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)(gammaD), 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 (proportional to)(alpha)(k(max)(2)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> less than or equal to 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.