## 3 Results
### Application Analysis
#### 3.5.2 Rheological Behavior  
In the field of cosmetic science, viscosity management is of paramount importance, having a profound impact on the overall perceived quality of the product as it determines its consistency and spreadability on the skin [59; 60]. This significance is particularly evident in surfactant-based cosmetic systems, where precise viscosity control is a routine and crucial practice in industrial settings. This control relies primarily on the use of rheology modifiers to fine-tune the product's consistency. Achieving a certain viscosity level is not just a technical requirement but a fundamental necessity to ensure optimal dispensing of  
Figure 5: Assessment of resazurin reduction via keratinocytes (HaCaT) after a 24 h exposure to the tested facial servums (at concentrations of 0.01, 0.1, and 1.0%). The black bar shows control cells (not treated with the tested preparations) for which the viability was assumed to be 100%. Data are the mean \(\pm\) SD of three independent experiments, each consisting of four replicates per treatment group. Figure 6: Assessment of the uptake of neutral red dye via keratinocytes (HaCaT) after 24 h exposure to the tested facial servums (at concentrations of 0.01, 0.1, and 1.0%). The black bar shows control cells (not treated with the tested preparations) for which the viability was assumed to be 100%. Data are the mean \(\pm\) SD of three independent experiments, each consisting of four replicates per treatment group. the product from the package and to facilitate even spreading on the skin. Moreover, in cosmetic preparations, high relative viscosity is perceived among users as corresponding to high concentrations of active ingredients and high efficacy. Rheological properties are often selected to align with the type of cosmetic product and its impact on the skin. Cosmetics designed for skin regeneration and irritation relief, especially those intended for dry and flaky skin, should linger on the skin's surface for an extended period [60; 61; 62], and these are commonly provided as creams and serums [63]. In this context, rheological modifiers play a pivotal role in adjusting the viscosity profile to meet desired specifications, thereby enhancing the consumer's experience with the product. The rheological characteristics of the model facial serums obtained are visually represented in Figure 7. This analytical representation aids in comprehending the flow and deformation properties of facial serums, elucidating their practical suitability and consumer perceptions. The observed rheological properties of the model facial serums show a non-linear trend between viscosity and shear stress with respect to increasing shear rates. This characteristic behavior is representative of non-Newtonian fluid dynamics, a common feature identified in all tested facial serum formulations. The addition of grapevine bud extract to the formulation of facial serums, instead of a pure extraction medium, resulted in an increase in apparent viscosity. This suggests that the extract is rich in bioactive compounds that contribute to the overall rheological profile of the formulations under development. The highest apparent viscosity was observed for the facial serum based on grapevine buds' micellar extract, indicating the highest amount of bioactive compounds extracted via micellar extraction from grapevine buds. The high concentration of active ingredients influenced the product viscosity. On the other hand, incorporating a pure solvent or pure micellar solution in the designed formulations during the LCE process had relatively little effect on apparent viscosity, as indicated by marginal significance (\(p>0.05\)). The observed relationship of shear stress increase with increasing shear rate in the model facial serum was fitted to the power-law model (Equation (4)), providing a quantitative representation of the rheological properties of the tested products. Figure 7: Rheogram of model facial serums: (**a**) log viscosity vs. log shear rate; (**b**) log shear stress vs. log shear rate.