Spectroscopic Studies of Anomalous Hydrodynamic Behaviour in Complex Fluids
Edington, David W. N.
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Brillouin spectroscopy probes the thermally generated pressure fluctuations (sound waves) which propagate in a material. The resulting information on sound velocity and absorption provides a fast and efficient method of monitoring high frequency (GHz) dynamics in the system being studied. In certain cases, structural information may also be inferred from changes in the Brillouin spectrum as a function of temperature, pressure or composition (in the case of multi-component systems). The aim of the work presented in this thesis was to integrate Brillouin spectroscopy into current soft condensed matter research projects at Edinburgh, namely (i) hydration in methanol-water mixtures and (ii) the behaviour of hard-sphere colloidal dispersions. A Brillouin spectrometer based on a Fabry-Perot interferometer was developed and tested, resulting in a high-resolution instrument operating at variable scattering vector (exchanged momentum), temperature and pressure. The technical aspects of this work were carried out in collaboration with a colleague. Data analysis routines were designed and implemented, enabling calibrated Brillouin spectra to be produced automatically from raw experimental data. Excellent agreement with results on several materials studied in the literature confirmed the accuracy and sensitivity of the spectrometer. The molecular details of hydration in methanol-water mixtures are of great interest due to the prototypical amphiphilic nature of the methanol molecule. The effect of deep cooling on the Brillouin spectrum across a wide range of methanol concentrations was studied in detail, resulting in the first observation of an anomalous increase in sound velocity and maximum in sound absorption at intermediate compositions. A similar effect was then found at higher temperature in aqueous tertiary butanol, and was identified in a brief survey of several other aqueous solutions. High pressure Brillouin spectra indicate that this anomalous behaviour may also be present in pure water. It is suggested that these novel effects may be due to the presence of a relatively unperturbed water structure in the aqueous solutions studied, even at quite high solute concentration. Preliminary results from a neutron diffraction experiment performed on a 40% by mass methanol-water mixture were consistent with this hypothesis. Brillouin spectroscopy was also used to study the propagation of high frequency sound in monodisperse colloidal suspensions of sub-micron hard spheres. A second longitudinal sound mode was observed for scattering vectors of magnitude greater than pi/d where d is the diameter of the spheres. These results are the first reproduction and extension of the pioneering work in the field, which identified the additional mode with a surface acoustic excitation, propagating between adjacent spheres via an evanescent wave in the solvent. The new results show that the second mode is extinguished at a particular scattering vector - an effect not reported previously. It is suggested that this extinction is due to the minimum in the form factor for elastic scattering from a single sphere.