  We study the geometrically induced cohesion of ensembles of granular
"u-particles" which mechanically entangle through particle interpenetration. We
vary the length-to-width ratio $l/w$ of the u-particles and form them into
free-standing vertical columns. In laboratory experiment we monitor the
response of the columns to sinusoidal vibration (frequency $f$, peak
acceleration $\Gamma$). Column collapse occurs in a characteristic time,
$\tau$, which follows the relation $\tau = f^{-1} \exp(\Delta / \Gamma)$.
$\Delta$ resembles an activation energy and is maximal at intermediate $l/w$.
Simulation reveals that optimal strength results from competition between
packing and entanglement.
