The flow of amorphous solids: from atomistic simulations to Earth Science applications?

Centre Blaise Pascal, ENS Lyon, France June 15-172016

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Panta rhei, “everything flows”. This famous aphorism was attributed to the Greek philosopher Heraclitus and indeed many systems in Earth Sciences are in motion ranging from sand dunes, land slides, rock or snow avalanches to sea ice, subglacial sediments, and earthquake fault gouge.

While often described by a continuum model these systems have all in common an arrangement of elementary building blocks or grains. These can be rocks (land slides [1]), blocks of ice or floes (sea ice [2]), depth hoar (snow avalanches [3]) sub-glacial sediment (glacier [4]) or microscopic crushed pieces of rock composing the gouge region (earthquakes [5]).

The physical laws that are used to describe this motion are often empirical - Glen law for ice sheet and glaciers dynamic, rate and state friction law in seismology - and can lack justification at the individual grain size - visco-plastic rheology of sea ice.

Alone such macroscopic laws provide limited physical insight into the deformation processes controlling the flow of these materials and tend to inadequately represent the temporal and spatial scaling laws. This in turn limits the predictive capabilities of these models with important consequences for human and structural safety but also for climatic and natural disaster risk assessments.

With the increase in computer power, discrete element simulations have emerged as ideal tools to address these shortcomings and validate the macroscopic continuum models by providing a microscopic view of the flow of amorphous solids.

Today, important progress has been achieved in material sciences, and granular or atomistic simulations can complement successfully meso-scale and continuum descriptions to provide a complete picture of the flow of amorphous solids (see introduction). On the other hand similar multi-scale approaches in the fields of Earth sciences and geophysics are only sporadically used.

In this context the aim of the proposed workshop is to bring together a group of leading experts in the field of computational modeling of amorphous plasticity at all scales (molecular dynamics, discrete element model, granular dynamics, mesoscopic elastoplastic models, finite element models) with leading experts in the relevant fields of Earth sciences.

This confrontation should help: (i) identify the concept of amorphous solid as a useful paradigm for several Earth science systems; (ii) popularize in the Earth science community the numerical methods used in material sciences; (iii) offer a consistent multi-scale approach of the flow of amorphous solids combining the micro, meso and macro scales; (iv) understand the mechanisms controlling the localization of the deformation; and (v) identify outstanding theoretical and numerical challenges.

References

[1] Y. Fortere and O. Pouliquen: Flows of dense granular media. Annu. Rev. Fluid. Mech. 40:1-24, 2008 [2] A. V. Wilchinsky, D. L. Feltham and M. A. Hopkins: Effect of shear rupture on aggregate scale formation in sea ice. J. Geophys. Res. 115 C10002 2010 [3] Heierli, J., Gumbsch, P., & Zaiser, M. (2008). Anticrack nucleation as triggering mechanism for snow slab avalanches. Science, 321(5886), 240-243. [4] Damsgaard, A., Egholm, D. L., Piotrowski, J. A., Tulaczyk, S., Larsen, N. K., & Tylmann, K. (2013). Discrete element modeling of subglacial sediment deformation. Journal of Geophysical Research: Earth Surface, 118(4), 2230-2242. [5] E. G. Daub and J. M. Carlson: “Friction, fracture, and earthquakes.” Annu. Rev. Condens. Matter Phys. 1.1 (2010): 397-418.