RHEOLOGICAL CHANGES IN COSMETICS 327 lOO •o i i ß i iiii ß i i i i iii 0.1 1 10 Viscosity I Pa-s Figure 5. Relationship between sample viscosity and friction using the block-type probe. Viscosity stan- dard liquids (5•40 pl) were measured under a probe load of 0.05 N and with reciprocation at 2.4 cm/s. The open circle (¸) represents the amount of sample (5 pl) that was applied on the stage the closed triangle ('), 10 pl the open square ([•), 20 pl the open triangle (/•), 40 pl. Apparent viscosity was measured at 32øC. 60 60 Z 50 4O o 30 .o_ 20 10 z 50 •40 • 30 ._o 20 10 B 0 5 10 15 20 0 5 10 15 Time / min Time / min Figure 6. Measurements of typical. O/W-type and W/O-type moisturizing creams using the block-type type probe (A) and the roller-type probe (B). The closed circle (0) represents the rich, sticky O/W-type cream the closed triangle ('), the rich, sticky W/O-type cream the open circle (¸), the light, non-sticky O/W-type cream the open square ([•), the light, non-sticky W/O-type cream. The probe load and recip- rocation were 0.05 N and 2.4 cm/s, respectively. Sample volume was 20 pl in (A) and 10 pl in (B). stant values viscous oils that give a rich skin sensation, such as pentaerythrityl tetra- octanoate, gave high friction, whereas light oils, such as dimethyl polysiloxane, gave low friction. Figure 7B shows a clear correlation between the frictional force and the spread- ing value, which can be defined as skin surface area covered spontaneously during a certain period of time (15). This is an interesting result, because the spreading values of oils have been reported to have a close relation to the sensation that the oils produce on the skin, and to be a useful index in selecting oils for cosmetic formulations (16). Figure 8 shows the results of measurements of moisturizing gels containing various humectants (Table II). A large frictional force was observed during the first 1-2 min when the roller of the probe slipped on the reciprocating stage. At that time, the probe

328 JOURNAL OF COSMETIC SCIENCE 10 8 10 • 8 6 B 0 20 40 60 80 0 5 10 15 20 Time / s Spreading value / cm 2 Figure 7. Measurements of oil components using the roller-type probe and its relation with spreading value. The measured friction for pentaerythrityl tetraoctanoate (¸), trioctanoin ('), and dimethyl polysi- loxane ([•) is shown in (A). The relation between the friction and spreading value of oils is shown in (B). 4O Z E 30 o 20 u. 10 A 0 2 4 6 8 10 Time / min z E 12 •- 10 E 8 o 6 • 4 o 2 ._u 0 -B Figure 8. Effect of humectants on friction using the roller-type probe. Moisturizing gels (10 I•l) containing various humectants (shown in Table lI) were measured under a probe load of 0.05 N and with reciprocation at 2.4 cm/s. Friction curves for gels containing 10% glycerol (O), 10% DPG (dipropylene glycol,/•), and 10% BG (1,3-butylene glycol, [•) are shown in (A). The detected friction levels at 10 min are summarized in (B). spread the sample over the sweep area. Differences in the frictional force gradually appeared as the slipping of the roller disappeared over time. At 10 min, the frictional force with the gel containing 10% glycerol was 5-7 times higher than that with the gel containing 10% 1,3-butylene glycol or 10% dipropylene glycol. The gel containing 5 %