Spatiotemporal dynamics of multiple shear-banding events for viscoelastic micellar fluids in cone-plate shearing flows

Abstract

We characterize the transient response of semi-dilute wormlike micellar solutions under an imposed steady shear flow in a cone-plate geometry. By combining conventional rheometry with 2-D Particle Image Velocimetry (PIV), we can simultaneously couple the temporal stress response with time-resolved velocimetric measurements. By imposing a well defined shear history protocol, consisting of a stepped shear flow sweep, we explore both the linear and nonlinear responses of two surfactant solutions: cetylpiridinium chloride (CPyCl) and sodium salicylate (NaSal) mixtures at concentrations of [66:40] mM and [100:60] mM, respectively. The transient stress signal of the more dilute solution relaxes to its equilibrium value very fast and the corresponding velocity profiles remain linear, even in the strongly shear-thinning regime. The more concentrated solution also exhibits linear velocity profiles at small shear rates. At large enough shear rates, typically larger than the inverse of the relaxation time of the fluid, the flow field reorganizes giving rise to strongly shear-banded velocity profiles. These are composed of an odd number of shear bands with lowshear-rate bands adjacent to both gap boundaries. In the non-linear regime long transients (much longer than the relaxation time of the fluid) are observed in the transient stress response before the fluid reaches a final, fully-developed state. The temporal evolution in the shear stress can be correlated with the spatiotemporal dynamics of the multiple shear-banded structure measured using RheoPIV. In particular our experiments show the onset of elastic instabilities in the flow which are characterized by the presence of multiple shear bands that evolve and rearrange in time resulting in a slow increase in the average torque acting on the rotating fixture.

Shear-banding fluid(s) under time-dependent shear flows. Part II: A test of the Moorcroft–Fielding criteria

“…39 ). Note that delayed formation of the shear-banding structure with respect to the fast overshoot relaxation was also reported for similar wormlike micelles systems in cone-and-plate geometry 16,22 . The velocity profiles were observed to become banded during the sigmoidal relaxation.…”

Shear Banding of Complex Fluids

“…It also pushes the shear banding flow to be made of only two bands, with the high shear rate band always near the inner cylinder (radius R). In contrast, in a CP flow, there can be two, three or even more bands, and their location is more variable [18,22,25]. TC flow is less prone to edge effects which can strongly limit the accessible range of flow strength in CP for instance [45].…”

Shear layers and plugs in the capillary flow of wormlike micellar gels

“…Shear banding in a pipe isn't as widely studied as shear banding in a Couette cell, 27,28,31,94,95 where shear-banding manifests as a velocity profiles with two (or more 96,97 ) shear rates characterized by a sharp kink(s) in the velocity profile. The analogue shear-banding velocity profile in confined channel and pipe flows is found to consist of a plug flanked by shearlayers and the transition from zero to finite _ g is sharp.…”