Clays, in particular the synthetic clay laponite, are commonly used as model systems for colloids.
Clay-water mixtures are viscoelastic materials, and of particular interested is fingering (in clay sols) and fracturing (in clay gels) instabilities in the vicinity of the continuous sol-gel transition, at which the elastic modulus of mixtures of clay-water gels increases from zero to a finite value when the concentration of the clay overpasses a critical value. The sol-gel transition in clay-water systems a lso involves a gelation time, i.e. after mixing clay and water, it takes some time (typically a few days) before the clay gels are formed, due to local self-organization taking place of the clay particles when they are suspended in water.
Such gelation a nd flocculation continueson the time scale of months and years. For high concentration of laponite and low ionic concentration the final state is recognized as a repulsive glass. At higher ionic strength a gel state is observed due to double layer screeni ng of the charged clay particles.
A very important issue we wish to pursue within the present project is to understand how interactions between particles control the mechanical properties of clays. The addition of a controlled quantity of salt to laponit e clay sol/gels should favour a transition from attractive to repulsive interactions. While repulsive systems are usually ductile, although with a strong localization of the deformation, attractive systems can fracture easily. To control and understand qu antitatively the brittle to ductile transition in such systems should give important clues for understanding the microscopic origin of the mechanical properties of viscoelastic materials.