2010 TRI/PRINCETON CONFERENCE 239 The resulting microbalance reading F includes the buoyancy force Fb on the fi ber, the weight of the fi ber Fg, and the wetting force Fω: ω b g F F F F (2) For a single human hair fi ber the buoyancy force is negligible versus the wetting force Fω. The weight of the hair is zeroed before the hair touches the surface of the liquid. Thus, the resulting force can be considered as equivalent to the wetting force. In addition to the dynamic single fi ber approach a new method was developed to deter- mine the "pseudo static" contact angle formed by water droplets on parallel aligned hair collectives. In order to reach the "pseudo-static" stage, hair should deliver contact angles above approx. 80°. If contact angles are lower, the water resorption by hair takes place too fast. The shape of the droplet was recorded from a perpendicular positioned directly after deposition (approx. 1s). The evaluation is based on the asymptotical fi tting of a tangent to the segment where the water contacts hair (Figure 2). The resorption time of the water droplet by the hair can be considered as an additional parameter to evaluate the damage degree of the hair surface. Thus the droplet was moni- tored over time and the elapsed time until the water was totally resorped by the hair was determined, as shown in Figure 3. The resorption kinetic differs from a spontaneous re- sorption for severely damaged hair to no detectable resorption for virgin hair. Water droplets remain for hours on virgin hair strands. They slowly shrink with evaporation. In agreement with these observations medium bleached hair was used for this study. The determined resorption times ranged from 10 to 100 s. Figure 2. Determination of the “pseudo-static” contact angle of a droplet of water deposited on an aligned hair strand. Figure 3. Resorption process of a droplet of water on an aligned hair strand recorded from a perpendicular camera angle.
JOURNAL OF COSMETIC SCIENCE 240 RESTORATION OF THE DAMAGED HAIR SURFACE The objective of this study was to restore the natural properties of the virgin hair surface. Considering the morphology of the outer hair surface, where lipids are covalently bound to a protein matrix, lipid-modifi ed proteins, the so called proteolipids, were tested as potential surface restoring agents. Assuming that the adhesion and organization of proteolipids on the hair surface is con- trolled by electrostatic and polar forces (protein-protein) as well as apolar interactions (hydrophobic forces), a lipophilic surface layer should be specifi cally formed, where pro- tein patches are exposed (Figure 4). The incorporation of proteolipids should increase the hydrophobicity of damaged hair, leading to higher contact angles. MATERIALS HAIR For all tests a mixture of light brown Caucasian hair (Color Code 7/0) from Kerling In- ternational Haarfabrik GmbH in Backnang (Germany) was used. The tests were per- formed on hair strands (width 4 cm, length 13 cm, weight 250 mg) and on single fi bers (length 30 mm). STANDARD HAIR CLEANSING An aqueous solution (pH 5.5) of 12.5% (w/v) sodium laureth sulfate was applied on hair (0.25 g / 1 g). The fi bers were rubbed against each other for 1 min. Afterwards it was washed out with tap water at approx. 30 °C for 1 min. Shampoo A. Aqua, sodium laureth sulfate, disodium cocoamphodiacetate, citric acid, so- dium chloride, sodium benzoate, salicylic acid. Figure 4. Schematic diagram of the hypothesis of the adhesion principle of proteolipids on damaged hair surfaces.
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