AsianScientist (Apr. 10, 2018) – A research group in Japan has used molecular dynamics simulations to gain greater clarity about the process of glass formation. Their findings are published in Physical Review X.
Glassy substances are everywhere, yet this state of matter poses many puzzles. The basic picture is clear enough—glasses are solids that lack the regular atomic structure of a crystal. How and why they form, however, are questions that have kept physicists busy for decades.
In the present study, researchers at the University of Tokyo’s Institute of Industrial Science have obtained detailed information on the structural changes during glass transition. Using molecular dynamics simulations, they were able to resolve whether the process is fundamentally thermodynamic (dependent on some form of static order) or dynamic (driven by random atomic motions).
The team simulated supercooled liquids near the transition point, the temperature where particle diffusion stops and an amorphous solid appears. The aim was to find a link between structural patterns and the slowing of atomic motion, that is, whether atoms in emerging structures are less mobile than in disordered regions. A structure-dynamics correlation would verify that thermodynamics controls the formation of glasses, just as for crystals.
In each simulation, the team quantified how well the atoms packed together in the cooling liquid by measuring a structural order parameter.
“We were careful to define order as any local packing that was sterically favored, not just crystalline packing. When the atoms were classified by this criterion and then quantified by their environments, known as coarse-graining, a clear correlation emerged between structural order and dynamics,” said study lead author Dr. Tong Hua. This indicated that more ordered atoms were indeed less mobile.
The researchers were also aware that glass formation occurs on two timescales: a slow alpha (α) process and a fast beta (β) process. They demonstrated that the α and β modes of glass formation have a common structural origin.
“First, using robust statistics, we show that glass formation really is thermodynamic. Despite their apparent randomness, glassy liquids show subtle ordering, though less directionally than in crystals. Second, we reveal an intrinsic link between two important dynamic modes. Now, the question is whether the structure-dynamics link is more than just a correlation. In the future, we hope to find a direct causality,” said study corresponding author Professor Hajime Tanaka.
The article can be found at: Tong & Tanaka (2018) Revealing Hidden Structural Order Controlling Both Fast and Slow Glassy Dynamics in Supercooled Liquids.
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Source: University of Tokyo; Photo: Hajime Tanaka.
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