| Summary: | The work reported in this Thesis involves studies of the extensional deformation of complex fluids. Complex fluids are so-called in terms of their composition, microstructure and rheology. Of particular interest in the work reported herein is the extensional viscosity of mobile (low shear viscosity) fluids in uniaxial extensional (or elongational flows) as this form of fluid determination is of wide concern within the process industries and is an important part of efforts in rheometry (the science of rheological measurement) to develop appropriate techniques for the measurement of extensional flow properties (in terms of a parameterisation of the resistance to extension). The principal experimental technique reported herein is a form of filament-stretching rheometer which generates a steady uniaxial extension of a fluid at the mid-point of a filament which is generated by the simultaneous motion of two confirming plate geometries. Difficulties which arise in this experiment and new approaches to their solution are described herein and the Thesis contains a comprehensive account of the design, construction, testing and refinement of such an instrument. Due to the relatively low rates of extension associated with filament-stretching rheometers, a new form of filament generation is described which involves fluid cavitation. Using this technique, which involves the generation of a rapidly stretching filament by the collapse of a gas-filled bubble, industrially relevant deformation rates are achieved and the technique has been successfully employed, for the first time, to provide a measure of resistance to extension of multigrade motor lubricants under process relevant conditions of fluid extension.
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