Multilamellar Vesicle Formation from a Planar LamellarPhase under Shear Flow.

The formation of multilamellar vesicles(MLVs) from the lamellar phase of nonionic surfactant system C12E5/D2O under shear flow is studiedby time-resolved small angle neutron and light scattering during shearflow. A novel small angle neutron scattering sample environment enablesthe tracking of the lamellae alignment in the velocity–velocitygradient (1–2) plane during MLV formation, which was trackedindependently using flow small angle light scattering commensuratewith rheology. During the lamellar-to-multilamellar vesicle transition,the primary Bragg peak from the lamellar ordering was observed totilt, and this gradually increased with time, leading to an anisotropicpattern with a primary axis oriented at ∼25° relativeto the flow direction. This distorted pattern persists under flowafter MLV formation. A critical strain and critical capillary numberbased on the MLV viscosity are demonstrated for MLV formation, whichis shown to be robust for other systems as well. These novel measurementsprovide fundamentally new information about the flow orientation oflamellae in the plane of flow that cannot be anticipated from thelarge body of previous literature showing nearly isotropic orientationin the 2,3 and 1,3 planes of flow. These observations are consistentwith models for buckling-induced MLV formation but suggest that theinstability is three-dimensional, thereby identifying the mechanismof MLV formation in simple shear flow. [ABSTRACT FROM AUTHOR]