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Granular materials – such as cereals, powders, and sand – are the most used form of matter by human beings, next only to water. Still, the dynamics of granular media is far from understood. In this talk, firstly I will present one of our recent discoveries: when some common cohesive powder materials are subjected to shear they crack and slip, often accompanied by mysterious but reliable electrical signals. More surprisingly, these electrical signals sometimes 'precede' the slip events, as early as 4.8s. This phenomenon is robust to change in many experimental parameters. Insights gained from our laboratory-scale experiments may help us deepen our understanding of, and even predicting, real world catastrophic slip events like earthquakes and landslides.
In the second part of the talk I will focus on the jamming-unjamming transition that lead to material failure and slip events. I will discuss the role of particle shape, inter-particle friction and their influence on the mechanical stability of the granular packing. To this end, we use tetrahedra shaped particles and employ different preparation protocols, which are then analyzed using 3D X-ray tomography to obtain the local geometric arrangement of these particles. It is well known that granular packings are history dependent. I will show that the packings prepared using two different protocols can have the same packing fraction but not the same constraint number, which determines the mechanical stability. Finally, I will conclude by discussing how our results lead to show that the 'packing fraction' φ solely can be inadequate to characterize the 'jamming transition' – the point at which materials become mechanically unstable, thereby unjam and slip.
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