Entangled Granular Media

We study the geometrically induced cohesion of ensembles of granular u particles that mechanically entangle through particle interpenetration. We vary the length-to-width ratio l/w of the u particles and form them into freestanding vertical columns. In a laboratory experiment, we monitor the response of the columns to sinusoidal vibration (with peak acceleration Gamma). Column collapse occurs in a characteristic time tau which follows the relation tau proportional to exp(Gamma/Delta). Delta resembles an activation energy and is maximal a intermediate l/w. A simulation reveals that optimal strength results from competition between packing and entanglement.