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In this Research Note the Zwanzig's formulation of the Stokes–Einstein (SE) relation for simple atomistic fluids is re-examined. It is shown that the value of the coefficient in SE relation depends...

The translational self-diffusion coefficient and the shear viscosity of water are related by the fractional Stokes–Einstein relation. We report extensive novel molecular dynamics simulations for the self-diffusion coefficient and the shear viscosity of water. The SPC/E and TIP4P/2005 water models are used in the temperature range 220–560 K and at 1 or 1,000 bar. We compute the fractional exponents t, and s that correspond to the two forms of the fractional Stokes–Einstein relation (Formula presented.) and (Formula presented.) respectively. We analyse other available experimental and numerical simulation data. In the current analysis two temperature ranges are considered (above or below 274 K) and in both cases deviations from the Stokes–Einstein relation are observed with different values for the fractional exponents obtained for each temperature range. For temperatures above 274 K, both water models perform comparably, while for temperatures below 274 K TIP4P/2005 outperforms SPC/E. This is a direct result of the ability of TIP4P/2005 to predict water densities more accurately and thus predict more accurately the water self-diffusion coefficient and the shear viscosity.

International audience; A series of classical molecular dynamics simulations of calcium aluminosilicate (CAS) (CaO-Al2O3)(1-x)(SiO2)(x) melts with varying silica content x, along the CaO/Al2O3 concentration ratio R=1, have been performed for the purpose of studying (i) the evolution of fragility with silica content and (ii) the temperature and composition evolution of self-diffusion coefficients, viscosity and -relaxation times. Our results indicate a decrease in the fragility of the CAS systems along R=1 from calcium aluminate to pure silica. It is found that upon cooling, the Stokes-Einstein relation breaks down at a temperature significantly higher than the critical temperature of the mode-coupling theory T-C for all compositions and also above the melting point.

Experiments have been performed at 5 GPa on liquid Fe-FeS in order to determine Fe and S self-diffusivity as a function of temperature. The viscosity of the sample was then obtained using the Stokes-Einstein relation. The results are in excellent agreement with previous experiments where the viscosity of a material of the same composition under similar conditions was measured directly. These results support high, near-metallic, values of diffusivity and low viscosity in liquid Fe-S up to a few hundred K above the eutectic temperature, in contrast with some previous studies. Moreover, these results fully confirm the validity of the Stokes-Einstein relation between viscosity and diffusion coefficients for Fe0.61S0.39.