The Structural Difference Between Strong and Fragile Liquids

TL;DR

This paper introduces a structural order parameter that distinguishes strong from fragile liquids based on local restraint, independent of dynamics, revealing fundamental differences in their atomic configurations.

Contribution

It proposes a novel structural measure based on local restraint that differentiates strong and fragile liquids without relying on their dynamic behavior.

Findings

Molten silica remains highly constrained at equilibrium.

Fragile liquids show an abrupt transition to restrained states below melting.

The structural restraint correlates with the potential energy landscape features.

Abstract

A structural order parameter for disordered configurations is defined, based, not on local topologies, but on the degree of local restraint imposed on each atom. This restraint parameter provides a clear distinction between a strong liquid (SiO2) and a fragile liquid (a binary Lennard-Jones mixture) without reference to dynamics. Where the fragile liquid exhibits an abrupt transition from unrestrained to restrain below the melting point, molten silica is highly constrained even at equilibrium. We determine the temperature dependence of the average number of particles restrained per pinned particle and consider the feature of the potential energy landscape responsible for determining the fragility.