Rheological studies to objectify sensations occurring when cosmetic emulsions are applied to the skin

When cosmetic emulsions are applied to the skin, different flow conditions arise because the film thickness of the emulsion decreases as it is rubbed in. The sensations experienced during application — the so-called skin feeling — therefore result in part from the flow properties of an emulsion subjected to varying conditions of stress. To take this into account when investigating the skin feeling, a more discriminating approach is developed in this study than simple correlation of the skin feeling with a stationary rheological parameter. In this study, the skin feeling is divided into two parts, a primary and secondary skin feeling, which are each correlated with different rheological variables. The primary skin feeling describes the sensations at the start of application, whereas the secondary skin feeling describes the sensations at the end of application when the product has been almost completely rubbed into the skin. The primary skin feeling correlates with the shear stress at the onset of flow τF and the dynamic viscosity ηdyn. Ranges are defined for these variables ΔτF and Δηdyn that encompass the values for products assessed as good in sensory panel tests. The secondary skin feeling correlates with the value of the stationary viscosity η for the rate of shear prevailing at the end of application to the skin. Ranges of the stationary viscosity Δη are determined for this rate of shear that will include all values corresponding to a good assessment of the secondary skin feeling. The application shear rate for the creams studied is in the magnitude of γ̇Appl=500s−1 and for the lotions γ̇Appl.=5000s−1. Furthermore, the results of this study show that the shear stress at the onset of flow alone τF is not an unambiguous criterion for distinguishing between types of products, as has been described in the literature. In the lotions and creams studied, the characteristic difference is the height of the maximum ηmax of a dynamic viscosity curve. The viscosity maximum for creams is in the order of magnitude of ηmax=1000 Pa s and for lotions ηmax=100 Pa s. Barry [1]correlated results of sensory testing of cosmetic products with the stationary viscosity for a defined rate of shear. The results of this work show, however, that sensory testing does not correlate with the stationary viscosity alone. To assess the primary skin feeling by means of rheological measurements, a dynamic viscosity curve is needed that must be evaluated at the characteristic points. The results of rheological measurements are correlated with those of field tests. For this we carefully considered the most important criteria of sex, age and skin type, so that the results of the field tests are representative of human skin.