Experimental Demonstration of Nonuniform Frequency Distributions of Granular Packings

We developed a novel experimental technique to generate mechanically stable (MS) packings of frictionless granular disks. We performed a series of coordinated experiments and numerical simulations to enumerate the MS packings in small 2D systems composed of bidisperse disks. We find that frictionless MS packings occur as discrete, well-separated points in configuration space and obtain excellent quantitative agreement between MS packings generated in experiments and simulations. In addition, we observe that MS packing probabilities can vary by many orders of magnitude and are robust with respect to the packing-generation procedure. These results suggest that the most frequent MS packings may dominate the structural and mechanical properties of granular systems. We argue that these results for small systems represent a crucial first-step in constructing a statistical description for large granular systems from the `bottom-up'.