Energy dissipation in granular materials

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Ever since Coulomb's work on equilibrium and the failure of granular slopes, friction and strength of granular soils became firmly linked. More than two centuries later, many theories have been advanced resulting in a near one-to-one proportionality between Coulomb's friction angle and the angle of friction between soil grains. Attempts to verify these theories experimentally yield a surprising indication that the link between macro and micro friction angles is far weaker than past theories implied. Although shear strength characteristics of granular soils appear analogous to those of a frictional system, the lack of a direct link to interparticle friction is not easily rationalizable. This points to a fundamental knowledge gap that possibly explains the many difficulties in describing engineering properties of soils through practically usable models. The proposed project is to substantially close this gap, both in terms of the fundamental understanding of the role of friction in the behavior of granular material and in developing a constitutive model based on adequate physics. Both objectives are connected to the broader topic of the physical mechanisms of energy dissipation in granular materials and its analytical description for modeling purposes. The project is theoretical in its nature but involves particle-level simulations, as well as grain-scale physical experimentation utilizing recent advances in sensing technology.***The role of inter-particle friction as a controlling factor of macroscopic strength has been clarified to some extent through particle-level simulations when it was shown that very little friction is required to effect significant macroscopic resistance of simulated assemblies. Moreover, increasing the friction coefficient to prevent interparticle slip changes little in the behavior of a granular assembly. As no energy is dissipated in frictional slip in this case, other dissipative mechanisms are clearly involved. Observations of particles motions in simulated systems suggest that forces are transmitted through granular assemblies in a highly heterogeneous manner so that some particles are highly stressed, while others do not carry any load. This causes spontaneous losses of local stability and structural rearrangements during which particles gain significant kinetic energy that eventually dissipates. Such motions are manifested in acoustic emissions during laboratory tests on sand, booming sound during sand avalanches, and the "singing" of desert dunes.****The experimental part of the project will involve utilizing highly sensitive laser interferometry to monitor motions of a single marked particle in "transparent soil" composed of oil-fused quartz particles with the pore fluid of a matching refraction index. It is expected that the tracked particle will experience spurious motions indicative of instabilities in the system.**** **