Paper Number
NF11 

Session
Non-Newtonian Fluid Mechanics & Instabilities

Title
Distinguishing shear banding from shear thinning in Taylor-Couette flows

Presentation Date and Time
February 15, 2017 (Wednesday) 1:30

Track / Room
Track 2 / Audubon A

Authors (Click on name to view author profile)

1. Cheng, Peng (University of California, Santa Barbara, Department of Chemical Engineering)
2. Burroughs, Michael (University of California, Santa Barbara, Department of Chemical Engineering)
3. Leal, Gary (University of California Santa Barbara)
4. Helgeson, Matthew E. (University of California, Santa Barbara, Department of Chemical Engineering)

Author and Affiliation Lines (in printed abstract book)
Peng Cheng, Michael Burroughs, Gary Leal, and Matthew E. Helgeson
Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, CA 93106-5080

Speaker / Presenter 
Cheng, Peng

Text of Abstract
Shear banding is typically attributed to mechanical instability due to non-monotonic or discontinuous stress-rate constitutive behavior. However, such behavior is obscured in experiment by the significant spatial gradients in the shear stress inherent to most rheometric geometries, such as Taylor-Couette flow, which results in an apparent stress plateau with increasing shear rate. Moreover, these same stress gradients can produce banding-like velocity profiles, even when the fluid possesses a monotonic constitutive curve. This raises an important practical question: how do we distinguish whether a fluid is truly shear banding as opposed to strongly shear thinning? To answer this question, we report a model-free numerical approach to identify steady state shear banding through higher-order derivatives of the velocity profile. The approach is validated using flow calculations of the diffusive-Giesekus model, and then applied to rheology and flow velocimetry measurements on shear thinning and shear banding wormlike micelles in a Taylor-Couette device with systematically varying curvature (i.e., ratio of gap size to cylinder radius). The results of both simulation and experiment are used to identify several dominant features of shear banding, whose presence and quantitative behavior are relatively insensitive both to the applied shear rate and the degree of curvature of the flow geometry. By contrast, we find that many shear thinning fluids exhibit flows that resemble banding, but with features that are highly sensitive to the flow conditions and geometry. Our results provide clear, unambiguous criterion for identifying shear banding in complex fluids, and clear guidelines for performing flow velocimetry measurements to identify shear banding in complex fluids.