Paper Number
IN13 

Session
Flow-induced Instabilities and Non-Newtonian Fluids

Title
Flow-induced concentration non-uniformity and shear banding in entangled polymer solutions

Presentation Date and Time
October 13, 2021 (Wednesday) 10:40

Track / Room
Track 4 / Meeting Room C-D

Authors (Click on name to view author profile)

  1. Burroughs, Michael C. (North Carolina State University, Chemical and Biomolecular Engineering)
  2. Shetty, Abhishek (Anton Paar USA, Rheology)
  3. Leal, L. Gary (University of California, Santa Barbara, Department of Chemical Engineering)
  4. Helgeson, Matthew E. (University of California, Santa Barbara, Department of Chemical Engineering)

Author and Affiliation Lines (in printed abstract book)
Michael C. Burroughs1, Abhishek Shetty2, L. Gary Leal1 and Matthew E. Helgeson1
1Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, CA 93106; 2Rheology, Anton Paar USA, Ashland, VA 23005

Speaker / Presenter 
Helgeson, Matthew E.

Keywords
experimental methods; theoretical methods; flow-induced instabilities; non-Newtonian fluids; polymer solutions

Text of Abstract
A foundational assumption of most rheological models is that an initially homogeneous fluid remains compositionally homogeneous when subjected to flow. A notable exception is the phenomenon of flow-enhanced concentration fluctuations in polymer solutions, which has been successfully modeled by the Helfand-Fredrickson mechanism where coupling of polymer viscoelasticity with osmotic stresses can lead to enhanced local fluctuations. However, it is unclear whether these local fluctuations can propagate to induce macroscopic demixing of the fluid, and whether this affects viscoelastic flows of polymer solutions. In this work, we address these questions for highly entangled polymer solutions. To do so, we present new rheo-fluorescence microscopy experiments in which the concentration profile of a labeled species can be directly imaged in situ in a rheometric flow. For polymer solutions that are sufficiently entangled and sufficiently close in temperature to the equilibrium cloud point, we observe steady state shear-induced concentration “bands” in Taylor-Couette flow for a range of imposed Weissenberg numbers. Simultaneous velocimetry measurements reveal that these concentration bands are concomitant with shear banding, whose existence for entangled polymers has been highly controversial and the subject of conflicting experimental reports. Comparison to a recently developed two-fluid model that couples the nonlinear polymer rheology to Helfand-Fredrickson type osmotic stresses shows near quantitative agreement with both the concentration and velocity profiles over a range of solution conditions, and suggests that banding is a result of unstable shear-induced demixing. The results provide a potential new mechanism for shear banding in entangled polymers. More broadly, they call for a re-examination of the basic assumption that homogeneous polymeric fluids remain macroscopically homogeneous in flow, and provide an initial exploration of new flow phenomena that emerge when this assumption is relaxed.