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

• Published in: Journal of non-Newtonian fluid mechanics Vol. 222; pp. 234 - 247
• Main Authors: Casanellas, Laura, Dimitriou, Christopher J., Ober, Thomas J., McKinley, Gareth H.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.08.2015; Elsevier
• Subjects: Chemical Physics; Condensed Matter; Elastic instabilities; Fluid Dynamics; Fluid flow; Fluids; Nonlinearity; Particle Image Velocimetry; Physics; Relaxation time; Shear banding; Shear flow; Soft Condensed Matter; Stress concentration; Stresses; Wormlike micelles; Elastic instabilities; Wormlike micelles; Shear banding; Particle Image Velocimetry
• ISSN: 0377-0257; 1873-2631
• DOI: 10.1016/j.jnnfm.2014.12.005

•We study the transient response of wormlike micellar solutions in shearing flows.•We perform simultaneous bulk rheometry and velocimetry measurements.•We explore the role of changing the degree of shear thinning of viscoelastic fluids.•We observe the formation of time-dependent, multiply-banded states.•We identify the onset of elastic instabilities in shear-banded flows. 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 correlate 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 low-shear-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.