Qualitative difference in rheology between fragile and network-forming strong liquids

Abstract

We elucidate a qualitative difference in rheology between fragile and network-forming strong liquids. In a flow field, the structural configuration is distorted in accordance with the flow symmetry, whereas the form of the interaction potential remains unchanged. The role of this mismatch in the relaxation mechanism under the flow field is crucial for understanding the shear-thinning mechanism and differs between strong and fragile glass formers. In fragile glass formers, shear thinning can be attributed to the shear-induced reduction of the {\it effective density}. In contrast, in strong glass formers, the shear-induced reduction of the {\it effective activation energy} is a possible origin of a significant acceleration in relaxation. Our simple predictions of the crossover shear rate, $\dot\gamma_{\rm c}$, from Newtonian to non-Newtonian behaviors can be expressed in terms of experimental observables: in fragile liquids, $\dot\gamma_{\rm c}=(\rho \partial \tau_\alpha/\partial \rho)^{-1}$, where $\rho$ and $\tau_\alpha$ are the density and structural relaxation time, respectively, and in strong liquids, $\dot\gamma_{\rm c}= (\tau_\alpha\Delta E_0 /T)^{-1} $, where $T$ and $\Delta E_0$ are the temperature and equilibrium activation energy, respectively. These predictions are consistent with the results of molecular dynamics simulations for four different glass formers: two fragile and two strong ones. This different route to the non-Newtonian flow response is related to differences in the role of density in the relaxation dynamics.