How and why do particles composing glass move just before it solidifies?

What is it about?

Large-scale simulations of dumbbell-shaped molecules have elucidated the full mechanism of the motion of molecular liquids just before they solidify as glasses, providing a bridge between theoretical and experimental studies on the glass transition. The long- and short-time motions are attributed to the rotational and translational motions of the molecules, respectively, providing a clear real-space understanding of these motions. As for the theoretically important energy landscape picture, we showed numerically that a two-step hierarchical structure gives rise to the above two processes, proving the correctness of the theory more than 25 years after it was proposed.

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Photo by Jan Canty on Unsplash

Photo by Jan Canty on Unsplash

Why is it important?

Glasses are formed by rapidly cooling liquids. Just before the glass transition, when the liquid solidifies as glass, the liquid molecules are undergoing complex motions on various time scales. Numerical studies have so far exclusively simplified the constituent particles of liquids with spherical shapes, such as spheres and disks. However, the Johari-Goldstein β relaxation, which has been widely observed in molecular liquid experiments, cannot be captured by spherical particles. As compensation for simplification, there has been little progress in understanding what kind of molecular motion this process corresponds to. Theoretically, a two-step hierarchical potential energy landscape interpretation was proposed more than 25 years ago and has been widely accepted by the community. However, for a similar reason, numerical evidence for this interpretation has not been available. This situation has been a major frustration with theories explaining glass transitions exhibited by real molecules. Our study solves these two major difficulties related to the glass transition of molecular liquids.

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