Is glass a state of matter?

TL;DR

This paper explores whether glass should be considered a distinct state of matter by reviewing theoretical, computational, and experimental perspectives on the nature of glass and its classification.

Contribution

It provides a pedagogical review of the debate on glass as a separate state of matter, including recent theoretical and simulation advances.

Findings

Recent theories suggest an 'ideal glass' as a separate equilibrium state.

Computer simulations support the existence of a distinct glass state.

Experimental perspectives are discussed for future research.

Abstract

Glass is everywhere. We use and are surrounded by glass objects which make tangible the reality of glass as a distinct state of matter. Yet, glass as we know it is usually obtained by cooling a liquid sufficiently rapidly below its melting point to avoid crystallisation. The viscosity of this supercooled liquid increases by many orders of magnitude upon cooling, until the liquid becomes essentially arrested on experimental timescales below the ``glass transition'' temperature. From a structural viewpoint, the obtained glass still very much resembles the disordered liquid, but from a mechanical viewpoint, it is as rigid as an ordered crystal. Does glass qualify as a separate state of matter? We provide a pedagogical perspective on this question using basic statistical mechanical concepts. We recall the definitions of states of matter and of phase transitions between them. We review recent theoretical results suggesting why and how an ``ideal glass'' can indeed be defined as a separate equilibrium state of matter. We discuss recent success of computer simulations trying to analyse this glass state. We close with some experimental perspectives.