Structuration and deformation of colloidal hydrogels

The aim of the present paper is to determine the optimum conditions for the formation of homogeneous colloidal silica hydrogels by aggregation and drying processes, avoiding mechanical instabilities at the surface. Aggregation is controlled by adding monovalent salt to the silica nano-particle suspension while the drying of the sol is also modulated by changing the evaporation rate. A phase diagram reveals two regions in the parameter plane, ionic strength versus evaporation rate: a region where the drop undergoes an isotropic shrinkage and forms the required homogeneous gel and a region where mechanical instabilities appear due to the formation of a solid skin at the gel surface. The frontier between these two regions can be determined by equating the following two characteristic times: the gelation time and the time for skin formation. Permeability measurements of the final gel provide an estimate of the drying stress which is compared to the yield stress of the material. In accordance with the determined phase diagram, our study shows that instabilities appear when the drying stress is larger than the yield stress.