PENETRATION OF OILS INTO HAIR FIBERS 289 Figure 5. Schematics of the oil liquid bridge between two fibers. Oil penetration decreases the contact area, A = 'TTR2, and increases the Laplace pressure, P = crcos0/r, where CJ is the surface tension of oil and 0 is the contact angle. The combined effect of these two results in a decrease in adhesive force (P 1 A 1 P 2 A2). for a hair fiber in the R-T and T-R modes at a given angle of incidence was due to the scale angle. Deposition of thick oil films eliminated this shift by masking the scale structure with the oil film. For such a system, the reflectance peaks in the R-T and T-R modes overlapped. In Figure 6A the GP curves recorded in the R-T and T-R positions for an untreated Indian single hair fiber and for the same fiber treated with a mineral oil are shown. For the untreated hair fiber the sharp specular reflectance peak occurs at an angle different from the angle of incidence, due to the inclination of scales relative to the axis of the fiber. For R-T and T-R scans, the specular peaks have a maximum at 38 ° and 52°, respectively. From these values a scale angle of 3.5° for untreated hair fiber is calculated. After application of mineral oil, the specular reflectance peaks overlap, both appearing at 45°. The angle of reflection being equal to the angle of incidence indicates the formation of a smooth mirror-like oil film on the fiber surface. Hair-fiber scale structure is no longer seen. Also, the sharper peaks with higher reflectance compared to untreated hair fiber illustrate the lustrous surface without diffuse reflectance. For mineral oil, the GP intensity scans, directly after oil treatment (not shown in Figure 6A) and after 24 hours, are similar regarding both the peak position as well as reflected light intensities. The additional shoulder on the specular reflectance peak for R-T at a higher angle is most probably caused by the discontinuity of film at certain locations. After heat treatment for five minutes, the intensity of reflectance has decreased by a factor of 1.6, as shown in Figure 6B. However, the shape of the GP curve remains similar to those obtained after 24 hours, with no separation indicative of the scale angle. Thus, even after the heat treatment, most of the mineral oil remains on the hair surface. It is worthwhile mentioning that the GP curves remained unchanged even with heat treatment over a longer period of time (20 minutes). Finally, in order to demonstrate the reproducibility and reliability of the results and the method, the hair fiber was dipped into acetone, removing the mineral oil film from the fiber surface. This resulted in the separation of scans from the R-T and T-R positions, giving a scale angle of 3.4°. Also, the reflection intensities are comparable to ones obtained from the initial measurements. To some

290 JOURNAL OF COSMETIC SCIENCE A 0.05 .......-----------------------------, 0.04 0.03 cti 0.02 � "in 0.01 0 0.030 0.025 0.020 � 0.015 "in 0.010 0.005 0.000 0 untreated R-T 20 40 60 80 Scattering angle (degrees) Oil removed R-T 20 I I 40 B Mineral oil 24 h and heat T-R R-T Oil removed T-R 60 80 Scattering angle (degrees) Figure 6. Goniophotometric intensity scans: (A) Individual untreated Indian hair fiber and 24 hours after mineral oil application. (B) 24 hours after mineral oil application with short-term heat treatment and after removal of oil film with acetone.