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 phases which arise from electromagnetic forces between atoms. More generally, the ...

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 me ...

, the terms viscous liquid, supercooled liquid, and glassforming liquid are often used interchangeably to designate liquids that are at the same time highly

viscous The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the in ...

(see Viscosity of amorphous materials), can be or are supercooled, and able to form a glass.

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 Soda or SODA may refer to: Chemistry * Some chemical compounds containing sodium ** Sodium carbonate, washing soda or soda ash ** Sodium bicarbonate, baking soda ** Sodium hydroxide, caustic soda ** Sodium oxide, an alkali metal oxide * S ...

Fragile-strong classification

In a widespread classification, due to chemist

Austen Angell Charles Austen Angell (14 December 1933 – 12 March 2021) was a renowned Australian and American physical chemist known for his prolific and highly cited research on the chemistry and physics of glasses and glass-forming liquids. He was intern ...

, a glass-forming liquid is called strong if its viscosity approximately obeys an

Arrhenius law In physical chemistry, the Arrhenius equation is a formula for the temperature dependence of reaction rates. The equation was proposed by Svante Arrhenius in 1889, based on the work of Dutch chemist Jacobus Henricus van 't Hoff who had noted in 18 ...

(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, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Bre ...

. Viscous flow in amorphous materials is characterised by deviations from the Arrhenius-type behaviour: the activation energy of viscosity 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 The Free University of Berlin (, often abbreviat ...

and collaborators since the 1980s. This theory describes a slowing down of

structural relaxation A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as ...

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