Viscoelastic Fluids in Parallel Shear Flows at low re: Instabilities, Bifurcations & Single Molecule Experiments

复杂流体是一类广泛的材料，通常在宏观尺度上是均匀的，但在中间尺度上具有结构（例如胶体、血液和聚合物）。这类流体的流变性和整体流动行为是其中间或结构尺度的强烈函数。一个典型的例子是流体流动中柔性聚合物分子的拉伸和排列，这与许多鲜为人知的现象有关，如湍流减阻、反常粘度增强和不规则流动。最近的研究表明，粘弹性流体在平行剪切几何（管道/通道）中的流动在低雷诺数（Re）的有限振幅扰动下是不稳定的。结果表明，亚临界转变类似于管道中的牛顿湍流，只不过动力是弹性应力而不是惯性。智力优势：本提案的主要目标是系统地研究平行剪切流中这种新发现的纯弹性亚临界转变的起源及其对复杂流体流动的广泛影响。粘弹性流体实验将在一个长而直的微通道中进行，使用测速仪和原位（局部）压力测量以及单分子示踪剂。特别是，使用荧光DNA分子的单分子实验将通过测量这种流动中的聚合物分子的构象动力学和统计数据来深入了解粘弹性不稳定性的分子起源。我们将能够解决许多重要的问题，如：i)导致这种非线性弹性不稳定性发生的主要分子机制是什么？在这种转变之前、期间和之后的分子构象动力学是什么？ii)在过渡过程中形成了什么样的流动连贯结构？iii)过渡是非线性/亚临界的，还是已知存在于牛顿管流中的线性增长放大的产物？iv)这种粘弹性亚临界转变会导致“弹性湍流”吗？v)压降与发生不规则流动的流量上下有何关系？流动阻力或压降是否增加？从科学的角度来看，这里提出的研究将提供非常需要的见解，通过使用分子示踪剂（荧光DNA分子）的直接可视化，柔性分子的构象动力学影响体积流动行为稳定性的机制。以往的此类研究大多集中在流体流动对聚合物动力学的影响上。对整体流动行为的并行研究和直接的分子可视化将使人们对分子与外加流体应力的相互作用有一个全面的认识。反过来，这将导致更现实和准确的理论和分子模型的发展，一般流动不稳定的开始。微流体的使用为单分子实验提供了一个很好的试验台，因为流动可以很好地控制。更广泛的影响：该提案概述了一个综合的研究和教育计划，包括：i)通过提供复杂流体、流变学和非线性动力学的研究生水平课程来培养研究生，以及在这些领域的研究机会。主要目标是增加历史上代表性不足的少数民族，如女性、非洲裔美国人、美洲原住民和西班牙裔美国人在研究中的参与；ii)从没有工程研究生课程的传统黑人学院和大学招募本科生进行暑期研究实习。PI还将利用宾夕法尼亚大学强大的外联基础设施，让来自西费城的K-12教师和高中生参与研究项目；Iii)最后，这项研究和教育计划的结果将被广泛传播，并将对社会产生潜在的重要利益。特别是，这些结果将为复杂流体流动现象提供新的认识。

Arratia, Paulo 1336171Complex fluids are a broad class of materials that are usually homogeneous at the macroscopic scale, but possess structure at an intermediate scale (e.g., colloids, blood, and polymers). The rheology and bulk flow behavior of such fluids are strong functions of their intermediate or structural scale. A prime example of this is the stretching and alignment of flexible polymer molecules in fluid flow, which has been connected to many poorly understood phenomena such as turbulence drag reduction, anomalous viscosity enhancement, and irregular flow. Recent studies have shown that the flow of viscoelastic fluids in parallel shear geometries (pipes/channels) is unstable to finite amplitude perturbations at low Reynolds numbers (Re). Results point to a subcritical transition that is akin to Newtonian turbulence in pipes except that elastic stresses rather than inertia is the driving force. Intellectual Merit :The main goal of this proposal is to systematically investigate origins this newfound purely elastic subcritical transition in parallel shear flows and its broad consequences to complex fluid flows. Experiments with viscoelastic fluids will be performed in a long, straight microchannel using velocimetry and in-situ (local) pressure measurements, and single molecule tracers. In particular, single molecule experiments using fluorescent DNA molecules will be used to gain insight into the molecular origins of viscoelastic instabilities by measuring the conformation dynamics and statistics of polymer molecules in such flows. We will be able to address many important questions such as: i) What are the main molecular mechanisms leading to the onset of such nonlinear elastic instability? What are the molecular conformation dynamics before, during, and after such transition? ii) What kind of flow coherent structures develops during the transition? iii) Is the transition nonlinear/subcritical or is it a product of a linear growth amplification which is known to exist in Newtonian pipe flows? iv) Does this viscoelastic subcritical transition lead to "elastic turbulence"? v) How is the pressure drop related to the flow rate below and above the onset of irregular flow? Is there an increase in flow resistance or pressure drop? From a scientific standpoint, the studies proposed here will provide much needed insight into the mechanisms by which the conformation dynamics of flexible molecules affects the stability of the bulk flow behavior using direct visualization of molecular tracers (fluorescent DNA molecules). Most previous investigations of this kind focus on the effect of fluid flow on polymer dynamics. The parallel pursuit of bulk flow behavior and direct molecular visualization will give rise to a comprehensive view of the molecular interactions with the applied fluid stresses. This, in turn, will lead to the development of more realistic and accurate theoretical and molecular models for the onset of flow instabilities in general. The use of microfluidics allows for an excellent test-bed for single molecule experiments since flows can be very well controlled.Broader Impacts :This proposal outlines an integrated research and educational program that includes: i) training graduate students by offering graduate level courses in complex fluids, rheology, and nonlinear dynamics as well as research opportunities in these areas. A main goal is to increase the participation of historically under-represented minorities such as females, African-Americans, Native-Americans, and Hispanics in research; ii) recruiting undergraduate students for summer research internships from Historically Black Colleges and Universities that do not possess an engineering graduate program. The PI will also take advantage of the University of Pennsylvania's strong outreach infrastructure to involve K-12 teachers and high school students from West Philadelphia in the research program; iii) finally, the results of this research and educational program will be broadly disseminated and will have potentially important benefits to society. In particular, the results will offer new knowledge in complex fluid flow phenomena.