Direct Measurements of Forces inside a 3D Random Granular Solid

Disordered solids differ tremendously in their microscopic detail, yet may exhibit similar response to applied forces. To test this idea in 3D, we measure positions, orientations and magnitudes of inter-particle forces in a dense pile of droplets. Confocal microscopy of a monodisperse emulsion stabilized by CdSe nanoparticles provides quantitative information in the interior of 3D samples. The contact areas between neighboring droplets are quantitatively related to the force (f) between the droplets. A method of image analysis was developed to find contact surfaces and thus map forces in 3D. We are interested in force autocorrelation functions and structure factors which quantify the spatial distribution of forces. In particular, the possibility of ! deg force chains!+/- and the distribution of forces are investigated in our experiment. Our results show an exponential tail of the probability distribution of f when f/ < f > > approx. 2. Applying uniform or point forces along chosen directions will provide additional insight. This work is supported by NSF (DMR-0305395).