Molecular dynamics study on the shear viscosity of molten Na₂O·2SiO₂

This paper describes the applicability of molecular dynamics simulation to the shear viscosity of silicate melts. Simulations of molten Na₂O·2SiO₂ have been performed on 756 and 189 particle systems to evaluate the viscosity by using three different methods: the Green-Kubo integrand, the Stokes-Einstein equation, and the non-equilibrium molecular dynamics (NEMD) simulation. Of these methods, the Green-Kubo treatment and the Stokes-Einstein equation, adopting the SiO₄^4- anion for the diffusing unit, gave the most reasonable results, and they qualitatively reproduced the temperature dependence of viscosity. The simulated values of viscosity were consistent with experimental values within an order of magnitude at temperatures, higher than 1500 K, but were smaller by more than one order of magnitude at lower temperatures. The NEMD simulation of shear flow showed a strong shear-thinning features and failed to reproduce the viscosity value at the experimental shear rate region. The discrepancy between the simulated and experimental values is attributed to the short sampling time and the small system size in the MD simulation.