(216ac) Flow Properties and Morphology of Multilamellar Structures in Surfactant Solutions

Surfactant systems morphology is a topic of high scientific interest that has various industrial applications. However, while many studies have been addressed to the phase behaviour of water-surfactants solutions, only limited information is available in literature on flow-induced morphology of such systems. The recent advances in rheo-optical techniques help now to elucidate the changes in microstructural conformation taking place in the lamellar phase during flow.

The focus of this work is to develop a quantitative investigation of the flow behaviour of single surfactant multilamellar vesicles (SMLVs). The fluid investigated is a solution of HLAS in water, a surfactant widely used in the detergent industry. Depending on concentration, different phases can be observed, such as planar lamellae, unilamerllar and multilamellar vesicles. We focused on a range of concentration where lamellar phase inclusions are surrounded by an isotropic matrix, and eventually form SMLVs. Vesicle shape has been quantified by image analysis techniques. Different setups allow the direct observation of the sample, either during the flow or in static conditions, by several microscopy imaging techniques including high speed, bright field, phase contrast and confocal. A simple shear flow cell, where the fluid is placed among two parallel transparent glass plates one of which is translating respect to the other, allow the sample observation both along the vorticity and velocity gradient directions, using two different setups, hence allowing the three-dimensional reconstruction of the system microstructure. A different flow cell can be used to investigate the Poiseuille flow using a cylindrical microcapillary.

The main result is that SMLVs 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). SMLV deformation shows some analogies with the behaviour of a single droplet immersed in an immiscible fluid, thus suggesting possible non-dimensional scaling parameters such as the Capillary number, that is the ratio between the shear and the interfacial stress. Furthermore, 3D shape reconstruction of confocal images of SMLVs, was used to identify the presence of both superficial and internal defect, whose role in the deformation dynamics is discussed.

A possible application of the physical insight provided by this work is in the rationale design of processing methods of surfactant-based systems.

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A. Pommella, S. Caserta, V. Guida, S. Guido, “Shear-induced deformation of surfactant multilamellar vesicles”, Physical Review Letters 2012 108, 138301.

A. Pommella, S. Caserta, S. Guido, “Dynamic flow behaviour of surfactant multilamellar vesicles under simple shear flow”, Soft Matter, submitted.