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Title: Local structure in crystallization
Author: Taffs, Jade
ISNI:       0000 0004 5917 3694
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2015
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In the work presented here, we use simulations of hard and nearly-hard spheres to examine the role of local structures in crystallisation. We are motivated by the work of Sir Charles Frank, who suggested that locally favoured structures, if incommensurate with crystalline ordering, could potentially suppress nucleation [1). Structural analysis is carried out using the topological cluster classification [2], which identifies those local arrangements of 5 < n < 13 particles which are structurally equivalent to the ground state structure for 11 particles considered in isolation. Initial work compares homogeneous crystallisation in simulation and experiment at the single particle level. Brownian dynamics simulations are carefully matched to a system of colloidal 'hard' spheres, taking both poly-dispersity and charge into account. In the regimes accessible, we find a reasonable agreement in the crystallisation rates, although the larger system size in experiments allows crystallisation to be observed at lower supersaturations than in simulation. Structural analysis of the metastable fluid finds a strong similarity between simulation and experiment, and in both we find large populations of five-fold symmetric local structures. We then move to examine what effect energetically favourable, five-fold symmetric local structures have on nucleation. A model hard sphere system with a novel many-body biasing potential is developed, which can be tuned to favour or disfavour the formation of pentagonal bipyramidal structures in the fluid . When the kinetic and thermodynamic effects of the biasing are decoupled, we find that Frank is correct, and that increasing the number of five-fold symmetric structures suppresses nucleation. Finally, in light of this finding, we explore the possibility of employing the structural biasing technique as a novel mechanism for a one component glass-former. Strongly biasing towards formation of five-fold symmetric structures results in unexpected phase behaviour and formation of an icosahedra-rich, Frank-Kasper phase is observed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available