Shear-induced transitions and instabilities in shear banding fluids

剪切带流体中剪切引起的转变和不稳定性

• 批准号: 1335653
• 负责人: Jeffrey Reimer
• 金额: $ 29万
• 依托单位: University of California-Berkeley
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-09-01 至 2017-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1335653&HistoricalAwards=false
• 关键词: Shear; induced; transitions; instabilities; shear

Muller, Susan 1335653An experimental study of transitions and instabilities in shear banding fluids using wormlike micellar solutions as model fluids and measurements of velocity fields and critical conditions is proposed. While shear banding, i.e. discontinuities in the velocity gradient in a flowing material, has been reported in a broad range of important materials, including suspensions, emulsions, foams, granular materials, and liquid crystalline polymers, wormlike micellar solution provide a convenient paradigm for studies of shear banding fluids for many reasons. Wormlike micellar solutions show simple viscoelastic behavior that is characterized by a single relaxation time, are easy to prepare, and, in contrast to polymers, are not susceptible to shear-degradation or chain scission. Recent theoretical and experimental work suggests that both interfacial and purely elastic instabilities may occur in these systems. Inertial instabilities are also likely, and these systems thus promise incredibly rich dynamics. Intellectual Merit :The intellectual merit of the proposal lies in understanding these interactions and scaling in shear banding materials. A Taylor-Couette (TC) flow apparatus, where the flow occurs between concentric, rotating cylinders, will be the model flow for these experimental studies. The TC problem offers a number of unique features that make it ideal for teasing apart the forces that drive instability. First, the role of inertial destabilization for Newtonian fluids has been well-studied, and inertial (centrifugal) destabilization can be completely eliminated if the flow is generated through rotation of only the outer cylinder. Counter-rotation of the cylinders leads to a nodal surface in the base flow where the angular velocity is zero and leads to essentially an "inner" TC geometry bounded by the rotating inner cylinder and the stationary nodal surface, and an "outer" TC geometry bounded by the stationary nodal surface and the rotating outer cylinder. Counter-rotation thus provides a convenient mechanism for introducing both a free (rather than hard, no slip) boundary and a continuously variable length scale for the gap between the cylinders. Purely elastic instabilities in the TC problem are also well-understood and progress has been made in understanding the combination of inertia and elasticity. Shear banding fluids introduce two additional wrinkles: the new length scale introduced by the shear band, and the potential for interfacial instabilities associated with the discontinuity in properties across the shear band. In contrast to elastic instabilities, interfacial instabilities are stabilized by increased curvature. Thus, the TC problem provides a platform for systematically isolating the effects of these three competing instability modes, for understanding the scaling of the modes with curvature, kinematic, and fluid parameters, and for understanding the role of rigid versus free boundary conditions. Broader Impacts :The broader impacts of the study lie in processing of shear banding materials, which is relevant to a range of industrially important systems, including powders, suspensions, oil sands, and foams. A postdoctoral scientist will receive training and mentoring, as will approximately 6 undergraduates who will be recruited to participate in this research. This research will also be integrated into graduate coursework on transport phenomena and complex fluids at Berkeley.

用蠕虫状胶束溶液作为模型流体，对剪切带状流体中的相变和不稳定性进行了实验研究，并测量了速度场和临界条件。虽然剪切带状流体，即流动材料中速度梯度的不连续性，在包括悬浮液、乳液、泡沫、颗粒材料和液晶聚合物在内的许多重要材料中都有报道，但蠕虫状胶束溶液由于许多原因为剪切带状流体的研究提供了一种方便的范例。蠕虫状胶束溶液表现出简单的粘弹性行为，其特征是松弛时间单一，易于制备，与聚合物相比，不易发生剪切降解或链断裂。最近的理论和实验工作表明，在这些系统中既可以发生界面不稳定性，也可以发生纯弹性不稳定性。惯性不稳定性也是可能的，因此这些系统提供了令人难以置信的丰富的动力学。智力价值：该提议的智力价值在于了解这些相互作用和剪切带材料的比例。泰勒-库埃特(TC)流动装置，其中的流动发生在同心，旋转的圆柱体之间，将是这些实验研究的流动模型。TC问题提供了许多独特的功能，使其成为梳理驱动不稳定的力量的理想选择。首先，牛顿流体的惯性失稳作用已经得到了很好的研究，如果仅通过外筒旋转来产生流动，则可以完全消除惯性(离心力)失稳。圆柱体的反向旋转导致了基流中角速度为零的节面，并实质上导致了由旋转的内柱和静止的节面所包围的“内”TC几何，以及由静止的节面和旋转的外柱所包围的“外”TC几何。因此，反向旋转提供了一种方便的机制，用于为气缸之间的间隙引入自由(而不是硬的、无滑动的)边界和连续可变的长度尺度。TC问题中的纯弹性不稳定性也得到了很好的理解，在理解惯性和弹性的组合方面也取得了进展。剪切带流体引入了两个额外的褶皱：剪切带引入的新的长度尺度，以及与剪切带上的性质不连续相关的界面不稳定的可能性。与弹性不稳定性相反，界面不稳定性通过增加曲率来稳定。因此，TC问题为系统地隔离这三种相互竞争的不稳定模式的影响，理解曲率、运动学和流体参数的模式的标度，以及理解刚性边界条件和自由边界条件的作用提供了一个平台。更广泛的影响：这项研究的更广泛影响在于剪切带材料的加工，这与一系列具有重要工业意义的系统相关，包括粉末、悬浮液、油砂和泡沫。一名博士后科学家将接受培训和指导，大约6名本科生将被招募参与这项研究。这项研究还将被整合到伯克利大学关于运输现象和复杂流体的研究生课程中。