Non-Technical Abstract: This project seeks to understand grain-scale structures that form and evolve in granular materials as they are pushed or pulled by applied strains such as shear or compression. Granular materials are everywhere. They are used in a multitude of applications from handling ores, coal and wheat to pellets used for forming many manufactured plastic products. However, the basic understanding of the way that granular materials respond when they are strained or when they flow is very limited. This lack of knowledge causes significant problems for industry that must handle massive amounts of granular materials. There are two reasons that make it difficult to understand granular materials that are in dense fluid-like and solid-like states. The first is the difficulty of looking inside these materials with tools that can probe key quantities, such as the forces that act between grains. The second is that particles carry forces in very unequal ways. Only a small subset of grains, the so-called force chains, carry the majority of the forces that arise from applied strains or stresses, and the rest are "spectators", bearing almost no force. Understanding the formation and evolution of the force chains is the key challenge for dense granular materials. This project addresses this challenge through experiments using special photoelastic particles that allow precise experimental determination of forces between particles, and hence throughout a granular material. Photoelastic materials, including many plastics, transmit polarized light differently when they are subject to forces. By making grains from photoelastic materials, and placing everything between crossed polarizers, the force chain particles become immediately obvious. With this technique, it is possible to track how the force chains form and change, particularly when they are sheared. Shear occurs when a layer of grains is pushed one way on its top, and in the opposite way on the bottom. Shear is one of the most important ways to change the state of a granular material, and creates long force chains. When shear is continuous in time, the force chains form and break, leading to average forces on the opposite sides of the sheared material. Recent work has shown that the average forces for steady shear depend in a very predictable way on how fast the shearing is carried out. The present project involves experiments to understand how the evolution of the force chains, which change rapidly, lead to the average forces reported earlier. The answers to how force chains form, and how they respond to shear have the potential to answer fundamental questions about the behavior of granular materials. The experimental work involves students at all levels, from high school through Ph.D. candidates. Particular care will be given to involve women and minorities in all aspects. The project will involve outreach to industry, in terms of applications, and to elementary and secondary school students through science demonstrations and visits. The PI is also involved in a number of professional activities that benefit the field, including being editor in chief of the journal Granular Matter, and organizing parts of national and international meetings. All aspects of STEM, science, technology, engineering, and mathematics are present in this project.
This project addresses two key intertwined questions for dense granular materials through a series of novel experiments. The experiments involve quasi-two dimensional photoelastic particles, enabling powerful methods to measure forces and other quantities at the particle scale. The key questions are: What are the microscopic processes that lead to the evolution of granular force networks? and What are the physical origins of stresses for steadily sheared granular materials, sometimes referred to as rheology? Key question 1 concerns self-organization of grain scale structures in response to applied protocols, for example stresses or strains. Granular materials form complex networks, force chains, whose evolution, starting at the microscale, is basic to the origin of jammed granular states and granular dynamics in flowing systems. Collective granular response is both surprising and poorly understood at microscopic scale. How can a stress-free state become jammed under shear, with no change in area? What microscopic processes enable memory effects under cyclic strains, or the Janssen effect? What determines the lowest packing fraction for a random loose packing? Force networks self-organize to create the observed responses, but little if anything is known about the processes that generate the networks. Key question 2 is: What are the physical origins of rheology? More precisely, what relates the effective friction, which is the ratio of shear stress and pressure, to a dimensionless shear rate, the inertial number, involving the pressure, density, and particle size? Empirical "local" models describe relatively simple flows, but are inadequate for more complex cases. Recent non-local models extend the local ones. But, in the past, there are few experiments that probe inside materials, determining local properties, to definitively test these models. Experiments in this project use high speed imaging of photoelastic particles in several shear-dominated flows to test these models to determine all properties of the shear flow, including the full stress tensor. For both questions 1 and 2, the experiments use a range of different quasi-2D photoelastic particles having different friction, shape and stiffness. This project involves: 1) determining local granular structures that self-organize in response to a range of protocols, and 2) relating the evolution of these structures to large scale response. The interconnection of macroscopic and specific collective microscopic processes in dry granular materials, specifically to understand how force networks form and evolve, has not been achieved previously to the principal investigator's knowledge. This understanding has the potential to explain many complex processes that are usually treated at a macroscopic phenomenological level. For shear flows, this work provides experimental tests and comparisons to recently proposed models of rheology. A key goal is understanding multi-scale dynamical processes that underlie these models. This project involves continued efforts to transfer knowledge derived from DMR- supported research to industrial applications through connections to the International Fine Particle Research Institute (IFPRI). In particular, flows in hoppers are an industrially relevant flow involving rheology. The principal investigator and other project participants regularly involve undergraduates and high school students in the lab. The project involves outreach to students in elementary and secondary schools through science activities, and demonstrations. The project involves lab members at all stages of intellectual development, with special attention to the recruitment of under-represented groups, in particular, women and minorities. The PI is involved in a number of activities including editorship of Granular Matter and conference organization that support research and foster collaboration in the granular and complex fluids communities.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
