Structural relaxation and rheological response of a driven amorphous system

TL;DR

This study investigates how structural relaxation and rheological behavior of a binary Lennard-Jones glass-forming system are affected by shear, revealing distinct dynamics in supercooled and glass states and indicating a transition toward non-ergodicity as shear diminishes.

Contribution

It demonstrates how shear influences structural relaxation and rheology in glassy systems, highlighting differences between supercooled and glass states under shear.

Findings

Supercooled state shows shear thinning at high shear rates and equilibrium relaxation at low shear rates.

Glass state exhibits shear-driven relaxation with viscosity diverging as shear rate approaches zero.

A transition toward non-ergodic behavior is observed in driven glasses as shear diminishes.

Abstract

The interplay between the structural relaxation and the rheological response of a binary LJ glass former is studied via MD simulations. In the quiescent state, the model is well known for its sluggish dynamics and a two step relaxation of correlation functions at low temperatures. An ideal glass transition temperature of $T_c = 0.435$ has been identified in the previous studies via the analysis of the system's dynamics in the frame work of the mode coupling theory of the glass transition [W. Kob and H.C. Andersen, PRE 51, 4626 (1995)]. Here, we test wether a signature of this ideal glass transition can also be found under shear. Indeed, the following distinction in the structural relaxation is found: In the supercooled state, the structural relaxation is dominated by the shear at relatively high shear rates, $\dot{\gamma}$, whereas at sufficiently low $\dot{\gamma}$ the (shear-independent) equilibrium relaxation is recovered. In contrast to this, the structural relaxation of a \emph{glass} is always driven by shear. This distinct behavior of the correlation functions is also reflected in the rheological response. In the supercooled state, the shear viscosity, $\eta$, decreases with increasing shear rate (shear thinning) at high shear rates, but then converges toward a constant as the $\dot{\gamma}$ is decreased below a (temperature-dependent) threshold value. Below $T_c$, on the other hand, the shear viscosity grows as $\eta \propto 1/\dot{\gamma}$ suggesting a divergence at $\dot{\gamma} =0$. Thus, within the accessible observation time window, a transition toward a non-ergodic state seems to occur in the driven glass as the driving force approaches zero.