Institute for Complex Systems - Sapienza - CNR

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ISC Sapienza Granular Systems Rehology of Disordered Materials

Rehology of Disordered Materials

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Disorder is crucial in determining the behavior of matter. At the macroscopic level it affects the response to mechanical solicitations, reflecting in physical properties like plasticity, elasticity, viscosity, etc. and affecting inherent phenomena, e.g. fracture generation and propagation. Presently our activity is mainly focused on the dynamics of granular matter. In both natural and industrial world, grains are much more frequently encountered than crystals, however their dynamics is much less understood. They also provide laboratory model systems for studying dissipative, non-linear and non-equilibrium processes, like earthquakes. One main facility of the lab is aimed at measuring the response of a horizontal granular bed to an applied shear stress. It consists of a mechanical device which can shear the samples at different imposed rotation speeds, a set of sensors with their dedicated channels for data acquisition, and a computer with codes specifically developed for the experiment control and the acquisition and storage of data.

The figure  shows the shearing part of the apparatus, consisting of a cylindrical cell made of transparent PMMI, which contains the granular sample. A mobile annular plate on the top can shear the sample in the horizontal and allowing its expansion in the vertical. The shearing rate can be imposed directly, ranging in a wide interval (from 10 -4 to 10 rads -1 ), or through a rotating torsion spring. For us this is the most interesting case in which,like in many natural phenomena, shearing can take place in a chaotic way and grains produce avalanches, whose statistical properties are the main object of investigation. Through data analysis, avalanche features like size, duration, and velocity are put in quantitative relation among them and with other quantities, like friction, an are used to devise and test stochastic models for the dynamics.


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