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Jensen, Katharine Estelle

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Jensen

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Katharine Estelle

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Jensen, Katharine Estelle
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    Structure and Defects of Hard-Sphere Colloidal Crystals and Glasses
    (2013-08-21) Jensen, Katharine Estelle; Weitz, David A.; Spaepen, Frans A.; Nelson, David
    Colloidal particles provide convenient and useful building blocks for creating ordered and disordered structures with length scales on the order of a micrometer. These structures are useful materials in their own right, and also serve as excellent scale models for exploring properties of atomic materials that would otherwise be inaccessible to direct experiment. In this dissertation, we explore structure formation in hard-sphere colloidal systems using templated sedimentation techniques, and then use colloidal crystals and glasses formed in this way to study the development of extended defects in single crystals and shear defects in glasses. We find that it is possible to form large, defect-free colloidal single crystals extremely rapidly by centrifugation onto a deterministic template. On non-deterministic templates, we find a critical deposition flux above which the material always crosses over to forming a glass. With this understanding of the effects of template and deposition flux, we designed and tested amorphous templates that allow us to make colloidal glasses by sedimentation under gravity, as well as more complex structures. In face-centered cubic colloidal single crystals grown on (100) templates, extended defects (dislocations and stacking faults) can nucleate and grow if the crystal exceeds a critical thickness that depends on the lattice misfit with the template spacing. We account for the experimental observations of the density of misfit dislocations using the Frank-van der Merwe theory, adapted for the depth-dependent variation of lattice spacing and elastic constants that results from the gravitational pressure. In the second part of the thesis, we report the first results of a detailed study of reversible and irreversible deformation of colloidal glasses. We show that shear defects exist and are active in both sheared and quiescent colloidal glasses and that these defects behave as Eshelby inclusions. We observe a decrease in the shear modulus of the glass, which corresponds to a small dilatation, which, in turn, lowers the activation barrier for shear.
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    Experimental Observation of the Crystallization of Hard-Sphere Colloidal Particles by Sedimentation onto Flat and Patterned Surfaces
    (American Physical Society, 2009) Ramsteiner, I.B.; Jensen, Katharine Estelle; Weitz, David; Spaepen, Frans
    We present a confocal microscopy study of 1.55micrometers monodisperse silica hard spheres as they sediment and crystallize at the bottom wall of a container. If the particles sediment onto a featureless flat wall, the two bottom layers crystallize simultaneously and layerwise growth follows. If the wall is replaced by a hexagonal template, only layerwise growth occurs. Our results complement earlier numerical simulations and experiments on other colloidal systems.