Dynamics of granular materials: from microscopic scale to collective effects at macroscopic scales
Main aims of this project:
The main goal of the project is understanding the properties of granular materials at different scales. The sound propagation through granular material is very sensitive to the structure of the internal forces network, a small vibration can trigger a catastrophic event. We intend to obtain information about the internal structure of the granular material by studying quantitatively the changes in the propagation of sound inside the material. The sound propagation has long used as a tool for assessing a wide range of mechanical properties of complex materials. However, little effort has been devoted to exploring its potential in the evaluation of micro-structural changes of granular systems. The program will contribute to the understanding of the intricate connection between local phenomena and large-scale dynamics of granular media.
Contact dynamics.
Macroscopic effects in granular media depend in the complex contact networks that are formed inside the material. This inhomogeneous chains of active contacts, give rise to a great number of phenomena. We use a one dimentional system to study the propagation of energy in one of this chains. Due to the nonlinear interaction potential determined by Hertz law, the compressing stage between two spherical beads, is limited by the rapidly increasing of the stiffness. This results in a fast repulsion between grains. If the beads are barely touching one another, the energy is propagated in the form of solitary waves (highly nonlinear limit). We have studied the interaction of this non linear waves with boundaries [1], mitigation of impacts using geometrical considerations [2], localization of energy in chains with small intruders [3], interaction of solitary waves [4]
Wave propagation in wet granuar media.
More recently, we have turned our atention to the propagation of waves in wet granular materials. It is known that de addition of a viscous fluid to a granular media results on the increase of sound velocity in the wet material. We have found experimental evidence that indicates that the main mechanism for this effect is due to viscoelasticity of the fluid. The contact geometry of two spherical beads creates a singular point as they approach. In turn, shear rate increases rapidly and reveals the viscoelastic nature of the fluid.
Instabilities under sonic perturbation of a granular material.
Due to the sismic nature of our country's territory, we are interested in studying the failure mechanism induced by the propagation of compressive waves in a granular material and the relationship between an earthquake event and the secondary events that, generally, that come after. For this, we study the behavior of a statically loaded granular system when a train of, small amplitude, acoustic waves propagates in the axis of loading. We have observed that a strong correlation exists between failure events and the amplitude of the propagating waves.
[1] S. Job, F. Melo, A. Sokolow and S. Sen, Phys. Rev. Lett. 94, 178002 (2005)
[2] F. Melo, S. Job, F. Santibañez and F. Tapia, Phys. Rev. E 73, 041305 (2006)
[3] S. Job, F. Santibañez, F. Tapia and F. Melo, Phys. Rev. E 80, 025602(R) (2009)
[4] F.Santibañez, R.Muñoz, A.Caussarieu, S.Job and F.Melo, Phys. Rev. E 84, 026604, Agosto 2011 (2011)
Researchers
Francisco Melo (ANR Director)
