J. Cosmet. Sci., 63, 33–41 (January/February 2012) 33 Differential wetting characterization of hair fi bers ABE VAYNBERG, MARK STUART, and XIANG-FA WU, Ashland Inc., 5200 Blazer Parkway, Dublin, OH 43017 (A.V., M.S.), and Department of Mechanical Engineering and Applied Mechanics, North Dakota State University, Fargo, ND 58108-6050 (X.-F.W.). Accepted for publication August 31, 2011. Synopsis Surface wetting is one of the key properties of human hair used to indicate the extent of chemical/mechanical damage and the outcome of conditioning treatment. Characterization of hair wetting property is a challeng- ing task due to the non-homogeneous nature of hair fi bers and the requirement for sensitive equipment. Motivated by these considerations, we developed a new methodology, termed a differential wetting charac- terization (DWC), which would allow rapid and reliable characterization of the wetting property of hair fi - bers. This method is based on observation of a number of droplets suspended on a pair of parallel fi bers stretched in a horizontal plane. The wetting behavior of the fi bers can be deduced from the shape assumed by the droplets. When the wetting properties of the two hair fi bers are identical, the droplets suspended between the fi bers assume a symmetric confi guration. In contrast, on the fi bers with dissimilar wetting characteristics, the droplets will assume a skewed confi guration towards a more hydrophilic fi ber. This makes it possible to differentiate the hydrophobicities of the tested fi bers. In this paper it is demonstrated that the proposed DWC method is capable of differentiating the changes in wetting property of hair surfaces in response to either chemical or physical treatment. Results of the paper indicate that the DWC method is applicable for broad wetting differentiation of various fi bers. INTRODUCTION The surface of undamaged, so-called virgin, hair is naturally hydrophobic due to the ex- istence of 18-methyl-eicosanoic acid in the outmost layer of epicuticle (1). This acid is covalently bound via a thioester linkage to the cell membrane complex (2) and can be removed as a result of weathering and chemical treatment. Typically, the loss of 18-methyl- eicosanoic acid is accompanied by noticeable coarsening of hair to the touch and an in- crease in combing forces in both wet and dry states. Cosmetic treatments aim to alleviate the negative effect of hair damage. For instance, treatment of damaged hair with silicones and quaternary surfactants can restore both the manageability and hydrophobicity of hu- man hair. Improvement of the hair wetting property has been the concern of cosmetic chemists. Consequently, a number of techniques have been practiced in determining the wetting properties of human hair fi bers (3–10). Address all correspondence to Abe Vaynberg at kvaynberg@ashland.com.

JOURNAL OF COSMETIC SCIENCE 34 Historically, Kamath et al. (3) were the fi rst to adopt Wilhelmy’s balance principle to characterize the wetting property of single human hair fi bers. By assuming the perfect elliptical cross section of the hair fi bers, the circumference of the hair fi bers was calculated from the lengths of the major and minor axes measured by means of optical microscopy. In the study, the contact angle of the hair fi bers in water was determined using the wet- ting force and the estimated circumference. More recently, Molina et al. (4) and Lodge and Bhushan (5) reported the contact angle measurements on human hair fi bers using Wilhelmy’s balance approach as well, but with the notable exception of estimating the hair fi ber circumference based on the fi ber wetting force measured in low-energy hydrocar- bon fl uids where complete wetting was assumed. These studies indicated lower contact angles on damaged vs undamaged hair, as expected. In addition, Lodge and Bhushan (5) measured an increase in contact angle on damaged hair treated with a conditioner. Contact angle can also be measured by a direct observation. Though it is a relatively simple process when a liquid droplet is sitting on fl at surface, the high curvature assumed by fi bers requires specialized equipment, such as that described by Jones and Porter (6). The technique mentioned therein was based on passing a fi ber horizontally through a stationary eyelet containing a droplet of water. The fi ber produces an advancing or receding contact angle that can be directly measured using a low-magnifi cation optical microscope. Another method of determining the contact angle on microfi bers is based on observing the barrel-shaped droplets as they envelop the surface of fi bers. In this case, the barrel dimensions, i.e., the diameter and length, accurately defi ne the contact angle as a func- tion of the wetting length and fi ber diameter (7–10). This method has been utilized to determine the contact angles of cholesterol-containing squalane on hair fi bers, in which the measurement was conducted in water (10). Furthermore, Carroll (10) reported a de- crease in contact angle with increasing cholesterol concentration, an outcome attributed to the decreasing water/squalane interfacial tension with increasing cholesterol concentra- tion. So far, the above methods have provided practicable measurements of contact angles on hair fi bers however, the measurements usually are time-consuming and rely on single-point measurements. In this paper we provide a novel method to determine the wetting property of human hair fi bers. This method is based on the observation that a droplet suspended between two stretched parallel fi bers of dissimilar wetting characteristics will invariably assume a skewed confi guration towards the fi ber of larger hydrophilicity. This paper demonstrates that such an observation can be utilized to develop an effi cient and reliable technique for fi ber wetting characterization, referred herein as fi ber differential wetting characteriza- tion (DWC). We provide detailed validation based on direct experimental observation of droplet confi guration as a function of fi ber wetting properties and numerical simulations. Although developed and validated for characterization of human hair fi bers, the proposed DWC method can be considered as a universal technique equally applicable for rapid characterization of the wetting property variation of other microfi bers. EXPERIMENTAL In this study, hair characterization was carried out using a 12-inch-long Caucasian brown virgin hair tress and a Caucasian brown hair tress bleached for one hour. All the