Molecular simulation of solid-liquid phase equilibria and shear viscosity
Creators: name: Owida-Morsi, Amal
Publisher: name: Swinburne
Year: 2024

description: The objectives of this thesis are to study the solid-liquid phase equilibria and the shear viscosity for both bounded and unbounded intermolecular potentials. A variety of molecular simulation techniques are used to calculate solid-liquid coexistence whereas nonequilibrium molecular dynamics algorithm is used to examine the shear viscosity. The solid-liquid coexistence properties are calculated using the GWTS algorithm which is self starting, independent of particle exchange mechanism and does not rely upon a prior free energy equation of state. A combination of equilibrium and nonequilibrium molecular dynamics simulation algorithms constructs the framework for the GWTS algorithm. It has been demonstrated by the solid-liquid phase coexistence data reported for 12-6 Lennard-Jones fluid that the GWTS algorithm is capable of calculating solid-liquid coexistence properties with comparable accuracy irrespective of temperature, density and pressure range. The solid-liquid phase transition is found not to be very sensitive to the choice of truncation and shifting schemes except close to the vicinity of triple point where the coexistence properties vary significantly compared to the entire melting line. The effects of repulsive component (n) on the solid-liquid coexistence properties of n - 6 Lennard-Jones potentials are reported. The estimated triple points of n - 6 Lennard-Jones potentials are reported for the first time. Scaling relationships for the triple point pressures and temperatures have been established with respect to n. The solid-liquid phase coexistence data for purely repulsive Weeks-Chandler-Andersen system are presented from very low to high temperatures and pressures. It has been observed that the WCA potential approaches zero-temperature limit which is in contrast to the 12-6 Lennard-Jones case. The data presented in this thesis also demonstrate that the GWTS algorithm can also generate the phase diagram of bounded Gaussian core model potential in presence of re-entrant melting scenario. The solid-liquid coexistence properties are also reported for the state points closer to the common point. It has been demonstrated how the GWTS algorithm is capable of calculating solidliquid coexistence properties for intermolecular potentials complex than 12-6 Lennard-Jones potential. For the first time, the strain rate dependent shear viscosity data are reported for the Gaussian core model fluid. It has been demonstrated that in the reentrant melting region shear viscosity decreases with increasing density at constant temperature whereas viscosity increases with increasing temperature. This behaviour has been found to be consistent with viscosity measurements of cationic surfactant solution and attributed to the 'infinite-density ideal-gas limit' of the Gaussian core potential. Extensive nonequilibrium molecular simulation data are reported for a wide range of temperature, pressure, density, shear viscosity and strain rate. A nonequilibrium steady-state equation of state has been developed from the strain rate dependent pressure and energy data. A generic viscosity model has also been developed to establish connections between the shear viscosity and the steady state variables such as temperature, pressure, density and strain rate via the nonequilibrium equation of state. The generic viscosity model has been compared with recommended experimental data.