LIGHT SCATTERING IN HAIR CUTICLES 359 b) ! c) Figure 7. Light-scattering level from glittering speckles of the cuticle just after blow-drying (70ø030 sec) from (a) wet (immersed in de-ionized water), (b) damp (moisture controlled at 75% RH/23øC), and (c) dry (dried in silica-gel desiccator) conditions. Figure 8 shows optical microscope images of the hair surface before blow-drying, just after blow-drying, and five minutes after blow-drying. The light-scattering level from glittering speckles was more intense for the hair standing for five minutes at a high- humidity condition after blow-drying. The effects of humidity on the relaxation process are shown in Figure 9. In the humidifying process after blow-drying, more glittering speckles were observed in the case of relaxation in a condition of higher humidity. All these facts from Figures 6-9 suggest that the generation of the speckles is essentially not caused by heat, but by an abrupt and dramatic change in moisture content. After that, these speckles almost disappeared within ten minutes, because the hair fiber was in a high-humidity equilibrium. Figure 10 illustrates the variation of moisture content in the hair fiber from just after blow-drying. Surface moisture was measured using the NIR-PAS method. The thermal diffusion depth is considered to be comparable with the total thickness of cuticle cells (about 2-3 micrometers). The rate of bulk moisture absorption was measured by moni- toring weight changes. The scale of 0% shown in the graph corresponds to the moisture content for the hair fiber that was dried in a silica-gel desiccator, and the 100% scale corresponds to hair equilibrium, with an atmosphere at 23øC/75% RH each. The a) b) c) 20 • m 20 m Figure 8. Variation in the amount of glittering speckles on the fiber surface (a) before, (b) just after blow-drying, and (c) five minutes after blow-drying. The gradual increase in glittering speckles was observed after stopping blow-drying at a high-humidity condition. The analysis was performed on the same part of the hair fiber.

360 JOURNAL OF COSMETIC SCIENCE a) b) Figure 9. Light-scattering level from glittering speckles of cuticle (a) just after blow-drying, and five minutes after blow-drying in relaxing (b) low-humidity (30% RH) and (c) high-humidity (75% RH) conditions. moisture contents were represented by the relative percentage from the original water content of the fiber. The bulk moisture content of the fiber increased gradually and reached equilibrium within an hour. On the other hand, the surface moisture content increased rapidly within two minutes after stopping the blow-drying process. The generation of glittering speckles on the fiber surface corresponded to moisture regain in the fiber surface region. In addition, these glittering speckles decreased as the bulk moisture content reached equilibrium (Figure 11). The influence of moisture change on the generation of the speckles was confirmed more precisely. The speckles were con- firmed to appear during the relaxation process in high humidity (23øC/75% RH) for five minutes after the fibers had been transferred from a dried state in a silica-gel desiccator (Figure 12). These results can be summarized as follows. The structural changes in the cuticle cell caused by blow-drying are closely related to the change in moisture content in the hair fiber. Although there are many hypotheses that can be proposed to explain this phe- nomenon, we believe that the generation of splitting spaces between the cuticle layers is caused by two different mechanisms. The first is that the abrupt desorption of moisture from the wet hair caused by blow-drying may cause mechanical stress on the cuticle cells, and, as a result, deformation of the cuticle layers occurs. The second mechanism is that the dramatic absorption of moisture from the hair in its dry state also causes mechanical stress on the cuticle cells by inhomogeneous swelling. Both proce- dures only result from rapid shrinkage or swelling of the cuticle cell, and therefore a gradual change in moisture does not cause these phenomena. The glittering speckles are observed to a high degree just after an abrupt change in moisture occurs. The moisture content will soon be in equilibrium, and thus these speckles almost disappear within ten minutes. These structural changes in hair cuticles are believed to occur in a daily hair care routine, but they have been unnoticed thus far because deformation of cuticle layers will disappear upon relaxing at high humidity conditions or wetting with water. HAIR DEGRADATION MECHANISM In most cases, structural changes in the hair cuticle by blow-drying are reversed in a