Shear-induced transitions and instabilities in shear banding fluids

提出了一种以蠕虫状胶束溶液为模型流体，并测量速度场和临界条件的剪切带流体的过渡和不稳定性的实验研究。虽然剪切带，即流动材料中速度梯度的不连续，已经在广泛的重要材料中被报道，包括悬浮液，乳液，泡沫，颗粒材料和液晶聚合物，蠕虫状胶束溶液为剪切带流体的研究提供了一个方便的范例，原因很多。虫状胶束溶液表现出简单的粘弹性行为，其特征是单一的松弛时间，易于制备，并且与聚合物相比，不容易受到剪切降解或链断裂的影响。最近的理论和实验工作表明，界面不稳定和纯弹性不稳定都可能发生在这些系统中。惯性不稳定也是可能的，因此这些系统承诺了令人难以置信的丰富动力学。知识价值：该建议的知识价值在于理解剪切带材料中的这些相互作用和结垢。泰勒-库埃特（TC）流动装置，其中的流动发生在同心，旋转圆柱之间，将是这些实验研究的模型流。TC问题提供了许多独特的特征，使其成为梳理驱动不稳定性的力量的理想选择。首先，惯性失稳对牛顿流体的作用已经得到了很好的研究，如果只通过外筒的旋转产生流动，惯性（离心）失稳可以完全消除。圆柱体的反旋转导致基底流中的节点面，角速度为零，并导致实质上由旋转的内圆柱体和静止的节点面组成的“内”TC几何形状，以及由静止的节点面和旋转的外圆柱体组成的“外”TC几何形状。因此，反向旋转提供了一种方便的机制，可以引入自由（而不是硬的、无滑移的）边界和连续可变的圆柱体间隙长度刻度。纯弹性不稳定性在TC问题中也得到了很好的理解，并且在理解惯性和弹性的结合方面取得了进展。剪切带流体引入了两个额外的褶皱：剪切带引入的新长度尺度，以及与剪切带性质不连续相关的界面不稳定性的可能性。与弹性不稳定性相反，界面不稳定性是通过增加曲率来稳定的。因此，TC问题提供了一个平台，可以系统地分离这三种相互竞争的不稳定模式的影响，可以理解曲率、运动学和流体参数对模式的影响，也可以理解刚性与自由边界条件的作用。更广泛的影响：该研究的更广泛的影响在于剪切带材料的加工，这与一系列工业上重要的系统有关，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，约6名本科生将被招募参与本研究。这项研究也将被整合到伯克利大学关于输运现象和复杂流体的研究生课程中。

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.