MORPHOLOGICAL CHANGES OF HUMAN HAIR 341 RESULTS AND DISCUSSION CHARACTERIZATION OF PIGMENTED AND NONPIGMENTED HAIR FIBERS The movement characteristics of human hair depend, aside from the shape of the hair collective, greatly on fi ber–fi ber interactions and the properties of the single hair fi bers. Therefore, the fi ber diameter, waviness, and Youngs’ modulus of pigmented and non- pigmented hair fi bers of the hair strands (Kerling International Haarfabrik GmbH) are analyzed. Nonpigmented hair fi bers have a signifi cantly higher equivalent fi ber diameter (p  0.001) in comparison with pigmented hairs as shown in Figure 3. Furthermore, nonpigmented hair fi bers are signifi cantly (p  0.001) wavier in comparison with pigmented hair as shown in Figure 4. The parameter to determine the waviness of single hair fi bers is the decrease of the fi ber length (%). The Youngs’ modulus is a material-dependent parameter that is measured in the linear elastic region during the deformation of the hair fi ber. The modulus for nonpigmented hair (2.44 ± 0.28 GPa) is not signifi cantly (p = 0.088) different in comparison with pig- mented hair (2.51 ± 0.31 GPa). DYNAMIC HAIR MOVEMENT OF STRANDS—RESULTS OF THE IN VITRO METHOD Pigmented and nonpigmented hair fi bers differ in their morphological and mechanical properties. It can be assumed that the movement characteristics change by ageing as well. Figure 3. Box and whisker plot of the equivalent diameter of pigmented and nonpigmented hair fi bers (n = 100). SE: standard error.

JOURNAL OF COSMETIC SCIENCE 342 Therefore, pigmented and nonpigmented human hair strands are investigated in vitro by means of driven and free oscillation. During the driven oscillation, the relative amplitude Arel, also called as “swing height,” is determined. Arel for a nonpigmented hair strand is signifi cantly lower in comparison with pigmented hair (p  0.001) as shown in Figure 5. This can be explained by higher bending stiffness and lower defl ection of the nonpigmented hair strand during motion. During the free oscillation, the natural frequency f0 and the logarithmic decrement - are determined. f0 is signifi cantly lower (p = 0.017) for nonpigmented hair in comparison with pigmented hair (Figure 6). The energy loss in a hair strand, given by the logarithmic decrement -, is signifi cantly higher (p  0.001) for nonpigmented hair in comparison with pigmented hair as shown in Figure 7. Figure 4. Decrease in the fi ber length due to the fi ber waviness of pigmented and nonpigmented hair (n = 50). SE: standard error. Figure 5. Box & whisker plot of the relative amplitude Arel of pigmented and nonpigmented hair (n = 3). SE: standard error.