Browsing by Author "Galvosas, P."
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- Planar lamellae and onions: a spatially resolved rheo-NMR approach to the shear-induced structural transformations in a surfactant model systemPublication . Medronho, B.; Brown, J.; Miguel, M. G.; Schmidt, C.; Olsson, U.; Galvosas, P.The shear-induced transformations between oriented planar lamellae and a state of closely packed multilamellar vesicles (MLVs) in a lyotropic nonionic surfactant model system were studied by the combination of nuclear magnetic resonance (NMR) spectroscopy and diffusometry with magnetic resonance imaging (MRI). (2)H NMR imaging confirmed the discontinuous nature of the transition from onions to planar lamellae, revealing the spatial coexistence of both states within the gap of the cylindrical Couette geometry. On the other hand, NMR diffusion measurements in three principal directions and at various values of strain strongly suggest that a multi-lamellar cylindrical or undulated intermediate structure exists during the continuous and spatially homogeneous transition from planar lamellae to MLVs.
- Transient and steady-state shear banding in a lamellar phase as studied by Rheo-NMRPublication . Medronho, B.; Olsson, U.; Schmidt, C.; Galvosas, P.Flow fields and shear-induced structures in the lamellar (L-alpha) phase of the system triethylene glycol mono n-decyl ether (C10E3)/water were investigated by NMR velocimetry, diffusometry, and H-2 NMR spectroscopy. The transformation from multilamellar vesicles (MLVs) to aligned planar lamellae is accompanied by a transient gradient shear banding. A high-shear-rate band of aligned lamellae forms next to the moving inner wall of the cylindrical Couette shear cell while a low-shear-rate band of the initial MLV structure remains close to the outer stationary wall. The band of layers grows at the expense of the band of MLVs until the transformation is completed. This process scales with the applied strain. Wall slip is a characteristic of the MLV state, while aligned layers show no deviation from Newtonian flow. The homogeneous nature of the opposite transformation from well aligned layers to MLVs via an intermediate structure resembling undulated multilamellar cylinders is confirmed. The strain dependence of this transformation appears to be independent of temperature. The shear diagram, which represents the shear-induced structures as a function of temperature and shear rate, contains a transition region between stable layers and stable MLVs. The steady-state structures in the transition region show a continuous change from layer-like at high temperature to MLV-like at lower temperature. These structures are homogeneous on a length scale above a few micrometers.