Absolute Rheological Measurements of Model Suspensions: Influence and Correction of Wall Slip Prevention Measures

Area of Science:

• Materials Science
• Fluid Dynamics
• Rheology

Background:

• Suspensions in industries like food, cement, and cosmetics often exhibit wall slip.
• Structured measuring surfaces in rheometers are widely used to mitigate wall slip effects.
• Traditional rheological calculations using parallel-plate geometries assume no flow beyond the measured gap, which is often inaccurate.

Purpose of the Study:

• To determine the effective gap extension (δ) for various structured surfaces and fluid types.
• To compare corrected rheological values with reference data.
• To investigate the applicability of a derived correction function across different fluid behaviors and structured systems.

Main Methods:

• Experimental determination of the effective gap extension (δ) using different structures and fluids (Newtonian, shear-thinning, model suspensions).
• Correction of the prescribed gap height (H) by adding the effective gap extension (δ) to obtain absolute rheological properties.
• Comparison of corrected rheological data against established reference values.

Main Results:

• A gap- and material-independent correction function was derived for Newtonian fluids for each measuring system.
• This correction function was found to be applicable to suspensions.
• The derived correction function was not applicable to shear-thinning fluids, indicating a dependence on flow behavior characteristics.

Conclusions:

• Accurate rheological property determination requires correcting the measuring gap (H) with the effective gap extension (δ).
• Structured rheometer surfaces can be calibrated using Newtonian fluids and this calibration is transferable to suspensions up to a certain particle concentration.
• The flow behavior of the fluid significantly influences the effectiveness of gap correction methods.