Flow velocity profiles and shear banding onset in a semidilute wormlike micellar system under Couette flow.
Flow velocity profiles and shear banding onset in a semidilute wormlike micellar system under Couette flow.
复制标题
库埃特流下半稀释蠕虫状胶束系统中的流速分布和剪切带起始。
DOI：
10.1021/jp9027996
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发表时间：
2009
影响因子：
3.3
通讯作者：
R. Castillo
中科院分区：
文献类型：
--
作者：
Jorge Delgado;H. Kriegs;R. Castillo
在由十六烷基三甲基溴化铵（CTAB）、水杨酸钠（NaSal）和水组成的蠕虫状胶束溶液中，在R（= [NaSal]/[CTAB]）= 2和R = 4; [CTAB] = 100 mM时，测量了Couette流中的速度分布。配置文件显示，一个胶束的解决方案（R = 2）变得不均匀的流动开始后很长一段时间，即使在非常低的施加剪切速率。然而，轮廓不对应于梯度剪切带的预期，因为流体在靠近移动圆柱体的一个部分中分裂，其中局部平均速度线性地取决于差距位置，并且在第二部分中呈现重要的速度波动。靠近静态圆柱体，有第三个部分，流体不流动;它表现得像一个滑动块。另一方面，在高施加的剪切速率下，前滑动块体流动并呈现线性轮廓。在这里，速度剖面是一致的梯度剪切带。观察到剪切带的出现。两个稳定剪切带被一个薄的稳定界面分开的图象并不总是有效的。可以观察到不均匀的流动，尽管它不能归类为剪切带。此外，可以发现的条件下，随着剪切速率的增加和剪切带出现之前，而不是薄的界面，可以观察到一个波动的中间带。相反，对于R = 4的解决方案，即使在高剪切速率下，流动也不会变得不均匀。在两种情况下，在剪切速率控制下的库埃特池中测量流动曲线，当用移动圆柱体感测应力时，以及当用池的静态圆柱体感测应力时。流动曲线之间的差异可以通过使用两种溶液的速度分布来解释。
Velocity profiles in Couette flow are measured in a wormlike micellar solution made of cetyltrimethylammonium bromide (CTAB), sodium salicylate (NaSal), and water, at R (= [NaSal]/[CTAB]) = 2 and at R = 4; [CTAB] = 100 mM. Velocity profiles were obtained by using a two-incident beam laser Doppler technique. Profiles reveal that one of the micellar solutions (R = 2) becomes heterogeneous a long time after flow inception, even at very low imposed shear rates. However, profiles do not correspond to what is expected for gradient shear banding, because the fluid splits in one section close to the moving cylinder where the local mean velocity depends linearly on the gap position and in a second section presenting important velocity fluctuations. Close to the static cylinder, there is a third section where the fluid does not flow; it behaves like a slipping block. On the other hand, at high imposed shear rates, the former slipping block flows and presents a linear profile. Here, velocity profiles are consistent with gradient shear banding. The onset of shear banding was observed. The picture of two stable shear bands separated by a thin steady interface is not always valid. Inhomogeneous flow could be observed, although it cannot be classified as shear banding. In addition, conditions can be found where, as shear rate is increased and before shear banding appears, instead of a thin interface, a fluctuating intermediate band can be observed. On the contrary, for the R = 4 solution, the flow never becomes heterogeneous even at high shear rates. Flow curves were measured in a Couette cell under shear rate control in two cases, when stress is sensed with the moving cylinder and when it is sensed with the static cylinder of the cell. Differences between the flow curves can be explained by using the velocity profiles for both solutions.