Effect of the second and the third normal stress difference on occurrence of instability in viscoelastic fluid flow

本文研究粘弹性流体流动中，正应力差与流动不稳定性发生条件的关系。特别地，采用双光束光学测量技术，测量了二维Poillille流动池和同心圆柱流动池中法向应力差对流动不稳定性的影响。使用表面活性剂CTAB（十六烷基三甲基溴化铵）的水溶液作为测试流体。添加NaSal（水杨酸钠）作为抗衡离子。CTAB和NaSal分别以0.23mol/L和0.03mol/L溶于蒸馏水中。该样品在室温下形成蠕虫状胶束。用应变流变仪测定了该流体的流动性能，结果表明，该流体在低剪切速率区与单分散聚合物溶液一样，为麦克斯韦型线性粘弹性流体。用同心圆柱形流动池澄清了在启动流动时， ...更多信息 在一定的应变下，流变液表现出流动诱导的结构变化，在较高的剪切速率下，流变液开始失稳。此外，设计并制作了一个由两块玻璃板组成的平行板流动池，用于产生二维Poiffille流。该流动池设计用于同时从两个方向测量流体双折射，并且玻璃板之间的间距可以根据不同的实验条件而改变。完成了流体双折射测量装置的搭建和数据采集软件的编制。当剪切速率和应变分别超过3l/s和25时，流体变得不透明。在此区域内，应力-光学系数随剪切速率的变化而变化，表明发生了流动诱导的结构变化。第二和第三法向应力差与应力光系数有关。因此，流动不稳定性的发生可能受到这些法向应力差的影响。具体情况还在根据实验结果不断研究中。少

The aim of this research is investigating the relation of the normal stress differences to the occurrence condition of the flow instability in viscoelastic fluid flow. Specially, the effect of the normal stress differences on the flow instability in the two-dimensional Poiseuille flow cell and the concentric cylinders flow cell was measured by the double-beams optical measurement technique. The aqueous solution of a surfactant CTAB (Cetyltrimethylammonium bromide) was used as a test fluid. NaSal (Sodium salicylate) is added as a counter ion. CTAB and NaSal were dissolved 0.23mol/l and 0.03mol/l in the distilled water. This sample forms wormlike micelles at a room temperature. The flow property of this fluid was measured by a strain-controlled rheometer and the result showed that this fluid is a Maxwell type linear viscoelastic fluid at low shear rate region as same as a mono-disperse polymer solution. It was clarified by using the concentric cylinder flow cell that at start-up flow thi … More s fluid showed the flow-induced structure change at certain strain and started the flow instability when shear rate was higher. In addition, a parallel plates flow cell made by two glass plates was designed and built up to generate two-dimensional Poiseuille flow. This flow cell is designed to measure the flow birefringence from two directions simultaneously and the space between the glass plates can be changed for various experimental conditions. The flow birefringence measurement device was finished to build and the data acquisition software was also completed. The fluid becomes opaque when shear rate and strain exceed 3 l/s and 25, respectively. In this region, the stress-optic coefficient is changed by shear rate and it means that the flow-induced structure change occurred. The second and the third normal stress differences are related with the stress-optic coefficient. So, the occurrence of the flow instability might be affected by these normal stress differences. The details are investigating by this experimental result continuously. Less