Nonlinear viscoelasticity shear thinning flow

Finally, it is also important to emphasize that the nonlinear rheology of viscoelastic surfactant solutions is characterized by the existence of normal stresses of nonnegligible magnitude. In steady-state flow, a nonzero first normal stress difference N has been detected once the first stable branch becomes shear-thinning. Ni was found to increase with 7 and a slight change of slope was observed at the... 

Although it is a powerful means of investigating molecular structure and of basic characterization, and provides a general indication of the influence of M on flow behavior, the restrictions imposed by linear viscoelasticity make it inapplicable to a wide variety of practical problems. Nonlinearity is often associated with the phenomenon of shear thinning , that is, a reduction of the viscosity with shear rate in steady flow, characteristic of many polymer melts at intermediate shear rates [15]. This contrasts with the Newtonian behavior implied by the Boltzman superposition principle for steady flow [Eq. (54), in a liquid, G(s) must vanish as s coj. 

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