Collaborative Research: Towards a Molecular-Scale Understanding of Flow-Induced Gelation in Thread-Like Micelle Solutions

尽管在理解许多类型的复杂流体(包括聚合物、胶体、玻璃、液晶和其他)的输运和流变学的微观结构基础方面取得了实质性进展，但最重要的一类复杂流体--表面活性剂溶液的流动行为仍然是个谜。具体地说，至少二十年来，人们已经知道丝状/棒状表面活性剂胶束的半透明溶液在足够强的剪切或拉伸流动下经历相变形成粘弹性凝胶。由于缺乏对这些结构的了解，它们被简单地命名为流动诱导结构，或FIS。虽然在分子水平上模拟线状/棒状(纳米尺度)胶束以及模拟FIS存在的宏观后果(如剪切流动中的带状结构)方面取得了进展，但在分子结构与这种纳米结构可能出现的可能性和条件之间几乎完全缺乏理论联系。这是两所大学之间的合作调查，旨在通过实验和一组多尺度的模拟来帮助弥合理解上的差距，模拟的范围从原子(和纳米)到连续介质的长度和时间尺度。将进行一系列实验，探索由微通道中的拉伸变形引起的暂态和新型永久流致结构的形态。同时，该项目提出了四种不同的模拟方法。1)原子分子动力学模拟。它们可以捕捉到周期框中一个或两个线状胶束碎片的结构和相互作用，周期框的一侧约为10 nm，原子水平的时间尺度为10纳秒。这足够长和足够大，以确定离子对胶束结构和胶束间相互作用的影响。2)粗粒(CG)分子动力学模拟。使用Marrink Martini模型，在每个珠子中聚集大约四个重原子，获得了相对于原子模拟的1000倍的速度，几乎达到毫秒时间尺度，同时通过适当选择的CG势保持分子尺度的性质。CG模型将允许确定胶束持续长度和线状胶束的稳定性作为盐浓度的函数。3)基于珍珠项链胶束模型的布朗动力学模拟。这一模型是由Ryck及其同事首创的，它将蠕虫状胶束视为一串可以端到端断裂和融合的珠子，并且速度足够快，可以在有流和无流的情况下平衡胶束长度分布。我们将在这个模型中加入胶束连接或交联键以及捆绑的可能性，从而首次允许对流动诱导凝胶形成的分子尺度进行模拟。4)动力学模型和本构方程。我们将试图从模拟中得出建立动力学模型所需的成分，如果可能的话，还将得出丝状胶束流动的完全非线性本构方程。通过这组相互关联的模拟，每个模拟针对不同的长度和时间尺度，并辅之以实验，研究人员制定了一份路线图，在分子性质和宏观流动效应之间架起桥梁，如流动诱导凝胶和剪切带。广泛的影响包括与宝洁公司的科学家合作，该公司最感兴趣的是理解、建模和控制线状胶束溶液的性质。他们计划宝洁科学家与我们的研究生、本科生和教职员工团队之间的年度会议，以及在宝洁为期一个月的学生实习。这将带来富有成效的思想交流，将实际的商业问题引起学生的注意，并将新的基本想法和新的建模方法带入企业界。他们还计划招募UG(REU)以及包括少数族裔学生在内的学校学生(通过华盛顿大学的STAR计划)，并让他们参与开发由FAST GROMACS和具有粗粒度潜力的马提尼发动机驱动的模块，以帮助学习在表面活性剂溶液中的自我组装。

0853662LarsonDespite substantial progress achieved in understanding the microstructural basis of transport and rheology in many classes of complex fluids, including polymers, colloids, glasses, liquidcrystals and others, the flow behavior of one of the most important classes of complex fluids, surfactant solutions, remains mysterious. In particular, it has been known for at least two decades that translucent solutions of thread/rod like surfactant micelles undergo a phase transition to form viscoelastic gels under sufficiently strong shear or extensional flows. Lacking an understanding even of what these structures are, they are simply given the name Flow-Induced Structures, or FIS. While progress has been made towards simulating thread/rod like(nanoscale) micelles at the molecular level, and towards simulating the macroscopic consequences of the presence of FIS, such as banded structures in the shear flow, there is an almost complete lack of theoretical connection between molecular structures and the possibility and conditions under which such nano structures might manifest.Intellectual Merit. This is a collaborative investigation between two universities to help close gaps in understanding through both experiments and a multi scale set of simulations encompassing length and time scales ranging from atomic (and nano) to continuum. A set of experiments exploring the regimes of transient and novel permanent flow-induced structures, induced by extensional deformation in micro channels, will be carried out. In parallel, the project proposes four different simulation methods. 1) Atomistic Molecular Dynamic Simulations. These can capture the structure and interactions of one or two thread like micelle fragments in a periodic box roughly 10 nm on a side, at the atomic level on timescales of 10 nanoseconds. This is long enough and big enough to determine ionic effects on micellar structure and intermicellar interactions. 2) Coarse Grained (CG) Molecular Dynamics Simulations. Using the Marrink MARTINI model that lumps around four heavy atoms into each bead, a 1000 fold speed up relative to atomistic simulations is attained, reaching nearly to the millisecond time scale, while preserving molecular scale properties through suitably chosen CG potentials. The CG model will allow for the determination of micelle persistence lengths and the stability of thread like micelles as a function of salt concentration. 3) Brownian Dynamics Simulations using pearl necklace micelle model. This model, pioneered by Ryckaert and coworkers, treats the wormlike micelle as a string of beads that can break and fuse end-to-end, and is fast enough to allow for the equilibration of micelle length distributions, with and without flow. We will incorporate into this model the potential for micelle junctions or cross links, and bundling, thereby allowing for the first time a molecular scale simulation of flow induced gel formation. 4) Kinetic Model and Constitutive Equation. We will attempt to draw from the simulations the ingredients necessary to build a kinetic model and, if possible, a full nonlinear constitutive equation for flow of thread like micelles. Through this set of interlocking simulations, each aimed at different length and time scales, complemented by experiments, the investigators have developed a roadmap to bridge between molecular properties and macroscopic flow effects such as flow induced gelation and shear banding.Broader impacts include a collaboration with scientists at Proctor and Gamble, whose nterest is in understanding, modeling, and controlling the properties of thread like micellar solutions. They plan annual meetings between P&G scientists and our team of graduate and undergraduate students and faculty as well as month long student interships at P&G. This will lead to fruitful exchange of ideas, bringing practical commercial concerns to the attention of students, and carrying novel fundamental ideas and new modeling methods into the corporate world. They plan to also recruit UG (REU) as well as school students including minority students (through STARS program at Washington University) and involve them in developing modules driven by fast GROMACS and MARTINI engines with coarse grained potentials to help learn self assembly in surfactant solutions.