Microscopic dynamics and rheological behavior of amorphous solids.

Luca Cipelletti
Laboratoire Charles Coulomb,
Universite' de Montpellier et CNRS

The mechanical properties of amorphous solids such as glasses or gels are currently a topic of intense research, with implications in material science as well in fundamental condensed matter physics. Traditionally, researchers have investigated the relationship between two macroscopic quantities: the stress applied to the sample and the resulting strain. Recent works aim at gaining a deeper understanding of the origin of the rheological properties, by coupling rheology to structural and dynamical measurements.

I’ll discuss experiments probing the relationship between rheology and microscopic dynamics in two amorphous systems: a semicrystalline polymer (PEEK) and a colloidal gel. For the PEEK, a remarkably simple relation is found between the macroscopic stress relaxation at a fixed strain and the temporal evolution of the microscopic dynamics. We furthermore provide evidence for the existence of dynamical precursors of failure: the microscopic dynamics is found to speed up dramatically before the macroscopic rupture of the material.

A similar behavior is found in the creep of a colloidal gel under a constant shear stress. The gel creep consists of three regimes. Deviations from a purely elastic (or affine) deformation are observed in the initial regime. These non-affine dynamics are fully reversible upon removing the applied stress, and are associated to the heterogeneity of the local gel elasticity. In the second regime, non-affine displacements grow much slower with strain, but are associated to irreversible rearrangements. In the third regime, a sharp acceleration of the dynamics at small length scale is observed. As for the semicrystalline polymer, these rearrangements are dynamic precursor of material failure.