Colloquium: Theoretical Studies on Dynamics in Complex Systems: Binary Mixtures, Electrolyte Solutions, Nanoconfined Water, Glass Transition Phenomena, and Boltzmann’s H-function 

 Theoretical and experimental studies of structure and dynamics of complex liquids and glasses are of importance in condensed matter science. It provides fundamental insights into the behaviour of materials that have significant implications across various scientific and industrial applications. Understanding the intricate interplay of molecular interactions, composition-dependent anomalies, confinement effects, glass transition phenomena, and non-equilibrium relaxation is essential for advancements in fields such as materials science, chemical engineering, and biophysics. [1]

In our studies, we employ molecular dynamics simulations and theoretical analyses to unravel the intricacies of complex liquids and glasses. In the first part, we investigate the microscopic origin of non-ideality in viscosity and transport properties of aqueous binary mixtures. We find that the non-idealitycan be explained through the formation of quasi-stable extended structures. Detailed analyses, including mode coupling theory (MCT) and inherent structure (IS)analysis, provide valuable insights into the anomalous composition dependence of viscosity. The investigation extends to composition-dependent anomalies in the viscosity of aqueous electrolyte solutions, highlighting the crucial role of cross-correlations, often overlooked in previous studies. Additionally, the research delves into nanoconfined water, revealing disparate static and dynamic correlation lengths and their implications.Frequency dependent viscosity provides further understanding of the structural aspects. [2-5]

In the second part of the thesis, we investigate the glass transition phenomena by introducing a novel class of binary mixtures of molecules that interact with each other through anisotropic (angle-dependent) interactions. This model system exhibits distinct thermodynamic and dynamic features of glass transition. The splitting of the rotational and translational spectrum reveals the dynamic heterogeneity in the system.These resemble the Johari-Goldstein bifurcation, well-known in glass transition literature. Sudden large changes in the diffusion coefficient and rotational correlation times in mesoscopic domains indicate first-order transitions between low and high-mobility domains. [6-8]In the third part of the thesis, the study analyses non-equilibrium relaxation, assessing sensitivity to interaction potential and dimensionality using Boltzmann’s H-function. The evaluation of the H-function for molecules with orientational degrees of freedom showcases its potential in understanding translational and rotational contributions to entropy, emphasizing translation-rotation coupling as a function of molecular shape. [9]This comprehensive exploration makes a substantial contribution to condensed matter science, providing valuable insights and paving the way for further progress.