Viscosity: Definition, Measurement and Importance in Cosmetic Formulation

What Is Viscosity?

Viscosity describes how strongly a liquid resists flowing. A low-viscosity liquid flows easily, while a high-viscosity material flows more slowly and feels thicker.

For example, water has a relatively low viscosity, while a cream or gel has a much higher viscosity.

Viscosity in Cosmetic Formulation

Viscosity is an important parameter when developing cosmetic products because it affects both product performance and consumer experience. It can influence:

• Texture and consistency
• Spreadability
• Pourability
• Ease of application
• Product appearance
• Filling and packaging
• Stability of the formulation

The desired viscosity depends on the type of product. A facial toner may require low viscosity, while a cream, gel or shampoo may require a higher viscosity.

How Is Viscosity Measured?

Viscosity can be measured using instruments such as rotational viscometers and rheometers. The measurement may be expressed in millipascal-seconds (mPa·s) or centipoise (cP).

For many cosmetic products, viscosity can also depend on the shear rate. This means that a product may behave differently when it is at rest, being stirred, pumped or spread on the skin.

Viscosity and Cosmetic Product Development

Viscosity is normally evaluated during formulation development and quality control. Changes in viscosity can indicate changes in the formulation and may be monitored during stability testing.

Factors such as temperature, emulsifier system, polymers, surfactants, oils and the concentration of other ingredients can all affect the final viscosity of a cosmetic product.

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