Dynamic behaviour of Multilamellar Vesicles under Poiseuille flow

TL;DR

This study investigates the flow behavior of multilamellar vesicles in surfactant solutions under shear flow, revealing their deformation, flow dynamics, and rheological properties relevant to industrial applications.

Contribution

It provides new insights into the single-vesicle flow dynamics and rheology of MLVs in shear flow, extending understanding beyond collective behavior studies.

Findings

MLVs deform into parachute and bullet shapes under flow

HLAS solution exhibits shear-thinning behavior with shear-dependent viscosity

Flow-focusing effect observed in lamellar phase within capillaries

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.