Wall Slip: An Apparent Low Viscosity May Need a Method Review

A low apparent viscosity can reflect a measurement-boundary effect. Preserve the geometry evidence before attributing it to the formulation.

Recognize the Boundary Effect

Anton Paar explains that wall-slip effects can produce measured responses that are too low. The selected measuring geometry and its boundary with the sample therefore belong to the interpretation. An apparent low response should not immediately be assigned to a thinner formulation.

Its primary guidance discusses surface comparisons using smooth and profiled or sandblasted fixtures to investigate wall-slip behaviour. The laboratory should choose the actual method and account for geometry dimensions and sample condition; this guide does not prescribe a fixture for every material. Primary rheology reference.

Illustrative Comparison

Suppose an invented sample gives different low-rate apparent responses on two identified fixture surfaces. The disagreement is a reason to review the boundary and method, not immediate proof that the product batch changed. Other experimental differences still need to be controlled.

The example contains no HM trial result or quoted yield-stress value. Keep the raw curves and exact fixture identities available to the measurement owner. A selected result should not be relabelled as true viscosity simply because it agrees with the buyer’s expectation.

Ask for a Bounded Method Conclusion

Request the laboratory’s evaluation of whether slip may affect the intended comparison and how it was assessed. Retain sample condition, surface, dimensions and the relevant low-rate range. If the cause remains unresolved, preserve that limitation rather than inventing a formulation explanation.

Complete the worksheet before sharing the selected curve in a filling enquiry. Separating a possible test-boundary effect from a material change improves the product description, while actual product-path compatibility and machinery performance remain separate questions. HM application context: Servo Piston Liquid Filling Machine.