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∝(T-Tc)γD, with γD=1.93±0.02 and Tc=0.458±0.002. The self-part of the intermediate scattering function Fs(kmax,t) exhibits a steplike behavior at the lowest temperatures. On cooling, the increase of the related relaxation time τα tracks the diffusivity, i.e., τα∝(kmax2D)-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, ψ(t), is exponential at higher temperatures. At lower temperatures a power-law divergence is evidenced at short times, ψ(t)∝tξ-1 with 0<ξ<~1, which is ascribed to intermittency. The shear viscosity is fitted by the power law η∝(T-Tc)γη, with γη=-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 Dη/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 ψ(t). A precursor effect of the breakdown is observed, which manifests itself as an apparent stick-slip transition.