40 JOURNAL OF COSMETIC SCIENCE Table I Desired Features and Corresponding Tribological Attributes of Conditioners Desired hair feature Smooth feel in wet and dry environments Shaking and bouncing during daily activities Easy combing and styling Tribological attributes Low friction between hair and skin in respective environment Low friction between hair fibers and groups of hair Low friction between hair and comb (plastic) and low adhesion* * More complex styles may require higher adhesion between fibers. Virgin or damaged hair 'cuticle layers ...,,-­ (negatively charged) Treated hair Conditioner deposits (positively charged) Figure 3. Negatively charged hair and the deposition of positively charged conditioner on the cuticle surface. Table II Combinations of Conditioner Ingredients and Their Benefits Towards Wet and Dry Feel Key ingredients Cationic surfactant Fatty alcohols Water Key ingredients Silicones Fatty alcohols Cationic surfactant Gel network chassis for superior wet feel Benefits Creamy texture Ease of spreading Slippery feel while applying Soft-rinsing feel Combination of "conditioning activities" for superior dry feel Benefits Moistness Softness Dry-combing ease the area surrounding the cuticle scale edges more so than on the general surface. Likewise, the thickness of the conditioner layer is expected to be on the order of a few nanometers (shown in the cross-section diagrams of Figure 2) and is critical to the friction and adhesion properties of the hair. Conditioner consists of a gel network chassis (cationic surfactants, fatty alcohols, and water) for superior wet feel and a combination of conditioning actives (cationic surfac- tants, fatty alcohols, and silicones) for superior dry feel. The benefits of the conditioner are shown in Table II. The wet-feel benefits are a creamy texture, ease of spreading, a slippery feel while applying, and a soft-rinsing feel. The dry-feel benefits are moistness, softness, and easier dry combing. Each of the primary conditioner ingredients also has

NANOTRIBOLOGICAL PROPERTIES OF HAIR 41 specific functions and roles that affect the performance of the entire product. Table III displays the functions of the major conditioner ingredients and Table IV displays their chemical structure. Cationic surfactants are critical to the forming of the lamellar gel network in conditioner, and also act as a lubricant and static control agent, since their positive charge aids in counteracting the negative charge of the hair fibers. Fatty alcohols are used to lubricate and moisturize the hair surface, along with forming the gel network. Finally, silicones are the main source of lubrication in the conditioner formu- lation. Recently AFM/FFM work has been done on the roughness, friction, and adhesion changes of various experimental and commercial treatments applied to the damaged hair surface (3,4). It was found that, in general, chemical and mechanical damage to the hair caused the outer lubricious layer of the cuticle to wear off, resulting in an increased nanoscale coefficient of friction. Conditioner treatment was found to slightly increase the nanoscale coefficient of friction for virgin hair (due to meniscus effects), while decreasing it for damaged hair (the higher negative charge on the surface caused more conditioner to deposit and therefore shear more easily). For both virgin and damaged hair, adhesive pull-off force increased with conditioner treatment, largely due to meniscus effects brought on by the interaction of the AFM tip and conditioner layer. Since conditioner is such a complex network of ingredients, it is necessary to also study the effects of these individual components to reveal the significance each has on nano- tribological properties when applied to the hair. In this study, AFM/FFM is used to conduct nanotribological studies of surface roughness, friction force, and adhesive force as a function of silicone type, silicone deposition level, and cationic surfactant type. Since most application of conditioner is typically done on wet hair, the coefficient of friction differences between dry and wet damaged hair (with and without commercial condi- tioner) are also discussed. EXPERIMENT AL DETAILS HAIR SAMPLES Caucasian hair samples were received from Procter & Gamble (Cincinnati, OH) and prepared per Appendix A. The samples arrived as hair swatches approximately 0.3-m long. Although the exact location from the root is unknown, it is estimated that hair samples used for testing were between 0.1 and 0.2 m from the scalp. All hair samples had undergone two rinse-wash cycles of commercial shampoo application (in the case of Table III Individual Conditioner Ingredients and Their Corresponding Purpose/Function Conditioner gel network chassis for desired viscosity, texture, and performance Conditioner ingredient Quanternary amine-based cationic surfactant Fatty alcohol Silicone Purpose/function Aids formation of lamellar gel network Lubricates and controls static Lubricates and moisturizes Aids formation of lamellar gel network along with cationic surfactant Primary source of lubrication Gives hair a soft and smooth feel