WETTING CHARACTERIZATION OF HAIR FIBERS 37 To validate the trend shown in Figure 2, we further considered the wetting behavior of another combination of hair fi bers, in which stripped hair fi bers were used as the reference fi bers. Images of droplets wetting between stripped vs stripped fi bers (set A) and stripped vs bleached fi bers (set B) appear in Figure 3. Images in Figure 3 confi rm the conclusion drawn from the droplet confi guration observed in Figure 2 that stripped hair fi bers are more hydrophobic than bleached hair fi bers. Images in Figure 3 also examine the sensitiv- ity of the proposed DWC method. Thus, it has been demonstrated that the present method is capable of distinguishing not only the virgin hair fi bers from the damaged fi - bers but also fi bers with different extents of damage. Next we demonstrate the use of the DWC method in evaluating the impact of condition- ing treatment. Once again, for the purpose of comparison and validation, column A in Figure 4 shows the droplet confi guration between two bleached hair fi bers column B shows the droplet confi guration between the bleached hair fi bers and the bleached hair fi bers treated with a conditioner. The droplets in Figure 4, as expected, are nearly symmetric for the bleached vs bleached fi bers. However, in the case of the bleached vs the bleached/treated fi bers, the droplets are systematically skewed toward the un- treated bleached fi ber. This implies the larger hydrophobicity of the hair fi ber after conditioner treatment. Finally, we demonstrate that in case of doubt as to the droplet orientation, one could simply allow the water droplets to evaporate while observing the shape evolution of the evaporating droplets. Figure 5 shows the shape evolution of an evaporating droplet Figure 2. Variation of droplet confi guration with extent of hair damage (I). Column A: virgin vs virgin fi bers. Column B: virgin vs stripped fi bers. Column C: virgin vs bleached fi bers. Note that the reference fi ber (virgin in this case) appears at the bottom of each image.

JOURNAL OF COSMETIC SCIENCE 38 between fi bers with different degrees of damage. The extent of skew becomes stronger as the droplet evaporates, with the fi nal image unequivocally illustrating which fi ber has a stronger affi nity to water. NUMERICAL VALIDATION OF THE DROPLET CONFIGURATION BY THE SURFACE FINITE ELEMENT METHOD (FEM) Surface FEM is employed to simulate droplet appearance as a function of droplet volume, fi ber separation, and diameter, as well as contact angles. The surface FEM method is based on minimization of the surface potential energy of a droplet-on-fi ber system, as detailed elsewhere (8,12). The purpose of the numerical simulations is to confi rm that water drop- lets suspended between fi bers with dissimilar contact angles would produce skewed ori- entation, with the degree of skew proportional to the difference in contact angles. The images based on the numerical simulations are shown in Figure 6. In each set of images, the fi rst illustrates a droplet as viewed from the top, like that in Figures 2–5, while the second and third show the droplets as viewed in the horizontal plane (side view), with the fi bers (not shown) located in front of the image. Figure 3. Variation of droplet confi guration with extent of hair damage (II). Column A: stripped vs stripped fi bers. Column B: stripped vs bleached fi bers.