Pseudoplastic Flow – Cosmetic Uses in Rheology and Product Formulation

What Is Pseudoplastic Flow?

Pseudoplastic flow is a type of non-Newtonian flow in which the apparent viscosity of a material decreases as the rate of shear increases. This behaviour is commonly referred to as shear thinning.

In contrast to Newtonian fluids, whose viscosity remains essentially constant regardless of shear rate, pseudoplastic materials change their flow behaviour when subjected to different levels of deformation. Most cosmetic products exhibit some form of non-Newtonian behaviour.

Pseudoplastic Flow in Cosmetic Formulations

Pseudoplastic behaviour is particularly useful in cosmetic products that need to remain relatively viscous at rest but become easier to flow when force is applied.

Examples include:

• Creams and lotions
• Gels
• Shampoos and conditioners
• Cleansing products
• Emulsions
• Suspensions
• Foundations and other liquid colour cosmetics

For example, a cream may maintain its structure while stored in a container but become less viscous when squeezed or spread over the skin. This can make dispensing and application easier while maintaining sufficient structure during storage.

Pseudoplastic Flow and Rheology

Pseudoplastic flow is one of the characteristic flow behaviours of non-Newtonian cosmetic systems, alongside plastic flow and dilatant flow.

The rheological behaviour of a formulation is important during manufacturing, filling, dispensing and application. It also influences sensory properties such as consistency, spreadability and smoothness, as well as the physical stability of the finished product.

The degree of pseudoplasticity can be influenced by the formulation's rheological modifiers, polymer concentration, phase composition and other ingredients. Small changes in the concentration or ratio of certain ingredients can result in significant changes in rheological characteristics.

Pseudoplastic Flow and Cosmetic Product Performance

A desirable pseudoplastic profile can provide a useful balance between stability at rest and ease of application. The formulation can remain sufficiently structured during storage while becoming more fluid when subjected to the shear generated during dispensing or spreading.

For emulsions, rheological behaviour is influenced by factors including the viscosity of the internal and external phases, phase-volume ratio, particle-size distribution, emulsifier type and concentration, and viscosity-modifying agents.

Rheological properties are commonly characterised using rotational or oscillatory instruments, both during formulation development and for quality-control purposes.

Source: Handbook of Cosmetic Science and Technology - André O. Barel, Marc Paye, Howard I. Maibach.