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From avalanches and erosion to the transport of grain or pharmaceuticals, granular materials are all around us. When working with them, it rapidly becomes clear that these "simple" systems produce complex behavior: networks of force chains support the material, flowing motion is localized in shear bands, and particles segregate by size or shape when you try to mix them. An important feature of granular materials is that the internal forces are not carried uniformly by the material, but instead through long chain-like strucutres whose density and orientation depend on the state and history of the sample. We are able to visualize these forces using a photoelastic disks and a polariscope, as seen in the image at right. Research projects in the group address visualizing sound wave propagation, effects of order and anisotropy on material properties, phase transitions, and developing and testing statistical mechanical models.
With their ability to act as solids, liquids, or gases, the behavior of granular materials begs analogy with much of what we first learned about conventional molecules, but there are imporant differences. One goal of our research is to understand the solid-liquid ("jamming") transition for dense granular materials. In particular, the dynamics of granular materials depend strongly on whether they are loosely or closely packed. In this movie we show the diffusion and braiding of trajectories in a 2D granular system. Additionally, we examined the fluctuations of the local volume fraction in dense driven granular systems. We found the fluctuations decrease as the system approaches the jamming transition independent of boundary condition (Constant pressure / Constant Volume) and inter-particle friction. To explore the universality of the relationship between local volume fraction and its fluctuations, we extended the recent granocentric model by including the separation distribution. Through the granocentric model, we observe that diverse functional forms of the separation distribution all produce the trend of decreasing fluctuations, but only the experimentally observed separation distribution provides quantitative agreement with the measured local volume fluctuations. Therefore, both the volume fraction and separation distribution encode similar information about the ensemble and are connected with the granocentric model. Publications
Granular materials are inherently heterogeneous, leading to challenges in formulating accurate models of sound propagation. In order to quantify acoustic responses in space and time, we perform experiments in a photoelastic granular material in which the internal stress pattern (in the form of force chains) is visible. Through high-speed imaging, we observe that the wave amplitude is on average largest within particles experiencing the largest forces. In addition, we are able to directly observe rare transiently-strong force chains formed by the opening and closing of contacts during propagation. Additionally, the speed of the leading edge of the pulse is in agreement with the speed of a one-dimensional chain, while the slower speed of the peak response suggests that it contains waves which have traveled over multiple paths even within just this near-field region. These effects highlight the importance of particle-scale behaviors in determining the acoustical properties of granular materials. Movie illustrating the technique. Publications
Granular materials of mixed sizes can segregate under shear, as seen in the image at right. Together with Michael Shearer in the NCSU Mathematics Department, we are developing models of how segregation happens in mixed samples and how mixing happens when the system is unstably stratified. Intringuingly, we observe that small particles do not necessarily exhibit faster mixing and segregation behavior. Publications
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