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School of Physics Nonlinear Science and Mathematical Physics WEBINAR Series: Presenting Jie Ren, Duke University
Loose sand particles can jam together to become a solid. Think of the dry sand on the beach: it supports your weight so you can walk on it – you don't need to swim in sand. Jammed sand indeed so much resists deformation that it even expands when you try to make it flow. This was already discovered by Osborne Reynolds in 1885. In fact, you can make sand particles jam by making them flow in a container that cannot expand. This effect is known as shear jamming and was discovered only recently. We have now studied the remarkable mechanics of these solid, shear-jammed structures.
We did this in a model system, composed out of plastic disks. With an inventive new experimental setup, we were able to perform uniform shear to a two-dimensional assembly of these disks in a container that does not expand. Deforming the collection of disks from a loose state with no forces between the disks, this model sand developed fascinating force structure. The plastic disks are optically sensitive to the forces acting on them, so these forces could be visualized as fringe patterns. With so much detail about the microscopic contacts in hand, we could for the first time fully establish and also quantify the existence of non-linear mechanical behavior of these shear jammed solids.
Moreover, even though particles in these shear jammed packings hardly had the space to move, we uncovered completely unexpected dynamics in the force networks as shown in the image, when they exposed the packing to repeated deformations. These results provide new perspective and important benchmarks for theoretical modeling of these shear jammed solids. And that's good, because at some point engineers may use the concept of shear jamming to build your new house. Perhaps even better is that it also shows the surprise and beauty in materials as common as sand on the beach.