Thickeners influence the product's perception, particularly what is called the primary and secondary skin feelings.55,62 The primary feeling is related to the viscosity perceived at the onset of the flow of the product (\(\eta_{1}\ \) , viscosity at low shear stresses), while the secondary feeling is related to much lower viscosity value, as perceived at the product's application on the skin (\(\eta_{2}\ \) , viscosity at high shear stresses). Under such circumstances, the ideal value of \(\eta_{1}\ \) is between 1350 and 3500 Pa·s for creams and between 120 and 500 Pa·s for lotions. In contrast, \(\eta_{2}\ \) should be between 0.023 and 0.500 Pa·s for both types of products. This last viscosity interval corresponds to a shear rate of around 500 s-1 if the application is over small areas (e.g., face or hands), or 5000 s-1 over large areas of the body.55