When tilted downhill, a pile of grain starts flowing at an angle θstart that is larger than the angle θstop at which it stops. That hysteresis is an important feature of the behavior of earthquakes, landslides, and industrial flows. But despite the many years researchers have investigated granular flows (see the article by Anita Mehta, Gary Barker, and Jean-Marc Luck, Physics Today, May 2009, page 40), the origin of the hysteresis has remained mysterious.
Recent work postulated that inertia—from the shock between grains and acoustic noise—is a key ingredient in the mechanism. But Yoël Forterre, his postdoc Hugo Perrin, and their colleagues at Aix-Marseille University in France and the Swiss Federal Institute of Technology in Lausanne have now upended that hypothesis. In particular, they have found a large hysteresis in the avalanche angle even when inertia was negligible.
The trick to their finding was controlling the interparticle friction in a suspension of silica microbeads held in a rotating drum. When immersed in water, the silica beads spontaneously develop negative surface charges, which electrostatically push them apart by a distance known as the Debye length λD and prevent them from making solid contact, as shown in the figure. Interparticle friction can be turned on and incrementally tuned simply by dissolving electrolytes (in this case, sodium chloride) in the water, which screen the surface charges.
In experiments at low salt concentration, the mean avalanche angle was small, just 6°, and close to that of frictionless spheres. At high concentration, the angle climbed to 22°, a value typical for frictional grains. What’s more, when the grains interacted through frictional contacts, sawtooth-shaped hysteretic avalanches emerged. The hysteresis completely disappeared when the friction was turned off. (H. Perrin et al., Phys. Rev. X 9, 031027, 2019.)
