Mechanical Engineering Department, University of Washington, Seattle, WA 98195, USA.
Proc Natl Acad Sci U S A. 2013 Apr 30;110(18):E1653-60. doi: 10.1073/pnas.1215353110. Epub 2013 Apr 8.
Surfactant molecules can self-assemble into various morphologies under proper combinations of ionic strength, temperature, and flow conditions. At equilibrium, wormlike micelles can transition from entangled to branched and multiconnected structures with increasing salt concentration. Under certain flow conditions, micellar structural transitions follow different trajectories. In this work, we consider the flow of two semidilute wormlike micellar solutions through microposts, focusing on their microstructural and rheological evolutions. Both solutions contain cetyltrimethylammonium bromide and sodium salicylate. One is weakly viscoelastic and shear thickening, whereas the other is strongly viscoelastic and shear thinning. When subjected to strain rates of ∼10(3) s(-1) and strains of ∼10(3), we observe the formation of a stable flow-induced structured phase (FISP), with entangled, branched, and multiconnected micellar bundles, as evidenced by electron microscopy. The high stretching and flow alignment in the microposts enhance the flexibility and lower the bending modulus of the wormlike micelles. As flexible micelles flow through the microposts, it becomes energetically favorable to minimize the number of end caps while concurrently promoting the formation of cross-links. The presence of spatial confinement and extensional flow also enhances entropic fluctuations, lowering the energy barrier between states, thus increasing transition frequencies between states and enabling FISP formation. Whereas the rheological properties (zero-shear viscosity, plateau modulus, and stress relaxation time) of the shear-thickening precursor are smaller than those of the FISP, those of the shear-thinning precursor are several times larger than those of the FISP. This rheological property variation stems from differences in the structural evolution from the precursor to the FISP.
表面活性剂分子在适当的离子强度、温度和流动条件组合下可以自组装成各种形态。在平衡状态下，随着盐浓度的增加，蠕虫状胶束可以从缠结状态转变为支化和多连通结构。在某些流动条件下，胶束结构转变遵循不同的轨迹。在这项工作中，我们考虑了两种半稀蠕虫状胶束溶液通过微柱的流动，重点研究它们的微观结构和流变演变。两种溶液都含有十六烷基三甲基溴化铵和水杨酸钠。一种溶液具有弱的粘弹性和剪切增稠性，而另一种溶液具有强的粘弹性和剪切变稀性。当受到约 10(3) s(-1)的应变速率和约 10(3)的应变量时，我们观察到稳定的流动诱导结构化相（FISP）的形成，其特征是缠结、支化和多连通的胶束束，这可以通过电子显微镜得到证明。在微柱中高的拉伸和流动排列增强了蠕虫状胶束的柔韧性并降低了其弯曲模量。当柔性胶束流过微柱时，最小化端盖数量同时促进交联形成变得具有能量优势。空间限制和拉伸流动的存在也增强了熵涨落，降低了状态之间的能量势垒，从而增加了状态之间的转变频率并促进了 FISP 的形成。虽然剪切增稠前体的流变性质（零剪切黏度、平台模量和应力松弛时间）小于 FISP，但剪切变稀前体的流变性质比 FISP 大几个数量级。这种流变性质的变化源于从前体到 FISP 的结构演变的差异。
