The thixotropy of cellulose nanocrystal (CNC) water suspensions is intrinsically dependent on the hierarchical structure of the suspension. The diverse hierarchies that comprise individual CNC nanoparticles and mesophase liquid crystalline domains, chiral nematic and nematic structures, contribute selectively to the rheological material response. Here, we combine rheology with polarized light imaging (PLI) to elucidate the thixotropic behavior of CNCs suspended in water. The simultaneous monitoring of PLI and rheological tests enables the observation of mesogens and their orientation dynamics. Creep, dynamic time sweep, ramped hysteresis loop, and thixotropic recovery tests combined with PLI aim to differentiate the contribution of the different hierarchical levels of CNC suspensions to their thixotropy. The range of concentrations investigated comprised biphasic (4 and 5 wt. %) and liquid crystalline phase suspensions (6, 7, and 8 wt. %). The CNC suspensions exhibited complex thixotropy behavior, such as viscosity bifurcations in creep tests and overshoot in ramped hysteresis loop tests. The restructuring and destructuring appeared to correspond to different levels of their hierarchical structure, depending mainly on the phase, in agreement with previous studies. Restructuring was attributed to re-organizations of an individual CNC, e.g., in the isotropic fraction of biphasic suspensions and at the mesogen interfaces in liquid crystalline phase suspensions. However, by increasing liquid crystalline fraction in the biphasic concentrations, restructuring could also involve mesogens, as indicated in the creep tests. For flow conditions above the yield stress, as evidenced by the ramped hysteresis and thixotropy recovery tests, destructuring was dominated by orientation in the flow direction, a process that is readily observable in the form of PLI “Maltese-cross” patterns. Finally, we show that a simple thixotropy model, while unable to capture the finer details of the suspension’s thixotropic behavior, could be employed to predict general features thereof.

中文翻译：

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纤维素纳米晶悬浮液的触变性

纤维素纳米晶 (CNC) 水悬浮液的触变性本质上取决于悬浮液的分级结构。包括单个 CNC 纳米粒子和中间相液晶域、手性向列和向列结构在内的不同层次结构对流变材料响应有选择性。在这里，我们将流变学与偏振光成像 (PLI) 相结合，以阐明悬浮在水中的 CNC 的触变行为。PLI 和流变测试的同时监测可以观察介晶及其取向动力学。蠕变、动态时间扫描、斜坡滞后回线和触变恢复测试与 PLI 相结合，旨在区分 CNC 悬架的不同层次级别对其触变性的贡献。研究的浓度范围包括双相（4 和 5 重量%）和液晶相悬浮液（6、7 和 8 重量%）。CNC 悬浮液表现出复杂的触变性，例如蠕变试验中的粘度分叉和斜坡滞后环试验中的过冲。重组和解构似乎对应于其层次结构的不同级别，主要取决于阶段，与先前的研究一致。重组归因于单个 CNC 的重组，例如，在双相悬浮液的各向同性部分和液晶相悬浮液的介晶界面处。然而，通过增加双相浓度中的液晶分数，重组也可能涉及介晶，如蠕变测试所示。对于高于屈服应力的流动条件，如斜坡滞后和触变性恢复测试所证明的那样，解构主要由流动方向的取向决定，这一过程很容易以 PLI“马耳他十字”图案的形式观察到。最后，我们展示了一个简单的触变模型，虽然无法捕捉悬浮液触变行为的更精细细节，但可以用来预测其一般特征。