Oscillating viscosity in a lyotropic lamellar phase under shear flow

Area of Science:

• Rheology and soft matter physics
• Materials science

Background:

• Lyotropic lamellar phases are complex fluids with ordered structures.
• Understanding their behavior under external forces like shear flow is crucial for material applications.

Purpose of the Study:

• To investigate the time-dependent behavior of lyotropic lamellar phases under shear flow.
• To characterize the dynamic transitions between different structural states.

Main Methods:

• Applying controlled shear stress to lyotropic lamellar phase samples.
• Measuring shear rate and viscosity over time.
• Developing a new stress-strain rate diagram incorporating temporal dynamics.

Main Results:

• Observed oscillating shear rates at fixed stress near a layering instability.
• Identified two distinct branches of stationary states: disordered and ordered multilamellar vesicle phases.
• Recorded sustained viscosity oscillations corresponding to periodic structural changes between disordered and ordered states.

Conclusions:

• Lyotropic lamellar phases exhibit complex, time-dependent rheological responses under shear.
• The study introduces a new phase diagram that captures dynamic behaviors.
• Periodic structural transitions between ordered and disordered states are key features under increasing shear stress.