Statistical models for granular materials

Granular materials, like sand or powders, have many peculiar properties, most of them not yet fully understood. These are due to the special physical properties of granular media like dissipation, friction, dilatancy and the possibilities of having grains of many different sizes and shapes. In the last 10 years much progress has been achieved in the basic understanding of granular media due to the use of tools from Statistical Physics, including disordered systems, critical phenomena, instabilities and chaos. We will present in this talk two examples: