Nonlinearities and shear banding instability of polyacrylamide solutions under large amplitude oscillatory shear

Abstract

The response of semidilute entangled and salt-free solutions of aqueous polyacrylamide under large amplitude oscillatory shear deformations was studied in this work. We systemically probed the effects of four polymer concentrations from 5 to 15 wt. % and two molecular weights (5–6 and 18 M) at De > 1. The mitlaos software package was utilized to analyze the nonlinear moduli, among other nonlinear parameters. We found that the polymer concentration is the dominant parameter controlling the progression into the nonlinear regime. The trends of elastic decomposition indicate an intensive strain-rate softening behavior under high strain amplitudes. At high strain rates and at higher concentrations, however, a gradual transition from shear thickening to shear thinning could be noticed for the viscous dissipation. In addition, the effect of the measuring geometry was also considered, since the rheometer was coupled with a particle image velocimetry (PIV) system in the second part of this study. The PIV results suggest that the flow field is significantly altered during the oscillatory cycle. Banded profiles were observed for both molecular weights studied.