Supercooled liquids relax at low temperatures through slow, thermally or mechanically activated molecular rearrangements. It has long been theorised that, as in crystals, structural defects underlie these relaxations in amorphous systems as well. However, a physically transparent structural signature of such defects has remained elusive, with candidate indicators so far emerging mainly as dynamical proxies, such as vibrational quasi-localised soft modes or supervised machine learning studies, rather than as an interpretable structural quantity in its own right.
Here, I introduce a structural order parameter (SOP) derived from microscopic density functional theory [1], which quantifies the softness and the depth of the mean-field caging potential experienced by each particle due to its neighbours. I will show that (i) the average dynamics of a supercooled liquid can be predicted directly from the average SOP [2]; (ii) local dynamics is causally linked to the local SOP [3]; and (iii) experiments on dense colloidal suspensions confirm a strong, quantitative correlation between the SOP and structural relaxation in real amorphous systems [4]. In quiescent suspensions, this correlation strengthens with increasing density, as rearrangements become rarer and more spatially localised. Under shear, the SOP reliably identifies incipient shear transformations: applied strain weakens the local caging potential through shear-induced structural distortion, driving plastic deformation preferentially at structurally weak (soft) regions.
Together, these results establish the SOP as an interpretable, transferable, and experimentally accessible structural framework for understanding relaxation and plasticity across a broad class of disordered systems.
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REFERENCES
[1] M. K. Nandi, A. Banerjee, C. Dasgupta and S.M. Bhattacharyya Role of the Pair Correlation Function in the Dynamical Transition Predicted by Mode Coupling Theory, Phys. Rev. Lett., 119, (2017), 265502.
[2] M. K. Nandi and S. M. Bhattacharyya, Microscopic Theory of Softness in Supercooled Liquids, Phys. Rev. Lett. 126, (2021) 208001.
[3] M. Sharma, M. K. Nandi, S. M. Bhattacharyya, Identifying structural signature of dynamical heterogeneity via the local softness parameter, Phys. Rev. E 105, (2022) 044604.
[4] R. Sahu, M. Sharma, P. Schall, S. M. Bhattacharyya, V. Chikkadi, Structural origin of relaxation in dense colloidal suspensions, Proc. Natl. Acad. Sci. 121 (2024) e2405515121.