Dynamic phase coexistence in glass-forming liquids

TL;DR

This study provides direct evidence of transient coexistence of two dynamical phases with different diffusivities in supercooled liquids, linking this phenomenon to dynamic heterogeneity and the breakdown of the Stokes-Einstein relation.

Contribution

It is the first to directly observe bimodal diffusivity distributions indicating phase coexistence in supercooled liquids, advancing understanding of their heterogeneous dynamics.

Findings

Bimodal diffusivity distribution observed in supercooled regime

Dynamical phase coexistence persists longer than relaxation time during cooling

Breakdown of the Stokes-Einstein relation linked to phase coexistence

Abstract

One of the most controversial hypotheses for explaining the heterogeneous dynamics of glasses postulates the temporary coexistence of two phases characterized by a high and by a low diffusivity. In this scenario, two phases with different diffusivities coexist for a time of the order of the relaxation time and mix afterwards. Unfortunately, it is difficult to measure the single-particle diffusivities to test this hypothesis. Indeed, although the non-Gaussian shape of the van-Hove distribution suggests the transient existence of a diffusivity distribution, it is not possible to infer from this quantity whether two or more dynamical phases coexist. Here we provide the first direct observation of the dynamical coexistence of two phases with different diffusivities, by showing that in the deeply supercooled regime the distribution of the single-particle diffusivities acquires a transient bimodal shape. We relate this distribution to the heterogeneity of the dynamics and to the breakdown of the Stokes-Einstein relation, and we show that the coexistence of two dynamical phases occurs up to a timescale growing faster than the relaxation time on cooling, for some of the considered models. Our work offers a basis for rationalizing the dynamics of supercooled liquids and for relating their structural and dynamical properties.