An experimental investigation of diffuse optical scattering from stationary and flowing dispersions

Wood, Peter Robert (1991) An experimental investigation of diffuse optical scattering from stationary and flowing dispersions. Doctoral thesis, City of London Polytechnic.


This Thesis describes the measurement and interpretation of diffuse optical scattering from concentrated colloidal dispersions. Novel measurement techniques using optical fibres are investigated. These are applied to monodisperse spherical latexes where the results are compared with theories of radiative transport, and also to plate-like Kaolinite particles where non-random particle orientation by shear flow is studied at small and large Reynolds number.

For the spherical latex dispersions various fibre configurations are investigated, the results being compared with multiple scattering predictions of the inverse scattering length S. All configurations give values of S which show the theoretical particle size dependence, but the single fibre bundle gives S in good absolute agreement with that calculated from an accepted approximate solution to the radiative transport equation. Comparisons of S between the single fibre bundle and the conventional integrating sphere method show good agreement, but only after making large specular reflectance corrections in the latter case. Measurements made on latex volume fractions (v) up to 0.4 showed a maximum in S at about v = 0.2, Interpreted as non-random destructive interference between ordered particles.

For the kaolilnite dispersions S is measured as a function of shear rate, volume fraction, particle size, pH and deflocculant concentration. Using the anomalous diffraction and Rayleigh-Gans light scattering approximations, calculations are given which support a mods of particle orientation expected below the critical Reynolds number from the particle hydrodynamics over a wide range of rotational Peclet number. The results for concentric cylinder flow above the critical Reynolds number are explicable in terms of Taylor vortices. The shear-induced changes in S in laminar flow are shown to be a useful practical measure of the state of flocculation of the dispersion.

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