Wallenberg Wood Science Center, Royal Institute of Technology, SE-100 44 Stockholm, Sweden.
DESY, Notkestrasse 85, Hamburg, Germany and Department of Fibre and Polymer Technology, Royal Institute of Technology, Teknikringen 56-58, SE-100 44 Stockholm, Sweden.
Soft Matter. 2020 Jun 17;16(23):5439-5449. doi: 10.1039/c9sm01975h.
Hydrodynamic alignment of proteinaceous or polymeric nanofibrillar building blocks can be utilized for subsequent assembly into intricate three-dimensional macrostructures. The non-equilibrium structure of flowing nanofibrils relies on a complex balance between the imposed flow-field, colloidal interactions and Brownian motion. The understanding of the impact of non-equilibrium dynamics is not only weak, but is also required for structural control. Investigation of underlying dynamics imposed by the flow requires in situ dynamic characterization and is limited by the time-resolution of existing characterization methods, specifically on the nanoscale. Here, we present and demonstrate a flow-stop technique, using polarized optical microscopy (POM) to quantify the anisotropic orientation and diffusivity of nanofibrils in shear and extensional flows. Microscopy results are combined with small-angle X-ray scattering (SAXS) measurements to estimate the orientation of nanofibrils in motion and simultaneous structural changes in a loose network. Diffusivity of polydisperse systems is observed to act on multiple timescales, which is interpreted as an effect of apparent fibril lengths that also include nanoscale entanglements. The origin of the fastest diffusivity is correlated to the strength of velocity gradients, independent of type of deformation (shear or extension). Fibrils in extensional flow results in highly anisotropic systems enhancing interfibrillar contacts, which is evident through a slowing down of diffusive timescales. Our results strongly emphasize the need for careful design of fluidic microsystems for assembling fibrillar building blocks into high-performance macrostructures relying on improved understanding of nanoscale physics.
水动力排列的蛋白质或聚合纳米纤维建筑模块可以用于随后组装成复杂的三维宏观结构。流动纳米纤维的非平衡结构依赖于复杂的平衡之间的强制流场、胶体相互作用和布朗运动。非平衡动力学的影响的理解不仅薄弱，而且还需要结构控制。对流动所施加的基础动力学的研究需要原位动态特性，并受现有特性化方法的时间分辨率限制，特别是在纳米尺度上。在这里，我们提出并展示了一种流动停止技术，使用偏光显微镜（POM）来量化剪切和拉伸流中纳米纤维的各向异性取向和扩散率。显微镜结果与小角 X 射线散射（SAXS）测量相结合，以估计运动中的纳米纤维的取向和松散网络中的同时结构变化。观察到多分散体系的扩散率在多个时间尺度上起作用，这被解释为表观纤维长度的影响，其中还包括纳米尺度的缠结。最快扩散率的起源与速度梯度的强度相关，与变形类型（剪切或拉伸）无关。在拉伸流中，纤维导致各向异性系统增强了纤维间的接触，这通过扩散时间尺度的减慢而明显体现出来。我们的结果强烈强调需要仔细设计流体微系统，以便将纤维状建筑模块组装成依赖于对纳米尺度物理的改进理解的高性能宏观结构。
