NSF Award
0907695
****NON-TECHNICAL ABSTRACT****<br/>This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Granular materials suffuse daily life; from the cereal eaten at breakfast, the coal used for fuel, the ingredients in the pharmaceuticals which return us to health, to the soil and rocks that make up avalanches and landslides. The inherently granular nature of these types of systems make them difficult to describe and predict. One of the fundamental questions regarding granular systems has been the way in which force is transmitted through the grains. In two-dimensions, using optical techniques, it was discovered that the force is concentrated and transmitted through filamentary chain-like structures that have become known as "force chains", with many of the grains carrying little or none of the force. More recently, using a novel variation of a magnetic resonance imaging (MRI) method called magnetic resonance elastography (MRE), the existence of such chains in three-dimensions was confirmed. This individual investigator award supports a project that will refine the MRE imaging method as applied to dense granular materials and extend it to studies of a wide variety of granular systems. Analyses of the statistics of the force chain structure, as determined from the MRE images, will characterize these complex granular systems and allow quantitative predictions to be made. The experiments will engage undergraduate students, allowing them a varied learning environment including hardware and software development in addition to the physics of Nuclear Magnetic Resonace and the engineering of granular materials. <br/><br/>****TECHNICAL ABSTRACT****<br/>This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Understanding the internal 3D force chain structure of dense granular materials is necessary for a comprehensive explanation of granular behavior. These structures have been studied in 2D using photoelastic techniques, and preliminary experiments were recently conducted in 3D to detect force chains using a novel magnetic resonance imaging method termed magnetic resonance elastography (MRE). MRE measures amplitudes of small periodic displacements having a specific frequency and maps them as images. Periodic stimuli that are transmitted predominantly along the force chains are made visible in MRE images as lines of particles oscillating in-phase with the largest amplitudes, therefore the highest image intensities. This individual investigator award supports a project to refine the MRE imaging method as applied to dense granular materials and extend it to studies of a wide variety of systems. These investigations will include an optimization of the confining containers, the forcing scheme and the forcing frequency and amplitude used for studies of the chain structure, as well as a probe of a variety of compression types and amounts in both 2D and 3D systems. Furthermore, analyses of the statistics of the force chain structure, as determined from the MRE images, will be conducted. These will include the distribution of force chain lengths and statistics of the Voronoi cells, e.g., volumes, faces, areas, and edges. Students will be included in these studies, presenting them an exciting and varied learning environment, from hardware and software development to conducting and analyzing experiments and from the physics of NMR to the engineering of granular materials.
