SKIN FRICTION MEASUREMENTS Table III Comparison of Friction Values on "Normal" and "Dry" Skin Load 98.9 g, Rotation Speed 2.03 cm/sec 49 Friction Force (g)a Friction Force (g)b Polished Probe Rough Probe Normal Skin 62.5 16.0 Dry Skin 37 11.6 aValues after 20 sec. bValues refer to kinetic friction force. EFFECT OF SKIN DRYNESS ON FRICTION PROPERTIES Because of the difficulty in inducing skin dryness under laboratory conditions, it was not possible to conduct an exhaustive study on the effect of dryness. We were able, however, to conduct some measurements on 1 panelist suffering from a severe case of skin dryness, which was obvious even to the untrained eye. The dry site was located on the dorsal forearm, and its extent was rather restricted. Friction measurements were made on a "normal" and the "dry" site using the polished and rough probes. The data are given in Table III. it can be seen that a substantially lower value of the force of fric- tion is found on the dry site with both probes. A much larger decrease is observed with the polished probe, however. EFFECT OF TALCUM POWDER Before-and-after friction measurements were conducted using talcum powder as a solid lubricant on 7 panelists. The polished and rough probes were used at a speed of 32.4 rpm and 98.9 g normal load. As expected, some wrinkling was observed on the untreated skin with the polished probe, and the force readings were arbi- trarily taken after 20 sec. A decrease averaging 50 per cent in the friction force was ob- served with the polished probe after application of talcum powder. No wrinkling of the skin or increase of the force with time was observed in the course of the measurement. Little or no decrease in the friction force was registered with the rough probe after ap- plication of talcum powder. The amount of talcum powder applied to the skin was enough to cover the surface with a thick film, such that direct contact between the probe and the skin substrate was not possible. The effect of talcum powder can be at- tributed to its low Sm value and adhesion to the stainless steel surface hence, a lower friction force would be expected. The fact that the same values for the friction force were obtained with the polished and rough probes after application of talcum powder, lends support to the notion that a film of talcum powder was transferred to the probe surface such that the measured values of the force reflect the property of the talcum/talcum system. EFFECT OF SILICONE OIL An investigation of the effect of silicone oil (polydimethyl siloxane) as a fluid lubricant, when applied as a thick film onto the skin surface, on the friction properties was

50 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table IV Effect of Silicone Oil Viscosity and Probe Speed on the Friction Force. Load 98.9 g. Polished Probe Viscosity of Silicone Oil (cks) Friction Force a (g) Speed (rpm) Untreated Skin 100 1,000 10,000 3.6 30.6 6.5 7.5 8.5 10.8 30.8 7.5 11.9 15.0 32.4 30.7 9.4 15.0 28.8 67.2 30.8 11.9 19.6 37.5 aValues at 20 sec. conducted on 10 panelists. The effects of speed of rotation of the probe and of the vis- cosity of the silicone oil were examined in some detail to identify the mechanism of lu- brication involved. The relevant data for 1 panelist have been compiled in Table IV. The results obtained indicate that the presence of silicone oil decreases substantially the friction force compared to untreated skin and that the mechanism involved is fluid or hydrodynamic lubrication, i.e., the friction force is dependent on the bulk properties (viscosity) of the lubricant. Both probes behave similarly in the presence of a fluid lubricant, both qualitatively and quantitatively. A number of important points emerge from the above findings regarding in vivo fric- tion measurements on skin as follows: 1. The effect of the surface condition of the probe. It has been shown that the type of finishing given to the probe surface has an important qualitative and quantitative ef- fect on the results. 2. Low friction values for untreated skin do not necessarily mean a smooth skin condi- tion. As has been shown in this work, obviously dry skin gave lower friction force values than seemingly normal skin. It is, therefore, necessary, before assigning any practical significance to the effect of product treatments on skin condition, to es- tablish a meaningful correlation between instrumental measurements and what consumers perceive as an acceptable skin condition. This can be achieved by using a large panel and trained judges to help determine the range of the friction coefficients which describe the different skin conditions. 3. In order to bring about perceptible changes in the friction properties of skin through product application, it is obvious that a sufficient amount of some beneficial ingredient should be deposited on the surface. The simplest approach is to use the product directly as in the case of creams and lotions, for example. As suggested by this work, the properties of the residual film will probably have a direct bearing on consumer acceptability. Such products will probably exhibit hydrodynamic lubrica- tion, and, hence, the viscosity of the applied film will be of considerable im- portance. Again, it will be necessary to define the optimum ranges of an acceptable friction coefficient under these conditions via panel testing. A very low value of the friction coefficient may be associated with "slippery feel" and too high a value will most likely be associated with "sticky feel."