Doctor of Philosophy
Dr. Colin Denniston
The behaviour and properties of colloidal suspensions strongly depend on the interactions arising between the immersed colloidal particles and the solvent. However, modelling such interactions is not at all straightforward; the larger time and length scales experienced by the colloidal particles compared to the solvent molecules makes all-atom molecular dynamics (MD) simulations of such systems completely impractical. Therefore a coarse-grained representation of the fluid is required, along with a method to couple this fluid to the colloidal particles.
In the first part of this thesis, we propose a new method for coupling both point and composite MD particles to an isotropic lattice-Boltzmann fluid. This coupling is implemented through the use of conservative forces, calculated by assuming elastic collisions between the particles and the fluid. With the implementation of a thermal lattice-Boltzmann method, the fluid acts as a heat bath for the MD particles without requiring external Langevin noise. This method has been implemented into the open source molecular dynamics package, LAMMPS, providing an efficient technique for explicitly including hydrodynamic interactions in MD simulations.
If a liquid crystal (LC) is used as a solvent instead of an isotropic fluid, anisotropic forces develop among the immersed colloidal particles even in the absence of flow. These forces arise due to a preferred orientation of the LC molecules on the colloidal surface, leading to the formation of topological defects in the bulk LC. A thorough understanding of the resulting forces is important, as their anisotropic nature could potentially be used to manufacture non-close packed photonic colloidal crystals.
In the second part of this thesis, we use a lattice-Boltzmann LC algorithm to investigate the interactions arising among colloidal particles in a LC. Using a cholesteric LC, we present results for a defect bonded particle chain, and a diamond colloidal crystal. In addition, as the defects and distortions generated in the LC result in a non-uniform pressure exerted on the particle surface, we also investigate the behaviour of 2D deformable particles in a nematic, as any potential shape change could have a significant impact on the resulting interactions.
Mackay, Frances E., "Hybrid Lattice Boltzmann - Molecular Dynamics Simulations With Both Simple and Complex Fluids" (2013). Electronic Thesis and Dissertation Repository. 1262.