Understanding the Nanoconfined Fluid via Absolute Free Energy Determination
In recent years, the development of microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) has received intense attention and effort. However, studies on nanoconfined fluid phase behavior has given rise to a vigorous debate that began in the early 1990s. While experiments by Klein and coworkers suggest the formation of a nanoconfinement-induced solid phase as the separation of two solid walls is reduced below seven molecular layers, experiments by Granick and coworkers instead suggest that a glassy state is formed, since no yield stress was observed in their experiment and the measured viscosity increase is significantly smaller. Simulation studies have intrinsic molecular resolution and hence no such observational problems, but the lack of a clear demonstration that the observed solid-like structure is an equilibrated structure, has hindered wide acceptance of the results of these studies. To determine whether a structure is metastable, the key measurement required is the free energy of the system. We concentrate our studies on development and applications of absolute free energy calculation methodology, specifically for nanoconfined systems, and uses the absolute free energy to reach a conclusive understanding of nanoconfined fluid phase behavior. Based on free energy measurements conducted in this dissertation research, we reveal the thermodynamic role of wall-fluid interaction, wall-wall separation and fluid-wall particle size ratio, in determining nanoconfined phase behavior. Finally, according to investigations on the central layers of nanoconfined system with artificially increased density, we demonstrate the potential of jammed atoms in destabilizing the nanoconfinement-induced solid structure. This finding suggests one possible explanation of the divergent experimental results. In this picture, we find there are discrete solid states at separations around integer numbers of ideal layers, as opposed to a conventional fluid-solid transition.