The Effects of Viscoelasticity on Filament Thinning & Drop Breakup in Microfluidic Devices: Single Molecule Experiments

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。复合流体是一类广泛的材料，通常在宏观尺度上是均匀的，在微观尺度上是无序的，但在中间尺度上具有结构（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，这与湍流阻力减少、流体动力学不稳定性和粘度增强有关。在两相流中液滴破碎的特殊情况下，聚合物分子的存在可以导致许多有趣的现象，例如增强流体丝的寿命和？串珠。流体长丝寿命的增加通常归因于分子的拉伸，而分子的拉伸与流体弛豫时间和拉伸粘度有关。在这项工作中，我们旨在通过可视化荧光DNA分子的构象动力学和统计来了解聚合物流体在简单微流控装置中的滴破碎过程。利用这些方法，我们将能够第一次解决许多悬而未决的问题，例如：i)两相流中线圈拉伸转变的临界应变率是多少？ ii)在细丝变薄过程中分子是完全拉伸还是部分拉伸？ iii)在迭代拉伸不稳定过程中分子的表现如何？这就产生了串珠现象iv)在细丝变薄和/或破裂过程中是否存在分子断裂v)流体细丝中分子的动力学与拉伸粘度和流体弛豫时间的测量有什么关系？实验将在交叉槽微流控装置中进行。液滴将通过流体动力聚焦形成。我们将使用矿物油作为连续相，稀释聚合物溶液和不同浓度的DNA悬浮液作为分散相。所有流体都将使用常规流变仪进行表征。DNA将使用荧光显微镜和CMOS相机进行可视化。我们将测量分子的延伸和构象作为应变率和粘性阻力的函数。通过将当前开发的单分子成像方法与受控流体流动相结合，可以评估流体细丝在变薄和破裂过程中分子拉伸的动力学。知识价值：本文提出的研究是第一个基于分子直接可视化的柔性分子构象动力学影响粘弹性流体细丝细化和液滴破碎过程机制的基础研究。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，这将导致粘弹性流体液滴破碎过程的更现实和准确的理论和分子模型的发展。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。此外，预计该实验将导致对流体松弛时间和拉伸粘度等重要流体流变特性的精确测量。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、流变学和多相流的新研究生课程来培养研究生，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，这些结果将为多相和复杂流体流动现象提供新的认识。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).0932449ArratiaComplex fluids are a broad class of materials that are usually homogeneous at the macroscopic scale and disordered at the microscopic 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 turbulence drag reduction, hydrodynamic instabilities, and enhanced viscosity. In the particular case of drop breakup in two phase flows, the presence of polymer molecules can lead to many interesting phenomena such as enhancement in fluid filament lifetime and ?beads on string. The increase in fluid filament lifetime is often credited to the stretching of molecules, which is related to both the fluid relaxation time and extensional viscosity. In this proposed work, we aim to understand the drop breakup process of polymeric fluids in a simple microfluidic device by visualizing the conformation dynamics and statistics of fluorescent DNA molecules. Using such methods, we will be able, for the first time, to address many outstanding questions such as: i) What is the critical strain rate for the coil stretch transition in two phase flows ii) Are the molecules fully stretched or only partially stretched during the filament thinning process iii) How do the molecules behave during the iterated stretching instability, which gives rise to the beads-on-string phenomenon iv) Is there molecular scission during the filament thinning and/or breakup process v) How do the dynamics of molecules in the fluid filament relate to measurements of extensional viscosity and fluid relaxation time. Experiments will be performed in a cross slot microfluidic device. Drops will be formed using hydrodynamic focusing. We will use mineral oil as the continuous phase and dilute polymeric solutions and DNA suspensions of various concentrations as the dispersed phase. All fluids will be characterized using conventional rheometers. DNA will be visualized using a fluorescent microscope and a CMOS camera. We will measure the molecule extension and conformation as a function of strain rate and viscous drag. By combining currently developed single molecule imaging methods with controlled fluid flow, it is possible to assess the dynamics of molecule stretching inside a fluid filament undergoing thinning and breakup.Intellectual Merit: The studies proposed here are the first fundamental investigations of the mechanisms by which the conformation dynamics of flexible molecules affects the filament thinning and drop breakup process of viscoelastic fluids using direct visualization of molecules. 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 drop breakup process of viscoelastic fluids. The use of microfluidics allows for an excellent test-bed for single molecule experiments since flows can be very wellcontrolled. In addition, it is expected that the experiments will lead to accurate measurements of important fluid rheological properties such as fluid relaxation time and extensional viscosity.Broader Impact: This proposal outlines an integrated research and educational program that includes: i) training graduate students by offering new graduate level courses in complex fluids, rheology, and multiphase flows 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 multiphase and complex fluid flow phenomena.