Vorticity Banding During the Lamellar-to-Onion Transition in a Lyotropic Surfactant Solution in Shear Flow

TL;DR

This study investigates the shear-induced transition from lamellar to onion phases in a surfactant solution, revealing shear banding and slow, irregular transformation dynamics during the phase change.

Contribution

It provides detailed rheological analysis and evidence of shear banding during the lamellar-to-onion transition in a lyotropic surfactant solution, highlighting the nucleation process.

Findings

Discontinuous transition between lamellar and onion phases.

Identification of shear banding with lamellae and onions coexisting.

Slow, irregular lamellae-to-onion transformation events.

Abstract

We report on the rheology of a lamellar lyotropic surfactant solution (SDS/dodecane/pentanol/water), and identify a discontinuous transition between two shear thinning regimes which correspond to the low stress lamellar phase and the more viscous shear induced multi-lamellar vesicle, or ``onion'' phase. We study in detail the flow curve, stress as a function of shear rate, during the transition region, and present evidence that the region consists of a shear banded phase where the material has macroscopically separated into bands of lamellae and onions stacked in the vorticity direction. We infer very slow and irregular transformations from lamellae to onions as the stress is increased through the two phase region, and identify distinct events consistent with the nucleation of small fractions of onions that coexist with sheared lamellae.