Column collapse of granular rods

We find the collapse of columns of granular rods to show range of behaviors that depends on particle aspect ratio (length $L$ to diameter $d$) and initial pile geometry (height/radius). For all aspect ratios $L/d$ below 24 there exists a critical height at $L/4$ below which the pile acts as a solid, maintaining its initial shape, and a second critical height at $3L/4$ above which the pile always collapses like an ordinary granular material. Separating the critical heights is a transition region in which the probability of collapse increases linearly from 0 to 1. This behavior is independent of particle length, width, or aspect ratio. When the pile does collapse, the runoff radius $r_f$ scales as a power-law with dimensionless height $\tilde H$, agreeing with previous experiments on ordinary sand. For low piles the scaling is linear, with $r_f\sim \tilde H^{1.2\pm 0.1}$. Above a critical pile aspect ratio (pile height/radius) this switches to a square-root scaling, with $H^{0.6\pm0.1}$.
Comment: Submitted to Physical Review E, 6/2010