The formation of multilamellar vesicles (MLVs) in the lyotropic lamellar phase of the system triethylene glycol mono n-decyl ether (CE)/water is investigated under large amplitude oscillatory shear (LAOS) using spatially resolved rheo-NMR spectroscopy and a combination of rheo-small angle light scattering (rheo-SALS) and conventional rheology. Recent advances in rheo-NMR hardware development facilitated the application of LAOS deformations in high-field NMR magnets. For the range of investigated strain amplitudes (10-50) and frequencies (1 and 2 rad s), MLV formation is observed in all NMR and most SALS experiments. It is found that the MLV size depends on the applied frequency in contrast to previous steady shear experiments where the shear rate is the controlling parameter. The onset of MLV formation, however, is found to vary with the shear amplitude. The LAOS measurements bear no indication of the intermediate structures resembling aligned multilamellar cylinders observed in steady shear experiments. Lissajous curves of stress vs strain reveal a transition from a viscoelastic solid material to a pseudoplastic material.

No abstract

A combined two-dimensional Rheo-NMR velocimetry and two-dimensional Rheo-USV approach is used to further elucidate the flow of a wormlike micellar solution in cylindrical Couette geometries. Recent experimental enhancements for both methods enable a more detailed description of the flow dynamics than available in the past. This enabled us to revisit and investigate shear banded flow utilizing improved spatial and temporal resolution, clearly confirming a departure from a simple lever rule for the micellar solution under study. Both experimental techniques observe different shear rates in the high shear rate band accompanied by a varying position of the interface between bands for different applied shear rates. Furthermore, spatially and temporally resolved velocimetry reveals various flow instabilities. Ultrasound measurements show vorticity undulations with wavelengths that scale linearly with the size of the high shear rate band. For high enough shear rates, the occurrence of turbulent bursts is detected in the USV case. However, no direct evidence of these bursts is found in the NMRmeasurements for equivalent shear rates, which we attribute so far to differences in the design of the shearing geometries.