Yield Stress – Cosmetic Uses in Rheology and Product Stability

What Is Yield Stress?

Yield stress is the minimum amount of applied stress required to initiate flow in a material. Below this level, the material behaves like a solid or structured system; once the yield stress is exceeded, the material begins to flow.

Yield stress is commonly observed in non-Newtonian cosmetic formulations. It is an important rheological property because many cosmetic products need to remain stationary in a container but flow easily when sufficient force is applied.

Yield Stress in Cosmetic Formulations

Yield stress is particularly relevant to products such as:

• Creams and lotions
• Gels
• Pastes
• Masks
• Foundations
• Toothpastes
• Cleansing products
• Emulsions
• Products containing suspended particles

For example, a cream with an appropriate yield stress can remain in a jar without flowing excessively under gravity, while still spreading easily when applied to the skin.

Yield Stress and Product Stability

Yield stress can contribute to the physical stability of cosmetic formulations. A sufficiently structured continuous phase can help reduce the movement of dispersed particles or droplets and therefore limit phenomena such as sedimentation and creaming.

Rheological additives, including clays, natural polymers and other viscosity-modifying ingredients, can be used to influence the rheological behaviour of a formulation. Their concentration and interactions with other ingredients can significantly affect yield stress and overall product texture.

Yield Stress and Consumer Experience

Yield stress also influences how a product behaves during dispensing and application. A product with excessive yield stress may be difficult to squeeze from a tube or pump from a container. Conversely, insufficient yield stress may cause a product to flow too readily and negatively affect its appearance, handling or application.

The desired yield stress therefore depends on the product type, packaging and intended method of application. Rheological properties should be evaluated as part of formulation development and quality control.

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