Collaborative Research: Towards a molecular-scale understanding of flow-induced gelation in rodlike micelle solutions

尽管在理解包括聚合物、胶体、玻璃、液晶和其它的许多种类的复杂流体中的输运和流变学的微观结构基础方面取得了实质性的进展，但最重要的一类复杂流体的流动行为，表面活性剂溶液，仍然是个谜。特别地，至少二十年来已知的是，线/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以形成粘弹性凝胶。由于缺乏对这些结构的理解，它们被简单地称为流致结构，或FIS。虽然在分子水平上模拟线/棒状（纳米级）胶束以及模拟FIS存在的宏观后果（如剪切流中的带状结构）方面取得了进展，但分子结构与这种纳米结构可能显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，以帮助通过实验和多尺度模拟集缩小理解差距，包括从原子（和纳米）到连续体的长度和时间尺度。一组实验探索瞬态和新的永久性流致结构，在微通道中的拉伸变形引起的制度，将进行。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。这些可以在10纳秒的时间尺度上在原子水平上捕获一个或两个线状胶束片段的结构和相互作用，这些胶束片段位于一个周期性的盒子中，大约10 nm。这是足够长和足够大，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，将大约四个重原子聚集到每个珠子中，相对于原子模拟，获得了1000倍的速度，几乎达到毫秒时间尺度，同时通过适当选择CG势保留分子尺度特性。CG模型将允许作为盐浓度的函数的胶束持久性长度和线状胶束的稳定性的测定。3)用珍珠项链胶束模型进行布朗动力学模拟。由Ryckaert及其同事开创的这个模型将蠕虫状胶束视为一串可以端对端断裂和融合的珠子，并且速度足够快，可以平衡胶束长度分布，无论是否有流动。PI将把胶束连接或交联和成束的可能性纳入该模型中，从而首次允许对流动诱导凝胶形成进行分子尺度模拟。4)动力学模型和本构方程。PI将尝试从模拟中提取建立动力学模型所需的成分，如果可能的话，还将提取用于线状胶束流动的完全非线性本构方程。通过这一系列相互关联的模拟，每个模拟都针对不同的长度和时间尺度，并辅以实验，研究人员已经制定了一个路线图，在分子特性和宏观流动效应（如流动诱导凝胶化和剪切带）之间架起桥梁。更广泛的影响包括与普罗克特和甘布尔的科学家合作，他们的兴趣是理解，建模，并控制线状胶束溶液的性质。他们计划在宝洁公司的科学家和我们的研究生和本科生和教师团队之间举行年度会议，以及在宝洁公司为期一个月的学生实习。这将导致思想的富有成效的交流，带来实际的商业问题，以学生的注意，并进行新的基本思想和新的建模方法到企业界。他们还计划招募UG（REU）以及包括少数民族学生在内的学校学生（通过华盛顿大学的STARS计划），并让他们参与开发由快速GROMACS和MARTINI引擎驱动的模块，这些引擎具有粗粒度的潜力，以帮助学习表面活性剂溶液中的自组装。

0853735SureshkumarDespite 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. The PI's 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. The PI's 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.