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

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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.