Paper Number
PO88 

Session
Poster Session

Title
Transient evolution of flow profiles in shear banding wormlike micellar fluids

Presentation Date and Time
October 23, 2019 (Wednesday) 6:30

Track / Room
Poster Session / Ballroom C on 4th floor

Authors (Click on name to view author profile)

1. Rassolov, Peter (FAMU-FSU College of Engineering, Department of Chemical and Biomedical Engineering)
2. Mohammadigoushki, Hadi (Florida State University, Chemical and Biomedical Engineering)

Author and Affiliation Lines (in printed abstract book)
Peter Rassolov and Hadi Mohammadigoushki
Department of Chemical and Biomedical Engineering, FAMU-FSU College of Engineering, Tallahassee, FL 32310

Speaker / Presenter 
Rassolov, Peter

Text of Abstract
We report experiments on spatiotemporal evolution of flow fields in model shear-banding wormlike micellar solutions based on hexadecyltrimethylammonium bromide and sodium salicylate in a Taylor-Couette cell. Our goal is to systematically study the effects of fluid elasticity on transient evolution of flow fields in wormlike micellar fluids. By varying surfactant concentration, salt concentration, and temperature, we vary the fluid elasticity number in the range 4.21 × 10⁴ to 8.57 × 10⁶ while keeping entanglement density fixed at 5.1 ± 0.7, viscosity ratio fixed at 40.3 ± 2.9 Pa∙s, and curvature fixed at 0.085. Our experiments show that these entangled solutions initially deform uniformly upon imposition of step shear, similarly to an elastic solid. As shear strain increases, shear stress shows an overshoot followed by a decay towards steady state. Simultaneously with shear stress decay, fluid moves in the opposite direction to that of the imposed motion in a subset of the gap (i.e., back flow). Consistently with theoretical predictions, the back flow strengthens as elasticity number increases. However, at very high elasticity numbers, the transient back flow disappears, contrary to the same theoretical predictions. In addition to the back flow, a multiple shear band structure forms in the transient flow at high elasticity numbers. These transient multiple bands persist to steady state. We surmise that at high elasticity numbers where back flow is not observed, the formation of transient multiple bands prevents formation of transient back flows. Furthermore, while such multiple banding has been reported for polymer solutions and in wormlike micellar fluids in the cone and plate geometry, and predicted by theory, it has not been reported in wormlike micellar fluids sheared in Taylor-Couette geometry.