Dynamic flow behaviour of surfactant vesicles under shear flow: role of a multilamellar microstructure

Surfactant based systems are ubiquitous in everyday life products, including food, pharmaceuticals, and detergents. In concentrated solutions, surfactants may assemble into multilamellar, onion-like vesicles, whose size distribution is known to be affected by the action of flow during processing and usage. However, very little is known about the dynamic behaviour and the mechanisms by which the packed microstructure of a surfactant multilamellar vesicle (MLV) is deformed under flow. In this work, the microstructure of surfactant MLVs is investigated both at rest by confocal microscopy and under flow by a rheo-optical parallel plate apparatus using bright field and polarized light microscopy. Small MLVs have a more homogeneous multilamellar structure extending to the vesicle centre and show small deformation regimes, such as tumbling and breathing. Larger MLVs have an inner isotropic core surrounded by a multilamellar shell and can also exhibit a tank-treading deformation mode depending on the flow conditions. The main result of this work is that tank-treading is associated with the formation of parabolic focal conic defects in the outer shell, which is driven by shear-induced bilayer dilation and undulation as in smectic A liquid crystals, and with convection-induced surfactant rearrangement at a constant shell volume. This finding is supported by the analysis of vesicle retraction upon cessation of flow, which scales with the inverse third power of radius and thus is consistent with the effect of a compressibility modulus.