Instabilities in stagnation point flows of polymer solutions
Instabilities in stagnation point flows of polymer solutions
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DOI：
10.1063/1.4818151
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发表时间：
2013-08-01
影响因子：
4.6
通讯作者：
McKinley, G. H.
中科院分区：
文献类型：
--
作者：
Haward, S. J.;McKinley, G. H.
一种新型的微流控装置——优化形状交叉槽拉伸流变仪[S]。J. Haward， M. S. N. Oliveira， M. A. Alves和G. H. McKinley，“微流体扩展流变学的优化交叉槽流动几何”，物理学报。[j]，研究了粘弹性聚合物溶液在理想平面滞止点流动中的稳定性。由不同分子量和浓度的聚（环氧乙烷）和透明质酸组成的聚合物水溶液的配制是为了提供具有广泛流变特性的流体。高分子量聚合物的半稀溶液提供了具有长松弛时间的高粘弹性流体，在雷诺数（Re）保持较低的流速下实现高魏森伯格数（Wi）；因此弹性系数El = Wi/Re较高。中等分子量聚合物的低浓度溶液只能提供弱粘弹性流体，其中惯性仍然很重要，El相对较低。当通过OSCER装置的体积流量稳步增加时，流动双折射观测用于可视化流体中流动不稳定性的性质。在低Wi和低Re条件下，所有流体都呈现出稳定、对称、均匀的“双折射链”，高度定向的聚合物分子沿着测试几何形状的流出对称轴排列，表明在这种条件下流场的稳定性。在El bbbb1流体中，我们观察到超过临界Weissenberg数Wicri t的稳定弹性流动不对称性，其特征与标准交叉槽几何中已经报道的相似[e]。g， P. E. Arratia， C. C. Thomas， J. Diorio和J. P. Gollub，“跨通道流动中聚合物溶液的弹性不稳定性”，《物理》杂志。[j].科学通报，2006(5)。然而，在El < 1的流体中，我们观察到一系列时间相关的惯性弹性不稳定性，超过临界雷诺数Recri t，其特征是双折射链的高频时空振荡。通过绘制Wi-Re操作空间中各种流体的临界稳定性极限，我们能够构建一个稳定性图，描绘出理想平面伸长流动装置中不同的稳定对称、稳定不对称和惯性弹性流动形式。(C) 2013 AIP出版有限责任公司
A recently developed microfluidic device, the optimized shape cross-slot extensional rheometer or OSCER [S. J. Haward, M. S. N. Oliveira, M. A. Alves, and G. H. McKinley, "Optimized cross-slot flow geometry for microfluidic extensional rheometry," Phys. Rev. Lett. 109, 128301 (2012)], is used to investigate the stability of viscoelastic polymer solutions in an idealized planar stagnation point flow. Aqueous polymer solutions, consisting of poly(ethylene oxide) and of hyaluronic acid with various molecular weights and concentrations, are formulated in order to provide fluids with a wide range of rheological properties. Semi-dilute solutions of high molecular weight polymers provide highly viscoelastic fluids with long relaxation times, which achieve a high Weissenberg number (Wi) at flow rates for which the Reynolds number (Re) remains low; hence the elasticity number El = Wi/Re is high. Lower concentration solutions of moderate molecular weight polymers provide only weakly viscoelastic fluids in which inertia remains important and El is relatively low. Flow birefringence observations are used to visualize the nature of flow instabilities in the fluids as the volumetric flow rate through the OSCER device is steadily incremented. At low Wi and Re, all of the fluids display a steady, symmetric, and uniform "birefringent strand" of highly oriented polymer molecules aligned along the outflowing symmetry axis of the test geometry, indicating the stability of the flow field under such conditions. In fluids of El > 1, we observe steady elastic flow asymmetries beyond a critical Weissenberg number, Wicri t, that are similar in character to those already reported in standard cross-slot geometries [e. g., P. E. Arratia, C. C. Thomas, J. Diorio, and J. P. Gollub, " Elastic instabilities of polymer solutions in cross-channel flow," Phys. Rev. Lett. 96, 144502 (2006)]. However, in fluids with El < 1 we observe a sequence of time-dependent inertio-elastic instabilities beyond a critical Reynolds number, Recri t, characterized by high frequency spatiotemporal oscillations of the birefringent strand. By plotting the critical limits of stability for the various fluids in the Wi-Re operating space, we are able to construct a stability diagram delineating the distinct steady symmetric, steady asymmetric and inertio-elastic flow regimes in this idealized planar elongational flow device. (C) 2013 AIP Publishing LLC.