Date of Award

2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Physics & Astronomy

First Advisor

Douglas J. Durian

Abstract

This work focuses on the clogging of granular materials. Dynamic arrest in granular systems continues to elude a comprehensive description. We consider 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. However, the nature of clogging remains poorly understood. A key point of debate is whether there exists a critical opening size Dc, for which when the opening size D > Dc, the flow never clogs. There has been little consensus about what material or hopper properties govern the clogging probability. To lay these issues to rest, we have investigated clogging through multiple approaches. We present several findings in this work. First, we demonstrate that clogging of hard granular media is controlled solely by an effective area Aeff, which is a function of the opening area and orientation. Second, we show that clogging is a Poisson process with a sampling rate set by the grain diameter and the effluent velocity. Finally, we show that in this picture, the only requirement for clog formation is for all of the grains in a region near the exit to be sufficiently “pre-clogged”. Clogging becomes highly unlikely for large openings simply because the number of grains which are required to be pre-clogged grows as D^alpha, where alpha is the system dimensionality. In a separate track, we have investigated the dynamics of granular flows, both with D < Dc and D > Dc. We find that the flow becomes more intermittent, with higher effective granular temperature, when it is more prone to clogging. However, we find no evidence of a clogging transition in the velocity fluctuations or intermittency. Indeed, the intermittency does not exhibit a diverging time scale as might be expected from a jamming or glass transition. Instead, the time scale of flow intermittencies is set entirely by the rate at which the flow samples for stable configurations.

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