Competition between shear banding and wall slip in wormlike micelles

Area of Science:

• Soft Matter Physics
• Rheology
• Complex Fluids

Background:

• Wormlike micellar systems exhibit complex flow behaviors, including shear banding.
• Boundary conditions play a critical role in dictating the dynamics of shear banding.
• Understanding shear banding is crucial for controlling the processing and application of these materials.

Purpose of the Study:

• To investigate the interplay between shear band formation and boundary conditions in wormlike micellar systems.
• To elucidate the influence of different boundary conditions (smooth vs. sandblasted) on shear banding dynamics.
• To correlate micellar behavior with observed flow profiles and rheological responses.

Main Methods:

• Utilizing ultrasonic velocimetry coupled with standard rheology in Couette geometry.
• Performing transient, strain-controlled experiments to capture time-resolved velocity profiles.
• Analyzing data from both smooth and sandblasted geometries to differentiate boundary effects.

Main Results:

• Standard shear banding is observed under stick boundary conditions, with temporal fluctuations in highly sheared bands depending on entanglement.
• Wall slip occurs under slip boundary conditions only above a critical shear rate.
• Shear band formation is shifted by a constant value (Delta gamma) at low entanglement, while at higher entanglement, shear bands exhibit a 'nucleate and melt' behavior.

Conclusions:

• Boundary conditions profoundly affect shear banding phenomena in wormlike micelles.
• Micellar orientation gradients at the walls are proposed as a mechanism explaining the observed behaviors.
• This research provides insights into the fundamental physics governing shear banding in complex fluids.