The Origin of Geometric Friction and Cohesion

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CBET - 1605178PI: Brown, EricDry granular materials are collections of small discrete particles that interact with each other through frictional contacts. Examples of granular materials from nature include sand, soil, and snow. Granular materials are also found in industrial processes, especially in the pharmaceutical, energy, and agricultural industries. These materials display complex spatial and temporal responses to external forces because they behave as collections of discrete grains instead of a pure fluid and because they are not in thermal equilibrium. The mechanical strength of these materials has been studied previously, but most fundamental research has considered spherical or nearly spherical particles. This project focuses on understanding the mechanical properties of packings of granular materials composed of highly nonspherical particles such as long rods, chains, and objects that can interlock, such as staples and bent rods. We will image the packings using x-rays to identify contacts and entanglements and connect this information about the microstructure to the strength of the packings in response to compression. The experimental data will be used in computer simulations to validate a physical model of packings of highly nonspherical grains.Uniaxial compression experiments will be performed on packings of nonspherical particles with different particle aspect ratios andtopologies. The experimental results will be compared with a theoretical model that predicts which particle shapes exhibitgeometric cohesion and which particle shapes produce packings that show significant strain-hardening stress-strain response. Comparisons for several particle shapes (e.g. rods and staples) will allow us to validate the model so that it can be employed to predict the mechanical response of granular packings for a wide parameter space of particles shapes. In addition, numerical simulations combined with x-ray micro computed tomography studies will allow us to investigate the microstructural properties of granular packings(e.g. interparticle contacts and entanglements) that give rise to geometric cohesion and strain hardening and how they are affected byfriction and bending stiffness.