Supercooled liquids analogous fractional Stokes-Einstein relation in NaCl solution above room temperature

The Stokes-Einstein relation D~T/η and its two variants D~τ^-1 and D~T/τ follow a fractional form in supercooled liquids, where D is the diffusion constant, T the temperature, η the shear viscosity, and τ the structural relaxation time. The fractional Stokes-Einstein relation is proposed to result from the dynamic heterogeneity of supercooled liquids. In this work, by performing molecular dynamics simulations, we show that the analogous fractional form also exists in sodium chloride (NaCl) solutions above room temperature. D~τ^-1 takes a fractional form within 300-800 K; a crossover is observed in both D~T/τ and D~T/η. Both D~T/τ and D~T/η are valid below the crossover temperature T_x, but take a fractional form for T>T_x. Our results indicate that the fractional Stokes-Einstein relation not only exists in supercooled liquids but also exists in NaCl solutions at high enough temperatures far away from the glass transition point. We propose that D~T/η and its two variants should be critically evaluated to test the validity of the Stokes-Einstein relation.