Reo-Optical investigation of shear induced deformation of multilamellar surfactant vesicles

The morphology of surfactant systems is a topic of high scientific and industrial interest. However, while many studies have been addressed to the phase behavior of surfactants in water, only limited information is available in literature on flow-induced morphology of such systems. Depending on concentration, different static morphologies have been observed, such as lamellae and vesicles. In this work, we present the first quantitative investigation of the flow behavior of single surfactant multilamellar vesicles using a video-microscopy rheo-optical apparatus. Two different setups allow the observation of the sample both along the vorticity and velocity gradient directions. The vesicles have been observed both in bright field and confocal microscopy, and the shape has been studied by image analysis techniques. The main results is that surfactant multilamellar vesicles are deformed and oriented by the action of shear flow while keeping constant volume, and exhibit complex dynamic modes (i.e., tumbling, breathing and tank-treading). In Fig. 1 are reported images of a vesicle in shear flow in tank-treading regime, as observed along the velocity gradient direction. The vesicle deformation can be quantified as a function of the flow intensity, the results show some analogies with the case of the deformation of a single droplet immersed in an immiscible fluid, thus suggesting possible nondimensional scaling parameters. Furthermore the internal structure of surfactant multilamellar vesicles has been also investigated through confocal microscopy, which provides information on the spatial arrangement of the typical multilayer microstructure by 3D shape reconstruction of vesicles observed in the experiments.