Cracking condition of cohesionless porous materials in drying processes

Three-Dimensional Modeling of Polygonal Ridges in Salt Playas

In this work, we investigate the formation of the curious polygonal salt ridges that tessellate salt playas worldwide using suitable three-dimensional modeling and simulation of the dynamical processes that are responsible for their formation. We employ the principles of fracture mechanics under cyclic wetting and drying, fluid and mass transport in fracture channels, and processes of crystallization and self-organization to finally replicate the almost Voronoidal pattern of salt ridge mosaics observed in playas. The model is generic and applicable to playas having different salt compositions, as the effect of the salt diffusion coefficient and critical salinity for crystallization are factored in. The final pattern of polygonal salt ridges obtained by simulation visually resembles the geometry of the salt ridges reported in the literature. A single equation describing the time of first crystallization of salt in terms of evaporation suction pressure P, diffusion coefficient D of salt, and relative salinity Δc with respect to critical salinity at saturation, has been proposed. The saturation of crystal growth rate is shown to be a dynamic equilibrium between advection and diffusion processes. We show that the stable polygonal geometry of the salt playas is an effort toward the total minimization of system energy.

Memory effect and anisotropy of particle arrangements in granular paste

It is known that pastes of fine powder, for example those of clay, retain memory of shaking applied early in a drying process. This memory results in the appearance of anisotropic patterns of desiccation cracks after drying. In this work, we find a similar behavior in pastes consisting of large granular particles, specifically cornstarch and Lycopodium spores. Because of the large particle size, we were able to observe particle arrangements in Lycopodium paste with micro-focus X-ray computerized tomography ( μ CT). We prepared pastes consisting of Lycopodium particles and water. Agar was added to the paste in order to allow for solidification during a drying process. In these samples, we found statistical anisotropy induced by shaking applied early in the drying process. This anisotropy possesses a feature that was predicted on the basis of results obtained in previous experimental and theoretical studies.

Water transfer and crack regimes in nanocolloidal gels

Direct observations of the surface and shape of model nanocolloidal gels associated with measurements of the spatial distribution of water content during drying show that air starts to significantly penetrate the sample when the material stops shrinking. We show that whether the material fractures or not during desiccation, as air penetrates the porous body, the water saturation decreases but remains almost homogeneous throughout the sample. This air invasion is at the origin of another type of fracture due to capillary effects; these results provide insight into the liquid dynamics at the nanoscale.

Surface wrinkling and cracking dynamics in the drying of colloidal droplets

The cracking behavior accompanied with the drying of colloidal droplets containing polytetrafluoroethylene (PTFE) nanoparticles was studied. During evaporation, due to the stretching effect of the liquid zone, the receding wet front leads to the formation of radialized surface wrinkling in the gel zone. This indicates the building of a macroscopic stress field with a similar distribution. As a result, the cracks in the deposited films are in a radial arrangement. The propagation velocity of the cracks depends on the thickness of the film, ∼ H (3/5). In addition, sodium dodecylsulfate (SDS) additives can be used to tune crack behavior by causing a reduction of the capillary force between particles. The results highlight the significance of the receding wet front in building the drying deposition stress field and may be helpful in other fields related to drying and cracking processes.