Abstract
The dynamics of a supercooled liquid near the glass transition is characterized by two-step relaxation, fast and slow relaxations. Because of the apparently disordered nature of glassy structures, there have been long debates over whether the origin of drastic slowing-down of the relaxation accompanied by heterogeneous dynamics is thermodynamic or dynamic. Furthermore, it has been elusive whether there is any deep connection between fast and slow modes. To settle these issues, here we introduce a set of new structural order parameters characterizing sterically favored structures with high local packing capability, and then access structure-dynamics correlation by a novel nonlocal approach. We find that the particle mobility is under control of the static order parameter field. The fast process is controlled by the instantaneous order parameter field locally, resulting in short-time particle-scale dynamics. Then the mobility field progressively develops with time , following the initial order parameter field from disorder to more ordered regions. As is well known, the heterogeneity in the mobility field (dynamic heterogeneity) is maximized with a characteristic length , when reaches the relaxation time . We discover that this mobility pattern can be predicted solely by a spatial coarse graining of the initial order parameter field at over a length without any dynamical information. Furthermore, we find a relation , indicating that the static length grows coherently with the dynamic one upon cooling. This further suggests an intrinsic link between and : the growth of the static length is the origin of dynamical slowing-down. These we confirm for the first time in binary glass formers both in two and three spatial dimensions. Thus, a static structure has two intrinsic characteristic lengths, particle size and , which control dynamics in local and nonlocal manners, resulting in the emergence of the two key relaxation modes, fast and slow processes, respectively. Because the two processes share a common structural origin, we can even predict a dynamic propensity pattern at long timescale from the fast pattern. The presence of such intrinsic structure-dynamics correlation strongly indicates a thermodynamic nature of glass transition.
- Received 21 September 2017
- Revised 10 January 2018
DOI:https://doi.org/10.1103/PhysRevX.8.011041
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Glass, despite its ubiquity, is still largely a mystery to physicists. In particular, researchers do not understand the nature of how liquid transitions to a glass as temperature drops or the accompanying slowdown in the dynamics of the material. A crucial and highly debated question is whether this comes from changes in liquid structure—and therefore can be characterized as a type of thermodynamic phase transition—or whether it is just a dynamical phenomenon. The difficulty arises from the complex disordered nature of glass structures, which show no obvious difference from a higher-temperature liquid. To overcome this, we introduce a set of mathematical quantities (known as order parameters) that describe the efficiency with which particles pack together. This allows us to reveal hidden structural order that controls the dynamics of glass-forming liquids, which suggests a universal thermodynamic nature for the glass transition.
Going beyond previous approaches that search for a link between local structure and dynamics, we show that correlated features of structural ordering emerge through progressive spatial coarse graining of an initial inherent structure. Based on such structural information at different length scales, we are able to predict the time evolution of dynamic heterogeneity with remarkable precision. We further establish intrinsic links between key timescales and length scales that characterize the glass structure and glassy dynamics, and we find a common static structural origin for both fast and slow dynamics of glass-forming liquids.
Our findings unveil the hidden structural ordering behind complex glassy dynamics, which we expect may shed light on the long pursuit of an ultimate theory for glass transitions.
