2D Friction-driven Granular Flow Mechanics

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.

Although granular impact dynamics have been studied extensively granular flow is still a topic of active research. Granular flow mechanics is not yet well understood and has applications to ballistics, meteor impacts and industrial processes. We have designed an experiment to study the mechanics of 2D friction-driven granular flow into a hopper as well as around a probe. We use a conveyor belt to drive a jammed packing of acrylic laser cut disks past a probe that is free to rotate about the center of mass axis. A laser cutter is used to construct different probes of varying geometry. We study, for instance, the effect of probe geometry on its rotational stability and the drag force it experiences. Using a force sensor and an optical encoder, we can measure the force on the probe and its rotational motion.