SKIN FRICTION 61 50 40 30 2O 10 O-- -10 EFFECT OF LUBRICANT COSMETIC INGREDIENTS ON SKIN FRICTION COEFFICIENT (mean of 5 subjects) I I I I I Baseline '• Petrolatu m ---• Glycerin ß Heavy mineral oil -20 -30 0 • 1 2 3 4 5 Time after application (hrs) Immediately after application Figure 3. Effect of lubricant cosmetic ingredients on skin friction coefficient. Amount applied of each material: approx. 2 mg/cm 2. Each point is the mean of five subjects measured in triplicate. ately after application (Figure 3). As the materials were absorbed into the skin surface layers and the hydrating effects overcame the diminishing lubricating effect, a gradual increase in friction coefficient was observed. EFFECTS OF MOISTURIZING FORMULATIONS AND SURFACE LUBRICANTS ON SKIN FRICTION We then compared the changes in friction coefficient induced by different products, immediately after application and for six hours thereafter, with the perceived sensory skin feel. The changes in skin friction coefficient induced by the six moisturizing formulations tested can be grouped into three main categories (Figure 4). Products A, B, and C, which were perceived by the sensory panel as essentially nongreasy, induced similarly high increases in friction coefficient immediately after application, reflecting a prompt hydration effect (Figure 4-a). These increases were the highest among the six products tested and, although somewhat lower than those induced by water, they were significantly longer lasting. The effect of water treatment lasted only 10 to 15 min, while those observed with products A, B, and C persisted for the six hours of the

62 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS •I CHANGES IN SKIN FRICTION COEFFICIENT AFTER TREATMENT WITH DIFFEREN 120]l • MOIS'rURIZING FORMULATIONS Jl (mean of 5 subjects) c õ oo •, 40 • zoJJ- Ill20 J-.•"'•b J- I-Y ] I I • I I I • I I I I O* 2 4 6 O* 2 4 6 O* 2 4 6 Time after application (hrs) . Immediately after application Figure 4. Changes in skin friction coefficient after treatment with different moisturizing formulations. A, B, C, D, E, and F are six commercial moisturizing products. Amount applied of each product: approx. 2 mg/cm 2. Each point is the mean of five subjects measured in triplicate. duration of the test, indicating the prolonged hydration induced by the products. The large increases in friction coefficient observed with these products indicate that the skin surface can generate an appreciable drag thus, it will be non-slippery and the product will be perceived as non-greasy. These conclusions are well substantiated by the results from the sensory evaluation. Products D and E, which were perceived by the sensory panel as moderately greasy, resulted in a second type of instrumental response (Figure 4-b). They induced a moderate increase in friction coefficient immediately after product application and this increase continued for the six hour period tested. This moderate increase in skin friction early after product application can be interpreted as the composite effect of an immediate hydration produced by the aqueous vehicle and some degree of slipperiness, related to the nature of the residue, which possibly produces some occlusive effects. As the occluded skin surface becomes more hydrated, this process overcomes the lubricant effect of the treatment and a steady increase in friction coefficient is observed. Product F was perceived in sensory evaluation as considerably greasy and it exemplifies a last type of products which induce an immediate decrease in friction coefficient due to the slipperiness (or greasiness) of the product (Figure 4-c). The