Heterogeneous diffusion, viscosity, and the Stokes-Einstein relation in binary liquids.
2016
We investigate the origin of the breakdown of the Stokes-Einstein relation (SER) between diffusivity and
viscosity in undercooled melts. A binary Lennard-Jones system, as a model for a metallic melt, is studied by
molecular dynamics. A weak breakdown at high temperatures can be understood from the collectivization of
motion, seen in the isotope effect. The strong breakdown at lower temperatures is connected to an increase
in dynamic heterogeneity. On relevant time scales some particles diffuse much faster than the average or than
predicted by the SER. The van Hove self-correlation function allows one to unambiguously identify slow particles.
Their diffusivity is even less than predicted by the SER. The time span of these particles being slow particles,
before their first conversion to be a fast one, is larger than the decay time of the stress correlation. The contribution
of the slow particles to the viscosity rises rapidly upon cooling. Not only the diffusion but also the viscosity
shows a dynamically heterogeneous scenario. We can define a “slow” viscosity. The SER is recovered as the
relation between slow diffusivity and slow viscosity.
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