The Janssen effect is a unique property of confined granular materials experiencing gravitational compaction in which the pressure at the bottom saturates with an increasing filling height due to frictional interactions with side walls. In this Letter, we replace gravitational compaction with frictional compaction. We study friction-compacted 2D granular materials confined within fixed boundaries on a horizontal conveyor belt. We find that even with high-friction side walls the Janssen effect completely vanishes. Our results demonstrate that gravity-compacted granular systems are inherently different from frictioncompacted systems in at least one important way: vibrations induced by sliding friction with the driving surface relax away tangential forces on the walls. Remarkably, we find that the Janssen effect can be recovered by replacing the straight side walls with a sawtooth pattern. The mechanical force introduced by varying the sawtooth angle θ can be viewed as equivalent to a tunable friction force. By construction, this mechanical friction force cannot be relaxed away by vibrations in the system.
Inspired by the lovely paper “Shock Waves in Weakly Compressed Granular Media” by van den Wildenberg, Rogier van Loo, and Martin van Hecke we investigate granular optics in the non-shock regime.
We experimentally probe wave propagation in compressed granular media. Due to the presence of a Hertzian contact potential between particles, granular media is unique in that small changes in confining pressure translate into large changes in the sounds speed. We use this property to externally construct gradients in the effective refractive index. This allows us to demonstrate refraction within a granular pack. We further use this property to build acoustic lenses within a granular system using nothing more than applied confining pressure. We measure the propagation of sound waves within the granular material using force sensitive resistors.
Our setup includes a 2’x2’x6” containment device (sandbox) and a variety of medium from mm glass beads (sand) to ½” stone material (rocks & gravel). By using a variety of granular medium we can further probe the refractive nature of the system. We apply steady confining pressure though metal plates.