[论文解读] Simultaneous measurement of extensional stress and flow birefringence field for uniaxially extending worm-like micellar solutions

Masakazu Muto, Tatsuya Yoshino+1 more|arXiv (Cornell University)|Apr 26, 2024

Surfactants and Colloidal SystemsChemistry被引用 3

一句话总结

本研究提出了一种新颖的流变-光学技术，结合液滴滴落法与高速偏振相机，实现了对一维拉伸状态下蠕虫状胶束溶液中拉伸应力与流动双折射的同步测量。该方法可实现实时可视化胶束取向，并证实了在拉伸与剪切流场下应力-光学系数具有可比性，且符合线性应力-光学定律。

ABSTRACT

The present study proposes a novel and simple rheo-optical technique to investigate the relation between the rheology of complex fluids and their internal structural deformation under uniaxial extensional flow. The macroscale results of viscoelasticity from rheological measurements and microscale results of birefringence from optical measurements are combined to evaluate the microstructural deformation and orientation state inside the fluids under extensional stress. The proposed technique combines a liquid dripping method with a high-speed polarization camera to measure the extensional stress and flow-induced birefringence field simultaneously. In the liquid dripping method, temporal evolution images of the liquid filament diameter for fluids dripping from a nozzle are measured to obtain the extensional stress loading on the liquid filament. These images are captured with a high-speed polarization camera connected to a micro polarization element alley, enabling high-speed imaging of the birefringent field. Worm-like micellar solutions of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) with varying concentrations of CTAB and NaSal are employed as the measurement targets. Consequently, we successfully visualized temporally developing images of the birefringence field of uniaxially extending worm-like micellar solutions induced by the orientation of micelles toward the extensional direction. Furthermore, the proposed technique supports investigating the conditions for establishing the stress-optical rule, which is the linear relation between stress and birefringence for complex fluids. The stress-optical coefficient, a proportionality constant indicating the sensitivity of birefringence to stress, is analyzed from these measurements. The stress-optical coefficient under uniaxial extensional flow is confirmed to be comparable to that under shear flow.

研究动机与目标

• 开发一种简单、高分辨率的流变-光学技术，用于同步测量复杂流体中宏观拉伸应力与微观流动双折射。
• 研究蠕虫状胶束在单轴拉伸流场下的结构形变与取向状态。
• 通过测量应力-光学系数，验证单轴拉伸流场下的应力-光学定律。
• 研究CTAB/NaSal溶液中应力-光学系数对胶束浓度的依赖关系。

提出的方法

• 采用液滴滴落法，通过喷嘴滴落生成蠕虫状胶束溶液中的单轴拉伸流场。
• 利用高速成像技术捕捉液丝直径随时间的演化，以表征拉伸应力的加载过程。
• 采用配备微型偏振元件阵列的高速偏振相机，实现对流动诱导双折射场的实时、高空间分辨率成像。
• 通过在液丝最小直径处提取光强的线性剖面，避免曲率引起的散射，从而获得相位延迟与取向角。
• 基于测得的拉伸应力与双折射之间的线性关系（δn = C(σ∥ − σ⊥)），计算应力-光学系数。
• 测试不同CTAB与NaSal浓度的溶液，以研究应力-光学系数对浓度的依赖性。

Figure 1: Birefringence induced by a change in the orientation state of worm-like micelles under stress loading. Without stress loading, the worm-like micelles in the solutions have random orientations in the coil state; thus, flow-induced birefringence does not occur. When applying stress to the so

实验结果

研究问题

• RQ1 能否以高时间与空间分辨率，在单轴拉伸的蠕虫状胶束溶液中同步测量拉伸应力与流动双折射？
• RQ2 应力-光学定律在单轴拉伸流场下是否成立？该条件下的应力-光学系数数值是多少？
• RQ3 CTAB/NaSal溶液在单轴拉伸与剪切流场下的应力-光学系数有何差异？
• RQ4 应力-光学系数如何随溶液中蠕虫状胶束浓度变化？

主要发现

• 所提出的实验方法成功实现了对单轴拉伸蠕虫状胶束溶液中随时间演化的双折射场的可视化，揭示了胶束沿拉伸方向的取向行为。
• 观察到拉伸应力与双折射之间存在线性关系，证实了在单轴拉伸流场下应力-光学定律的有效性。
• 单轴拉伸流场下的应力-光学系数在数值上与剪切流场下的系数相当，表明双折射对应力的敏感性在不同流场类型间保持一致。
• CTAB/NaSal溶液的应力-光学系数随CTAB浓度变化，表明其与溶液中蠕虫状胶束总量存在依赖关系。
• 随着CTAB/NaSal浓度升高，液丝断裂时间延长，与结构完整性的增强及更长的应力加载时间相关。
• 通过按放大倍数优化分析区域，选择光强分布恒定的区域，有效减小曲率引起的误差，确保测量准确性。

Figure 2: Conditions of index ellipsoids under (a) shear and (b) uniaxial extensional flows. Here, the major axis ( $n_{\perp}$ ) and minor axis ( $n_{\parallel}$ ) of refractive indices in the index ellipsoid correspond to $n_{y}$ and $n_{x}$ , respectively. $x,y,z$ denote the coordinate system bas