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We have investigated the uniaxial elongation behavior of six different wormlike micelle systems covering a broad range of surfactant concentrations cs and salt/surfactant ratios R using the capillary breakup elongational rheometry (CaBER). In the fast-breaking limit (high cs and R), filament lifetime tfil is controlled by the equilibrium shear modulus G0 and the breakage time λbr obtained from small oscillatory shear according to tfil/G0∝λbr2/3 and relaxation time ratios λe/λs≈1 are found. When reptation dominates (high cs, low R) λe/λs<1 is observed similar as for solutions of covalently bound polymers. In this concentration regime, the micellar structure seems not to be affected by the strong elongational flow. In contrast, high filament lifetimes up to 1000 s and λe/λs values up to 10 are observed at low cs irrespective of R. This indicates the formation of elongation-induced structures (EISs). A minimum viscosity and a minimum initial diameter are required for creating EIS. Additional filament stretching experiments indicate that a critical total deformation has to be exceeded for structure build-up. Finally, our experiments reveal a distinct difference regarding the dependence between solutions of linear and branched micelles of filament lifetime on viscosity suggesting that CaBER is a versatile means to distinguish between these structures.

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In the field of organic and printed electronics (e.g., polymer solar cells, OLEDs, or Li-ion batteries), there is a growing demand for thin functional layers with highly homogeneous surface topology. If these layers are coated from the liquid phase, the coating and drying steps affect the surface quality. As a result of inhomogeneous drying rates, the solvent concentration can vary along the top surface and the thickness of a solidifying solution, leading to local differences in surface tension. In turn, Marangoni convection, as the balancing mechanism, can occur and cause surface inhomogeneity. The in situ reconstruction of the free surface during drying has been presented elsewhere. During this investigation phenomena occurred that could not be completely understood without knowledge of the respective flow field. In the present work, the visualization of the flow field in thin polymer films [methanol-poly(vinyl acetate) solution with 67 wt% methanol] due to inhomogeneous drying is presented. To resolve the flow field, we apply fluorescent particle tracking (lPTV). Since both measurement techniques cannot easily be applied at the same time, the boundary conditions were adapted to the way of observation of each experimental setup. In the case of the setup for surface reconstruction of the free surface, locally different evaporation rates were realized by drying on a structured substrate (varying material). To force similar variation of the drying conditions in the case of the lPTV setup, the drying film was partially covered. As expected, both boundary conditions result in a propagating wave front towards regions of high surface tension. Combining both experimental setups, we were able to visualize the free surface and the flow structures up-and downstream of the wave front and found different flow regimes.