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

合作研究：在分子尺度上理解棒状胶束溶液中流动诱导的凝胶化

• 批准号: 1049454
• 负责人: Radhakrishna Sureshkumar
• 金额: $ 17.5万
• 依托单位: Syracuse University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-02-15 至 2012-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1049454&HistoricalAwards=false
• 关键词: Collaborative; Research; Towards; molecular; scale

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.

尽管在了解包括聚合物、胶体、玻璃、液晶等在内的许多复杂流体的输运和流变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一个谜。特别是，至少二十年来人们已经知道，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下经历相变形成粘弹性凝胶。由于不了解这些结构是什么，它们被简单地命名为流诱导结构（Flow Induced structures，简称FIS）。虽然在分子水平上模拟线/棒状（纳米尺度）胶束方面取得了进展，并在模拟FIS存在的宏观后果方面取得了进展，例如剪切流中的带状结构，但分子结构与这种纳米结构可能表现的可能性和条件之间几乎完全缺乏理论联系。知识价值。这是两所大学之间的合作研究，通过实验和多尺度的模拟，包括从原子（和纳米）到连续体的长度和时间尺度，帮助缩小理解上的差距。将进行一系列实验，探索由微通道中拉伸变形引起的瞬态和新型永久流致结构的机制。同时，该项目提出了四种不同的仿真方法。1)原子分子动力学模拟。它们可以在10纳秒的时间尺度上，在原子水平上捕捉到一条或两条像胶束碎片一样的线的结构和相互作用。这足够长，足够大，可以确定离子对胶束结构和胶束间相互作用的影响。2)粗粒度（CG）分子动力学模拟。使用Marrink MARTINI模型，将大约四个重原子集中到每个球中，相对于原子模拟，速度提高了1000倍，达到接近毫秒的时间尺度，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丝状胶束的稳定性作为盐浓度的函数。3)珍珠项链胶束模型的布朗动力学模拟。这个模型是由Ryckaert和他的同事首创的，它将蠕虫状胶束视为一串珠子，可以端到端断裂和融合，并且速度足够快，可以在有和没有流动的情况下实现胶束长度分布的平衡。PI将把胶束连接、交联和捆绑的可能性纳入该模型，从而首次实现了流体诱导凝胶形成的分子尺度模拟。4)动力学模型及本构方程。PI将尝试从模拟中得出建立动力学模型所需的成分，如果可能的话，还将建立一个完整的线状胶束流动的非线性本构方程。通过这组针对不同长度和时间尺度的联锁模拟，并结合实验，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（如流动诱导凝胶和剪切带）之间建立桥梁。更广泛的影响包括与宝洁公司（Proctor and Gamble）的科学家合作，他们的兴趣是理解、建模和控制像胶束溶液这样的螺纹的性质。他们计划P&amp；G科学家与我们的研究生、本科生和教师团队之间的年度会议，以及在P&amp；G为期一个月的学生实习。这将导致富有成效的思想交流，将实际的商业问题引起学生的注意，并将新颖的基本思想和新的建模方法带入企业界。他们还计划招募UG （REU）和学校学生，包括少数民族学生（通过华盛顿大学的STARS项目），让他们参与开发由快速GROMACS和MARTINI发动机驱动的模块，这些模块具有粗粒度势，有助于学习表面活性剂溶液中的自组装。