| Author: | Lee, Chun Shing |
| Title: | Computational and theoretical studies on structural glasses based on void-induced particle hopping motions |
| Advisors: | Lam, Chi-hang (AP) |
| Degree: | Ph.D. |
| Year: | 2022 |
| Subject: | Glass -- Analysis Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Applied Physics |
| Pages: | xx, 127 pages : color illustrations |
| Language: | English |
| Abstract: | Understanding the microscopic relaxation dynamics in structural glasses is important and has yet to be achieved. When a glassy system is cooled to a low temperature, stringlike hopping motions of particles become dominant. Stringlike motions have been studied intensively in experimental colloidal glasses, and also by molecular dynamic (MD) simulations of different kinds of systems such as binary Lennard-Jones liquid and bead-spring polymer. In addition, the free volume theory was proposed long ago to study dynamics in glasses. Alternatively, the structural relaxation of glasses has also been proposed to be a facilitation phenomenon. These studies serve as milestones in a possible understanding of structural glasses, and provide the basic ingredients for our approach on the microscopic origin of glasses. Recently, our group proposed a distinguishable-particle lattice model (DPLM) for capturing the essential physics of void and particle dynamics in the presence of random energy landscapes of disorder systems. The model is capable in reproducing many characteristics of glass formers, including a super-Arrhenius behaviour in diffusion coeffcient, a stretched exponential decay in the self-intermediate scattering function, and a growing length scale of dynamic heterogeneity based on the four-point susceptibility upon lowering the temperature. On the other hand, our group have also studied structural relaxation in glasses both by experiments on glassy colloidal systems and by MD simulations. Results justify the picture of void-driven particle hopping dynamics of structural glasses adopted in the DPLM. We believe that voids play a pivotal role in the dynamics of glasses. Thus, the objective of my research is to study glassy systems based on void-induced particle hopping motions. In this thesis, the DPLM is adopted to study an important concept: the glass fragility. Equilibrium DPLM simulations have been performed. By tuning the particle-particle pair interaction distribution and the hopping energy barrier, an extraordinarily wide range of kinetic fragility can be obtained. The stretching exponent from the self-intermediate scattering function decreases with the kinetic fragility, which is also found in most experimental systems. The most fragile glasses are found to be those with low hopping barriers and the most dramatic drops of entropy when the temperature decreases towards the glass transition temperature. The drop in the entropy implies the reduction of possible kinetic pathways, which slows down the dynamics dramatically. Next, an out-of-equilibrium study is carried out by applying a cooling-heating temperature protocol to an initially equilibrated DPLM system. A large heat capacity hysteresis is obtained for fragile glass, which is one important feature of realistic glasses that traditional kinetically constraint models cannot intrinsically reproduce. The result can be explained by a dramatic transfer of probabilistic weight from high-energy particle interactions to low-energy ones in a fragile system as temperature decreases. On the other hand, we have extended our previously proposed configuration-tree theory, which is a microscopic theory of structural glasses based on void-induced particle hopping dynamics, in order to calculate the mean-square displacement (MSD) and the self-intermediate scattering function (SISF) at any finite time analytically. Rate equations are formulated for system evolution in the particle configuration space of a local region which is well approximated by a tree. By solving the rate equations, the probabilities for the local configuration to visit different tree levels at any time are calculated, which allow us to calculate the particle mean-square displacement as a function of time. Furthermore, dynamics at low temperature is found to be dominated by mobile void clusters (MVC). By considering the movements of MVC across different local regions, we have calculated analytically the probability distribution of the total duration that some MVC resides inside a local region and this allows the calculation of the SISF. As a test on the applicability of the theory, the theoretical results of both the MSD and SISF are compared with those obtained from DPLM simulations, and satisfactory agreements have been obtained. |
| Rights: | All rights reserved |
| Access: | open access |
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