36 JOURNAL OF COSMETIC SCIENCE greater than for rubbing in a tipwise direction. This directional friction effect (or DFE) can be readily demonstrated by gently rubbing a hair between the thumb and forefinger, whereupon the hair will move towards contact with its tip, an effect first reported by Monge in 1790 (52). The DFE is a principal determinant for tangling in arrays ofkeratin fibers (see below). A convenient descriptor, which accords well with propensity for tangling (or felting shrinkage in the case of wool fabrics), is the difference between the two frictional coefficients as a proportion of the sum of the two coefficients (53): DFE = (p• - pw)/(p• + Pw) (Eq. 1) where p• and p• are, respectively, the against-scales and with-scales coefficients for friction. There has never been any doubt that the main reason for directional differences in friction in mammalian keratin fibers lies with the architectural asymmetry of their surfaces, viz., of cuticular scales overlapping like tiles on a roof. What has been uncer- tain, however, are the underlying microscopic and molecular processes of frictional interaction with the fiber surfaces, and how these are influenced by topographic asym- metry. Many different methods have been used to measure frictional interaction between hair fibers and to demonstrate DFE (54-56), but meaningful analysis of the results for modeling frictional processes, as they might relate to realistic interactions between hairs on the head or during the felting of wool, undoubtedly have been confounded by the use of high experimental loading levels. Significant understanding of friction in human hair, however, has been provided by the elegant work of Adams eta/. (57), who investigated the forces of interaction in the motion of a lightly tensioned hair at right angles across a cantilevered counter fiber, where the type and diameter of the counter fiber was varied. They concluded that friction in hair does not involve major mechanical engagement or a ploughing process such as against the scale edges, as had previously been suggested (54). Convincing evidence was presented that the directional friction effect arises ac- cording to the concepts of an adhesion model where interface shear frictional processes occur in different geometric configurations according to the direction of motion. The layer of 18-MEA at the surface of undamaged hair undoubtedly has a critical influence on its frictional behavior, the high molecular mobility of the outwardly presented anteiso-configuration serving to maintain a low free surface energy and a low interfacial shear strength in frictional contacts with other surfaces and with other hairs. When 18-MEA is specifically cleaved from the surface of wool with anhydrous potas- sium tertiary butoxide in tertiary butanol, the surfaces become anionic and hydrophilic and there is a dramatic increase in friction (58). Despite this, the fibers continue to exhibit a DFE, as is reflected by the continuing capacity of the wool fabric to undergo felting shrinkage, albeit much less than in the untreated fabric (59). Examination of the butoxide-treated wool fibers in the SEM showed there was little or no change to the fiber's pattern of overlapping scales (59). What was particularly significant about Leeder and Rippon's (59) work was their demonstration that felting shrinkage, interfiber fric- tion, and wettability of the butoxide-treated wool were almost completely restored to those of the untreated wool by further treatment with a cationic-substituted stearamide. By these processes, the covalently bound 18-MEA at the surface of the original wool had been effectively replaced by an alternative fatty chain, now attached by ionic interaction to the fiber surface. In this regard, there seems little doubt that the advantage of cationic

HUMAN HAIR CUTICLE 37 surfactants, such as disteryl dimethyl ammonium chloride or cetyl trimethyl ammonium chloride, as conditioners for human hair lies in their ionic binding at surface sites from which 18-MEA has been removed by environmental or other damaging processes, i.e., restoration in continuity of the low coefficient-of-friction surface. Taking the aforementioned facts into consideration, the compelling conclusion is that the DFE in normal hair involves both asymmetry of surface architecture (as dictated by overlapping scales) and an outer surface covered with a robust low coefficient-of-friction material (18-MEA). Overlapping scales are the underlying cause of the effect, without which there would be no DFE (57), and this is reflected by the value of the numerator in Equation 1. The surface 18-MEA serves to enhance this initial effect by maintaining a low average frictional resistance and is reflected by the denominator in Equation 1. 18-MEA AS A BOUNDARY LUBRICANT The condition of having one end of a long-chain molecule firmly attached to an under- lying solid substrate, as with 18-MEA at the hair's surface, is a well-known characteristic of boundary lubricants that serve to efficiently maintain surfaces of low frictional resis- tance (60,61). With conventional boundary lubricants, the attachment is usually ionic, but in hair nature has conspired in the use of a covalent linkage. Straight-chain mol- ecules are prone to ordering, which dramatically increases their bulk viscosity and frictional resistance under high loads, but efficient boundary lubricants are branched so as to remain liquid-like and of low frictional resistance. The two chains of di-alkyl quaternaries ionically adsorbed onto an anionic surface typically provide for this chain disorder and are good boundary lubricants. This is undoubtedly the reason why such molecules are good conditions for hair, binding occurring between the cationic groups of the conditioner at the anionic sites on the hair surface from which the 18-MEA has been removed by various damage processes. In 18-MEA, nature has arranged for chain disorder, high molecular mobility, and low frictional resistance by using anteiso methyl- branching of the aliphatic chain. CONTRIBUTIONS OF THE CUTICLE TO THE HAIR'S OVERALL BEHAVIOR HAIR TANGLING A common property of all mammalian hairs (including human hair), once they have been shorn from the animal and the untethered fibers are mechanically agitated, is their propensity for becoming tangled and increasingly so if the fibers are immersed in water. While in the textile industry this is used to advantage in the preparation of woolen felts, in former times this was a serious handicap and the cause for the washing shrinkage of knitted or woven woolen garments. The DFE is the underlying cause of the tangling process, and indeed its elimination, typically by surface oxidation of the fibers coupled with impregnation by reactive polymers, is a common route for producing shrink- resistant woolen garments. It is now generally recognized that, as well as the presence of a DFE amongst the fibers, free root-ends are a critical feature of tangling in mammalian hair (54). In this respect