The emergence of rigidity in desiccated granular materials.

The formation of clusters of granular particles during desiccation, such as sand on a beach, is predominantly determined by the pressure difference in the liquid capillary bridges between the grains. Even when fully dry, clusters can remain rigid, meaning that the contacts between grains retain frictional and adhesive forces. We reproduce this phenomenon with spherical glass particles (tens to hundreds of microns). We observe similar rigidity when clean particles are suspended in pure water and allowed to dry through evaporation. Moreover, these particles form a particularly strong dry cluster when placed in an autoclave. We found that cluster strength (and adhesion of glass particles) depended on the drying method, the packing fraction of the sample during the drying process, and, importantly, the solvent. We immersed glass particles in polar (water, ethanol) and non-polar (hexane, chloroform) solvents and dried them in an oven. After drying, we measured the yield force of the resulting clusters using a robotic arm equipped with a force sensor. We found that the strongest clusters were formed by placing the samples in an autoclave (high-pressure steam), followed by those in polar liquids and weaker ones in non-polar liquids. These results demonstrate that granular contacts formed through desiccation can form strong adhesive forces due to physical or chemical changes by the evaporated liquid.