Advanced rheological studies of glass-forming colloidal dispersions: Combining experiment and theory

胶体分散体提供了广泛的技术应用，从化妆品到食品科学到建筑材料。了解它们在外加强场下的力学行为对于发展基础理论以及最终产品的加工和应用条件是至关重要的。虽然胶体溶液在均质剪切流中的流动特性在基于颗粒的理论中得到了很好的理解，但到目前为止，对非均匀、边界驱动、非剪切或混合流动的方法仍然在很大程度上是唯象的。近年来，液体结构动力学的微观模式耦合理论(MCT)已被推广到均相流的本构形式，使其能够预测散体浓缩悬浮液的流动控制非线性流变性。在这个项目中，我们的目标是将MCT推广到包括应力控制和非均匀流在内的强非简单流。我们将结合MCT的推广和对模型胶体悬浮液(由温度敏感的PNIPAM微凝胶组成)的高级流变学研究，以获得一套一致的测量结果。为了实现这一目标，开发或改进先进的流变特性将是该项目的一部分，例如，有意提高在剪切作用下确定所产生的法向力的灵敏度。具体来说，我们将在合作项目中研究以下主题：1.拉伸流和正应力的测量将使我们能够测试MCT的应力-应变关系的张量性质，而不仅仅是简单的剪切流动。2.研究应变和应力控制下的大振幅振荡剪切、傅里叶变换流变学和高次谐波的测量，以了解迟滞和亚稳态的磁滞现象。变形模式的叠加将提供对流动驱动的流态化、与历史相关的力学性能或残余应力的更多洞察。毛细管流的限制将提供对流动下的空间相关性、非均质性和不稳定性(如剪切带)的洞察。同样，将详细研究对准备历史的依赖。在整个过程中，将发展一个统一的理论方法，并在胶体模型系统上进行实验验证。这样就会产生一致的基本描述，为合理设计浓缩复杂分散体的加工工艺提供依据。

Colloidal dispersions offer a wide range of technological applications ranging from cosmetics over food science to building materials. Understanding their mechanical behaviour under applied strong fields is crucial for developing a fundamental theory as well as for processing and the application conditions of the final products. While the flow properties of colloid solutions in homogeneous shear flows are rather well understood in particle based theories, approaches to inhomogeneous, boundary-driven, non-shear or mixed flows remain largely phenomenological up to now. Especially important are high concentrations, where viscoelastic effects dramatically change the dispersion properties.Recently, the microscopic mode coupling theory (MCT) of the structural dynamics in liquids has been extended to a constitutive form for homogeneous flows, enabling it to predict the flow-controlled nonlinear rheology of bulk concentrated suspensions. In this project, we aim to generalize MCT to strong non-simple flows including stress-controlled and inhomogeneous ones. We will combine the generalization of MCT with advanced rheological studies on model colloidal suspensions (consisting of thermosensitive PNiPAM microgels) to obtain a consistent set of measurements. To achieve this goal, the development or improvement of advanced rheological characterizations will be part of the project, e.g. an intended increase in the sensitivity for the determination of resulting normal forces under shear. In detail, the following topics will be studied in our cooperation project:1. Extensional flows and measurements of normal stresses will enable us to test the tensorial nature of stress-strain relations from MCT beyond simple shear flows. 2. Large amplitude oscillatory shearing, Fourier transform rheology and measurements of the higher harmonics will be investigated under strain and stress control in order to understand hysteresis in sluggish and metastable states. Superposition of deformation modes will provide additional insights into flow-driven fluidization, history dependent mechanical properties or residual stresses.3. Confinement in capillary flows will provide insights into spatial correlations, heterogeneities and instabilities (like shear-banding) under flow. Again, the dependence on preparation history will be studied in detail. Throughout, a unified theoretical approach will be developed and verified by experiment on the colloidal model system. Thus a consistent fundamental description will emerge, which can serve as basis for a rational design of processing technologies of concentrated complex dispersions.