SKIN FRICTION 59 Ta'ble III Attribute Rating of Moisturizing Formulations • Sensory Attribute A vs B A vs C A vs E A vs F B vs D Product disappears 2 2.84 1.9 3.24 2.2 3.04 2.3 2.24 1.5 3.2 2.7 Skin feels tacky/sticky 3 1.! 1.! 1.4 1.4 1.5 2.0 2.1 2.2 1.14 2.0 Skin feels greasy 3 0.8 1.0 0.7 4 1.3 0.84 2.9 1.8 4 4.2 1.2 4 2.8 Overall preference 10/20 10/20 10/20 9/20 14/20 5/20 18/20 2/20 16/20 4/20 Mean values of 2 Rating scale: 3 Rating scale: 4 P _ .05 20 subjects 0--not at all 0--not at all 1--very slowly 1--very slight 2--somewhat slowly 2--slight 3--somewhat quickly 3--moderate 4--moderately quickly 4--considerable 5--very quickly 4--very much so (A vs. F), a rather different value (1.8) was assigned to A. The other product in this cell, F, received a very high score of 4.2 ("Considerable" to "Very much so"). It is possible that the high level of greasiness perceived for F, distorted the rating of A, bringing it up from "very slight" to the "slight" level. Preference data elicited as part of the above evaluations, and also included in Table III, suggest that, in this type of products the perceived greasiness of a product affects its preference. When two products are given similar greasiness scores (as in the first two cells), overall preference scores are distributed evenly. But as the magnitude of the difference in greasiness scores grows, preference for the less greasy product becomes more pronounced. EFFECTS OF WATER APPLICATION ON SKIN FRICTION The effects of water on skin surface friction were measured as a function of time after application. Results (Figure 2) are expressed as percent change over baseline values. In all the subjects tested, there was a sharp increase in the coefiqcient of friction immediately after water application. However, this effect was short-lived and the skin returned to its baseline state within 10 min. This immediate, pronounced increase in friction coefiqcient upon skin hydration has been observed by other investigators (1,3,5-7). Highley et al. (7) observed two frictional peaks after water application, and proposed that the initial increase in friction reflects the surface tension of the water layer interposed between the surface of the wheel and the skin, while the second peak, which occurs 10 min later, reflects an increase in the adhesiveness of the stratum corneum brought about by hydration of the surface cell layers. In our studies, we observed only one frictional increase following water application. We used approximately 2 mg of water per cm 2 of skin, while Highley et al. applied almost 8 mg/cm 2. It seems likely that, due to the smaller dosage employed in this study, there was not enough liquid to form a continuous layer between the wheel of the instrument and the skin and, therefore, we could not detect the effects of the surface tension of the liquid. Instead, we measured directly changes in coefiqcient of friction that possibly reflect the increased adhesiveness induced by rapidly hydrating the stratum corneum.

60 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 150 0 5 10 1 2 3 4 min hrs Ti me after application -x-Immediately after application CHANGES IN SKIN FRICTION COEFFICIENT INDUCED BY WATER APPLICATION (mean +SE of 5 subjects) 5 6 Figure 2. Changes in skin friction coefficient induced by water. Amount applied: approx. 2 mg/cm 2. Each point is the Mean + SE of five subjects measured in triplicate. This surface phenomenon can also be explained by other changes in the physical- chemical state of the stratum corneum. The water absorbed by the stratum corneum can increase the size and surface characteristics of its cells. This could bring about an increase in the contact area between the probe and the skin cells and lead to an increase in friction coefficient. As water is lost through evaporation, the stratum corneum returns to its original state and this is associated with a return of the friction coefficient to baseline value. EFFECT OF VISCOUS LUBRICANTS ON SKIN SURFACE FRICTION Viscous lubricants such as petrolatum, heavy mineral oil, and glycerin may hydrate the skin surface by reducing the rate of transepidermal moisture loss this brings about a subsequent increase in friction coefficient induced by the slow and prolonged hydrating effect. However, the initial lubricating effects of the materials would be expected to result in a reduction in friction. Accordingly, when petrolatum, heavy mineral oil, and glycerin were tested, they decreased the friction coefficient immedi-