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J. Cosmet. Sci., 68, 173–182 (March/April 2017) 173 Model-based analysis of the torsional loss modulus in human hair and of the effects of cosmetic processing FRANZ J. WORTMANN, GABRIELE WORTMANN, HANS-MARTIN HAAKE, and WOLF EISFELD, School of Materials, University of Manchester, Manchester M13 9PL, United Kingdom (F.J.W., G.W.) and BASF Personal Care and Nutrition GmbH, 40589 Duesseldorf, Germany (H.-M.H., W.E.). Accepted for publication February 1, 2017. Synopsis Torsional analysis of single human hairs is especially suited to determine the properties of the cuticle and its changes through cosmetic processing. The two primary parameters, which are obtained by free torsional oscillation using the torsional pendulum method, are storage (G′) and loss modulus (G″). Based on previous work on G′, the current investigation focuses on G″. The results show an increase of G″ with a drop of G′ and vice versa, as is expected for a viscoelastic material well below its glass transition. The overall power of G″ to discriminate between samples is quite low. This is attributed to the systematic decrease of the parameter values with increasing fi ber diameter, with a pronounced correlation between G″ and G′. Analyzing this effect on the basis of a core/shell model for the cortex/cuticle structure of hair by nonlinear regression leads to estimates for the loss moduli of cortex (G″co) and cuticle (G″cu). Although the values for G″co turn out to be physically not plausible, due to limitations of the applied model, those for G″cu are considered as generally realistic against relevant literature values. Signifi cant differences between the loss moduli of the cuticle for the different samples provide insight into changes of the torsional energy loss due to the cosmetic processes and products, contributing toward a consistent view of torsional energy storage and loss, namely, in the cuticle of hair. INTRODUCTION The behavior of human hair under torsional stresses and strains is an important contribut- ing factor for the formation and maintenance of a hair style ( 1). Because of the nature of torsional deformation, the results for a fi ber are biased toward contributions from its outer regions ( 2). For human hair, the method is thus especially suited to investigate the properties of the cuticle. In a recent publication ( 3), we presented a set of data from in- vestigations on untreated and cosmetically treated human hair fi bers using the torsional pendulum technique. For that investigation, we concentrated on considerations of the storage modulus G′, which is derived from the frequency of the free torsional oscillation. A basic core/shell model of cortex and cuticle was applied to model the observed decrease Address all correspondence to Franz J. Wortmann at franz.wortmann@manchester.ac.uk.