Shear Rate – Cosmetic Uses in Rheology and Flow Behaviour

What Is Shear Rate?

Shear rate describes the rate at which adjacent layers of a fluid move relative to one another during deformation. It is an important parameter in rheology and is commonly expressed in reciprocal seconds (s⁻¹).

In cosmetic formulations, shear rate can vary considerably depending on how a product is handled. A formulation experiences different shear rates when it is at rest, being poured or pumped, spread across the skin, or sprayed.

The viscosity of many cosmetic formulations changes with shear rate. These materials are therefore described as non-Newtonian fluids.

Shear Rate in Cosmetic Formulations

Different stages of a product's use correspond to different shear-rate regions.

At low shear rates, the product is close to a resting condition. This region is relevant to phenomena such as sedimentation and levelling. A higher viscosity at low shear can help keep dispersed particles suspended and maintain the structure of a formulation.

At moderate shear rates, viscosity may decrease as shear rate increases. This behaviour can facilitate pouring, dispensing and pumping.

At high shear rates, such as those generated during rubbing or spraying, some formulations exhibit substantially lower viscosity. This reduced resistance to flow can contribute to easier spreading and a smoother sensory experience during application.

Shear Rate and Cosmetic Product Performance

Shear rate is important when evaluating how a cosmetic behaves during actual use. A cream may have relatively high viscosity while stored in a container but become easier to spread when subjected to the higher shear generated during application.

Similarly, a shampoo, shower gel or lotion must be sufficiently structured at rest while still being able to flow through its packaging and spread easily during use.

For this reason, measuring viscosity at only one shear rate may not fully describe the flow behaviour of a non-Newtonian cosmetic formulation. Rheological measurements across a range of shear rates can provide a more complete understanding of product behaviour.

Shear Rate and Rheological Modifiers

The relationship between shear rate and viscosity is also important when selecting and optimising rheological additives. The molecular weight and concentration of rheological additives can strongly influence viscosity, particularly in the low-shear region.

Understanding shear-rate behaviour therefore helps formulators balance stability at rest, dispensing performance and sensory properties during application.

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