Paper Number
PO69 

Session
Poster Session

Title
Evolution of shear bands in Carbopol gel under transient and oscillatory shear

Presentation Date and Time
October 17, 2018 (Wednesday) 6:30

Track / Room
Poster Session / Woodway II/III

Authors (Click on name to view author profile)

  1. Wei, Yufei (University of Michigan, Chemical Engineering)
  2. Liu, Luofu (Beihang University, School of Chemistry)
  3. Solomon, Michael J. (University of Michigan, Chemical Engineering)
  4. Larson, Ronald G. (University of Michigan, Department of Chemical Engineering)

Author and Affiliation Lines (in printed abstract book)
Yufei Wei1, Luofu Liu2, Michael J. Solomon1, and Ronald G. Larson1
1Department of Chemical Engineering, University of Michigan, Ann Arbor, MI; 2School of Chemistry, Beihang University, Haidian Dist, Beijing 100191, China

Speaker / Presenter 
Wei, Yufei

Text of Abstract
Shear banding often occurs in yield-stress materials and greatly alters the flow field as well as the bulk rheological properties. Understanding the onset and evolution of shear bands is crucial to correctly interpreting the rheological response of yield-stress materials and is important in the design of processing flows that enhance or mitigate shear banding as needed. This poster presents our experimental results of rheological tests and the simultaneous velocimetry measurement. The testing material is a Carbopol gel, a model yield-stress fluid, seeded with fluorescent tracer particles for particle image velocimetry. The rheological experiments include shear startup tests, flow reversal tests, and large amplitude oscillatory shear (LAOS) tests. We used a parallel plate geometry with rough surfaces to prevent wall-slip. We calculated the degree of banding (DOB) based on the results of time-resolved velocimetry. We found that upon shear startup, the DOB increases, reaches an overshoot, and then gradually decays towards a plateau. The overshoot of the DOB typically occurs shortly after the stress overshoot, but shear banding starts before the stress overshoot is reached. The banded flow becomes uniform if the shearing direction is suddenly changed. Shear bands gradually develop after shear reversal. In LAOS tests, as the amplitude increases, shear bands occur along with the emergence of a nonlinear rheological response. For large amplitudes, the DOB exhibits a hysteresis loop when plotted versus shear strain or shear rate. We examine the experiments in light of recent theoretical studies.