Coupled nonhomogeneous flows and flow-enhanced concentration fluctuations during startup shear of entangled polymer solutions
Coupled nonhomogeneous flows and flow-enhanced concentration fluctuations during startup shear of entangled polymer solutions
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DOI：
10.1103/physrevfluids.5.043301
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发表时间：
2020-04
影响因子：
2.7
通讯作者：
Michael C. Burroughs;A. Shetty;L. Leal;M. Helgeson
中科院分区：
文献类型：
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作者：
Michael C. Burroughs;A. Shetty;L. Leal;M. Helgeson
在简单剪切流动中，纠缠聚合物溶液的流动行为通常被认为是均匀的，无论是速度梯度还是聚合物浓度。然而，越来越多的证据表明，缠绕的聚合物溶液存在非均匀的“带状”瞬变或稳态流动。目前的工作考虑了一个明显的现象，即瞬时带状伴随着局部剪切增强浓度涨落的存在。采用流变学、粒子跟踪测速仪和显微镜(流变学)相结合的方法，对聚苯乙烯在边缘溶剂邻苯二甲酸二辛酯中缠绕的聚合物溶液在不同缠结数()下的Taylor-Couette流动启动剪切过程进行了实验观察。当流速较高时，水流在较宽的附加剪切率范围内形成瞬变的、非均匀的“带状”速度剖面，这与Taylor-Couette流的预期剪切率相反，然后在稳态时松弛到几乎均匀的剪切率。流变显微镜显示，这些瞬时带状态伴随着强烈的剪切增强浓度波动，集中在局部最低剪切速率区域。我们假设，这种有限幅度的浓度波动会导致耗散增加，从而导致更高的局部“有效”流体粘度，进而可能产生观察到的流动不均匀。尽管流体的流动和浓度波动是不均匀的，但测量到的流体的表观流变性与先前的实验研究和基于Rolie-Poly模型的模型预测在性质上是一致的，这表明只有通过对流体流量和浓度的空间分辨测量才能观察到这种行为。
The flow behavior of entangled polymer solutions in simple shear flow is often assumed to be uniform, both in terms of the velocity gradient and polymer concentration. However, there is growing evidence of nonuniform “banded” transient or steady state flows of entangled polymer solutions. The present work considers a distinct phenomenon, whereby transient banding is concomitant with the presence of local shear-enhanced concentration fluctuations. Experimental observations are made using combined rheological measurements with simultaneous particle tracking velocimetry and microscopy (rheomicroscopy) during startup shear in Taylor-Couette flow of entangled polymer solutions of polystyrene in a marginal solvent, dioctyl phthalate, at various entanglement numbers (). At high, the flow develops transient, nonhomogeneous “banded” velocity profiles over a wide range of imposed shear rates that are inverted from those expected for Taylor-Couette flow, which then relax to a nearly uniform shear rate at steady state. Rheomicroscopy reveals that these transient banded states are accompanied by strong shear-enhanced concentration fluctuations localized to the region of lowest local shear rate. We hypothesize that such finite-amplitude concentration fluctuations lead to increased dissipation that contributes a higher local “effective” fluid viscosity that, in turn, could produce the observed flow nonuniformity. In spite of the nonuniformity of the flow and the concentration fluctuations, the measured apparent rheology of the fluids is in qualitative agreement with both prior experimental studies and model predictions based on a Rolie-Poly model under the assumption that the fluid remains homogeneous, suggesting that this behavior is only observable through spatially resolved measurements of fluid flow and concentration.