Title: Velocity fluctuations and intermittency in hopper flow near clogging

Author (Poster): Charles Thomas, University of Pennsylvania

Abstract:

Dynamic arrest in granular systems continues to elude a comprehensive explanation. We consider the gravity-driven flow from a hopper as a quintessential example of a system that can spontaneously evolve from a freely flowing state to a jammed state. With a large enough opening of size D, grains flow out freely. When D is smaller, however, grains flow for a period and then stop, and the entire hopper has clogged. A critical opening size Dc is defined as the smallest D for which the flow will never clog, and marks the clogging transition. We investigate the grain motion in a quasi-2D hopper as a function of D when D > Dc. Using a high-speed camera, we track the particles and find their instantaneous velocities. Several significant features of the velocity fields as a function of vertical position and opening size D are reported. The distributions are systematically more skewed for smaller values of D and for regions higher up in the hopper. Additionally, the relative granular temperature increases as D approaches Dc. Finally, the average flow becomes more intermittent both for smaller opening sizes and at higher regions in the hopper. Intermittent dynamics are a key feature of both of glassy and jammed systems. The similarities and contrasts between the clogging and the jamming transitions will further illuminate systems that undergo dynamic arrest.