Onset of shear thinning in glassy liquids: Shear-induced small reduction of effective density

摘要

We propose a simple mechanism for describing the onset of shear thinning in a high-density glassy liquid.In a shear flow, along the compression axis, the overlap between neighboring particles is more enhanced thanthat at equilibrium, meaning that the “effective” size is reduced along this axis. On the other hand, along theextension axis perpendicular to the compression axis, the average structural configurations are stretched, butit does not indicate the expansion of the “effective” size itself. This asymmetric shear flow effect for particlesresults in a small reduction of the “effective” density. Because, in glass-forming liquids, the structural relaxationtime τ_α strongly depends on the density ρ, even a very small reduction of the effective density should lead toa significant decrease of the relaxation time under shear flow. We predict that the crossover shear rate fromNewtonian to non-Newtonian flow behaviors is given by γ˙c = [ρ(∂τ_α/∂ρ)]~（−1）, which can be much smaller than1/τ_α near the glass transition point. It is shown that this prediction is consistent with the results of moleculardynamics simulations.