Speaker Institution: Department of Physics - University of Pennsylvania
Title: Dynamics of Granular Clogging
Abstract: The gravity-driven flow of grains from a hole in a hopper is an iconic granular phenomenon. It’s different from a fluid in that the rate is constant also in that it can suddenly and unexpectedly clog. How does the the susceptibility to clogging decrease with increasing hole size, and is there a well-defined clogging transition above which the system never clogs? This problem is distinct from jamming due to presence of boundaries and gradients. We show how the fraction F of flow configurations that cause a clog may be deduced from the average mass discharged between clogs. We construct a simple model to account for the observation that F decays exponentially in hole width to the power of dimensionality. Thus the clogging transition is not sharp but rather is defined by observation limits similar to the glass transition. When the system is immersed in water, F barely changes. Therefore, the crucial microscopic variables are the grain positions; grain momenta play only a secondary role in destabilizing weak incipient arches. There is also a surprising effect whereby the discharge causes water to be pumped downwards, faster than the grains.