Automated Measurement of the Profile of an Avalanching Conical Bead Pile

A conical bead pile subject to slow driving is used as a model system to experimentally investigate critical behavior in a granular system. The pile is composed of roughly 20 000 steel beads, 3 mm in diameter; we drive the pile by adding one bead at a time to the pile apex. We record the changes in pile mass over the course of tens of thousands of bead drops to characterize the distribution of avalanche sizes. To better understand the dynamic effect of individual avalanches, we now capture a profile image of the pile at every bead drop. By automating analysis of the images in Matlab, we can track changes in the overall height of the pile as well as variations in the angle of repose as the pile responds to each bead drop. In our previous work, we have characterized the changes in avalanche size distribution as we tune the cohesion in the system. Specifically, as cohesion is added, the size and number of the largest avalanches in the system increase. Using the new profile measurements, we investigate the effect of cohesion on the overall angle of repose as well as the variation in the angle. We find larger variations in the angle of repose as the cohesion is increased, and we correlate these variations with the mass of individual avalanches.