Microstructural evolution of a model, shear-banding micellar solution during shear startup and cessation.
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.
ExxonMobil Chemical Company, Baytown, Texas 77520, USA.
Carlos R
CR
López-Barrón
Center for Neutron Science, Department of Chemical & Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, USA.
A Kate
AK
Gurnon
Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
Aaron P R
AP
Eberle
Institut Laue-Langevin, BP 156, F-38042 Grenoble Cedex 9, France.
Lionel
L
Porcar
Center for Neutron Science, Department of Chemical & Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, USA.
Norman J
NJ
Wagner