PENETRATION OF OILS INTO HAIR FIBERS 287 hours. The major decrease in withdrawal force for sunflower oil-treated hair was found to occur within 24 hours, whereas for coconut oil-treated hair the short-term heat treatment had the largest effect. The lowest withdrawal force, i.e., the closest to the untreated hair, was found for the coconut oil-treated hair after 24 hours with heat treatment. In Figures 2-4 the typical withdrawal force curves are shown for mineral, coconut, and sunflower oil-treated hair fibers after 5 min, 24 h, and with heat treatment. The decrease in withdrawal force is associated with changes in fiber-surface condition and can be explained as follows. The force of adhesion is determined by the effective area of the liquid bridges formed between fibers and by capillary pressure, which in turn depends on the surface tension of the oil, the contact angle of the oil on the fiber, and the curvature of the liquid bridge. An equation for the adhesive force due to capillarity for two parallel fibers has been derived by Brooks et al. (7). A decrease in the amount of oil on the fiber surface by penetration results in an increase in liquid bridge curvature and a decrease in contact area, shown in Figure 5. From this combined effect, an initial increase followed by a decrease in the withdrawal force is expected. Thus, decreased withdrawal forces found for sunflower and coconut oil-treated hair fibers indicate the penetrability of these oils into the hair fibers, leading to thinning of surface oil films. Unchanged withdrawal forces for mineral oil imply the possible lack of penetration and film thinning. These findings are in good agreement with fiber interior studies per­ formed by secondary ion mass spectrometry (SIMS) in combination with a time-of-flight (TOF) mass spectrometer, where mineral oil was not detected within the hair fiber cross section, whereas coconut oil was found to penetrate partially or completely (2). EFFECT OF MINERAL AND COCONUT OILS ON GP MEASUREMENTS It was the work of Stamm et al. (4) that showed that the shift in the reflectance peaks 160 - Cl) � � 140 120 100 80 60 40 20 0 A 5mm Length of Fiber Moved Figure 2. Typical withdrawal force curves for Indian hair fibers treated with coconut oil after 5 minutes (A), after 24 hours (B), and after 24 hours with heat treatment (C). Curves shifted arbitrarily.

288 JOURNAL OF COSMETIC SCIENCE 160 140 en 120 .§. G) 100 80 l! 60 "CS .c 3i 40 20 0 Length of Fiber Moved 5mm Figure 3. Typical withdrawal force curves for Indian hair fibers treated with mineral oil after 5 minutes (A), after 24 hours (B), and after 24 hours with heat treatment (C). Curves shifted arbitrarily. 250 200 - C, §. G) 150 l! 100 .c 50 Length of Fiber Moved 5mm Figure 4. Typical withdrawal force curves for Indian hair fibers treated with sunflower oil after 5 minutes (A), after 24 hours (B), and after 24 hours with heat treatment (C). Curves shifted arbitrarily.