Effect of size polydispersity vs. particle shape in dense granular media

D-H Nguyen, E Azéma, F Radjai and P Sornay

We present a detailed analysis of the morphology of granular systems composed of frictionless pentagonal particles by varying systematically both the size span and particle shape irregularity, which represent the size and shape polydispersities of the system, respectively. The packing fraction increases with both shape and size parameters. But we find that the effect of shape polydispersity for all the structural properties investigated in this paper is significant only at low size polydispersity where crystalline structures characterized by positional and/or orientational ordering of the particles can be identified. We show that the proportion of side/side contacts is nearly independent of the polydispersity parameters. They side/side contacts do not percolate but define clusters of increasing size as a function of size polydispersity and decreasing size as a function of shape polydispersity. The clusters have anisotropic shapes but with a decreasing aspect ratio as polydispersity increases. This feature is argued to be a consequence of strong force chains mainly captured by side/side contacts. Finally, the force transmission is intrinsically multiscale with a mean force increasing linearly with particle size. The probability density of forces is increasingly broader as size polydispersity increases with a well-defined exponential fall-off of the number of forces.