Microscopic Theory of Softness in Supercooled Liquids

We introduce a new measure of the structure of a liquid which is the softness of the mean-field potential developed by us earlier. We find that this softness is sensitive to small changes in the structure. Then, we study its correlation with the supercooled liquid dynamics. The study involves a wide range of liquids (fragile, strong, attractive, repulsive, and active) and predicts some universal behaviors such as the softness being linearly proportional to the temperature and inversely proportional to the activation barrier of the dynamics with system dependent proportionality constants. We establish a master equation between the dynamics and the softness parameter and show that, indeed, the dynamics, when scaled by the temperature and system dependent parameters, show a data collapse when plotted against softness. The dynamics of fragile liquids show a strong softness dependence, whereas that of strong liquids show a much weaker softness dependence. We also connect the present study with the earlier studies of softness involving machine learning (ML), thus, providing a theoretical framework for understanding the ML results.

Automated summary

The study introduces a new measure of the structure of liquid, softness of mean-field potential, which is sensitive to small changes. It correlates this softness with supercooled liquid dynamics and predicts universal behaviors such as linear proportionality with temperature and inverse proportionality with activation barrier. The research shows a strong dependence of fragility liquid dynamics on softness, while strong liquids have weaker dependence on softness.