Title: Dynamic behaviour of Multilamellar Vesicles under Poiseuille flow,
Abstract: Surfactant solutions exhibit multilamellar surfactant vesicles (MLVs) under
flow conditions and in concentration ranges which are found in a large number
of industrial applications. MLVs are typically formed from a lamellar phase and
play an important role in determining the rheological properties of surfactant
solutions. Despite the wide literature on the collective dynamics of flowing
MLVs, investigations on the flow behavior of single MLVs are scarce. In this
work, we investigate a concentrated aqueous solution of linear alkylbenzene
sulfonic acid (HLAS), characterized by MLVs dispersed in an isotropic micellar
phase. Rheological tests show that the HLAS solution is a shear-thinning fluid
with a power law index dependent on the shear rate. Pressure-driven shear flow
of the HLAS solution in glass capillaries is investigated by high-speed video
microscopy and image analysis. The so obtained velocity profiles provide
evidence of a power-law fluid behaviour of the HLAS solution and images show a
flow-focusing effect of the lamellar phase in the central core of the
capillary. The flow behavior of individual MLVs shows analogies with that of
unilamellar vesicles and emulsion droplets. Deformed MLVs exhibit typical
shapes of unilamellar vesicles, such as parachute and bullet-like. Furthermore,
MLV velocity follows the classical Hetsroni theory for droplets provided that
the power law shear dependent viscosity of the HLAS solution is taken into
account. The results of this work are relevant for the processing of
surfactant-based systems in which the final properties depend on flow-induced
morphology, such as cosmetic formulations and food products.