Doctor of Philosophy
John R de Bruyn
The behaviour of complex fluids is fundamentally interesting and important in many applications. This thesis reports on three experiments on the thermal and rheological behaviour of complex fluids. The first is a study of the rheological properties of and heat transport in a saline solution of hydroxyethyl cellulose. This material has been used as a tissue phantom in testing the behavior of medical devices in MRI scanners. We find it behaves as a typical entangled polymer, and flows in response to local heating, such as could occur due to eddy-current heating of metallic devices in an MR scanner. We use laboratory experiments and numerical simulations to determine the convective and conductive contributions to the heat transport in a simple model of this system. Our results indicate that convective heat transport is of the same order of magnitude as conductive transport under conditions typical of MRI device tests. The second project is an investigation of the start-up flow and yielding of a simple yield-stress fluid (Carbopol 940) in a vertical pipe. The Carbopol was displaced from below by an immiscible Newtonian liquid (Fluorinert FC-40) injected at a constant, controlled rate. Rough and smooth-walled pipes were used to study the effects of wall boundary conditions. In the rough-walled pipe, the yielding involved a long transient with several steps: elastic deformation, the onset of wall slip, yielding at the wall, and finally a steady-state plug flow that is well-described by the predictions of the Herschel-Bulkley model. In contrast, in the smooth-walled pipe, the wall shear stress never exceeded the yield stress. In the third project, we study the flow of Carbopol solutions confined to square microchannels with sides ranging from 500 down to 50 um. In the larger channels, the measured velocity profiles agreed well with simulations based on the bulks-scale rheology of the Carbopol and the Herschel-Bulkley model. In contrast, in microchannels with sides less than 150 um the velocity profiles could not be fitted by a model with a finite yield stress, but instead were described by a power-law model with zero yield stress. We explain the vanishing of the yield stress in terms of the confinement of the Carbopol’s microstructure by the microchannels.
Liu, Yang, "Three Experiments on Complex Fluids" (2017). Electronic Thesis and Dissertation Repository. 5118.