Shear Stress – Cosmetic Uses in Rheology and Flow Behaviour

What Is Shear Stress?

Shear stress describes the force applied parallel to a material's surface that causes its layers to move relative to one another. It is commonly expressed in pascals (Pa).

In cosmetic formulations, shear stress is generated whenever a product is subjected to a force that causes it to flow or deform. This occurs during manufacturing, pumping, pouring, spreading and application to the skin or hair.

Shear stress is closely related to shear rate. Shear stress describes the applied force, while shear rate describes the resulting rate of deformation.

Shear Stress in Cosmetic Formulations

Shear stress is particularly relevant when evaluating creams, lotions, gels, emulsions, shampoos, conditioners and other structured cosmetic products.

When sufficient shear stress is applied, a structured formulation can begin to flow. The minimum stress required to initiate flow is referred to as yield stress.

This property is important for products that need to remain stable and structured while at rest but become fluid when sufficient force is applied during dispensing or application.

Shear Stress and Viscosity

The relationship between shear stress and shear rate provides important information about the rheological behaviour of a cosmetic formulation.

For a Newtonian fluid, shear stress increases proportionally with shear rate and the viscosity remains essentially constant. Many cosmetic products, however, are non-Newtonian and their viscosity changes depending on the applied shear.

For example, a cream may have a relatively high resistance to flow at rest but become progressively easier to spread as shear stress is applied. This behaviour can contribute to both product stability and consumer handling.

Shear Stress During Product Development

Rheological measurements can be used to assess how a formulation responds to different levels of applied shear stress. This can help formulators optimise:

• Product structure and viscosity
• Pumping and dispensing behaviour
• Spreadability
• Sensory characteristics
• Suspension stability
• Manufacturing and processing conditions

The appropriate rheological profile depends on the type of cosmetic product and its intended packaging and application method.

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