QUASI-STATIC TORSIONAL DEFORMATION OF SINGLE HAIR FIBERS 393 deformation nor the swelling of these substructures. Further exploration and optimiza- tion of this modeling approach would prove insightful. PRACTICAL APPLICATIONS OF TORSION A number of studies to illustrate the use of torsion in cosmetic applications have been conducted using the Dia-Stron FTT950 system. The exaggerated difference of the torsional modulus of bleached and virgin hair at low humidity prompted the decision to conduct the studies at 20% RH. Double-bleached fi bers were treated with either an oil or a market leading shampoo and conditioner system. Abyssinian oil, a naturally derived oil rich in unsaturated fatty acids and long chain triglycerides which claims to improve hair softness and manageability, was selected for the study. A 5-min treatment with the oil resulted in a signifi cant reduction in the torsional modulus compared with both virgin and Figure 8. (A) Boxplot of the mean tensile elastic moduli for virgin, 2 × bleached and 2 × bleached hair treated with Abyssinian oil and treated with a commercial shampoo and conditioner. (B) Boxplot of the mean torsional moduli for virgin, 2 × bleached and 2 × bleached hair treated with Abyssinian oil and treated with a commercial shampoo and conditioner.

JOURNAL OF COSMETIC SCIENCE 394 untreated, bleached hair (Table III, Figure 8B). The lower modulus is indicative of a soften- ing effect on the cuticles either by a plasticizing effect of the oil or through the formation of an occlusive barrier which retains and prevents moisture loss at lower humidity. Simi- larly, treatment of bleached hair with the commercial shampoo and conditioner regime reduced the torsional stiffness compared with that in the untreated, bleached hair, although to a lesser extent than the oil (Table III, Figure 8B). The elastic tensile modulus was also measured on the same fi bers using the FTT950 which reveals the different discrimination potential of the two modes of deformation. The tor- sional modulus identifi es a signifi cant difference between all of the treatments, whereas for tensile deformation only fi bers treated with the product regime exhibit a signifi cant change. CONCLUSIONS The Dia-Stron FTT950 allows for an automated and direct measurement of fi ber torsional properties. The application of a theoretical two-phase model to single fi ber torsional data showed that under a few assumptions, the cuticle layers may play a signifi cant role in the torsional mode of deformation, whereas their contributions in tensile deformation can be assumed to be negligible. We have also shown the impact of relative humidity on the torsional modulus of both virgin and bleached fi bers. The differentiation between these treatments is maximized at low humidity levels therefore, the sensitivity of the method could be increased by run- ning experiments under low humidity conditions. Finally, we have demonstrated the positive effect of an oil-based treatment and a market leading hair care system (shampoo and conditioner) on hair torsion behavior, whereas less discrimination was observed in the tensile measurements. Although there is much still to explore with such a high-throughput torsion testing technique, we have illustrated the ability of this mechanical method to highlight clear and signifi cant effects from cosmetic treatments, opening the way to developing new ingredients and hair care formulations tar- geting the cuticle layers. REFERENCES (1) F. J. Wortmann and H. Zahn, The stress/strain curve of alpha keratin fi bres and structure of the inter- mediate fi laments, Text. Res. J., 69, 737–743 (1994). (2) F. J. Wortmann and S. De-Jong, Analysis of the humidity-time superposition for wool fi bres, Text. Res. J., 55, 750–756 (1985). (3) F. J. Wortmann and I. Souren, Extensional properties of human hair and permanent waving, J. Soc. Cosmet. Chem., 38, 125–140 (1987). (4) F. J. Wortmann, G. Wortmann, and C. Popescu, Assessing the properties if thermally treated human hair by tensile testing or DSC: are the complementary or equivalent methods, 20th International Hair Science Symposium HairS’17, Sept 6-8, Dresden, 2017. (5) E. G. Bendit, There is no hookean region in the stress strain curve of keratin (or other viscoelastic poly- mers), J. Macromol. Sci., Part B: Phys., 17, 129–140 (1980). (6) F. I. Bell, P. Carpenter, and S. Bucknell, Advantages of a high-throughput measure of hair fi bre torsional properties, J. Cosmet. Sci, 63, 81–92 (2012). (7) A. N. Parbhu, W. G. Bryson, and R. Lal, Disulfi de bonds in the outer layers of keratin fi bers confer higher mechanical rigidity: correlative nano-indentation and elasticity measurements with an AFM, Biochemistry, 38, 11755–11761 (1999).