Shear Jamming, Diffusion, Reversibility, Anisotropy, and Fluctuations in Dense Granular Materials

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****Technical Abstract****This project involves novel experiments to understand the dynamical, static and statistical properties of granular materials near jamming. These materials are of great technical importance, yet their properties remain poorly understood. The Duke group and collaborators at Brandeis, recently showed that near-jamming properties of frictional granular systems are much richer than previously thought, and exhibit an order-disorder transition, shear jamming, involving complex networks of contacts and forces. This project explores these networks, the physical processes involved in their formation, and the impact that these networks and processes have on static and dynamical granular properties. Experiments in two and three dimensions, using novel photoelastic and laser scanning techniques, will address several issues: 1) structural and mechanical properties of granular materials near shear jamming and their dynamical response to strain; 2) the role of friction in shear jamming; 3) variables needed for a good statistical characterization of states near jamming; 4) particle dynamics, diffusive motion, and time scales near (shear) jamming; 5) near-jamming rheological response of granular material; 6) differentiation in the structure and response of 3D vs. 2D granular systems. Post-docs, Ph.D. students and undergraduates will be actively involved at all stages of research. The PI, post-docs and students will be actively involved in outreach to elementary and middle schools.****Non-Technical Abstract****Sand, cereal, salt, and pills are commonplace examples of a special type of matter, granular materials. Besides being part of daily life, granular materials, e.g. coal, grain, plastic feed stock, among many others, form a large part of our economy. They have properties that are both fascinating and poorly understood. Under the right conditions, they flow like a fluid, but they also can become solid and support weight. The fluid-like to solid-like transition is particularly important for practical applications, and it is the focus of the present project which focuses on fundamental questions. The solid granular state is controlled by networks of inter-grain contacts. In recent work, the PI and collaborators at Brandeis showed that these networks, which control the fluid-solid jamming transition, have much richer and more novel properties than was previously thought. This project seeks to understand the mechanisms leading to the formation of the networks, and the impact that these mechanisms have on both fluid and solid granular states. This project integrates training at all levels. Ph.D. students and post-docs learn by leading research projects. Undergraduate and high school students routinely participate via related projects. The PI, Ph.D. students and post-docs are actively involved in outreach involving elementary and middle school students. The PI is also actively involved with technology transfer through interactions with industrial users of granular materials, for instance through the International Fine Particle Research Institute.