Shear-banding in a lyotropic lamellar phase, Part 1: Time-averaged velocity profiles

TL;DR

This study provides experimental evidence of shear-banding in a lyotropic lamellar phase, showing structural band formation near a shear-induced transition and confirming classical shear-banding assumptions through velocity profile measurements.

Contribution

First to experimentally demonstrate shear-banding in a lyotropic lamellar phase using velocity profiles and structural measurements, confirming classical shear-banding models.

Findings

Presence of structural shear-banding near shear transition

Confirmation of shear-banding interface at a specific stress

Detection of wall slip and flow fluctuations

Abstract

Using velocity profile measurements based on dynamic light scattering and coupled to structural and rheological measurements in a Couette cell, we present evidences for a shear-banding scenario in the shear flow of the onion texture of a lyotropic lamellar phase. Time-averaged measurements clearly show the presence of structural shear-banding in the vicinity of a shear-induced transition, associated to the nucleation and growth of a highly sheared band in the flow. Our experiments also reveal the presence of slip at the walls of the Couette cell. Using a simple mechanical approach, we demonstrate that our data confirms the classical assumption of the shear-banding picture, in which the interface between bands lies at a given stress $\sigma^\star$. We also outline the presence of large temporal fluctuations of the flow field, which are the subject of the second part of this paper [Salmon {\it et al.}, submitted to Phys. Rev. E].