Column Collapse of Rod-like Granular Materials

We study the collapse of piles of rod-like granular materials, in particular how the particle aspect ratio (length/width) and coefficient of friction affect the runoff. Rod particles can maintain the shape of their container, something round particles cannot, and we find transitional pile heights that determine the onset of collapse. For low aspect ratios, pile heights of less than a particle length do not collapse, implying that vertically oriented rods are anchoring the pile and providing stability. There is a broad transition range of pile heights in which the probability of collapse grows linearly from 0 to 1. The scaling of the runoff distance in and above this region is independent of aspect ratio and friction, depending only on the initial pile geometry. This work could have significant implications for construction of stable structures and understanding avalanches of needle-like snow crystals (hoar).