Viscous liquid
Encyclopedia
In condensed matter physics
and physical chemistry
, the terms viscous liquid, supercooled liquid, and glassforming liquid are often used interchangeably to designate liquid
s that are at the same time highly viscous
(see Viscosity of amorphous materials), supercooled
, and able to form a glass
.
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:
, 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.
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 QH at low temperatures (in the glassy state) to a low value QL 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 QH-QLL or fragile when QH-QL≥QL. The fragility of amorphous materials is numerically characterized by the Doremus’ fragility ratio RD=QH/QL . Strong melts are those with (RD-1) < 1, whereas fragile melts are those with (RD-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 .
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.
Condensed matter physics
Condensed matter physics deals with the physical properties of condensed phases of matter. These properties appear when a number of atoms at the supramolecular and macromolecular scale interact strongly and adhere to each other or are otherwise highly concentrated in a system. The most familiar...
and physical chemistry
Physical chemistry
Physical chemistry is the study of macroscopic, atomic, subatomic, and particulate phenomena in chemical systems in terms of physical laws and concepts...
, the terms viscous liquid, supercooled liquid, and glassforming liquid are often used interchangeably to designate liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
s that are at the same time highly viscous
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
(see Viscosity of amorphous materials), supercooled
Supercooling
Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its freezing point without it becoming a solid....
, and able to form a glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
.
Working points in glass processing
| designation | viscosity (Pa.s) | temperature (deg C, in soda lime glass) |
|---|---|---|
| melting point |
101 | 1300 |
| working point | 103 | 950-1000 |
| sink point | 103.22 | |
| flow point | 104 | ~900 |
| softening point (Littleton) | 106.6 | 600 |
| softening point (dilatometric) | ~1010.3 | >~500 |
| annealing point Annealing (glass) Annealing is a process of slowly cooling glass to relieve internal stresses after it was formed. The process may be carried out in a temperature-controlled kiln known as a Lehr. Glass which has not been annealed is liable to crack or shatter when subjected to a relatively small temperature change... |
~1012 | <~500 |
| transition point Glass transition The liquid-glass transition is the reversible transition in amorphous materials from a hard and relatively brittle state into a molten or rubber-like state. An amorphous solid that exhibits a glass transition is called a glass... |
1012..1012.6 | <~500 |
| strain point Annealing (glass) Annealing is a process of slowly cooling glass to relieve internal stresses after it was formed. The process may be carried out in a temperature-controlled kiln known as a Lehr. Glass which has not been annealed is liable to crack or shatter when subjected to a relatively small temperature change... |
~1013.5 | <~500 |
Fragile-strong classification
In a widespread classification, due to chemist Austen AngellAusten Angell
Charles Austen Angell is a chemist known for his prolific and highly cited research on the physics of glasses and glass-forming liquids.His most cited contribution is probably the "strong–fragile" classification of viscous liquids...
, 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.
Viscous flow in amorphous materials is characterised by deviations from the Arrhenius-type behaviour: the activation energy of viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
Q changes from a high value QH at low temperatures (in the glassy state) to a low value QL 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 QH-QL
Mode-coupling theory
The microscopic dynamics at low to moderate viscosities is addressed by a mode-coupling theory, developed by Wolfgang GötzeWolfgang Götze
Wolfgang Götze is a theoretical physicist. He is an emeritus professor of the Technical University of Munich....
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.
Text books
- 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 translation.