Model-based analysis of the torsional loss modulus in human hair and of the effects of cosmetic processing.

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 fiber 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. Significant 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.