Role of attractive interactions in the dynamics of a supercooled binary mixture

Abstract

Liquid under fast cooling or fast compression undergoes a glass transition; while, a glass is an unconventional phase unlike liquid, gas, and solid. These phases are formed during the change of the thermodynamic parameters such as temperature or pressure at infinitely slow rate where the system relaxes to its lowest Gibbs free-energy minima and remains there. When thermodynamic parameters change, all systems move away from equilibrium, which at a long time relaxes to new minima. This simple view of the phase changes considerably get altered when the relaxation process slows down. Glasses are formed when a system is cooled or compressed at a higher rate than the relaxation can occur. The specification of how fast the compression or cooling is required to form glasses depends on the relaxation time of the liquids, which vary with intermolecular potentials. Glass transition occurs in the dense systems and glass has the same structure as that of a liquid with arrested dynamics. Many systems with different complex potentials show similarities in the relaxation process irrespective of complexity in the potential. One of the ways to develop theories of glass transition is by extending the theories of the liquid state. Investigations of Weeks-ChandlerAndersen (WCA) (J. Chem Phys. 54,5237(1971)) showed that dense Lennard-Jones systems can be well described by the repulsive part of the potential. A test of this by Berthier and Tarjus (Phys. Rev. Lett. 103, 170601(2009)) on the Kob-Andersen (Phys. Rev. Lett. 73, 1376(1994)) glass-forming binary mixture (A (80%) and B(20%) components) and its WCA variant (without the attractive part of the interaction ) show that dynamics considerably vary at lower temperatures. There are many following investigations which looks into various aspect of the role of attractive interactions in glass transition. Inspired from results of these earlier studies, we attempt to explain the origin of the difference in dynamics as the interplay of the barriers of three interactions, namely, A-A, A-B, and B-B. We have looked into various aspects of glass transition and its density dependence with an emphasis on the role of attractive interaction. Chapter 1: Presents a short introduction to the present understanding of the glass transition and difficulties in various theoretical formalisms, especially the relation between structure and dynamics etc. which is missing from the earlier studies. This is followed by a discussion on correlations on basic liquid state theory and their relevance in understanding the relaxation and connection to glassy domain formation without attractive interactions. Basic theories of glass transition that are relevant to understand the problems addressed in this thesis is discussed: the schematic mode-coupling theory (MCT), the phenomenological Vogul-Fulcher-Tammann relation, Adam-Gibbs theory, random first-order transition theory, and free-volume theory. Next, the motivation for the studies presented in the thesis which details earlier studies that look into the role of attractive interactions in the system. Chapter 2:Computer simulations bridges the gap between the theoretical models and experimental observations. A comprehensive theoretical understanding of glass transition is still elusive,while,different experiments support different theoretical models;thus,a detailed microscopic understanding is desirable to build theories that can explain the glass transition. Despite the complexity of the system that undergoes glass transition, many features of the glass transitions have an underlying universal features such as steps like slow relaxation. Simple computational models, such as Kob-Andersen models have given light on many aspects of the glass transition. To look at density-dependent features of glass transition and also the effect of attractive interaction on the properties of glass transition,we have simulated Kob-Andersen model binary mixture and its WCA variant from low to higher density i.e. ρ =0.8 to 1.8, to understand, how various glass transition properties vary with density. The simulations are from high to low temperature in density grids in microcanonical ensemble to obtain dynamics driven by the unperturbed Hamiltonian. The lowest temperature in the grid in each set of density is set close to the mode-coupling theory glass transition temperature. Chapter3: One of the simple methodology to understand the difference in the dynamics of KA and KAWCA model is in terms of the inter-molecular interactions,this is because in the Kob-Andersen binary mixture, the minority component is introduced to induce frustrations that prevent crystallization, which is smaller in the size in its interaction to the major component. We have looked at this qualitative view and attempted a quantitative relation between structure and dynamics. In these studies lower densities ρ =0.8 and 1 are where attractive interactions lead to a phase-separated system at a lower temperature when attractive interactions are present. The relative difference in the pressure between KA and KAWCA models to total pressure reduces as density increases.