Pseudoplastic flow
A pseudoplastic material becomes less viscous when it is subjected to increasing shear.
This behavior is common in cosmetic formulations containing polymers, emulsions and structured surfactant systems.
For example, a cream may appear relatively thick in a container but become easier to spread when rubbed across the skin.
Cosmetic Importance
Shear thinning can improve both dispensing and application. It allows a product to maintain structure at rest while becoming more fluid during use.
The degree of shear thinning affects the sensory profile and the practical behavior of the product.
Formulation Considerations
Polymer type, polymer concentration, electrolyte concentration and formulation pH can all influence pseudoplastic behavior.
Rheological measurements at multiple shear rates are therefore more informative than a single viscosity measurement.
References
Barnes, H. A. A Handbook of Elementary Rheology.
ISO 3219-1. Rheology — Part 1: General principles and rheometry.
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