Experimental study of some properties of the strong and weak force networks in a jammed granular medium

We are experimentally investigating the force network of a monolayer of grains through the jamming transition. Once the packing has been compressed beyond the volume fraction of jamming phi(J), we use particle tracking and photoelastic techniques to determine the network of forces in the grains. We experimentally quantify the pressure inside each grain and highlight the two networks of forces existing in jammed granular materials: the load bearing network (force chains strong network) and the dissipative network (weak network). For the strong network, we study the statistical properties of the stress state of the grains and in particular the exponential tails of the probability density function of pressure and shear stress. As the distance to jamming increases, the probability density function of local shear in the system evolves and tends to look like a normal distribution while the probability density function of pressure does not evolve. For the weak network, we study the contact cycles in the granular packing. By calculating their numbers, we determine some statistical properties of these cycles. The number of cycles of order n = 3 which carries the dissipative network increases linearly with the number of contacts.