WETTING CHARACTERIZATION OF HAIR FIBERS 39 Figure 5. Morphology evolution of an evaporating droplet between two fibers with dissimilar hydrophobicities. Figure 6 shows the simulated shapes of droplets on fi bers with the identical diameter of 75 μm the fi ber separation is fi xed at 0.75 mm, and the droplets, with a volume of 0.3 μl, are suspended between the fi bers. Variation of the droplet shape is explored with respect to the contact angles: 60° vs 100° (set A) and 80° vs 100° (set B). As expected, the simulations yielded skewed droplet orientation, with the largest extent of skew observed in the case of set A (60° vs 100°). Figure 4. Variation of droplet confi guration with hair treatment. Column A: one-hour bleached vs bleached fi bers. Column B: bleached vs bleached/treated fi bers with conditioner.

JOURNAL OF COSMETIC SCIENCE 40 CONCLUDING REMARKS The DWC method proposed in this study has provided an effi cient and reliable technique for the characterization of the surface wetting property. This method is based on the di- rect observation of a number of droplets sitting on a fi ber pair, thus effectively suppress- ing the possible experimental errors resulting from single-point determination. The current method has been validated by controlled tests in the present study, in which the hair fi bers followed the same hydrophobicity trend as characterized by means of Wilhelmy’s method. REFERENCES (1) C. R. Robbins, Chemical and Physical Behavior of Human Hair, 4th Ed. (Springer-Verlag, New York, 2002). (2) L. N. Jones and D. E. Rivett, The role of 18-methyleicosanoic acid in structure and formation of mam- malian hair fi bers, Micron, 28, 469–485 (1997). (3) Y. K. Kamath, C. J. Dansizer, and H. D. Weigmann, Wetting behavior of human hair fi bers, J. Appl. Polym. Sci., 22, 2295–2306 (1978). (4) R. Molina, F. Comelles, M. R. Juliá, and P. Erra, Chemical modifi cation on human hair studied by means of contact angle determination, J. Colloid Interface Sci., 237, 40–46 (2001). (5) R. Lodge and B. Bhushan, Wetting properties of human hair by means of dynamic contact angle mea- surements, J. Appl. Polym. Sci., 102, 5255–5265 (2006). (6) W. C. Jones and M. C. Porter, A method for measuring contact angles on fi bers, J. Colloid Interface Sci., 24, 1–3 (1967). (7) B. J. Carroll, The accurate measurement of contact angle, phase contact areas, drop volume, and Laplace excess pressure in drop-on-fi ber systems, J. Colloid Interface Sci., 57, 488–492 (1976). (8) G. McHale, M. I. Newton, and B. J. Carroll, The shape and stability of small liquid drops on fi bers, Oil Gas Sci. Tech., 56, 47–54 (2001). (9) X. F. Wu and Y. A. Dzenis, Droplet on fi bres: Geometrical shape and contact angle, Acta Mech., 185, 215–225 (2006). Figure 6. Simulated 0.3-μl droplet shapes between 75-μm fi bers at 0.75-mm separation with contact angles of (A) 100° vs 60° and (B) 100° vs 80°.