Westermeier Fabian, Pennicard David, Hirsemann Helmut, Wagner Ulrich H, Rau Christoph, Graafsma Heinz, Schall Peter, Lettinga M Paul, Struth Bernd
Max Planck Institute for the Structure and Dynamics of Matter, CFEL, Luruper Chaussee 149, 22761 Hamburg, Germany.
Center for Free-Electron Laser Science, DESY, Notkestrasse 85, 22607 Hamburg, Germany.
Soft Matter. 2016 Jan 7;12(1):171-80. doi: 10.1039/c5sm01707f.
Structural distortion and relaxation are central to any liquid flow. Their full understanding requires simultaneous probing of the mechanical as well as structural and dynamical response. We provide the first full dynamical measurement of the transient structure using combined coherent X-ray scattering and rheology on electrostatically interacting colloidal fluids. We find a stress overshoot during the start-up of shear which is due to the strong anisotropic overstretching and compression of nearest-neighbor distances. The rheological response is reflected in uncorrelated entropy-driven intensity fluctuations. While the structural distortion under steady shear is well described by Smoluchowski theory, we find an increase of the particle dynamics beyond the trivial contribution of flow. After the cessation of shear, the full fluid microstructure and dynamics are restored, both on the structural relaxation timescale. We thus find unique structure-dynamics relations in liquid flow, responsible for the macroscopic rheological behavior of the system.
结构畸变和弛豫是任何液体流动的核心。要全面理解它们，需要同时探测力学以及结构和动力学响应。我们首次使用相干X射线散射和流变学相结合的方法，对静电相互作用的胶体流体的瞬态结构进行了完整的动力学测量。我们发现在剪切启动过程中存在应力过冲，这是由于最近邻距离的强烈各向异性过度拉伸和压缩所致。流变响应反映在不相关的熵驱动强度涨落中。虽然稳态剪切下的结构畸变可以用斯莫卢霍夫斯基理论很好地描述，但我们发现粒子动力学的增加超出了流动的平凡贡献。在剪切停止后，完整的流体微观结构和动力学在结构弛豫时间尺度上都得以恢复。因此，我们在液体流动中发现了独特的结构-动力学关系，这决定了系统的宏观流变行为。
