Structural relaxation and rheological response of a driven amorphous system

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.

12 pages, 9 figures

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