Extensional rheology and flow-induced structure formation of wormlike micelle solutions

蠕虫状胶束溶液不仅具有很高的技术价值，而且可以作为研究聚合物物理基本问题的模型体系。这种溶液的结构、胶束动力学和流动行为之间的关系是非常重要的。现在人们对WLM溶液的线性粘弹性性质有了很好的了解，而对非线性流动性质的研究则集中在我们的知识仍然非常有限的地方。特别是，在伸展流动中，流动诱导的WLM溶液的自组装和结构的变化是一个主要的挑战，为了有针对性地设计先进的表面活性剂体系用于各种重要的技术应用，有必要对这一现象有更深入的了解。例如，在过去几年里，由于全球石油需求的增加，使用表面活性剂溶液提高石油采收率(EOR)重新引起了人们的极大兴趣。要成功地将知识和产品从研究转移到油田实施，需要对这些流体在伸展流场中的结构和动力学有基本的了解。到目前为止，我们希望进行第一个系统的研究，重点是WLM溶液的流变学和伸长流动引起的结构变化之间的关系。将研究广泛的表面活性剂浓度和盐表面活性剂比率，包括线形和支链胶束的稀溶液和浓溶液、缠绕溶液。基于我们最近对毛细管破裂弹性流变仪(CABER)方法的技术改进，现在可以确定WLM溶液的真实拉伸粘度和流动活化能。对特定表面活性剂体系的初步测量表明，细丝寿命，即此类实验中流体对拉伸变形的阻力，即使溶液的线性粘弹性性质相似，也可能发生数量级的变化。这表明流动引起的胶束结构的强烈变化取决于表面活性剂的浓度和盐/表面活性剂的比例。在伸展流动中，胶束可能破裂或形成更大的聚集体。在产生平面拉伸流场的双曲流道中对选定的样品进行结构研究(小角光散射、中子散射、浊度、双折射和粒子图像测速)将定量地揭示这种流动诱导的结构变化作为延伸率和总变形的函数。拟议的研究项目将披露发生流动诱导偏离平衡胶束结构的表面活性剂类型、表面活性剂以及盐浓度和流动条件，并对这些结构进行定量表征。实验结果将被用来建议对预测WLM溶液流动行为的理论模型进行修改，该模型考虑了流动诱导的胶束破裂和复合速率的变化。

Solutions of wormlike micelles (WLM) are of great technical interest but also serve as model systems to study fundamental issues of polymer physics. The relationship between structure, micelle dynamics and flow behavior of such solutions is of major importance. The linear viscoelastic properties of WLM solutions are now well understood and research focuses on non-linear flow properties where our knowledge is still very limited. In particular, flow-induced changes in self-assembly and structure of WLM solutions in extensional flows are a major challenge and a deeper understanding of this phenomena is necessary for a targeted design of advanced surfactant systems to be used in various important technical applications. For instance, using surfactant solutions for enhanced oil recovery (EOR) has re-gained great interest in the last few years stimulated by the increasing global oil demand. A successful transfer of knowledge and products from research to oilfield implementation requires fundamental insight into structure and dynamics of these fluids subjected to extensional flow fields.We want to perform the first systematic study focusing on the relationship between rheology and elongational flow-induced structural changes of WLM solutions so far. A broad range of surfactant concentrations and salt surfactant ratios will be investigated, including dilute and concentrated, entangled solutions of linear and branched micelles. Based on our recent technical improvements of the Capillary Breakup Elongational Rheometry (CaBER) method, the true elongational viscosity and flow activation energy of WLM solutions can now be determined. Preliminary measurements on specific surfactant systems have shown that filament lifetime, i.e. the resistance of the fluid to elongational deformation in this type of experiment can vary by orders of magnitude even if the linear viscoelastic properties of the solutions are similar. This indicates strong flow-induced changes in micellar structure depending on surfactant concentration and salt/surfactant ratio. Micelles may either break or form larger aggregates in extensional flows. The structural investigations (small angle light- and neutron scattering, turbidity, birefringence and particle image velocimetry) on selected samples in a hyperbolic flow channel generating a planar extensional flow field will quantitatively reveal such flow-induced structural changes as a function of elongation rate and total deformation. The proposed research project will disclose for which surfactant types, surfactant as well as salt concentrations, and flow conditions flow-induced deviations from the equilibrium micellar structure occur and these structures will be characterized quantitatively. Experimental results will be used to suggest modifications of a theoretical model predicting the flow behavior of WLM solutions taking into account flow-induced changes of micellar scission and recombination rates.