Transient shear banding in a simple yield stress fluid

Area of Science:

• Rheology
• Soft Matter Physics

Background:

• Yield stress fluids are crucial in many industrial applications.
• Understanding their flow behavior, especially transient dynamics, is essential for process optimization.
• Simple yield stress fluids lacking aging or thixotropy provide a fundamental model system.

Purpose of the Study:

• To investigate the transient shear banding in simple yield stress fluids.
• To characterize the relationship between shear rate and the duration of the transient regime.
• To explore the influence of experimental parameters like gap width and boundary conditions.

Main Methods:

• Experimental rheometry on carbopol dispersions.
• Application of controlled shear rates.
• Observation and measurement of velocity profiles over time.

Main Results:

• Simple yield stress fluids exhibit transient shear banding before achieving a steady state with a homogeneous velocity profile.
• The duration of this transient regime follows a power-law decay with increasing shear rate.
• This power-law behavior is independent of gap width and boundary conditions for a given sample preparation.
• Heterogeneous flows were observed for up to 10^5 seconds at shear rates below 0.1 s^-1.

Conclusions:

• Transient shear banding is a fundamental characteristic of simple yield stress fluids.
• The observed power-law scaling provides a quantitative description of the transient regime duration.
• The long-lived heterogeneous flows at low shear rates are responsible for ultraslow stress relaxation phenomena.