Rheology of Creams and Lotions

The application behavior and long-term stability of skin creams and lotions in both cosmetic and pharmaceutical applications are important factors for the acceptance of the product by the consumer. Rheological measurements ensure that the products feel and behave on the skin as they should. Read this article and learn how rheology contributes to producing products with the perfect flow.

The rheological properties of creams and lotions are largely influenced by the ingredients and by the manufacturing process. Whether the consumer perceives the product as being pleasant must first be investigated in sensory tests, directly on the skin [1, 4]. As soon as the ideal consistency of a cream has been found, this consistency needs to be determined quantitatively and documented in order to use this as a reference for later products. This requires parameters such as structure or spreadability, which can be measured in rotational and oscillatory tests with rheometers.

Evaluating the structure with rheology

The structure is an important parameter for the evaluation of the skin cream’s behavior when it is extracted (e.g. pressed out of a tube) and for the skin feel [3]. The structure is also significant for processing and when planning the production plant. In fact, the structural strength of a sample can be determined with an oscillation measurement, in the form of an amplitude sweep. Whereas the parameter G′ (storage modulus) describes the elastic behavior of a sample, G″ (loss modulus) represents the viscous portion. If G′ is above G″ in the linear-viscoelastic range* (LVE range, see fig. 1), the sample shows gel-like behavior at the preset frequency. This means that the sample will only start to flow when influenced by additional external forces.

Figure 1 shows two measurements on a lotion and a skin cream. The measurement reflects what can be felt immediately: The cream has a higher structural strength. The measurement shows that the values for storage and loss modulus of the lotion are clearly lower than the values for the cream (see table 1). It is surprising that even lotions – which seem quite fluid – show gel-like behavior. This is an intended property as the product should be easily applicable on the skin and not drip off.

Table 1: Overview of the test measuring results from fig. 1 and 2
Table 1: Overview of the test measuring results from fig. 1 and 2

*The linear viscoelastic range describes the part of the measuring curve in an oscillatory measurement in which the structure of the sample is not destroyed. This means that the sample does not change during the measurement and G′ and G″ remain constant.

The yield point τf is an appropriate parameter to evaluate the structural strength. The structural strength correlates with the force that is required to be applied on the sample to make it flow. The evaluation is done via the cross-over point of G′ and
G″ versus shear stress τ.

The results show that the samples can be clearly differentiated and in this way their applicative behavior can be predicted. In practice it has proved useful to have good and known samples as reference measurements.

amplitude sweep versus the shear stress τ
Figure 1: Results of the amplitude sweep versus the shear stress τ, measured on a body lotion and a skin cream at 37 °C. The yield point τf is determined via the cross-over point of G′ and G

Evaluating the spreadability

The flow behavior of creams can be evaluated using viscosity measurements. Thereby it is assumed that the sample can be spread easier at a low viscosity value [1, 2].

Cosmetic creams and lotions generally show shear-thinning behavior. This means that the viscosity is not a constant value, but depends on the shear intensity. This correlation is described using the shear rate. How strong the shear is during spreading depends on the sample and application. A sun cream, for example, is applied more quickly and with more force than an ointment used on wounds and in this way the sun cream is exposed to considerably higher shear rates.

Figure 2 shows the viscosity curve of the already mentioned samples. Both samples were measured at 37 °C to simulate their behavior on skin. The viscosity values of the skin cream are higher during the whole measuring process. At the beginning, at lower shear rates, the difference is quite clear but it decreases with increasing shear rates (see table 1). It can therefore be expected that the difference in spreadability of the cream and lotion decreases with increasing shear stress.

Shear-rate-dependent viscosity curves of a skin cream and a body lotion
Figure 2: Shear-rate-dependent viscosity curves of a skin cream and a body lotion at 37 °C

Summary

With rheology it is possible to evaluate the applicative behavior of cosmetic creams and body lotions. Measurements in oscillation are ideal for describing the structural properties of samples. These correlate with the skin feel when applied. By using viscosity curves it is possible to evaluate the flow behavior and spreadability of samples.

The best choice for this task is a rheometer which provides both rotational and oscillatory measurements. With these kinds of measuring devices, samples for both quality control and research can be tested and evaluated within a short time.

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Rheology Literature

[1] Yao, M.L. and J.C. Patel, 2001, Rheological characterization of body lotions, Appl. Rheol. 11, 83-88.
[2] Min-Sun Kwak, Hye-Jin Ahn and Ki-Won Song, Rheological investigation of body cream and body lotion in actual application conditions, Korea-Australia Rheology Journal, 27(3), 241-251 (August 2015)
[3] Brummer, R. and S. Godersky, 1999, Rheological studies to objectify sensations occurring when cosmetic emulsions are applied to the skin, Colloids Surf. A 152, 89-94.
[4] Moravkova, T. and P. Stern, 2011, Rheological and textural properties of cosmetic emulsions, Appl. Rheol. 21, 35200

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