Non-monotonic dynamic correlations beneath the surface of glass-forming liquids

TL;DR

This paper uncovers a non-monotonic temperature dependence of dynamic correlation lengths near the surface of glass-forming liquids, revealing a crossover temperature linked to changes in cooperative particle rearrangements and decoupling of fast particle motion.

Contribution

It demonstrates the existence of a non-monotonic dynamic correlation length in glass-forming liquids and connects it to a morphological change in cooperative regions and a decoupling phenomenon.

Findings

Dynamic correlation length peaks at a crossover temperature T_*

Surface dynamic correlations are distinct from static correlations

Crossover T_* exceeds the mode-coupling critical temperature T_c

Abstract

Collective motion over increasing length scales is a signature of the vitrification process of liquids. We demonstrate the emergence of distinct static and dynamic length scales probed near the free surface in fully equilibrated glass-forming liquid films, and their connection to the bulk properties of the system. In contrast to a monotonically growing static correlation length, the dynamic correlation length that measures the extent of surface-dynamics acceleration into the bulk, displays a striking non-monotonic temperature evolution that is robust also against changes in detailed interatomic interaction. The maximum of dynamic correlations defines a cross-over temperature $T_*$ that we show to agree with a morphological change of cooperative rearrangement regions (CRR) of fast particles in the bulk liquids. The cross-over occurs at a temperature larger than the critical temperature Tc of mode-coupling theory (MCT). We link it to the point where fast-particle motion decouples from structural relaxation that can be defined rigorously within a recent extension of MCT, the stochastic $\beta$-relaxation theory (SBR).