Measuring excitations, mobile regions and local structure in deeply supercooled liquids

Abstract

When cooling down a glassforming liquid fast enough, it does not crystallise but becomes a super-cooled liquid until eventually falling out of equilibrium forming a glass. Despite the importance of glasses and amorphous solids in everyday life and decades of research, there is still no widely accepted theory of the glass transition. It is challenging to study supercooled liquids and determine if there is a genuine thermodynamic glass transition at deep supercooling. The dynamics slow down by 14 orders of magnitude with only a moderate decrease in temperature. Therefore, waiting times in experiments and numerical simulations quickly become unmanageable. Despite this large change in dynamics, the structure of the liquid changes very little. There are many very different theories of the glass transition. In this thesis, we focus on numerical tests for some aspects of two main theories: the random first-order transition theory, which describes the glass transition as a thermodynamic phenomenon, and the dynamic facilitation theory, which claims that all observation can be explained by the dynamics only.

We obtained deeply supercooled data with relaxation times three orders of magnitude longer than most other particle-resolved data from numerical simulations and colloidal experiments. We developed an algorithm to detect excitations and confirmed the predictions from dynamic facilitation theory about the concentration and energy barriers of excitations in the simulations and the experiments. Moreover, we show an increasing spatial correlation between excitations, and an anticorrelation between excitations and locally favoured structures, suggesting that facilitation spans larger distances at deeper supercooling and is related to local structure. We also find support for the thermodynamic theory of the glass transition: Mobile regions become larger and more compact with deeper supercooling, and the relaxation time follows the scaling with the size of the mobile regions and with configurational entropy predicted by the theory. We investigate the commonly used implementations of abstract quantities of the theories (excitations and cooperatively rearranging regions) and discuss their advantages and problems. Finally, we compare the mechanical properties of the supercooled liquid with those of the corresponding crystal and confirm that with deeper supercooling, the yielding behaviour of the supercooled liquids becomes more similar to that of the crystal.

Date of Award	21 Mar 2023
Original language	English
Awarding Institution	University of Bristol
Supervisor	Paul Bartlett (Supervisor) & Francesco Turci (Supervisor)