Low-friction soft particles add intermittency and avalanches to the flow of rigid particles in a hopper

Instabilities and avalanches in granular flows represent hallmarks of failure: they can both disrupt industrial process flows and signal dangerous conditions, like those in grain silos and snowy mountaintops. We investigate intermittency and avalanches in the gravity-driven flow of granular materials through a quasi-2D hopper. Mixtures of rigid polypropylene and soft polyacrylamide particles flow through a hopper constriction. The soft particles are nearly frictionless while the rigid ones are frictional. A combination of high-speed imaging, particle identification and tracking analyses enable us to measure quantities including particle velocities, outflow rates, the time intervals between consecutive particle exits, and the geometric properties of any temporary arches that form during a flow test. As the fraction of rigid particles increases in the mixture, the velocity of exiting particles increases. So too, however, does the probability of complete clogging. While low-friction soft particles exhibit slower velocities at the hopper exit, they also facilitate greater overall discharge rates. Simultaneously, the presence of low-friction soft particles induces both intermittency and avalanches in the flow. In this case, arches that could permanently block the flow are more likely to be temporary in nature. Slight adhesion between the low-friction soft particles may help explain this intermittency. Interestingly, low-friction soft particles, despite their correlation with flow instabilities, are not significantly more likely to fall first from a temporary arch. Investigating the arch geometries suggests that the particle forming the largest bond angle with its neighbors is the one that causes an arch to fail, regardless of its material properties. • Adding low-friction soft particles to rigid systems reduces clogs, speeds discharge. • Low-friction soft particles added to rigid ones cause flow intermittency, avalanches. • Slight adhesion may explain greater flow intermittency in low-friction soft particles. • The geometry of temporary arches reveals the origin of subsequent avalanches.