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

Abstract

The breakdown of the Stokes-Einstein (SE) law in fragile glassformers is examined by Molecular-Dynamics simulations of atomic liquids and polymers and consideration of the experimental data concerning the archetypical ortho-terphenyl glassformer. All the four systems comply with the universal scaling between the viscosity (or the structural relaxation) and the Debye-Waller factor ⟨u⟩, the mean square amplitude of the particle rattling in the cage formed by the surrounding neighbors. It is found that the SE breakdown is scaled in a master curve by a reduced ⟨u⟩. Two approximated expressions of the latter, with no and one adjustable parameter, respectively, are derived.

Fast Vibrational Modes and Slow Heterogeneous Dynamics in Polymers and Viscous Liquids

“…Nonetheless, given the scaling expressed by Equation (2), it is legitimate to wonder if DH and fast vibrational dynamics exhibit correlations. Working in that direction, we have found positive answers and the present paper collects and reviews a selected part of our results, with a focus on the breakdown of the SE law [29,30]. Even if strictly related, we will not discuss here a study concerning ultrathin molecular films with strong mobility gradients analogous to DH, where the same scaling observed in bulk, Equation (2), has been revealed [61].…”

“…In fact, a common feature of several fragile glass formers is the SE breakdown for increasing viscosity. The failure manifests itself as a partial decoupling between the diffusion and the viscosity, in the sense that D−1 increases less than η [4,11,27,28,29,30]. The decoupling is well accounted for by the fractional SE D∼η−κ [31], where the non-universal exponent κ falls in the range 0.5≤κ<1 [15].…”

“…Despite the huge difference in time scales, earlier [33] and later theoretical studies [5,34,35,36,37,38,39,40], and experimental ones [41], addressed the rattling process within the cage to understand the slow dynamics, rising a growing interest on the DW factor [8,29,30,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61]. Within this context, most interest has been devoted to the correlations between DW factor and the structural relaxation time τα, which are found to be strong and encompassed by a universal master curve [47]:τα=scriptF(〈u2〉)…”

“…Alternative forms of the master curve are reported by Douglas and coworkers [8,9,10]. Correlations between DW factor and the structural relaxation time τα are found in polymers in bulk [30,47,48,49] and thin films [61], binary atomic mixtures [48,55], colloidal gels [52], antiplasticised polymers [8,9], water [57] and water-like models [59,60]. The DW factor also provided an alternative interpretation of the so-called thermodynamic (or temperature/density) scaling [58].…”

Applications of Dynamic Mechanical Analysis in the Engineering of Amorphous Solid Dispersions

“…4) 32 and self-diffusion (Eq. 5) [48][49][50] have been validated with various experimental measurements at both elevated temperatures and near T g . In ►Fig.…”