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Title: Colloidal liquid crystals : phase behavior, dynamics and confinement
Author: Cortes, Louis
ISNI:       0000 0004 7971 7097
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2019
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In this thesis, we report on the phase behaviour, dynamics and confinement of micron-sized colloidal silica rods with small gravitational length. In sedimentation-diffusion equilibrium, distinct isotropic, nematic, smectic and crystal phases are found to coexist within tens of micrometers. By combining confocal microscopy and tracer particles, the rods' phase behavior and dynamics are characterized across the whole phase diagram in a single sedimentation-diffusion experiment. Our experimental approach allows to quantitatively compare the sedimentation-diffusion equilibrium of charged rods with theoretical predictions. All predicted phases are observed. However, the thickness of the upper sediment - counting the isotropic, nematic and smectic regions - is found to be inflated with respect to the hard rod prediction, up to a factor 2. We consider the possible effect of polydispersity, electrostatic repulsion and the entropy of counterions (Donnan effect). We conclude that only the Donnan effect can explain the observed inflation. To support the hypothesis, we independently determine the surface charge of silica rods and discuss the results. Subsequently, we measure the self-diffusion of rod-like particles in sedimentation-diffusion equilibrium. Firstly, a molecular dynamic simulation is used to test the method. The self-diffusion coefficients are measured across the whole phase diagram and compared to bulk simulations -without gravity. We then present experimental results for an aspect ratio of 10, which has not been yet reported upon in literature. We reproduce most trends predicted by simulations. In particular, we find evidence of strong correlation between rotational and translational diffusion approaching the nematic phase, and a crossover between diffusion parallel and perpendicular to the nematic director in the smectic phase. However, we measure smaller diffusion coefficients than predicted by simulation, which is consistent with work on viral colloidal liquid crystals with larger aspect ratio. Finally, we study sedimentation-diffusion equilibrium in confinement, with a focus on the smectic phase, and consider three geometries: the square, the lozenge and the annulus. The interplay between boundary conditions, elasticities and defects lead to a host of different structures. In squares and disks, we mostly observe the smectic bridge state. However, by transforming the shapes into annuli and lozenges, we force new configurations. The transitions are characterized in light of phenomenological models to extract the ratios between: the energy of an edge dislocation, the energy of a disclination line and the bending energy of smectic layers.
Supervisor: Aarts, Dirk Sponsor: European Union Horizon 2020
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Colloids ; Liquid crystals