Microstructural evolution of a model, shear-banding micellar solution during shear startup and cessation

Abstract

We present direct measurements of the evolution of the segmental-level microstructure of a stable shear-banding polymerlike micelle solution during flow startup and cessation in the plane of flow. These measurements provide a definitive, quantitative microstructural understanding of the stages observed during flow startup: an initial elastic response with limited alignment that yields with a large stress overshoot to a homogeneous flow with associated micellar alignment that persists for approximately three relaxation times. This transient is followed by a shear (kink) band formation with a flow-aligned low-viscosity band that exhibits shear-induced concentration fluctuations and coexists with a nearly isotropic band of homogenous, highly viscoelastic micellar solution. Stable, steady banding flow is achieved only after approximately two reptation times. Flow cessation from this shear-banded state is also found to be nontrivial, exhibiting an initial fast relaxation with only minor structural relaxation, followed by a slower relaxation of the aligned micellar fluid with the equilibrium fluid's characteristic relaxation time. These measurements resolve a controversy in the literature surrounding the mechanism of shear banding in entangled wormlike micelles and, by means of comparison to existing literature, provide further insights into the mechanisms driving shear-banding instabilities in related systems. The methods and instrumentation described should find broad use in exploring complex fluid rheology and testing microstructure-based constitutive equations.

Effect of the salt-induced micellar microstructure on the nonlinear shear flow behavior of ionic cetylpyridinium chloride surfactant solutions

“…This is exemplified by butterflylike patterns as observed earlier by Refs. [10,31,[47][48][49]. Our rheometric SANS experiments agree with this behavior.…”

The rheology and microstructure of branched micelles under shear

“…The shear thickening is highly repeatable amongst additional sample preparations and is also observed in samples with higher salt concentrations. Upon shear startup, it is well known that shear banding can take hundreds of seconds to develop in a worm-like micellar solution [Grand et al (1997), Britton and Callaghan (1999), López-Barrón et al (2014)]. In the low salt sample, the steady-state stress response is observed only after 50 -100s, at minimum, for the shear rates in the stress plateau, which is consistent with shear band formation.…”

Measuring Material Microstructure Under Flow Using 1-2 Plane Flow-Small Angle Neutron Scattering

“…These measurements have recently been successfully extended to time-dependent deformations by time resolved neutron scattering methods as described in section 5.2 of this work and these results have been submitted for publication . This instrument functions for both steady shear and time dependent flows, such as oscillatory or start-up shear flows, the latter using a stroboscopic methodology and time-resolved neutron scattering techniques 11,12,21 . An advantage to using SANS is that contrast matching methods can be employed to explore the individual components in complex mixtures and materials that are opaque, or lack contrast necessary for X-ray scattering.…”