Thixotropy – Cosmetic Uses in Rheology and Product Stability

What Is Thixotropy?

Thixotropy is a rheological property in which a material becomes less viscous when subjected to shear and gradually recovers its original structure when the shear is removed.

It is particularly relevant to non-Newtonian cosmetic formulations. Many cosmetic products are structured at rest but become more fluid when subjected to the stresses generated during pumping, pouring, spreading or application.

The time-dependent nature of thixotropy distinguishes it from simple shear-thinning behaviour. A thixotropic formulation not only becomes less viscous under shear but also requires time to rebuild its internal structure after the shear is removed.

Thixotropy in Cosmetic Formulations

Thixotropic behaviour can be desirable in many semisolid and liquid cosmetic products. An ideal topical product, for example, can remain sufficiently structured in its container, flow when pressure is applied, and become more fluid when spread over the skin.

This behaviour can be useful in:

• Creams and lotions
• Gels
• Emulsions
• Suspensions
• Masks
• Foundations
• Other semisolid topical products

In suspensions, a thixotropic structure can help reduce sedimentation during storage while allowing the product to become sufficiently fluid during shaking or application.

Thixotropy and Product Stability

Thixotropy can contribute to the physical stability of formulations by providing a structured system at low or zero shear. A higher viscosity under resting conditions can reduce the movement of dispersed droplets or particles.

When shear is applied, the structure breaks down and viscosity decreases, facilitating dispensing and application. Once the product is at rest again, the structure gradually rebuilds.

The degree and rate of structural recovery are important formulation characteristics. If recovery is too slow, the product may remain excessively fluid after application. If the structure rebuilds too rapidly or is too strong, dispensing and spreading may become difficult.

Factors Affecting Thixotropic Behaviour

The rheological behaviour of a cosmetic formulation can be influenced by:

• Type and concentration of rheological additives
• Molecular weight of polymers
• Emulsion structure
• Internal and external phase viscosity
• Phase-volume ratio
• Particle size distribution
• Emulsifier system
• Electrolyte concentration
• Processing conditions and shear history

Even relatively small changes in ingredient concentration or ratios can substantially alter the rheological characteristics of a formulation.

Measuring Thixotropy

Thixotropic behaviour can be evaluated using rheological measurements that apply controlled shear and monitor the resulting changes in viscosity or structure. Rotational and oscillatory instruments are commonly used to characterise non-Newtonian cosmetic systems.

During product development, understanding thixotropy helps formulators balance stability at rest, dispensing, spreadability and sensory performance during application.

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