Shear banding in entangled polymer solutions is an elusive phenomenon in polymer rheology. One recently proposed mechanism for the existence of banded velocity profiles in entangled polymer solutions stems from a coupling of the flow to banded concentration profiles. Recent work [Burroughs et al., Phys. Rev. Lett. 126, 207801 (2021)] provided experimental evidence for the development of large gradients in concentration across the fluid. Here, a more systematic investigation is reported of the transient and steady-state banded velocity and concentration profiles of entangled polybutadiene in dioctyl phthalate solutions as a function of temperature , number of entanglements (), and applied shear rate (), which control the susceptibility of the fluid to unstable flow-concentration coupling. The results are compared to a two-fluid model that accounts for coupling between elastic and osmotic polymer stresses, and a strong agreement is found between model predictions and measured concentration profiles. The interface locations and widths of the time-averaged, steady-state velocity profiles are quantified from high-order numerical derivatives of the data. At high levels of entanglement and large , a significant wall slip is observed at both inner and outer surfaces of the flow geometry but is not a necessary criterion for a nonhomogeneous flow. Furthermore, the transient evolution of flow profiles for large Z indicate transitions from curved to “stair-stepped” and, ultimately, a banded steady state. These observed transitions provide detailed evidence for shear-induced demixing as a mechanism of shear banding in polymer solutions.
Flow-concentration coupling determines features of nonhomogeneous flow and shear banding in entangled polymer solutions
Michael C. Burroughs, Yuanyi Zhang, Abhishek Shetty, Christopher M. Bates, Matthew E. Helgeson, L. Gary Leal; Flow-concentration coupling determines features of nonhomogeneous flow and shear banding in entangled polymer solutions. J. Rheol. 1 January 2023; 67 (1): 219–239. https://doi.org/10.1122/8.0000469
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