Doctor of Philosophy
de Bruyn, John R.
This thesis consists of two projects on the behaviour of a novel vibrating-wire rheometer and a third project studying the gelation dynamics of aqueous solutions of Pluronic F127. In the first study, we use COMSOL to perform two-dimensional simulations of the oscillations of a wire in Newtonian and shear-thinning fluids. Our results show that the resonant behaviour of the wire agrees well with the theory of a wire vibrating in Newtonian fluids. In shear-thinning fluids, we find resonant behaviour similar to that in Newtonian fluids. In addition, we find that the shear-rate and viscosity in the fluid vary significantly in both space and time. We find that the resonant behaviour of the wire can be well described by the theory of a wire vibrating in a Newtonian fluid if the viscosity in the theory is set equal to the viscosity averaged over the circumference of the wire and over one period of the wire’s oscillation at the resonant frequency. In the second study, we present the design and operation of our vibrating-wire rheometer and use it to measure the properties of Newtonian and viscoelastic fluids. We find that our device can accurately measure small viscosities. In homogeneous polymer solutions, we find that the viscoelastic moduli measured using our device are consistent with measurements at lower frequencies using a shear rheometer. In addition, we find that our device can measure the microrheological properties of an aging Laponite clay suspension that is heterogeneous on the micron scale. In the third project, we study the gelation dynamics of aqueous solutions of Pluronic F127. Using shear rheometry we find that when slowly heating the solutions, they undergo a transition from sol to gel around room temperature, followed by a gel to sol transition at a higher temperature. Our results show that these transitions take place over a temperature range of a few degrees. When cooling the solutions, the reverse transitions occur over a larger range of temperature and the width in temperature of the gel phase is larger. At temperatures near the phase transitions, we find that the rheological relaxation time becomes very long.
Hopkins, Cameron C., "Vibrating-wire Rheometry" (2018). Electronic Thesis and Dissertation Repository. 5584.