Reviewed by Lexie CornerApr 8 2025
For centuries, glass has been integral to human art, tools, and technology, yet its microscopic properties have remained difficult to explain through traditional physical theories. Koun Shirai of The University of Osaka addresses this gap by combining conventional physical theory with the study of nonequilibrium materials, providing a comprehensive thermodynamic framework to better understand glass.
(a) Random arrangement of atoms. This figure is the same as (b), except the lines are hidden. Image Credit: University of Osaka
Most materials exist in a state of equilibrium, where the forces and torques acting on their atoms are balanced. However, glasses are an exception. They are amorphous solids in which atoms are constantly, though very slowly, rearranging towards an equilibrium state, but never actually reach it.
If you compare the atomic structures of glasses and crystals, they are actually very different. Crystals have atoms that are arranged in neat lattices, whereas glasses have atoms that are more disorganized, similar to a liquid. Thus, one traditionally thinks of glasses as out-of-equilibrium liquids that flow very, very slowly.
Koun Shirai, The University of Osaka
The absence of an equilibrium state has been a significant challenge for physicists for generations. Since glasses are not in equilibrium, the standard principles of thermodynamics cannot be directly applied to them, complicating their analysis.
In thermodynamics, systems are usually characterized in terms of variables called order parameters. Order parameters can be used to describe what state a material is in or how close it is to changing states. However, the paradox is that glasses are inherently disordered, so how do we meaningfully define such parameters for these materials?
Koun Shirai, The University of Osaka
Shirai redefined the concept of "equilibrium." In his framework, a material is considered in equilibrium if no energy can be extracted from it without affecting its surroundings. From this perspective, glasses are considered to be in equilibrium, allowing the application of thermodynamic principles with some adjustments.
It can be shown that the order parameters are nothing more than time-averaged positions of the atoms. In this way, we can unify the thermodynamic description of glasses with that of other solid materials such as crystals.
Koun Shirai, The University of Osaka
Shirai anticipates that this novel formulation will provide insights into the physics of other materials typically categorized as non-equilibrium systems, such as biological systems. Thanks to his research, a complete thermodynamic understanding of other non-periodic and complex materials may soon be within reach.
Journal Reference:
Shirai, K. (2025) Nature of the Order Parameters of Glass. Foundations. doi.org/10.3390/foundations5010009