Paper Number
IN25
Session
Flow-induced Instabilities in Non-Newtonian Fluids
Title
Evolution of shear banding instabilities in high elasticity polymeric wormlike micelles (WLMs)
Presentation Date and Time
October 13, 2022 (Thursday) 9:05
Track / Room
Track 5 / Sheraton 2
Authors
- McCauley, Patrick J. (University of Minnesota, Chemical Engineering and Materials Science)
- Kumar, Satish (University of Minnesota, Department of Chemical Engineering and Materials Science)
- Calabrese, Michelle A. (University of Minnesota, Chemical Engineering and Materials Science)
Author and Affiliation Lines
Patrick J. McCauley, Satish Kumar and Michelle A. Calabrese
Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455
Speaker / Presenter
McCauley, Patrick J.
Keywords
flow-induced instabilities; surfactants
Text of Abstract
Nonionic polymer WLMs exhibit elastic flow instabilities including shear banding, where the flow separates into bands of distinct structure and viscosity. Here, the formation and evolution of shear banding and related instabilities are examined in a highly-elastic, polymeric WLM solution composed of 15% wt aqueous triblock poloxamer P234 (P234) in 2M NaCl. In addition to the high elasticity which drives unique flow behaviors, the slow dynamics and long relaxation times of these WLM solutions versus traditional surfactant WLMs enable clear identification of both transient and steady state flow phenomena with high resolution. Using a combination of linear and nonlinear shear rheology, rheo-particle tracking velocimetry (rheo-PTV) and flow-small angle neutron scattering (flow-SANS), we find that the P234 WLMs exhibit temperature-dependent signatures of both shear banding and fracture, due to the gel-like nature of the WLMs. Shear startup experiments reveal significant elastic recoil after the stress overshoot, the degree to which decreases with increasing shear rate. The strong elastic recoil results in transient flow reversal of nearly shear rate-independent duration. As flow reversal subsides, the fluid in the high shear rate band continues flowing while that in the low shear rate band becomes nearly immobilized, showing similarities to both shear banding and fracture. Interestingly, elastic recoil is not associated with significant wall slip; however, significant fluctuations in the band interface position are observed during band formation when slip increases. Flow-SANS measurements corroborate shear banding at lower shear rates, and reveal transient shear banding instabilities at high shear rates before homogeneous flow is realized. Finally, fracture is shown to preclude shear banding with minor changes to the thermal history, suggesting close competition between these mechanisms. These experiments lend insight into elastic instability formation and propagation in gel-like wormlike micelles for the first time.