Flow behaviour of entangled surfactant micelles
Flow behaviour of entangled surfactant micelles
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DOI：
10.1088/0953-8984/8/47/006
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发表时间：
1996-11-18
影响因子：
2.7
通讯作者：
Cates, ME
中科院分区：
文献类型：
--
作者：
Cates, ME
许多粘弹性表面活性剂溶液含有巨大的自组装胶束。这些可以被描述为‘活性聚合物’，其链受到可逆的断裂和重组的影响。它们在纠缠区的动力学相应地从传统高分子链的翻转图进行了修正。对于快速断裂动力学，线性粘弹性光谱接近单指数(Maxwell)行为：可以测量与此的微小偏差，并且用于推导关于胶束动力学(典型胶束破裂前的寿命)和结构(平均胶束长度)的信息的模型。这些想法适用于几个系统，但对于其他系统，这些数量的不合理趋势被发现。最可能的原因是胶束分支效应(就旋转反应模型而言)，它引入了等于分支点之间平均距离的有效胶束长度。另一个可能的差异来自于胶束反应的平均持有平均值的崩溃。旋转-反应模型得到了一个非线性本构方程，在简单的稳态剪切下，应力对应变率的非单调依赖关系。这导致了预期的流动不稳定性，并(在进一步的假设下)表明应该出现稳定的剪切带方式，其中共存着不同剪切速率的宏观流体层。一些实验观察支持这一总体情况，尽管同样的不稳定性可能会导致壁面滑移或非定常流动。
Many viscoelastic surfactant solutions contain giant, self-assembled micelles. These can be described as 'living polymers', whose chains are subject to reversible scission and recombination. Their dynamics in the entangled regime is accordingly modified from the reptation picture for conventional polymer chains. For rapid scission kinetics, the linear viscoelastic spectrum approaches a single-exponential (Maxwell) behaviour: small departures from this can be measured, and the model used to deduce information both on the micellar kinetics (the lifetime of a typical micelle before breaking) and on the structure (the mean micelle length). These ideas work for several systems, but for others, unreasonable trends for these quantities are found. The most likely reason for this is micellar branching effects, which (as far as the reptation-reaction model is concerned) introduce an effective micellar length equal to the mean distance between branch points. Another possible discrepancy comes from the breakdown of mean-held averaging for the micellar reactions. The reptation-reaction model yields a non-linear constitutive equation which shows a non-monotonic dependence of stress on strain rate, in simple steady shear. This leads one to expect flow instabilities, and (with further assumptions) suggests that steady shear-banded hows should arise, in which macroscopic layers of fluid of different shear rates coexist. Several experimental observations support this general picture, although the same instability could instead lead to wall slip, or unsteady flows.