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Title: Order in concentrated colloidal dispersions of anisotropic particles under shear
Author: Brown, A. B. D.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1999
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The properties of colloidal dispersions, particularly under flow, are important in many areas of life, and yet present many fascinating and profound scientific problems. This dissertation presents an investigation of the positional and orientational order found in stable dispersions of anisotropic colloidal particles under shear. In order to understand the reasons behind the order present, it is necessary to study simple systems. To keep the dispersions simple, an emphasis is placed upon knowing and controlling the shape, size distribution and interactions of the particles investigated. A number of dispersions of anisotropic particles were prepared and stabilised to give an approximation to 'hard' particles. A technique was developed to measure the orientation distribution using the diffraction from mono-crystalline colloidal particles. The positional order was measured using small angle scattering. New shear geometries were designed and tested for shearing samples in a neutron beam. Two dispersions of plate shaped particles were studied. Plates with an aspect ratio of 5 and a polydispersity of 13 % (nickel (II) hydroxide) displayed an equilibrium phase transition to a columnar phase. Under low shear an aligned columnar phase was observed with columns oriented near the flow direction, and plate normals in the compressional quadrant at 20 degrees to the flow direction. As the shear rate was increased a 'phase transition' was observed to a smectic structure, with layers and particles normal to the gradient direction. Upon ceasing shear the structure returned to the columnar phase without the particles reorienting. Plates with a higher aspect ratio and a broader size distribution (kaolinite) were observed to align with normals in the compressional quadrant, moving towards the gradient direction at higher shear rates. At volume fractions below 0.13 the orientational order increased with concentration, while at higher concentrations the orientational order decreased with increasing concentration.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available