NANOTRIBOLOGICAL PROPERTIES OF HAIR 51 the damaged hair caused a slight decrease in coefficient of friction and a large increase in adhesive force. The decreased coefficient of friction may be explained by the fact that the damaged hair accumulates much of the positively charged conditioner on the surface due to its highly negative charge, which in turn makes it easier to shear the liquid on the surface, causing a lower coefficient of friction. However, the nanoscale pull-off force (adhesive force) is much larger than on the untreated hair because of meniscus effects. In general, adhesive force varied widely, but typically showed a significant increase in the presence of conditioner. As discussed previously, this is a clear sign that meniscus effects are influencing the pull-off force between the tip and the sample. In most cases, the macroscale and microscale coefficient of friction followed the same trend, in which a decrease was observed with the addition of the PDMS blend or amino silicones to the BTMAC surfactant. The silicones are typically used as a major source of Table VII Observations and Corresponding Mechanisms Regarding Coefficients of Friction and Adhesion for Various Hair Treatments Observation Mechanism Damaged vs. damaged treated hair Damaged hair shows a decrease in coefficient of friction but an increase in adhesion from the application of commercial conditioner. The conditioner layer deposited on the surface of the damaged hair results in a lower shear strength, which in turn lowers the coefficient of friction, while meniscus effects increase the pull-off (adhesive) force between the tip and hair sample. PDMS blend vs. amino silicone (at high deposition) Amino silicones interact strongly with negatively charged hair surface, especially at high deposition levels. Amino silicone thickness distribution on hair is less uniform than with PDMS blend. A stronger electrostatic attraction exists, which results in stronger binding forces (which leads to higher adhesion) for high-deposition amino silicone. Less mobility with amino silicones, and so molecules attach to hair at contact and do not redistribute easily. Surfactant vs. surfactant plus addition of silicone Adhesion remains approximately the same while coefficient of friction decreases with addition of PDMS-blend silicone and amino silicone (low- deposition). Coefficient of friction remains about the same us that of high-deposition amino silicone. Mobility becomes easier with addition of PDMS-blend silicone, which leads to lower coefficient of friction. At high deposition of amino silicones, mobility ceases and coefficient of friction becomes high again. BTMAC surfactant vs. BAPDMA surfactant BTMAC surfactant has lower adhesion but higher coefficient of friction than BAPDMA surfactant. Interaction of surfactants with damaged hair surface causes inherent differences in coefficient of friction and adhesion. BAPDMA has both amino and amine groups, which increases polarity.

52 JOURNAL OF COSMETIC SCIENCE lubrication and thus give the conditioner more mobility on the hair surface compared to just surfactants and fatty alcohols. The inverse trend was seen only for the amino silicone group. The amino silicones have a strong electrostatic attraction to the negatively charged hair surface, which in turn creates higher binding forces and less mobility. The dampened mobility of the amino silicone at high deposition levels, with respect to hair surface and tip, may account for this wide variation in coefficient of friction and large adhesive force values. In terms of adhesive force, it was previously observed in Figure 6 that the amino silicone treatments showed much more distinct regions of higher and lower adhesion compared to PDMS-blend silicones. This nonuniform amino silicone thickness distri- bution on hair is also believed to be caused by the inhibited mobility, as the molecules immediately attach to the hair at contact and do not redistribute as a uniform coating. The increased polarity of the amino silicones compared to the PDMS blend can also be a major contributor to the higher friction and adhesion at high deposition levels. If we look at the hair only with the BTMAC surfactant and fatty alcohols, and then add low deposition levels of PDMS blend (blend of low and high MW) or amino silicone, we see that the coefficient of friction decreases, while adhesion remains approximately the same. If we increase the deposition level, it is observed that the PDMS blend is still lower, but now the coefficient of friction for the amino silicone is about the same as for the BTMAC-only samples. The mobility of the conditioner layer is again a major issue, as this easier mobility accounts for the lower coefficient of friction. However, as is seen when there is a large amount of the amino silicones on the hair surface, the mobility ceases and the coefficient of friction becomes high again. The BAPDMA surfactant typically showed a higher adhesive force than the BTMAC surfactant. However, there may be an increased accumulation of the BAPDMA surfac- tant such that the conditioner layer is more easily sheared by the tip, as is the case for damaged treated hair. The inherent differences in surfactant composition and how they interact with the damaged hair surface most likely account for the coefficient of friction differences. In respect to roughness, the vertical standard deviation decreased slightly with most treatments, although the standard deviations were similar. The spatial parameter in- creased slightly with the treatments, but the variation also became extremely high. EFFECT OF SOAKING ON COEFFICIENT OF FRICTION FOR VIRGIN, DAMAGED, AND DAMAGED TREATED HAIR Virgin, damaged, and treated hair samples were soaked in de-ionized water for five minutes. Their corresponding coefficient of friction was measured and compared to coefficient of friction values for dry samples that were adjacent to the wet samples on the respective hair fiber. Figure 8 shows the results for two hair samples of each hair type. Virgin hair exhibits a decrease in the coefficient of friction after soaking. Virgin hair is more hydrophobic, and so more of the water is present on the surface and results in a lubrication effect after soaking. Damaged hair tends to be less hydrophobic due to the chemical degradation of the cuticle surface, and absorbs water after soaking. This softens the hair and increases the real area of contact with the tip, which leads to higher friction,