Ordering fluctuations in a shear-banding wormlike micellar system

Area of Science:

• Soft Matter Physics
• Materials Science
• Physical Chemistry

Background:

• Wormlike micellar systems exhibit complex flow behavior, including shear-thinning and phase transitions.
• Understanding orientational order fluctuations is crucial for elucidating non-linear rheology and shear banding.
• Lecithin-water-cyclohexane systems serve as model systems for studying living polymers.

Purpose of the Study:

• To investigate the non-linear flow properties and transient orientational-order fluctuations in a lecithin-water-cyclohexane wormlike micellar system.
• To explore the behavior near the zero-shear isotropic-nematic phase transition.
• To elucidate the microscopic mechanisms underlying shear banding.

Main Methods:

• Rheological measurements to determine stress plateau and shear-thinning behavior.
• Rheo-small angle neutron scattering (Rheo-SANS) experiments to probe orientational order.
• Development of a theoretical model to explain observed oscillatory dynamics.

Main Results:

• A stress plateau was observed at very low shear rates, differing from typical concentrated living polymer systems.
• Rheo-SANS revealed periodic fluctuations in the order parameter P(2) and angular deviation phi.
• Oscillation periods were independent of the applied shear rate (gamma).

Conclusions:

• The study provides the first investigation into the non-linear flow and orientational fluctuations in this specific micellar system.
• A theoretical model successfully explained the oscillatory dynamics via standing waves of director orientation.
• These findings offer significant new insights into the microscopic mechanism of shear banding.