MORPHOLOGICAL CHANGES OF HUMAN HAIR 343 The increased logarithmic decrement of the nonpigmented hair strands is attributed to higher friction forces between the single hair fi bers and increased air resistance due to a higher collective volume. Figure 6. Box & whisker plot of the natural frequency f0 of pigmented and nonpigmented hair (n = 3). SE: standard error. Figure 7. Box & whisker plot of the logarithmic decrement - of pigmented and nonpigmented hair (n = 3). SE: standard error.
JOURNAL OF COSMETIC SCIENCE 344 DYNAMIC HAIR MOVEMENT OF PONYTAILS—RESULTS OF THE IN VIVO METHOD The relative amplitude Arel (swing height) of ponytails that contain ≥5% nonpigmented hair fi bers is signifi cantly lower (p 0.001) in comparison with pigmented ponytails ( 5% nonpigmented hair fi bers) as shown in Figure 8. The lower swing height of “gray ponytails” relates to the increasing fi ber curvature and bending stiffness with increasing age of unpigmented hairs (1,2). Free oscillation is analyzed by dropping the ponytail from a 90° angle until the resting position is reached. Logarithmic decrement - is signifi cantly higher (p 0.001) for “gray ponytails” (≥5% nonpigmented hair fi bers) in comparison with pigmented ponytails as shown in Figure 9. This can be explained by increased fi ber–fi ber interactions within the ponytail and in- creased frictional forces due to an increased fi ber curvature (1,2). SUMMARY AND CONCLUSIONS Ageing of the hair follicle leads to various morphological and mechanical changes of the fi bers. The nonpigmented hair fi bers used in this study are signifi cantly thicker, signifi - cantly wavier, and have a slightly, but not signifi cantly, lower Youngs’ modulus in com- parison with the used pigmented hair. The new in vitro method allows the quantifi cation of the hair movement of hair strands. Thereby, nonpigmented hairs have a signifi cantly lower relative amplitude in comparison with pigmented hair. The natural frequency is signifi cantly lower and the damping sig- nifi cantly higher for nonpigmented hair in comparison with pigmented hair. Figure 8. Box & whisker plot of the relative amplitude A of pigmented and “gray” ponytails (n = 39). NP: nonpigmented, SE: standard error.
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