Håkansson Karl M O, Lundell Fredrik, Prahl-Wittberg Lisa, Söderberg L Daniel
Wallenberg Wood Science Center and ‡Linné FLOW Centre, Department of Mechanics, KTH Royal Institute of Technology , SE-100 44 Stockholm, Sweden.
J Phys Chem B. 2016 Jul 14;120(27):6674-86. doi: 10.1021/acs.jpcb.6b02972. Epub 2016 Jun 30.
Alignment of anisotropic supermolecular building blocks is crucial to control the properties of many novel materials. In this study, the alignment process of cellulose nanofibrils (CNFs) in a flow-focusing channel has been investigated using small-angle X-ray scattering (SAXS) and modeled using the Smoluchowski equation, which requires a known flow field as input. This flow field was investigated experimentally using microparticle-tracking velocimetry and by numerically applying the two-fluid level set method. A semidilute dispersion of CNFs was modeled as a continuous phase, with a higher viscosity as compared to that of water. Furthermore, implementation of the Smoluchowski equation also needed the rotational Brownian diffusion coefficient, which was experimentally determined in a shear viscosity measurement. The order of the nanofibrils was found to increase during extension in the flow-focusing channel, after which rotational diffusion acted on the orientation distribution, driving the orientation of the fibrils toward isotropy. The main features of the alignment and dealignment processes were well predicted by the numerical model, but the model overpredicted the alignment at higher rates of extension. The apparent rotational diffusion coefficient was seen to increase steeply as the degree of alignment increased. Thus, the combination of SAXS measurements and modeling provides the necessary framework for quantified studies of hydrodynamic alignment, followed by relaxation toward isotropy.
各向异性超分子结构单元的排列对于控制许多新型材料的性能至关重要。在本研究中，利用小角X射线散射（SAXS）研究了纤维素纳米原纤维（CNFs）在流动聚焦通道中的排列过程，并使用Smoluchowski方程进行建模，该方程需要已知流场作为输入。使用微粒跟踪测速技术并通过数值应用双流体水平集方法对该流场进行了实验研究。将CNFs的半稀释分散体建模为连续相，其粘度比水高。此外，实施Smoluchowski方程还需要旋转布朗扩散系数，该系数是在剪切粘度测量中通过实验确定的。发现在流动聚焦通道中延伸过程中纳米原纤维的有序度增加，之后旋转扩散作用于取向分布，使原纤维的取向趋于各向同性。数值模型很好地预测了排列和去排列过程的主要特征，但该模型在较高延伸率下对排列的预测过高。随着排列程度的增加，表观旋转扩散系数急剧增加。因此，SAXS测量和建模的结合为流体动力学排列的定量研究以及随后向各向同性弛豫提供了必要的框架。
