RHEOLOGICAL CHANGES IN COSMETICS 325 was equipped with a probe-lifting function as described in Materials and Methods. Improved spreading status of samples on the stage was obtained with this function (Figure 3). Figure 4 shows the results of measurements with the block-type probe using O/W-type massage gels (Table I) that drastically change their spreading properties during mas- saging. The changes in the measured frictional force reflected the changes in spread- ability experienced during massaging. Since the measured frictional force decreased at almost the same rate that the spreadability decreased on the skin, the device could reproduce and detect the same phenomenon that occurred on the skin. To examine the relationship between the measured frictional force and the viscosity of samples, viscosity standard oils were used for measurement (Figure 5). A high correlation between the frictional force and the viscosity was found over a broad range, with only a minor dependence on the sample volume, which possibly could decrease over time due to evaporation if conventional cosmetics are measured. In addition, large differences in viscosity resulted in relatively small differences in friction (e.g., a 70-fold increase in viscosity yielded only an 8- to 9-fold increase in friction). This is due to the variation of sample thickness according to viscosity. The relation between the sample thickness and viscosity made it possible to detect the spreading changes that arise from large viscosity changes without changing the attenuation of the strain gauge amplifier. This is favorable for detection of dynamic changes occurring on the skin, since changing the attenuation of the amplifier is generally associated with zero-point shifting in measurement. Con- versely, it may become more difficult to detect changes in skin sensation that arise from small viscosity changes. With respect to another important sensory attribute, namely stickiness, no clear corre- lation was found between the results of the sensory evaluation and the frictional force of conventional moisturizing creams measured with the block-type probe (Figure 6A). The frictional force with the rich, sticky W/O-type cream was smaller than the force of the other two (light, non-sticky) creams. The frictional forces fell to almost the same level (about 0.01 N) 10 rain after the beginning of measurements, irrespective of the sticki- ness properties of the measured samples. The baseline friction might hinder the evalu- ation of stickiness. MEASUREMENT USING THE ROLLER-TYPE PROBE To evaluate the stickiness of cosmetics, another type of probe, i.e., the roller-type probe, was developed and used for the measurement of the same moisturizing creams described Figure 3. Improved spreading of samples on the stage owing to the probe-lifting function Sample distribution after 1 rain of reciprocation with probe lifting (A) and without lifting (B). A W/O-type foundation was used as a sample. Sample volume was 20 lal probe load, 0.05 N reciprocation, 2.4 cm/s. The scale bar in (B) represents 10 mm.

326 JOURNAL OF COSMETIC SCIENCE 4O z l:: 30 o 20 ii mllm a mllml '10 o o 50 lOO 15o Time / s Figure 4. Measurements of massage gels using the block-type probe. Sample volume was 20 lal probe load, 0.05 N reciprocation, 7.2 cm/s. The open (C)) and closed (0) circles represent the frictional force of gels A and B, respectively. The formulae of gels A and B are given in Table I. Table I Composition of Massage Gels and Their Spreading Profile on Skin Component Massage gel A Massage gel B Deionized water 11.32 15.75 Liquid paraffin 25.5 21.07 Trioctanoin 20 20 Dimethyl polysiloxane 15 15 Glycerol 5 5 Sorbitol 12.6 12.6 PEG 400 5 5 Sodium alginate 0.03 0.03 Dimethicone copolyol 0.5 0.5 PEG-60 hydrogenated castor oil 0.5 0.5 Sucrose stearate 2.4 2.4 Poloxamer 184 2 2 Sodium methyl cocoyl taurate 0.15 0.15 Spreading profile on the skin Simple decrease Decrease (about 65 s) (about 45 s) after a temporary increase above. The frictional force of the rich, sticky W/O-type cream was higher than that of the light, nonsticky W/O-type cream (Figure 6B). Similarly, the frictional force of the rich, sticky O/W-type cream was higher than that of the light, non-sticky W/O-type cream. These results are in accordance with the stickiness properties on skin. To study the properties of this measuring system, typical ingredients that cause sticki- ness of cosmetics (oils, humectants, and thickening polymers) or those added in standard formulations were used as samples. Figure 7A shows the results of measurements of oils commonly used for cosmetic products. The frictional forces immediately reached con-