MORPHOLOGICAL CHANGES OF HUMAN HAIR 339 Andover SP10 5NY, UK). At a relative humidity of 55%, 100 pigmented and nonpigmented hair fi bers were stretched at a constant speed rate of 10 mm/min up to the breaking point. The Youngs’ modulus is determined in the Hookean region between 0.3% and 1.2% strain. OSCILLATION OF HAIR STRANDS–IN VITRO A new measurement method has been developed by the University of Manchester (UK) and Henkel AG & Co. KGaA (Hamburg, Germany) to analyze the movement characteristics of human hair (4,5). A round hair strand is placed vertically into a sample holder that is connected at the top to a rotation axis. The latter enables impartment of a defi ned driving frequency (0–5 Hz) and a defi ned rotational angle (0–160°) by an electronic motor (V1.0, voltage: 220 V, power: 0.1 kW). The swinging movement of the strand is recorded by a high-speed camera (VW-9000 Keyence Corp., Osaka, Japan) at 1,000 frames per second. Before measurement, the hair strand is combed up to 5× by hand, discharged (Static Line LC, HAUG GmbH & Co. KG, Leinfelden-Echterdingen, Germany) and attached to the sample holder. Ten driven oscillations with an angle of 60° and a frequency of 1.40 Hz are imposed. With these parameters, a harmonic-driven oscillation and a subsequent harmonic-free oscillation can be established. The video recording starts with the rotation of the axis and stops after the free oscillation when no further movement of the strand can be subjectively noticed. To account for the 3D shape of the hair strand, it is recorded in four defi ned lateral orien- tations. The measurement of the hair movement takes place at 24 ± 3°C and 40% ± 10% relative humidity. OSCILLATION OF PONYTAILS—IN VIVO To differentiate between various hair movements in a realistic setting, the in vitro approach was modifi ed to assess the motion of ponytails of female volunteers while walk- ing on a treadmill (Paragon 308, Drive motor 2.5 CHP Digital System, 150 × 50-cm running area, Horizon Fitness, Taichung, Taiwan). To enable a stable hair movement, the women’s hair is tied up in a ponytail (middle center of the back of the head) and the women are instructed to walk (not run) on the treadmill. To generate a comparable walking frequency, the driven oscillation or “walking swing” is measured at a specifi ed velocity v that depends on the leg length L (pelvic bone to ankle) of the woman: ¯ ¡ ° ¡ ° ¸ ¡ ° ¡ ° ¢ ± ¢ ± 1¯ v =2.0 . m L m s s (5) The ponytail is combed up to 5× by hand and af ter a warm-up (walking for 1 min with an individual velocity and 2 min with at the specifi ed velocity on the treadmill), the hair movement is recorded for 24 s with a high-speed camera from the back (VW-9000 Keyence Corp., Osaka, Japan). Besides the driven oscillation, the free oscillation performance of each ponytail is also determined by dropping the ponytail from a 90° angle. The motion is recorded until the resting position is reached.
JOURNAL OF COSMETIC SCIENCE 340 QUANTIFICATION OF HAIR MOVEMENT The analysis of the movement characteristics is based on single frames of the video record- ing (VW-9000 Motion Analyser Keyence Corp.). The length l of the hair strand/ponytail is measured in the resting position at t = 0 s, and the position of the hair tips in the rest- ing position is also used as a baseline. During the driven and free oscillation, the amplitude A is tracked and the corresponding time t of the hair tips is determined in the area of the maximum swing height (Figure 2). The distance between the maximum of the tracking point and the baseline is defi ned as the maximum displacement. In addition, for the determination of the amplitude of the ponytail, the movement of the head is taken into account. During the driven oscillation, six amplitudes are determined (in vitro method: amplitudes number 6–11 in vivo method: constant swing height shown by an automatically gener- ated motion curve). The ratio of the amplitude A and the length of the hair collective l is defi ned as the relative amplitude Arel: = . A Arel l (6) The free oscillation is recorded when the excitati on stops (in vitro) or by dropping the ponytail from a 90° angle (in vivo). During the free oscillation, the fi rst six amplitudes are determined. The natural frequency f0 [equation (2)] and the logarithmic decrement - [equation (3)] of the strand is calculated with the corresponding times. Figure 2. Schematic representation of the determination of the amplitude A of a hair strand (in vitro) by means of tracking the left hair tips and measuring the length l of the strand at the resting position. The gray arrows represent the motion of the rotation axis and the hair strand.
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