Collaborative Research: Stress-Tensor Tomography in 3D Fluid-Coupled Granular Media

This grant supports research that looks to advance the knowledge of how fluid-coupled granular media behave. Because granular media are ubiquitous in natural systems (such as soils in riverbeds and landslides) and infrastructure (such as concrete and ballast), this research will promote both the progress of science and engineering, as well as advance national prosperity. When a load is applied to a granular medium, such load is transmitted via a network of forces among grains that are in contact. This network of forces, or force chains, is the ultimate determinant of how granular media behave under external loading (e.g., compression and shear) and under fluid injection and withdrawal. Understanding the spatial structure and temporal evolution of force chains constitutes a fundamental goal of granular mechanics. However, the current knowledge of granular media is limited by the experimental observations on force chains, which are either on two-dimensional packs or on three-dimensional packs with limited grain shapes or loading conditions. The coupling between the solids and fluids in granular media adds yet additional complexity for observations and modeling. This award supports fundamental research looking to advance experimental techniques and the associated theory for fluid-coupled granular media, enabling the observation of the transmission of external loads on both the single-grain scale and the granular pack scale. The outcomes of this research intend to provide new knowledge of the organization of contact forces in fluid-coupled granular media at the grain scale, and help predict their behavior in natural systems like landslides and earthquakes, as well as engineering applications like construction materials, infrastructure and robotics. The outcomes of the research will be integrated into undergraduate and graduate courses and multiple well-organized outreach activities, such as the Simons STEM Scholars program, the Simons Summer Research Program, and Engineering Academy for grade 6-12 students, with an expectation to engage a broad group of students, thus positively impacting engineering education in the US. This research looks to advance the fundamental understanding of the mechanical behavior of granular media by developing innovative experimental and theoretical techniques that will enable accessing, quantitatively, the stress-tensor field, and associated force chains, in 3D granular packs of round and angular particles, under various load conditions and fluid-coupling scenarios. This research seeks to develop the experimental apparatus and associated theory for the tomographic reconstruction of stress tensors in fluid-coupled granular media under external loads in 3D, based on interference optical projection tomography. This new method intends to advance the understanding of the tensor nature of effective stress on the grain scale, which results from the normal and tangential contact forces between particles of various shapes and moduli, as well as elucidate the spatial structure and temporal evolution of force chains in 3D packs of angular and round particles under various stress conditions and fluid-coupling scenarios.. 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.