A perspective on the fragility of glass-forming liquids

TL;DR

This paper examines extraneous factors affecting the measurement of fragility in glass-forming liquids, proposing refined definitions and scaling methods to better isolate genuine super-Arrhenius relaxation slowdown.

Contribution

It introduces a refined fragility measure and modified Angell plot to account for density and high-temperature effects, improving characterization of glass-forming liquids.

Findings

Density effects can obscure fragility measurements.

Refined scaling methods better isolate super-Arrhenius behavior.

Proposed modifications improve understanding of glass transition dynamics.

Abstract

We discuss possible extraneous effects entering in the conventional measures of "fragility" at atmospheric pressure that may obscure a characterization of the genuine super-Arrhenius slowdown of relaxation. We first consider the role of density, which increases with decreasing temperature at constant pressure, and then the potential influence of the high-temperature dynamical behavior and of the associated activation energy scale. These two effects involve both thermodynamic parameters and the strength of the "bare" activation energy reflecting the specific bonding between neighboring molecules. They vary from system to system with, most likely, little connection with any putative collective behavior associated with glass formation. We show how to scale these effects out by refining the definition of fragility and modifying the celebrated Angell plot. We dedicate this note to our great and so inspiring friend, Austen Angell, and associate in this tribute another dear colleague who died too soon, Daniel Kivelson.