Disentangling the dynamics of shear banding in entangled polymer solutions

塑料在现代社会中无处不在，用于食品包装、电子设备、交通运输、运动器材和医疗器械。所有成型塑料的前体都是聚合物流体。聚合物流体是一种独特的流体类型，因为链状聚合物分子的行为就像弹簧，在流体分子的集合中，聚合物弹簧变得缠绕在一起。与水和许多其他流体不同，这种复杂的流体不会以与所施加的力成比例的方式响应应力和剪切力。这种非比例响应被归类为非牛顿响应。一般的流体流动工程模型还不能很好地描述非牛顿流体。在没有通用模型的情况下，对工业加工中流量的预测仅限于试错法。在某些情况下，如缓慢流动或低浓度，这些复杂性中的许多可以忽略不计。然而，这些并不是工业上最相关的条件，在这些条件下，高浓度和快速流动非常普遍。该项目支持对浓缩聚合物流体快速流动的基础实验研究，并使用最先进的成像技术直接成像单个聚合物分子，以揭示复杂流体流动的微观动力学。理解高浓度聚合物流体动力学的挑战在于缠绕聚合物溶液的非线性粘弹性，特别是在高剪切速率下高浓度聚合物溶液中剪切带的形成。这些现象很难通过实验来探讨。本研究项目结合高速共聚焦显微镜和定制的剪切池来研究模型系统的剪切带流动的来源，即浓缩DNA溶液。这些独特的实验工具在这一问题上的应用将解决关于聚合物流体中剪切带的争议。绘制了DNA浓缩液剪切诱导动力学的相图。该项目的目的是在DNA分子的微观动力学和聚合物流体的宏观流动行为之间建立直接联系。该项目正在阐明快速剪切下浓缩液中单个聚合物分子的动力学，并通过实验验证关于聚合物流体剪切带的相互竞争的理论。这些结果揭示了人们对快速流动下浓缩聚合物流体的理论认识的缺失，并最终揭示了在最具工业相关性的条件下聚合物流体的实用工程模型。加强对聚合物流动的控制和预测将提高产量和效率，从而对美国制造业产生重大影响。这项研究项目还包括针对当地高中生的K-12外展计划，特别是那些在科学和工程领域代表性不足的学生。

Plastics are ubiquitous to modern society, used in food packing, electronic devices, transportation, sports equipment, and medical devices. The precursor to all formed plastics is a polymer fluid. Polymer fluids are a unique type of fluid, as the chain-like polymer molecules act like springs, and within a collection of fluid molecules, the polymer springs become entangled. Unlike water and many other fluids, this complex fluid does not respond to the forces of stress and shear in a manner that is proportional to the applied force. This non-proportional response is classified as Non-Newtonian. Non-Newtonian fluids have not been adequately described by general engineering models of fluid flow. Without general models, prediction of flow in industrial processing is limited to trial-and-error. Under some circumstances, such as slow flow or low concentration, many of these complexities can be ignored. However, these are not the most industrially relevant conditions, where high concentration and fast flow are prevalent. This project supports a fundamental experimental study on the fast flow of concentrated polymer fluids, and uses state-of-the-art imaging techniques to directly image single polymer molecules to reveal the microscopic dynamics of complex fluid flow. The challenge in understanding the dynamics of concentrated polymer fluids is the nonlinear viscoelasticity of entangled polymer solutions, particularly the formation of shear-banding in concentrated polymer solutions at high shear rates. These phenomenon are difficult to probe experimentally. This research project combines high-speed confocal microscopy with a custom shear cell to investigate the origin of shear-banding flows of a model system, namely, concentrated DNA solutions. The applications of these unique experimental tools to the problem will resolve a controversy regarding on shear-banding in polymer fluids. The phase diagram of the shear-induced dynamics of concentrated DNA solutions is being mapped. The project aims to establish a direct link between the microscopic dynamics of DNA molecules and the macroscopic flow behavior of polymer fluids. The project is elucidating the dynamics of single polymer molecules in concentrated solutions under fast shear and experimentally validating competing theories on shear banding of polymer fluids. These results are uncovering the missing theoretical understanding of concentrated polymer fluids under fast flows and, ultimately, practical engineering models for polymer fluids under the most industrially relevant conditions. Increased control and prediction of polymer flow will increase throughput and efficiency, and therefore greatly impact the U.S. manufacturing sector. This research project also involves K-12 outreach programs with local high school students, especially those underrepresented in science and engineering.