Evidence of a one-dimensional thermodynamic phase diagram for simple glass-formers

TL;DR

This study demonstrates that the dynamics of simple glass-formers across a broad timescale range can be unified along a single thermodynamic phase diagram, confirming the isomorph theory and simplifying the understanding of the glass transition.

Contribution

It provides the first simultaneous neutron and dielectric spectroscopy evidence that the phase diagram for certain glass-formers is effectively one-dimensional, aligning dynamics across multiple timescales.

Findings

Lines of identical dynamics exist on the phase diagram for two van der Waals liquids.

The phase diagram reduces from two to one dimension for these liquids.

Dynamics on different timescales are governed by the same underlying mechanisms.

Abstract

The glass transition plays a central role in nature as well as in industry, ranging from biological systems such as proteins and DNA to polymers and metals. Yet the fundamental understanding of the glass transition which is a prerequisite for optimized application of glass formers is still lacking. Glass formers show motional processes over an extremely broad range of timescales, covering more than ten orders of magnitude, meaning that a full understanding of the glass transition needs to comprise this tremendous range in timescales. Here we report on first-time simultaneous neutron and dielectric spectroscopy investigations of three glass-forming liquids, probing in a single experiment the full range of dynamics. For two van der Waals liquids we locate in the pressure-temperature phase diagram lines of identical dynamics of the molecules on both second and picosecond timescales. This confirms predictions of the isomorph theory and effectively reduces the phase diagram from two to one dimension. The implication is that dynamics on widely different timescales are governed by the same underlying mechanisms.