Esmaeili Mohsen, George Kyle, Rezvan Gelareh, Taheri-Qazvini Nader, Zhang Rui, Sadati Monirosadat
Department of Chemical Engineering, University of South Carolina, Columbia, South Carolina 29208, United States.
Biomedical Engineering Program, University of South Carolina, Columbia, South Carolina 29208, United States.
Langmuir. 2022 Feb 22;38(7):2192-2204. doi: 10.1021/acs.langmuir.1c01881. Epub 2022 Feb 8.
Studying the flow-induced alignment of anisotropic liquid crystalline materials is of major importance in the 3D printing of advanced architectures. However, in situ characterization and quantitative measurements of local orientations during the 3D printing process are challenging. Here, we report a microfluidic strategy integrated with polarized optical microscopy (POM) to perform the in situ characterization of the alignment of cellulose nanocrystals (CNCs) under the shear-flow condition of the 3D printer's nozzle in the direct ink writing process. To quantify the alignment, we exploited birefringence measurements under white and monochromatic light. We show that the flow-induced birefringence patterns are significantly influenced by the initial structure of the aqueous CNC suspensions. Depending on the CNC concentration and sonication treatment, various structures can form in the CNC suspensions, such as isotropic, chiral nematic (cholesteric), and nematic (gel-like) structures. In the chiral nematic phase, in particular, the shear flow in the microfluidic capillary has a distinct effect on the alignment of the CNC particles. Our experimental results, complemented by hydrodynamic simulations, reveal that at high flow rates (Er ≈ 1000), individual CNC particles align with the flow exhibiting a weak chiral structure. In contrast, at lower flow rates (Er ≈ 241), they display the double-twisted cylinder structure. Understanding the flow effect on the alignment of the chiral liquid crystal can pave the way to designing 3D printed architectures with internal chirality for advanced mechanical and smart photonic applications.
研究各向异性液晶材料的流动诱导排列对于先进结构的3D打印至关重要。然而，在3D打印过程中对局部取向进行原位表征和定量测量具有挑战性。在此，我们报告了一种与偏振光学显微镜（POM）集成的微流体策略，以在直接墨水书写过程中3D打印机喷嘴的剪切流条件下对纤维素纳米晶体（CNC）的排列进行原位表征。为了量化排列，我们利用了白光和单色光下的双折射测量。我们表明，流动诱导的双折射图案受CNC水性悬浮液的初始结构显著影响。根据CNC浓度和超声处理，CNC悬浮液中可以形成各种结构，如各向同性、手性向列（胆甾相）和向列（凝胶状）结构。特别是在手性向列相中，微流体毛细管中的剪切流对CNC颗粒的排列有明显影响。我们的实验结果，辅以流体动力学模拟，表明在高流速（Er≈1000）下，单个CNC颗粒与流动方向对齐，呈现出弱手性结构。相反，在较低流速（Er≈241）下，它们呈现出双扭曲圆柱结构。了解流动对手性液晶排列的影响可为设计具有内部手性的3D打印结构以用于先进的机械和智能光子应用铺平道路。
