WETTING CHARACTERIZATION OF HAIR FIBERS 35 hair samples were supplied by International Hair Importers (Valhalla, NY). One sample of brown virgin hair stripped of 18-methyl-eicosanoic acid was produced by soaking the hair tress in a 0.1 M KOH/methanol solution for 30 minutes as described by Swift and Smith (11). Hereafter, we designate the hair with 18-methyl-eicosanoic acid removed as “stripped.” All the hair tresses were cleaned by rinsing with methylene chloride followed by methanol and deionized (DI) water, as described by Molina et al. (4). The one-hour- bleached hair tress was treated with a commercial rinse-off conditioner (double applica- tion of 0.5 gram of conditioner to three grams of hair, followed by 30 seconds of hair massaging and an extensive rinse with 40°C tap water for approximately two minutes and DI water for one minute). All hair characterization was carried out at approximately the middle of 12-inch-long hair fi bers. The advancing contact angles were measured using a Cahn DCA-315 tensiometer. The fi bers were fi rst submerged into iso-octane and, assuming complete wetting, the fi ber circumference was determined based on the measured force. The contact angles in water were then calculated using the force and the estimated fi ber circumference. During the measurement, the fi bers were submerged to a depth of 2 mm with a rate of 20 μm/s. The wetting properties of the hair fi bers were determined by mounting the hair fi bers on an in-house built stage (see Figure 1) that carried a pair of parallel hair fi bers in a hori- zontal plane with a separation of ∼0.75 mm. Water droplets of the volume of 0.2∼0.4 μl were then applied onto the stretched fi ber pair by using a microsyringe with a 33-gauge needle. Droplets (10∼15) were placed along the fi ber pair with a length ∼6 cm as shown in Figure 1. The droplets were observed by an Olympus BX 40 microscope under 20× magnifi cation, and the images were taken using a Nikon 4500 digital camera. RESULTS AND DISCUSSION CONTACT ANGLE BY WILHELMY’S METHOD The contact angles on the virgin, stripped, bleached, and bleached/treated fi bers were measured using Wilhelmy’s method. The determination was carried out at the middle of Figure 1. Experimental assembly used for differential wetting characterization (DWC). The upper image is the top view of the droplet placement between hair fi bers. The lower image is the side view of the assembly to show the droplets suspended between parallel hair fi bers.

JOURNAL OF COSMETIC SCIENCE 36 three randomly selected hair fi bers. The fi bers were fi rst prepared on an aluminum foil and neutralized with a Milty Zerostat* anti-static gun to eliminate the possible electro- static effect that may be induced in the processes of packaging and handling. The circum- ference of each fi ber was determined by dipping the fi ber into iso-octane. Following this determination, the fi ber segments exposed to the iso-octane were trimmed and then submerged in DI water. By assuming that the circumference of hair fi bers does not vary appreciably over a short fi ber length, the contact angle can be calculated such that (1): T J cos( ) F L (1) where F is the measured force, γ is the surface tension of water, and L is the fi ber circumference. As expected, removal of 18-methyl-eicosanoic acid by methanolic KOH solution resulted in a decrease in the contact angle from 88° ± 2° of the virgin hair down to 83° ± 2° (the stripped hair). The bleached hair fi bers exhibited a lower contact angle of 78° ± 1°. Con- ditioner treatment of the bleached hair fi bers yielded an increase in the contact angle to 83.5° ± 0.2°. DIFFERENTIAL WETTING CHARACTERIZATION The differential wetting property was characterized by placing DI water droplets between two parallel hair fi bers stretched in a horizontal plane on the test stage as shown in Figure 1, where the fi ber separation was ∼0.75 mm and the volume of water droplets was 0.2 ∼0.4 μl. It was found that the majority of the droplets assumed a symmetrical confi gura- tion against the horizontal plane, which transected the fi bers. In contrast, the droplets with relatively large volume sagged below the hair fi bers. Occasionally, for example, in the case of the hair fi bers with large contact angles, droplets were found to “sit” on the top of fi bers, as shown in cell A6 in Figure 2. The droplet confi gurations shown in Figure 2 are three sets of parallel fi bers made up of (A) virgin vs virgin, (B) virgin vs stripped, and (C) virgin vs bleached fi bers. Note that the “reference” fi bers, i.e., the fi bers in reference to which the hydrophobicities are deter- mined, appear as bottom fi bers in all images. In addition, both the reference and the test fi bers were randomly extracted from the corresponding hair tresses, and therefore it is reasonably assumed that the fi bers carried the average wetting property. Images shown in column A of Figure 2 show the droplet confi gurations between two virgin hair fi bers. In this case, the droplets assumed nearly symmetrical confi gurations. This indicates, as ex- pected, nearly identical wetting properties of the two fi bers. The “unusual” appearance of the droplet as shown in image A6 of Figure 2, as noted, is due to the droplet resting on the top of the parallel fi bers—a phenomenon occasionally observed with larger droplets on fi bers with higher contact angles. Columns B and C of Figure 2 show the droplet con- fi gurations being progressively skewed. This indicates the sequence of decreasing hydro- phobicity, in which the virgin hair fi bers exhibited the highest extent of hydrophobicity, followed by the stripped (i.e., 18-methyl-eicosanoic acid removed) and bleached hair fi - bers, in full agreement with contact angle measurements.