Browsing by Subject "shear banding"

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    Understanding self-assembly and flow heterogeneities in poloxamer wormlike micelles
    (2023-06) McCauley, Patrick
    Wormlike micelles (WLMs) are elongated, self-assembled structures formed from amphiphilic molecules in solution. Although the structure of WLMs resembles that of polymers, WLMs are differentiated by their ability to break and recombine at rest and in response to deformations. This unique property has led to their ubiquitous use in a variety of applications such as consumer products and oilfield recovery. Additionally, entangled WLMs exhibit unique nonlinear flow behavior such as the formation of shear bands and other flow heterogeneities, which have garnered considerable scientific interest. While often formulated using small molecule ionic surfactants, WLMs can also form in solutions of amphiphilic block polymers. The most well-studied of these are poloxamers, ABA block polymers formed from two polyethylene oxide (PEO) end blocks and one polypropylene oxide (PPO) midblock. Poloxamers have the potential to form WLMs with a range of rheological properties due to the tunability block composition. The self-assembly of poloxamers into spherical micelles is well characterized, but the formation of poloxamer WLMs is poorly understood, leaving this class of WLMs underutilized. The first goal of this thesis is to formulate guidelines for varying poloxamer composition, temperature, salt type, and salt concentration to induce the formation of WLMs and tune their rheological properties. Small-angle neutron scattering, light transmittance measurements, and linear and nonlinear rheology were performed to characterize the temperature-induced rod formation, local micelle structure, and bulk mechanical properties of a large variety of poloxamer WLM formulations. This characterization revealed that the local microstructure of poloxamer WLMs is fairly insensitive to the poloxamer block composition, molecular weight, and the presence of salts, but the rheological properties varied greatly among formulations. Higher viscosity solutions were produced in poloxamers with higher molecular weights, lower PEO content, and added sodium chloride. Leveraging the insights from this self-assembly characterization, the second goal of this thesis is to study the nonlinear flow behavior of a highly elastic, high-viscosity WLM solution formed using poloxamers. These WLMs exhibited rheological behaviors typically observed in gels, including a yield stress and viscoelastic aging. Combined shear rheology and particle tracking velocimetry (rheo-PTV) measurements revealed this solution of WLMs formed shear bands in startup flows, accompanied by elastic instabilities and wall slip. The mechanism of shear-band formation was unlike any WLMs studied previously and more closely resembled the mechanism in yield stress fluids. Exploring the evolution of shear bands in this WLM gel with rheology similar to both canonical viscoelastic WLMs and yield stress fluids provided new insights on shear banding in both of these complex fluids. Shear bands in poloxamer WLMs are frequently accompanied by wall slip, which is also prevalent in nonlinear flow studies of many other WLMs. The third goal of this thesis is to explore the impact of wall slip on the spatiotemporal evolution of the flow field during shear banding. Cylindrical Couette flows of WLMs were simulated with the German-Cook-Beris (GCB) model and two phenomenological slip boundary conditions. Introducing wall slip was shown to delay the onset of shear banding and reduce the width of the high-shear-rate band, consistent with experiments. During shear band formation, the evolution of the flow field was sensitive to the form of the slip boundary condition; flow reversal prior to shear-band formation was enhanced with shear-rate-dependent wall slip and diminished with shear-stress-dependent wall slip. These results demonstrated that the qualitative agreement between shear-banding models of WLMs and experiments can be improved by incorporating wall slip into shear-banding simulations. The final goal of this thesis is to re-examine the peculiar shear-band formation in poloxamer WLM gels and verify the proposed yield-stress-driven shear banding mechanism. A new technique was developed to characterize flow heterogeneity that combines cessation of flow protocols with rheo-PTV. To successfully implement this technique, new methods to analyze rheo-PTV data were developed, which improved the calculation of the velocity by fitting entire particle trajectories described in cylindrical coordinates. Before performing experiments, theoretical flow problems were analyzed to demonstrate that fluid retraction accompanies stress relaxation in viscoelastic fluids with flow heterogeneity. The proposed evolution of flow heterogeneity in poloxamer WLM gels was confirmed by measuring fluid retraction in cessation of flow. This technique also revealed another poloxamer WLM formulation developed shear bands via the canonical shear-band formation mechanism. These findings demonstrate the utility of cessation of flow combined with PTV to characterize flow heterogeneity in viscoelastic complex fluids. Overall, this thesis combines computational and experimental approaches to gain a deeper understanding of the self-assembly and nonlinear flow behavior of WLMs formed from poloxamers. Fundamental insights are also uncovered about nonionic surfactant self-assembly, wall slip, shear banding, and flow heterogeneity in complex fluids in general. The unique rheology and highly tunable self-assembly of poloxamer WLMs make these solutions ideal candidates for future investigations about shear banding.