J. Phys. II France

**7**(1997) 1071-1086

## Slow Transients and Metastability in Wormlike Micelle Rheology

**C. Grand**

^{1, 2}, J. Arrault^{1}and M.E. Cates^{1}^{1}Department of Physics and Astronomy, University of Edinburgh, JCMB King's Buildings, Mayfield Road, Edinburgh EH9 3JZ, UK

^{2}Centre de Recherche Paul Pascal (CNRS), avenue Schweitzer, 33600 Pessac, France

(Received 28 February 1997, accepted 5 May 1997)

** Abstract **

The steady-state nonlinear rheology of wormlike micellar systems is thought to be subject to shear banding (the underlying
shear stress *vs.* strain rate curve
is nonmonotonic). Shear banding may result in a plateau (
) in the *measured* flow curve (at controlled *mean* strain rate
). We present new rheological data for aqueous CPyCl/NaSal (100 mM/60 mM). Steady-state flow curves published previously for
this system (Rehage H. and Hoffmann H., *Mol. Phys.*
**74** (1991) 933) have since been interpreted as shear-banded flow with "top-jumping", in which the steady-state shear rate
in the low shear band is the largest possible (
,
). That would rule out the existence of a metastable branch with a stress larger than
. We show that such a branch does, however, exist (for temperatures in the range 20 - 25
C). Similar results are found for a 100 mM/75 mM system. The time scale for relaxation of a metastable state onto true steady
state flow,
, is far longer than the Maxwell time of the fluid; this is consistent with shear banding. We observe
in the metastable regime
, with
*p* an exponent that depends on composition and temperature. The "critical" shear rate
is in some cases less than
so that no actual divergence of
occurs. In at least one case, though, there is evidence for a physical divergence (
) accompanied by a small window of shear rates,
, for which
is effectively infinite. In some respects the observed behaviour resembles that reported previously (Berret J.-F., Roux D.C.
and Porte G., *J. Phys. II France*
**4** (1994) 1261) for equimolar CPyCl/NaSal in 0.5 M NaCl. Those results were interpreted in terms of nucleation and growth of
a shear-induced nematic phase. However the same explanation is unlikely for the low weight fractions (
5%) used in our study.

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*Les Editions de Physique 1997*