Non linear rheology and dynamic yielding of a simple glass: A molecular dynamics study

Steady state non linear rheology and dynamic yielding of a well established simple glass forming model (a 80:20 binary LJ mixture first introduced by Kob and Andersen, PRE51 1995) is studied via molecular dynamics simulations. It is observed that the rheological response of the system changes qualitatively as the temperature is varied from the supercooled state towards the glassy phase. In the supercooled state, the system response is marked by a shear thinning behavior at high shear rates followed by a cross over to linear response in the limit of weak external drive. In contrast to this behavior, a stress plateau forms in the glass at low shear rates. We define this stress plateau as the dynamic yield stress of the model and study its dependence on temperature. Furthermore, the relation between a generalized stress based on the relaxation time (multiplied by shear rate) and the actual (''real'') shear stress is studied. It is found that such a generalized stress exhibits qualitatively the same features as the ''real'' stress. On a quantitative level, however, a gap opens between these two quantities deep in the glassy phase. This aspect is crucial for a determination of the temperature dependence of the dynamic yield stress.