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Granular Friction : from Building the Pyramids to the Anatomy of Individual Contacts at the Nanoscale
I will discuss the rheology and mechanical properties of wet granular materials, and show why the behavior can be very subtle. Once one understands the mechanical properties, I will show that one can use this knowledge to construct the perfect sandcastle, or to understand why the ancient Egyptians wetted the desert sand with water before sliding heavy stones over it. I will then go on to show some new results on friction at the microscopic scale, between two grains. Amonton’s famous friction law states that the friction force is proportional to the normal force since both are proportional to the area of contact. However for spherical grains, the contact area is not proportional to the normal force, as shown by Hertz long ago. We use a new fluorescence technique that allows us to probe the real area of contact between two rough surfaces. In our case, we conclude that important deviations from Amonton’s law are observed.