Extensional rheology and flow-induced structure formation of wormlike micelle solutions

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Solutions of wormlike micelles (WLM) are of great technical interest but also serve as model systems to study fundamental issues of polymer physics. The relationship between structure, micelle dynamics and flow behavior of such solutions is of major importance. The linear viscoelastic properties of WLM solutions are now well understood and research focuses on non-linear flow properties where our knowledge is still very limited. In particular, flow-induced changes in self-assembly and structure of WLM solutions in extensional flows are a major challenge and a deeper understanding of this phenomena is necessary for a targeted design of advanced surfactant systems to be used in various important technical applications. For instance, using surfactant solutions for enhanced oil recovery (EOR) has re-gained great interest in the last few years stimulated by the increasing global oil demand. A successful transfer of knowledge and products from research to oilfield implementation requires fundamental insight into structure and dynamics of these fluids subjected to extensional flow fields.We want to perform the first systematic study focusing on the relationship between rheology and elongational flow-induced structural changes of WLM solutions so far. A broad range of surfactant concentrations and salt surfactant ratios will be investigated, including dilute and concentrated, entangled solutions of linear and branched micelles. Based on our recent technical improvements of the Capillary Breakup Elongational Rheometry (CaBER) method, the true elongational viscosity and flow activation energy of WLM solutions can now be determined. Preliminary measurements on specific surfactant systems have shown that filament lifetime, i.e. the resistance of the fluid to elongational deformation in this type of experiment can vary by orders of magnitude even if the linear viscoelastic properties of the solutions are similar. This indicates strong flow-induced changes in micellar structure depending on surfactant concentration and salt/surfactant ratio. Micelles may either break or form larger aggregates in extensional flows. The structural investigations (small angle light- and neutron scattering, turbidity, birefringence and particle image velocimetry) on selected samples in a hyperbolic flow channel generating a planar extensional flow field will quantitatively reveal such flow-induced structural changes as a function of elongation rate and total deformation. The proposed research project will disclose for which surfactant types, surfactant as well as salt concentrations, and flow conditions flow-induced deviations from the equilibrium micellar structure occur and these structures will be characterized quantitatively. Experimental results will be used to suggest modifications of a theoretical model predicting the flow behavior of WLM solutions taking into account flow-induced changes of micellar scission and recombination rates.