Systematic Variation of the Rheology of Wormlike Micelles - Experiment and Theoretical Description

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Aim of this project is a substantial improvement of our understanding of the flow behaviour of rod-like (worm-like) micelles, and to explain their rheological properties, especially the occurrence of spatio-temporal instabilities, on the basis of the molecular composition of the systems. For that purpose comprehensive experimental rheological investigations will be done on surfactant systems with a systematic variation of the molecular structure. The experimental results will become compared to newly developed theoretical models. The flow behaviour is determined by structure and dynamics of the mesoscopic aggregates and these properties are in turn controlled by the constitution of the amphiphilic molecules and will further be modulated in a systematic fashion by addition of cosurfactants and amphiphilic copolymers (where these hybrid systems of hydrophobically modified polyelectrolytes and wormlike micelles are already interesting novel systems by themselves). Structure and dynamics of these systems will be characterised by means of light and neutron scattering (SANS, NSE) as well as electric birefringence (EBR). The rheological behaviour and especially the occurrence of spatio-temporal inhomogeneities will be studied in detail, primarily via optical methods and particle image velocimetry (PIV). The precise determination of the flow field and of the spatio-temporal changes (e. g. formation of shear bands) are in the focus of the investigations. Complementary Rheo-SAXS und –SANS measurements will be done in order to determine orientation and structure on the mesoscopic level. These comprehensive experimental investigations will be compared to the predictions of newly developed theoretical models for the dynamics of macroscopic variables, especially the viscoelastic shear stress in the frame of Fielding-Olmsted model. Especially this direct comparison of experimental parameters and observations with the theoretical description and a subsequent adaption of these theoretical models shall lead to a systematic and direct understanding of these viscoelastic systems. Here the parameters from theory (especially exponents for describing shear-induced changes in the chain length, stress diffusion coefficient) will be put into direct correlation to experimental quantities, which then in turn are coupled to the molecular build-up of the systems. In this way, we will generate a fundamental understanding of the rheological behaviour on the basis of the molecular composition of the viscoelastic systems in a way as it does not yet exist. Such a much deepened understanding of the properties of surfactant-based viscoelastic systems is not only of importance from the point of view of fundamental physico-chemical research, but will also be the basis for improved applications of such systems, which are employed in a large number of formulations, as this project will deliver a sound scientific background based on a molecular understanding.