Granular materials are large aggregates of macroscopic, individually solid particles, or "grains". Understanding and controlling their properties is of tremendous importance for industrial processes across a wide range of fields from agriculture to civil and chemical engineering to materials science. One of the most remarkable behaviors of granular matter is the transition from a jammed to an unjammed state under an applied shear stress. At this transition granular materials change character from solid- to fluid-like and exhibit characteristic properties such as the formation of shear bands. This also is an experimentally and theoretically challenging regime for which the current level of understanding is far less developed than for other phase transitions such as those occurring in ordinary fluids or solids.
Intellectual Merit. In addition to the industrial, economic value associated with proper control of particulate matter, the study of granular materials provides insights into fundamental scientific issues. A series of experiments in granular media has been designed to investigate the nature of the transition from static packing to a flowing regime. Emphasis is placed on studying the formation of shear bands and techniques that make it possible to measure some of the most important physical quantities that were previously inaccessible to experiment. In particular, direct measurements of the time-evolving forces and stresses within shear bands will be made using a newly developed optical force-visualization apparatus. A new type of linear, parallel plate shear cell is introduced that can measure the onset and evolution of structure and viscosity within a shear band from its initial transient behavior to it final steady-state character. Finally, a modified split-bottom Couette-type shear cell with wide and tunable shear band will be used to test some of the most important current theories concerning the applicability of effective temperatures and determine the importance of fluctuations generic to these systems. These studies will help provide an experimental underpinning for some of the key ideas that have so far only been tested in theory or simulation.
Broader Impact: The experimental research on sheared granular materials will be closely integrated with a multi-faceted set of education and outreach activities. It includes the active participation of a diverse group of high-school interns and undergraduates in the research, a close collaboration with two science museums to develop hands-on activities, and the development of a new course on granular materials. In addition, we plan an interdisciplinary workshop on granular matter and the jamming transition.
