Jamming in long rod-like granular materials

The physics of large aspect-ratio (L/D$>>$10) granular materials is relatively unexplored. Such materials form connected networks significantly more rigid than those found in ordinary round granular materials, a feature well-known to anyone who has moved hay with a pitchfork. I will describe recent experiments that investigate the response of two- and three-dimensional piles to an object forced through the pile, with an emphasis on the transition to a jammed state in which the pile acts as a solid body. In 2D the spatial extent of the disturbance scales with particle length and grows linearly with packing fraction. When the disturbance size becomes comparable with the container, solid-body motion occurs in piles of larger aspect ratio particles. In 3D the transition from stick-slip to solid-body occurs through an intermediate region where the time scale of particle rearrangements becomes comparable with that for the test object to move an appreciable distance. This behavior depends both on particle length, aspect ratio, and container dimension, and we are mapping out the behavior in the multi-dimensional parameter space.