NANOTRIBOLOGICAL PROPERTIES OF HAIR 53 Coefficient of friction Virgin hair 0.100 .-- ---------------- 5 0.075 � O 0.050 c GI ·o � 0.025 (.) Hair# 1 Hair#2 0 ------�-�---�---� Unsoaked Soaked Unsoaked Soaked Chemically damaged hair 0.100.------------------- 0 0.075 � - 0 c 0.050 GI ·o GI o 0.025 0 Hair# 1 Hair#2 0 ..____, __ ...,_ _ ____._ _____ ........_ _ ____._ __ ...____. Unsoaked Soaked Unsoaked Soaked Chemically damaged treated hair (1 cycle) 0.100 .------------------ C 'fl 0.075 ;s 'o c 0.050 GI ·o � 0.025 (.) Hair# 1 Hair#2 Unsoaked Soaked Unsoaked Soaked Adhesive force Virgin hair 50 ----------------- Z 40 .s GI � 30 ,E GI "iii 20 GI � 10 Hair# 1 Hair#2 Unsoaked Soaked Unsoaked Soaked 50 r------------r----------,hairdamagedChemically Z 40 .s � 30 ,E GI -� l3 20 Hair# 1 Hair# 2 0 ..____, __ ...,_ _ ____,_ _ ___.__....._ _ __. __ ....____, Unsoaked Soaked Unsoaked Soaked Chemically damaged treated hair (1 cycle) 50 .------------------- Z 40 .s � 30 ,E GI -� Ill 20 GI ( 10 Hair# 1 Hair#2 Unsoaked Soaked Unsoaked Soaked Figure 8. Effect of soaking in de-ionized water on the coefficient of friction and adhesive force for virgin, damaged, and damaged treated hair. even with conditioner treatment. This is yet another indication that virgin and damaged hair have significantly different surface properties, which in many cases results in op- posite trends for their nanoscale tribological properties. Adhesive force for virgin hair remained approximately the same before and after soaking, while it decreased for dam- aged and damaged treated hair after soaking. CONCLUSIONS The main conclusions are as follows: 1. In general, friction force maps of hair damaged by various treatments show a high friction force in the area surrounding the bottom cuticle edge, which is due to an increased meniscus force contribution between the AFM tip and the conditioner layer. 2. In most cases, the macroscale and microscale coefficients of friction followed the same

54 JOURNAL OF COSMETIC SCIENCE 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 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 at high deposition. The dampened mobility of the amino silicone at high depo- sition levels, with respect to hair surface and tip, may account for this wide variation in coefficient of friction. 3. Adhesive force varied widely, but typically showed a significant increase with the presence of conditioner ingredients. This is a clear sign that meniscus effects are influ- encing the pull-off force between the tip and the sample. At high deposition levels, the amino silicones showed much more distinct regions of high and low friction and adhe- sion, which shows that there is less mobility for these molecules and much less redis- tribution as they coat the hair. 4. Virgin (undamaged) hair exhibits a decrease in coefficient of friction after soaking, while damaged and damaged treated hair both show an increase. Virgin hair is more hydrophobic, and so more water is present on the surface and results in a lubrication effect after soaking. Damaged hair absorbs water, which softens the hair and increases the real area of contact (and thus friction) with the tip. This is yet another indication that virgin and damaged hair have significantly different surface properties, which in many cases results in opposite trends for their nanoscale tribological properties. ACKNOWLEDGMENTS The financial support for this project was provided by the Procter & Gamble Company (Cincinnati, OH) and Procter & Gamble Far East (Kobe, Japan). The authors thank Yu jun Li and Hoyun Kim of Procter & Gamble Far East for preparing all of the damaged treated hair samples. A special thanks is given to Rob Willicut and Matt Wagner of P&G for insightful discussions, and to Nianhuan Chen for helpful suggestions and the contribution of chemical structure diagrams of conditioner ingredients. REFERENCES (1) J. A. Swift and J. R. Smith, Atomic force microscopy of human hair, Scanning, 22, 310-318 (2000). (2) R. McMullen and S. Kelty, Investigation of human hair fibers using friction force microscopy, Scanning, 23, 337-345 (2001). (3) C. LaTorre and B. Bhushan, Nanotribological characterization of human hair and skin using atomic force microscopy, Ultrarnicroscopy, 105, 155-175 (2005). (4) C. LaTorre and B. Bhushan, Nanotribological effects of hair care products and environment on human hair using atomic force microscopy,]. Vac. Sci. Technol. A., 23, 1034-1045 (2005). (5) C. Robbins, Chemical and Physical Behavior of Hurnan Hair, 3rd ed. (Springer-Verlag, New York, 1994). (6) C. Bolduc and]. Shapiro, Hair care products: Waving, straightening, conditioning, and coloring, Clin. Derrnatol., 19, 431-436 (2001). (7) J. Gray, Hair care and hair care products, Clin. Derrnatol., 19, 227-236 (2001). (8) R. Molina, F. Comelles, M. R. Julia, and P. Erra, Chemical modifications on human hair studied by means of contact angle determination,]. Colloid Interface Sci., 237, 40-46 (2001). (9) B. Bhushan and Z. Burton, Adhesion and friction properties of polymers in microfluidic devices, Nanotechnology, 16, 467-478 (2005). (10) C. Jalbert, J. T. Koberstein, I. Yilgor, P. Gallagher, and V. Krukonis, Molecular weight dependence