Schubert Beth A, Wagner Norman J, Kaler Eric W, Raghavan Srinivasa R
Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716, USA.
Langmuir. 2004 Apr 27;20(9):3564-73. doi: 10.1021/la035810d.
Polymer solutions in the vicinity of the theta-point are known to undergo shear-induced turbidity or phase separation. The present study shows that a similar phenomenon also occurs for certain wormlike micellar solutions. Wormlike micelles are the self-assembled counterparts of polymers and are characterized by their ability to reversibly break and recombine. In the system of interest, the micelles are formed by the cationic surfactant erucyl bis(hydroxyethyl)methylammonium chloride (EHAC), in conjunction with a salt such as sodium chloride (NaCl) or sodium salicylate (NaSal). Micellar samples that become turbid under shear show evidence of critical concentration fluctuations and may contain predominantly branched micelles. The shear-induced turbidity in these samples correlates with the appearance of flow-dichroism in rheooptic experiments and with an increase in low-q scattering in small-angle light scattering under flow (flow-SALS) experiments. The characteristic "butterfly" pattern, with enhanced scattering in the flow direction and a dark streak perpendicular to the flow direction, is typically observed in flow-SALS. The results suggest that the turbidity is due to a shear-induced growth of concentration fluctuations, which in turn manifests as large anisotropic domains, typically oriented along the vorticity axis.
已知在θ点附近的聚合物溶液会发生剪切诱导的浊度变化或相分离。本研究表明，某些蠕虫状胶束溶液也会出现类似现象。蠕虫状胶束是聚合物的自组装对应物，其特点是能够可逆地断裂和重新结合。在本研究的体系中，胶束由阳离子表面活性剂芥酸双（羟乙基）甲基氯化铵（EHAC）与氯化钠（NaCl）或水杨酸钠（NaSal）等盐形成。在剪切作用下变浑浊的胶束样品显示出临界浓度波动的迹象，并且可能主要包含支化胶束。这些样品中的剪切诱导浊度与流变光学实验中流动二色性的出现以及流动（流动小角光散射，flow-SALS）实验中低q散射的增加相关。在流动小角光散射实验中通常会观察到特征性的“蝴蝶”图案，即流动方向上散射增强，垂直于流动方向有一条暗条纹。结果表明，浊度是由于剪切诱导的浓度波动增长所致，这反过来表现为大的各向异性区域，通常沿涡度轴取向。
