Rheological effects on purely-elastic flow asymmetries in the cross-slot geometry

Area of Science:

• Fluid dynamics
• Rheology
• Polymer science

Background:

• Viscoelastic flows in cross-slot geometry can transition from symmetric to asymmetric states.
• This transition is often attributed to elastic effects at critical flow rates (Weissenberg number, Wi).
• The role of shear thinning in this transition remains an area of investigation.

Purpose of the Study:

• To investigate the combined effects of fluid elasticity and shear thinning on flow asymmetry in cross-slot geometry.
• To determine how the interplay between these properties influences the onset and development of asymmetric flows.
• To map flow states based on elasticity and shear thinning parameters.

Main Methods:

• Experimental study using polymer solutions with varying rheological properties.
• Flow velocimetry to measure flow asymmetry (I) as a function of Wi and shear thinning parameter (S).
• Numerical simulations using the linear simplified Phan-Thien-Tanner model.

Main Results:

• Flow asymmetry increases with Wi beyond a critical value, but is also dependent on the degree of shear thinning (S).
• For some solutions, flow symmetry is recovered at higher Wi values when shear thinning is minimal (low S).
• A flow state diagram in Wi-S space illustrates the relationship between flow asymmetry and fluid rheology.

Conclusions:

• Both fluid elasticity and shear thinning, along with their interaction, are critical factors governing elastic instabilities in cross-slot flows.
• The findings highlight the importance of considering rheological complexity beyond pure elasticity.
• Results provide a framework for understanding and predicting flow behavior in such geometries.