2007 ANNUAL SCIENTIFIC SEMINAR 581 AN ANALYSIS OF THE WET-DRY TRANSITION COMBING FORCES OF HAIR AND TANGLING PEAK FORCE ASYMMETRY Manuel Gamez-Garcia, Ph.D. Ciba Specialty Chemicals Polymer Effect Research 540 White Plains Road, Tarrytown, NY 10591 USA Abstract A study has been made of the different adhesive and friction processes involved in the transition of combing forces from wet to dry in virgin and bleached hair. The analysis shows that the wet and dry tangling peaks can be separated into two main components, namely: one asymmetric and the other symmetric with respect to the maximum force value. For various degrees of tangling the ratio between the areas of these two peak components appears to be constant for a particular hair treatment. Furthermore, the analysis shows that both peak components contain important information about variations in adhesion, friction, and number of tangled fibers. The experiments also show that the irregularities frequently observed in the plateau and tangling peak forces of wet and dry hair are due to a stick•slip mechanism that arises from decreasing differences between the static and dynamic friction coefficients as the hair dries. Amonton's law of friction via a modified Capstan equation are considered when analyzing the effect of cuticle sheath visco•elasticity on the observed differences between the static and dynamic friction coefficients of the drying hair surface. Discussion and Analysis It is well agreed within the hair care community that the main difficulty in interpreting force combing measurements is related to the inherent reproducibility error of the various combing trials. In fact, it is perhaps this issue that has hindered researchers from unlocking the large amount of potential information that may be extracted from combing force experiments. It is certain that once this issue is solved the challenge will focus on dissecting the different friction contributions to the total force/comb displacement arising from the static and dynamic friction forces between comb and hair, and between the hair fibers themselves. Substantial progress has already been made in the past and it has been proposed that the total combing force stems from the various force contributions arising from the comb to hair friction forces, hair to hair friction forces, and fiber to fiber adhesion forces (1·2). However, so far no efforts have been made to quantify the contributions from comb to hair friction, hair to hair adhesion, and hair to hair friction by separate. Also, there is lack of information about the changing nature of the friction processes as the hair transitions from wet to dry. Finally, even though when it is known that that most polymers do not follow Amonton's Law of friction and that the hair is a biopolymer with bulk and surface visco•elastic characteristics no analysis has been made to verify the validity of this assertion for hair. This paper presents an analysis of various key experimental results that may help to elucidate some of these issues. Fig. 1 shows, for instance, that cutting the tip of a hair swatch at an angle of 45° eliminates almost completely the force tangling peak observed under dry conditions. This experiment together with the observation that there is always a tangling peak no matter how short or long we cut the hair tress, shows that the main contribution to the tangling peak stems not from increases in the fiber friction coefficient at the swatch tip, as it is commonly assumed, but rather to a hair fiber Capstan effect related to an increase in the contact angle between fibers. Fig. 2, on the other hand, shows a plot of the tangling peak area values v .s. peak heights obtained for various degrees of tangling. Line (A) represents the plot for the untreated hair swatch, while line (B) the plot for the same swatch after treatment with a commercial conditioner. Fig. 3a and 3b show half plots of the tangling peaks for the same untreated and treated swatches, while Fig. 4 and 5 show the partitioning analysis of a single peak into two area sections and into two peak components, respectively. Incidentally, it was fond that the ratio between the values of the two area sections was almost constant for all hair runs and only dependent on the type of hair treatment. The experiments described in Figs. 2 to 5 show that certain physical parameters related to hair comb and hair to hair friction remain constant during the combing process regardless of the degree of tangling introduced intentionally during combing. This observation is as expected as the only process susceptible of influencing the friction coefficients is the type of hair treatment and not the degree of tangling. Higher degrees of tangling will have, thus, the effect of increasing the contact angle and the contact area between, both, fiber and comb and between fiber and fiber.

582 JOURNAL OF COSMETIC SCIENCE Finally, Fig. 6 shows combing force plots obtained with a hair swatch as it transitions from wet to dry. A close inspection to any one of these plots shows the typical jumping irregularities that characterize a force v.s. comb displacement measurement. The irregularities appear all along the curve and resemble rather the shape of a ''tooth saw". This type of mechanical noise observed almost in any friction process is well docwnented and is due to a stick-slip mechanism typical of systems that present large differences between their static and dynamic friction coefficients. The plots show that as the hair swatch dries the size of the irregularities decrease indicating a decrease in the magnitude of the stick-slip effect. In fact, as it will be discussed later, during the combing friction process the hair fibers are also deformed elastically during the combing friction process and this effect accounts for the appearance of the stick-slip phenomenon. References 1) R Robbins, "Chemical and Physical Behavior of Human Hair" 3th. Ed. Springer-Verlag, New York, 1994, pp. 211-206 2) Y K Karnath, HD Weigmann, "Measurement of Combing Forces", J. Soc. Cosmet., Chem., vol. 37, n°3, pp. 111-124, (1986) . ... t ;,alha,QA{jf MU �- fiU I � !N ,., , ... ..,. ___ ,.,._,✓ ... .. ·•·◄ a,7 It.I nu 11U' 1"-i ,.,., C.�t !11l1 Fig. 1) Comparison of plots of comb/force displacement of hair swatches with normal cut at 90° and cut a 45°. .. .... ffl .. t 1- i- - ""' »II M * - ,i., ""' :m �� Fig. 2) Plots of Peak Areas v.s. corresponding Peak Force for various degrees of tangling for a hair swatch untreated (control) and treated with a commercial conditioner.