****Technical Abstract**** The main goal is to develop an understanding of the kinetics of jamming in granular systems, through measurement of jamming fronts and of grain-scale dynamics across the fronts. The simplest situation is the stationary upward propagation of a jamming front during sedimentary deposition, where the grain concentration profile will be measured by medical x-ray imaging, and the granular temperature profile (Tg) will be measured by Speckle-Visibility Spectroscopy (SVS). Key interest is in the deviations from step-function profiles, the vanishing of Tg, and spatiotemporally correlated fluctuations of dynamics akin to dynamical heterogeneities but now in presence of gradients and time evolution. Another situation is the clogging behavior for various grain types discharged from tall hoppers. Macroscopically, the clogging transition will be studied by the divergence of average discharge mass as hole size increases. Microscopically, it will be studied by spatially-correlated fluctuations of Tg around the hydrodynamic time-average flow field; unsteadiness of Tg is hypothesized to increase on approach to the clogging transition. This will be studied by particle tracking and by SVS through the sidewall. A separate thrust is planned for the dynamics of 2d air-fluidized grains within an array of pinned grains. These projects will bring modern physics and measurement techniques into the educational experience of graduate and undergraduate students. And the knowledge to be created will be helpful for understanding and controlling jamming phenomena in industrial and natural settings, where there is often a moving boundary between jammed and unjammed regions.
The flow of almost anything other than a fluid has an annoying tendency to "jam" and come to rest in spite of applied forces. Familiar examples include traffic on a highway, coal in a chute, and sand in an hourglass. Such problems confound the processing of diverse granular materials like foods, seeds, minerals, ores, building materials, and pharmaceutical pills & powders. In the natural world, jamming phenomena underlie horrific events such as mudslides and earthquakes. In none of these cases can the behavior yet be truly controlled. But thanks to recent theoretical and experimental science, there is now a reasonable understanding of the jamming transition for uniform systems, where all the grains are subjected to the same conditions and are all either jammed or unjammed. This award will support groundbreaking research on how jams develop in situations where there is an interface or "jamming front" that separates jammed and unjammed regions and that changes with time. Think of what happens when a clog forms over the outlet of an hourglass, or when grains settle out of from a fluid. Though the materials may seem mundane, the science and the experimental methods are sophisticated. This will be ideal for the educational experience of graduate and undergraduate students. The knowledge to be created will be of interest to physicists, engineers, and geologists, and will help industry avoid inefficiencies and process failures due to jamming. It may ultimately even help save lives and property through control of mudslides and erosion.
