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。0932449 Arratia复杂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，但在中等尺度上具有结构（例如，胶体、血液和聚合物）。这种流体的流变学和整体流动行为是其中间或结构尺度的强函数。这方面的一个主要例子是柔性聚合物分子在流体流动中的拉伸和排列，这与湍流阻力减小、流体动力学不稳定性和粘度增加有关。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，如增强流体细丝的寿命和？串在绳子上的珠子。流体细丝寿命的增加通常归功于分子的拉伸，这与流体松弛时间和拉伸粘度两者有关。在这项工作中，我们的目标是了解一个简单的微流体装置中的聚合物流体的液滴破碎过程，通过可视化的构象动力学和荧光DNA分子的统计。 使用这些方法，我们将首次能够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）在细丝变细过程中分子是完全拉伸还是仅部分拉伸iii）在迭代拉伸不稳定性期间分子如何表现，这引起串珠串现象iv）在细丝变细和/或断裂过程中是否存在分子断裂v）流体细丝中分子的动力学如何与拉伸粘度和流体弛豫时间的测量相关。实验将在交叉槽微流体装置中进行。液滴将使用流体动力学聚焦形成。我们将使用矿物油作为连续相和稀释的聚合物溶液和各种浓度的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。将使用荧光显微镜和CMOS相机观察DNA。我们将测量作为应变率和粘滞阻力的函数的分子延伸和构象。通过将目前开发的单分子成像方法与受控流体流动相结合，可以评估正在变细和断裂的流体细丝内分子拉伸的动力学。这里提出的研究是第一个基本的调查机制，通过这种机制，柔性分子的构象动力学影响的细丝变细和液滴破碎过程的粘弹性流体使用直接可视化，分子。对整体流动行为和直接分子可视化的并行追求将产生对分子与所施加的流体应力的相互作用的全面看法。反过来，这将导致更现实和准确的理论和分子模型的粘弹性流体的液滴破碎过程的发展。微流体的使用为单分子实验提供了一个极好的测试平台，因为流量可以得到很好的控制。 此外，预计这些实验将导致重要的流体流变性能，如流体松弛时间和拉伸粘度的精确测量。更广泛的影响：该提案概述了一个综合的研究和教育计划，其中包括：i）通过提供新的研究生课程，复杂流体，流变学和多相流以及在这些领域的研究机会，培养研究生。一个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在研究中的参与; ii）从历史上黑人学院和大学招收本科生暑期研究实习，不拥有工程研究生课程。PI还将利用宾夕法尼亚大学强大的外展基础设施，让来自西费城的K-12教师和高中学生参与研究计划; 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，结果将提供新的知识，多相和复杂的流体流动现象。

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.