Viscous flow and jump dynamics in molecular supercooled liquids. I. Translations.
The transport and relaxation properties of a molecular supercooled liquid on an isobar are studied by molecular dynamics. The molecule is a rigid heteronuclear biatomic system. The diffusivity is fitted over four orders of magnitude by the power law D proportional to (T-T(c))(gamma(D)), with gamma(D)=1.93+/-0.02 and T(c)=0.458+/-0.002. The self-part of the intermediate scattering function F(s)(k(max),t) exhibits a steplike behavior at the lowest temperatures. On cooling, the increase of the related relaxation time tau(alpha) tracks the diffusivity, i.e., tau(alpha) proportional to (k(2)(max)D)(-1). At the lowest temperatures, fractions of highly mobile and trapped molecules are also evidenced. Translational jumps are also evidenced. The duration of the jumps exhibits a distribution. The distribution of the waiting times before a jump takes place, psi(t), is exponential at higher temperatures. At lower temperatures a power-law divergence is evidenced at short times, psi(t) proportional to t(xi-1) with 0<xi<or=1, which is ascribed to intermittency. The shear viscosity is fitted by the power law eta proportional to (T-T(c))(gamma(eta)), with gamma(eta)=-2.20+/-0.03 at the lowest temperatures. At higher temperatures the Stokes-Einstein relation fits the data if stick boundary conditions are assumed. The product D eta/T increases at lower temperatures, and the Stokes-Einstein relation breaks down at a temperature which is close to the one where the intermittency is evidenced by psi(t). A precursor effect of the breakdown is observed, which manifests itself as an apparent stick-slip transition.
De Michele
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Dipartimento di Fisica, Università di Pisa, Via Filippo Buonarroti, 2, I-56100 Pisa, Italy.
Leporini
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