Communication: Fast and local predictors of the violation of the Stokes-Einstein law in polymers and supercooled liquids

Abstract

The violation of the Stokes-Einstein (SE) law is investigated in a melt of linear chains by extensive molecular-dynamics simulations. It is found that the SE breakdown is signaled (with 5% uncertainty) by the monomer mean-square displacement u 2 on the picosecond time scale. On this time scale the displacements of the next-next-nearest neighbors are uncorrelated. It is shown that: (i) the SE breakdown occurs when u 2 is smaller than the breadth of the distribution of the square displacements to escape from the first-neighbors cage, (ii) the dynamical heterogeneity affects the form of the master curve of the universal scaling between the structural relaxation and u 2 .

Communication: Fast dynamics perspective on the breakdown of the Stokes-Einstein law in fragile glassformers

“…Alternative definition of the SE product as K ′ SE = Dη/T (or K ′ SE = Dτ α /T ) virtually does not alter the quality of the scaling, as shown in the bottom inset of Fig.4 for the BM and MA systems. Notice that Fig.4 presents results for polymers with different lengths since K SE /K 0 is independent of it [6].…”

“…Remarkably, despite its macroscopic derivation, SE accounts also well for the self-diffusion of many monoatomic and molecular liquids, provided the viscosity is low ( 10 P a · s) [2]. Distinctly, a common feature of several fragile glass formers is the breakdown of SE for increasing viscosity, that manifests as a partial decoupling between the diffusion and viscosity itself [3][4][5][6]. The decoupling is well accounted for by the fractional SE (FSE) D ∼ η −κ [7] where the non-universal exponent κ falls in the range [0.5 − 1] [8].…”

Weak links between fast mobility and local structure in molecular and atomic liquids

“…After proper account of the molecularweight dependence Eq.1 extends to longer time scales in polymers where states with equal fast mobility exhibit also equal chain reorientation rate [30,31] and diffusivity [31,38]. Correlation between diffusion and fast mobility was observed in atomic mixtures too [32].…”

Cage rattling does not correlate with the local geometry in molecular liquids

“…Diffusion scaling was also observed in atomic mixtures [55]. More recently, the influence of free volume and the proper time scales to observe the genuine fast dynamics have been considered [58,59] as well as the breakdown of the Stokes-Einstein (SE) law [60], the relation with the elastic modulus [15] and the spatial extension of the involved particle displacements at short-times [61,62].…”