Abstract
The mechanical response of glasses remains challenging to understand. Recent results indicate that the oscillatory rheology of soft glasses is accompanied by a sharp non-equilibrium transition in the microscopic dynamics. Here, we use simultaneous x-ray scattering and rheology to investigate the reversibility and hysteresis of the sharp symmetry change from anisotropic solid to isotropic liquid dynamics observed in the oscillatory shear of colloidal glasses (D. Denisov, M.T. Dang, B. Struth, A. Zaccone, P. Schall, Sci. Rep. 5 14359 (2015)). We use strain sweeps with increasing and decreasing strain amplitude to show that, in analogy with equilibrium transitions, this sharp symmetry change is reversible and exhibits systematic frequency-dependent hysteresis. Using the non-affine response formalism of amorphous solids, we show that these hysteresis effects arise from frequency-dependent non-affine structural cage rearrangements at large strain. These results consolidate the first-order-like nature of the oscillatory shear transition and quantify related hysteresis effects both via measurements and theoretical modelling.
Highlights
The flow and relaxation of glasses is important for a wide range of materials including metallic glasses, polymer and soft glasses, but remains challenging to understand
As we have shown in [8], our setup combining x-ray scattering and rheology allows us to reveal a sharp symmetry change upon increasing strain amplitude: we observed a sharp transition from twofold symmetry characteristic of a sheared solid to isotropic symmetry characteristic of a slowly sheared liquid
This sharp symmetry change reminds of first-order equilibrium transitions, but in the case here is induced by the applied oscillatory shear
Summary
The flow and relaxation of glasses is important for a wide range of materials including metallic glasses, polymer and soft glasses, but remains challenging to understand. Our own x-ray scattering measurements during the oscillatory rheology of a colloidal glass showed a sharp symmetry change from anisotropic solid to isotropic liquid-like response [8] at the rheological yielding of the glass, suggesting a non-equilibrium first-order transition under the applied oscillatory shear. By employing a mean-field model based on the non-affine response formalism for amorphous solids [37,38,39,40], we identify this non-monotonous behavior as due to the competition of dissipation and finite-rate non-affine motion leading to entropic changes of the material These results provide deeper insight into the nature of this non-equilibrium firstorder transition in oscillatory sheared colloidal glasses.
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