condensed matter physics Condensed matter physics is the field of physics that deals with the macroscopic and microscopic physical properties of matter, especially the solid and liquid State of matter, phases, that arise from electromagnetic forces between atoms and elec ...

and

physical chemistry Physical chemistry is the study of macroscopic and microscopic phenomena in chemical systems in terms of the principles, practices, and concepts of physics such as motion, energy, force, time, thermodynamics, quantum chemistry, statistical mech ...

, the terms viscous liquid, supercooled liquid, and glass forming liquid are often used interchangeably to designate

liquid Liquid is a state of matter with a definite volume but no fixed shape. Liquids adapt to the shape of their container and are nearly incompressible, maintaining their volume even under pressure. The density of a liquid is usually close to th ...

s that are at the same time highly

viscous Viscosity is a measure of a fluid's rate-dependent resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for example, syrup h ...

(see

Viscosity of amorphous materials Viscosity is a measure of a fluid's rate-dependent resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for example, syrup h ...

), can be or are

supercooled Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid below its freezing point without it becoming a solid. Per the established international definition, supercooling means ''‘cooling a substance be ...

, and able to form a

glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...

Working points in glass processing

The mechanical properties of glass-forming liquids depend primarily on the viscosity. Therefore, the following working points are defined in terms of viscosity. The temperature is indicated for industrial soda lime glass:

Fragile-strong classification

In a widespread classification, due to chemist Austen Angell, a glass-forming liquid is called strong if its viscosity approximately obeys an Arrhenius law (log η is linear in 1/''T'' ). In the opposite case of clearly non-Arrhenius behaviour the liquid is called fragile. This classification has no direct relation with the common usage of the word "fragility" to mean

brittleness A material is brittle if, when subjected to stress (physics), stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of h ...

. Viscous flow in amorphous materials is characterised by deviations from the Arrhenius-type behaviour: the activation energy of

viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

Q changes from a high value Q_H at low temperatures (in the glassy state) to a low value Q_L at high temperatures (in the liquid state). Amorphous materials are classified accordingly to the deviation from Arrhenius type behaviour of their viscosities as either strong when or fragile when Q_H-Q_L≥Q_L. The fragility of amorphous materials is numerically characterized by the Doremus’ fragility ratio R_D=Q_H/Q_L . Strong melts are those with (R_D-1) < 1, whereas fragile melts are those with (R_D-1) ≥ 1. Fragility is related to materials bond breaking processes caused by thermal fluctuations. Bond breaking modifies the properties of an amorphous material so that the higher the concentration of broken bonds termed configurons the lower the viscosity. Materials with a higher enthalpy of configuron formation compared with their enthalpy of motion have a higher Doremus fragility ratio, conversely melts with a relatively lower enthalpy of configuron formation have a lower fragility. More recently, the fragility has been quantitatively related to the details of the interatomic or intermolecular potential, and it has been shown that steeper interatomic potentials lead to more fragile liquids.{{cite journal, last1=Krausser, first1=J., last2=Samwer, first2=K., last3=Zaccone, first3=A., date=2015, title=Interatomic repulsion softness directly controls the fragility of supercooled metallic melts, journal=Proceedings of the National Academy of Sciences of the USA, volume=112, issue=45, pages=13762–13767, doi=10.1073/pnas.1503741112, pmid=26504208, pmc=4653154, doi-access=free

Mode-coupling theory

The microscopic dynamics at low to moderate viscosities is addressed by a mode-coupling theory, developed by

Wolfgang Götze Wolfgang Götze (born 11 July 1937 – 20 October 2021) was a German theoretical physicist. He began his physics education at Humboldt University of Berlin and the Free University of Berlin, after which he obtained his doctorate at the Technic ...

and collaborators since the 1980s. This theory describes a slowing down of structural relaxation on cooling towards a critical temperature Tc, typically located 20% above Tg.

Notes and sources

Textbooks

*Götze,W (2009): Complex Dynamics of glass forming liquids. A mode-coupling theory. Oxford: Oxford University Press. *Zarzycki,J (1982): Les Verres et l'état vitreux. Paris: Masson. Also available in English translations.

References

Glass physics Glassforming liquids and melts