Shear-banding structure orientated in the vorticity direction observed for equimolar micellar solution |
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Authors: | Peter Fischer Elizabeth K Wheeler Gerald G Fuller |
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Institution: | (1) Department of Chemical Engineering Stanford University, Stanford CA 94305-5025 e-mail: peter.fischer@ilw.agrl.ethz.ch, CH;(2) Permanent address: ETH Zürich Institute of Food Science/Food Process Engineering and National Competence Center of Rheology (Swiss Rheo Center), ETH Zentrum 8092 Zürich, Switzerland, CH |
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Abstract: | The non-monotonic shear flow of a viscoelastic equimolar aqueous surfactant solution (cetylpyridinium chloride-sodium salicylate)
is investigated rheologically and optically in a transparent strain-controlled Taylor Couette flow cell. As reported before,
this particular wormlike micellar solution exhibits first a shear thinning and then a pronounced shear-thickening behavior.
Once this shear-thickening regime is reached, a transient phase separation/shear banding of the solution into turbid and clear
ring-like patterns orientated perpendicular to the vorticity axis, i.e., stacked like pancakes, is observed (Wheeler et al.
1998; Fischer 2000). The solution exhibit several unique features as no induction period of the shear induced phase, no structural
build-up at the inner rotating cylinder, jumping pancake structure of clear and turbid ringlike phases, and oscillating shear
stresses appear once the pancake structure is present. According to our analysis this flow phenomenon is not purely a mechanical
or rheological driven hydrodynamic instability but one has to take into account structural changes of the oriented micellar
aggregates (flow induced non-equilibrium phase transition) as proposed by several authors. Although this particular flow behavior
and the underlying mixture of shear induced phases and mechanical instabilities is not fully understood yet, some classification
characteristics based on a recent theoretical approach by Schmitt et al. (1995) and Porte et al. (1997) where a strong coupling
between the flow instability (non-homogeneous flow profile due to the bands) and the structural changes causes the observed
transient phenomena can be derived. In reference to the presented model the observed orientation of the rings is typical for
complex fluids that undergo a spinodal phase separation coupled with a thermodynamic flow instability. In contrast to other
shear banding phenomena, this one is observed in parallel plate, cone-plate, and Couette flow cell as well as under controlled
stress and controlled rate conditions. Therefore, it adds an additional aspect to the present discussion on shear banding
phenomena, i.e., the coupling of hydrodynamics and phase transition of rheological complex fluids.
Received: 8 January 2001 Accepted: 15 May 2001 |
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Keywords: | Equimolar viscoelastic surfactant solution Shear banding Shear induced phase Flow instability Taylor-Couette flow Stress-/strain-controlled flow |
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