362 JOURNAL OF COSMETIC SCIENCE =-. - 't IZOREAL R E C H E R C H E Figure 5. Sample preparation device. Top: four samples in preparation with aligned hair fibers. Bottom: four metallic supports before preparation. RESULTS STABILITY FOR REPEATED MEASUREMENTS The pendulum test was performed on four series (A, B, C, and D), each series composed of four virgin hair samples prepared as described above. Each sample was characterized twice (first and second measurements). Statistical results for the different series are represented on the same graph (Figure 6). © For a same series of four samples, the strokes number N remained constant when the measurement was repeated twice with the same series, as can be seen in comparing for each series the results obtained for the first and the second measurement. It was thus shown that the method is non-destructive. © The variability of data between series of four samples (for example, comparison between A and B) is greater than between two repeated measurements on the same series (comparison for series A between the first measurement and the second mea- surement). Therefore, to take account of the variability of hair fibers, it is necessary to test a minimum of two series, each composed of four samples mounted with the same 39 hair fibers for each tested hair-care product. INFLUENCE OF THE NUMBER OF FIBERS PER SAMPLE From Equation 8, and assuming that all of the hair fibers are geometrically and me- chanically homogeneous (i.e., R = RME,•t½ and E = E•4E,•t • are constants), it can be

BENDING PROPERTIES OF HAIR FIBERS 363 24O 23O 220, 210 200 . . seriesA series B seriesC series D measurement F•l st measurement •-'] 2rid measurement Figure 6. Repeated measurements for virgin hair. Each series (A, B, C, or D) corresponds in four samples prepared once with the same 39 fibers (see Sample Preparation). inferred that the number of strokes N is proportional to the inverse of the number of fibers according to the relation (11) The validity of the influence of the number of fibers n on the number of strokes N was checked. The bending test was repeated using four samples prepared with 39 fibers each. Between two successive tests, one fiber was cut on each sample. The test was stopped when the number of intact fibers reached 23. The graph (Figure 7) confirms that the number of swings is a linear function of the inverse of the number of fibers of the sample. The fact that the relationship is not purely proportional between N and n-t is due to friction occurring on the rotation axis. INFLUENCE OF FIBER DIAMETER The rigidity of different virgin hair fibers obtained from several subjects of two different ethnic origins (European and Asiatic) was evaluated in terms of stroke numbers. The mean number of strokes of the pendulum can be related to the mean diameter of the fibers, as measured by a laser scanning system (Zimmer Gmbh©). For each subject, ten