Does shear banding exist in polymer solutions?

Monday, September 14, 2009 - 2:20pm - 3:05pm
EE/CS 3-180
In 1975 Doi and Edwards predicted that entangled polymer melts and
solutions can have a constitutive instability, signified by a
decreasing stress for shear rates greater than the inverse of the
reptation time. Early experiments did not support this, and more
sophisticated theories were developed that incorporated Marrucci's idea (1996) of
removing constraints by advection; this produced a monotonically
increasing stress and thus stable constitutive behavior. Recent
experiments have suggested that entangled polymer solutions may
possess a constitutive instability after all, and have led some
workers to question the validity of existing constitutive models.
Based on this intense interest we have revisited some of the phenemology present in state of the art tube models for entangled polymers, and performed calculations that take into account the stress inhomogeneity inherent in rotating rheometers (cone and plate and cylindrical Couette). Using the Rolie-Poly model with an added solvent viscosity, we show that (1) instability and shear banding is
captured within this simple class of models; (2) shear banding
phenomena is observable for weakly stable fluids in flow
geometries that impose a sufficiently inhomogeneous total shear
stress; (3) transient phenomena can possess inhomogeneities that
resemble shear banding, even for weakly stable fluids. Many of these
results are model-independent.
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