Rheology

What Is Rheology?

Rheology describes the flow characteristics of liquids and the deformation of solids. In cosmetic formulations, rheological properties are particularly important for liquid and semiliquid products because they influence mixing, processing, filling, application, consistency, spreadability, and smoothness. Rheology can also affect the physical stability and biological availability of a product.

Newtonian and Non-Newtonian Flow

Cosmetic systems can generally be classified as Newtonian or non-Newtonian.

Newtonian fluids have a constant viscosity when subjected to shear, meaning that flow rate is directly proportional to the applied shear stress. Examples include water and mineral oil.

Most cosmetic products, however, exhibit non-Newtonian behaviour, where viscosity changes depending on the applied shear. This can include plastic, pseudoplastic, and dilatant flow, as well as thixotropy.

Thixotropy in Cosmetic Products

Thixotropic behaviour can be particularly desirable in topical products. For example, a cream or gel may remain in place inside a tube and not flow through the opening until pressure is applied. During application, the increased shear can make the product temporarily more fluid, allowing it to spread easily over the skin.

Once the shear is removed, the formulation can recover its higher viscosity and remain in place.

Measuring and Controlling Rheology

Rheological properties are evaluated during product development to characterise the formulation and are also important for quality control after manufacturing. Rotational and oscillatory instruments are commonly used to measure the rheological behaviour of non-Newtonian systems.

Rheology can be adjusted through the selection and concentration of viscosity-modifying agents and other formulation components. Electrolytes can also significantly affect viscosity; for example, polyacrylate-based systems can be sensitive to ions.

In emulsions, rheology depends on factors including the viscosity of the internal and external phases, phase volume ratio, particle size distribution, emulsifier system, and viscosity-modifying ingredients. Even relatively small changes in ingredient concentration or ratios can therefore produce significant changes in the final rheological properties.

References

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