colloidal gels during desiccation under controlled air flow highlight the role of crack formation in drying. The density of cracks and their rate of appearance depend on the initial solid fraction of the gels and their adherence to the substrate. While under certain conditions cracking leads to an increase of the drying rate, in other cases cracking allows for its conservation over an extended period of the drying process. Nevertheless, as long as the sample is saturated with water, each piece within the sample shrinks isotropically as if it were an independent drying system. By simulating the airflow around the sample and inside the crack cavities, we show the existence of a perturbation in the air velocity in the vicinity of the crack cavity whose scale depends on the aspect ratio (depth/width) of the latter. On this basis, we propose a simple model which predicts the observed drying rate variations encountered while the sample cracks; and further enables to simulate the desiccation for a designated crack density.

Strengthening and drying rate of a drying emulsion layer

“…Thus, during a first period, a soft colloidal solid shrinks and the drying rate remains constant as liquid is still surrounding the solid phase [34]. In a second stage, the particles cannot approach closer anymore, so that the liquid has to find a way through the rigid network now formed; air penetrates the pores and the drying rate decreases [18], [34]. Finally, by analogy with these materials, we expect a constant drying rate for emulsions.…”

“…The situation of emulsions rather resembles that of soft colloidal solids for which, as we withdraw the liquid from the matrix, the solid particles tend to approach further from each other, since creating some air-liquid interface between two such small solid elements would induce very large capillary effects that such a material could not resist. Thus, during a first period, a soft colloidal solid shrinks and the drying rate remains constant as liquid is still surrounding the solid phase [34]. In a second stage, the particles cannot approach closer anymore, so that the liquid has to find a way through the rigid network now formed; air penetrates the pores and the drying rate decreases [18], [34].…”

Drying regimes in homogeneous porous media from macro- to nanoscale

“…For bead diameter of 40 nm and below the samples are initially prepared as gels with a solid fraction of 20%. These gels are further dried on a non-adhering substrate allowing them to shrink in a homogeneous manner under the action of capillary forces, [25]. During shrinkage the gels keep saturated [24][25], however when shrinkage stops, the sample starts to desaturate; this regime constitute the stage of interest within the present frame.…”

“…These gels are further dried on a non-adhering substrate allowing them to shrink in a homogeneous manner under the action of capillary forces, [25]. During shrinkage the gels keep saturated [24][25], however when shrinkage stops, the sample starts to desaturate; this regime constitute the stage of interest within the present frame. Using MRI, we previously demonstrated our ability to distinguish between the saturated and non-saturated regimes during this process [24].…”

Convective drying of a porous medium with a paste cover

“…the paste remains saturated until the end of this regime. This simple shrinkage, which contrasts with fracturing or 3D homogeneous shrinkage observed in other colloidal systems [38][39][40][41][42][43], likely results from the fact that kaolin particles do not develop significant colloidal interactions, so that inside the paste the particles are essentially piled in disorder with various orientations. Then, under the action of capillary forces resulting from evaporation and slight air-liquid interface withdrawal inside the medium around the free surface, they can rearrange in a more compact way along the sample axis During a second period (regime A), the paste keeps a constant uniform water amount, but the water amount in the grain packing decreases homogeneously (see Fig.5).…”