CAREER: Kinetics and Molecular Origin of Shear Banding in Wormlike Micellar Fluids

胶束是表面活性剂的聚集体，可以形成各种形状的结构，包括称为蠕虫状胶束的长柔性链。 胶束用于各种产品中，以控制材料在加工和使用过程中的物理性质。 在流动下，蠕虫状胶束溶液有时会表现出一种奇怪的现象，称为剪切带，其中胶束分离成多个薄带。 剪切带非常重要，因为它会在加工过程中强烈影响溶液的性质，并影响最终产品的质量。 这个CAREER奖将支持蠕虫状胶束溶液剪切带的全面调查。 多种实验技术将被用来了解蠕虫状胶束的浓度，它们的端到端的长度，和胶束溶液的物理性质的影响剪切带。 这些结果将对处理各种材料的工艺工程师和科学家有用，因为在其他工业重要材料中也观察到剪切带，如乳液，泡沫，油砂和胶体悬浮液。 研究小组将让不同的学生群体参与研究。 为了向年轻学生展示这项研究的重要性，该团队将为教师和学生制作FlowKits，以探索教室中复杂流体的不寻常特性。由于传统技术在探测流体微观结构方面的局限性，理解蠕虫状胶束溶液中剪切带的机制仍然是一个关键的挑战。该CAREER奖将绕过这些限制，并通过实验研究a）流动诱导的胶束破裂和B）流动诱导的浓度梯度和c）粘弹性不稳定性对剪切带机制的影响，通过Rheo-DOSY-NMR和Rheo-qNMR技术。这将通过测量胶束长度的时空演变和跨越泰勒-库埃特流（即，两个同心圆柱体之间的流）的差距的浓度来实现，用于宽范围的系统和条件。 该研究将建立流体性质（弹性，惯性，胶束缠结），加工条件（流动斜坡和几何曲率）和剪切带流动形成的宏观动力学之间的联系。实验和模拟相结合，将提供第一个系统的比较实验和各种模型的剪切带的局部胶束长度，浓度梯度，速度分布和剪切应力。这些实验结果将为验证现有理论和/或开发新的预测模型提供必要的新数据。研究结果将帮助工程师合理设计新型剪切带材料，改善其性能，用于提高石油采收率、减阻和许多消费品。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Micelles are aggregates of surfactants that can form structures of various shapes, including long flexible strands that are called wormlike micelles. Micelles are used in a variety of products to control the physical properties of the material during its processing and use. Under flow, solutions of wormlike micelles sometimes exhibit a curious phenomenon known as shear banding in which the micelles segregate into multiple thin bands. Shear banding is important, because it can strongly influence the properties of the solution during processing and affect the quality of the final product. This CAREER award will support a comprehensive investigation of shear banding of wormlike micellar solutions. Multiple experimental techniques will be used to understand how the concentration of the wormlike micelles, their end-to-end lengths, and the physical properties of the micellar solution influence shear banding. The results will be useful to process engineers and scientists who work with a wide variety of materials, because shear banding is also observed in other industrially important materials such as emulsions, foams, oil-sands and colloidal suspensions. The research team will involve diverse groups of students in the research. To demonstrate the importance of the research to younger students, the team will produce FlowKits for teachers and students to explore the unusual properties the complex fluids in the classroom.Understanding the mechanisms of shear banding in wormlike micellar solutions remains a critical challenge due to limitations of the traditional techniques to probe fluid microstructures. This CAREER award will circumvent these limits and experimentally examine a) flow-induced micellar breakage and b) flow-induced concentration gradients and c) the effects of viscoelastic instabilities on the mechanisms of shear banding via Rheo-DOSY-NMR and Rheo-qNMR techniques. This will be accomplished by measuring spatio-temporal evolution of micellar length and concentrations across the gap of a Taylor-Couette flow, i.e. flow between two concentric cylinders, for a broad range of systems and conditions. The research will establish the connections among fluid properties (elasticity, inertia, micellar entanglement), processing conditions (flow ramp up and geometry curvature) and macroscale kinetics of shear banding flow formation. A combination of experiments and simulations will provide the first systematic comparisons between experiments and various models of shear banding in terms of the local micellar length, concentration gradients, velocity profiles and shear stresses. These experimental results will provide new data which are essential for verifying existing theories and/or developing new predictive models. Results will help engineers to rationally design novel shear banding materials with improved properties for enhanced oil recovery, drag reduction, and many consumer products.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.