Dilatant Flow – Cosmetic Uses in Rheology and Product Formulation

What Is Dilatant Flow?

Dilatant flow is a type of non-Newtonian flow in which the apparent viscosity of a material increases as the applied shear rate increases. It is also commonly referred to as shear thickening.

This behaviour is the opposite of pseudoplastic flow, where viscosity decreases as shear rate increases.

Dilatant behaviour is typically associated with concentrated dispersions containing a high volume of solid particles. As shear increases, the particles can interact more strongly and the system becomes increasingly resistant to flow.

Dilatant Flow in Cosmetic Formulations

Dilatant flow is less common as a deliberately targeted behaviour in conventional cosmetic formulations than pseudoplastic or thixotropic behaviour. However, it can occur in highly concentrated suspensions and dispersions.

Potential examples include formulations containing high concentrations of:

• Pigments
• Mineral powders
• Clay particles
• Solid particulate materials
• Other dispersed insoluble particles

The concentration, particle size and shape, surface properties and interactions between particles can all influence whether a dispersion exhibits shear-thickening behaviour.

Dilatant Flow and Cosmetic Product Performance

Dilatant behaviour can affect how a cosmetic product behaves during manufacturing, pumping, dispensing and application.

A formulation that becomes significantly more viscous under high shear may be more difficult to process or dispense. This can be particularly important when selecting pumps, filling equipment and mixing conditions.

During application, excessive shear thickening may also make a product feel less spreadable or more resistant to movement across the skin.

For this reason, understanding the flow behaviour of concentrated dispersions is important when developing products containing high levels of particulate ingredients.

Measuring Dilatant Flow

Dilatant behaviour can be identified through rheological measurements in which viscosity is measured over a range of shear rates. An increase in apparent viscosity with increasing shear rate indicates shear-thickening behaviour.

The rheological profile should be evaluated under conditions that represent the intended manufacturing and consumer-use processes. This helps determine whether the formulation will provide suitable processing, dispensing and application characteristics.

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