364 JOURNAL OF COSMETIC SCIENCE WITHDRAWAl FORCE (rag) .. t60 t20 - i0C- 80- o oJ o.2 o.5 Sebum treated Untreated I I I I I I 1 0.4 0.5 0.6 0.7 O.B 0,9 4.0 PACKING DENSITY (g/cm 3) Figure 14. Withdrawal forces as a function of packing densities for untreated and sebum-treated human hair. Error bars at 95% confidence level. Cutting the fibers after packing into the cylinder compacts the swatch in that region. Although an attempt was made to reverse this compaction using a dissecting needle, the accuracy of the results in Table VI and Figure 14 below packing densities of 0.15 appears questionable. It is therefore recommended to make measurements in a range where the withdrawal forces are essentially independent of packing density, i.e., 0.25-0.3 g/cm 3. Efj•ct of sebum add-on. Fiber withdrawal forces for various hair swatches treated with different sebum concentrations are shown in Table VII and Figure 15, together with values for the corresponding untreated swatches. It should be noted that the abscissa represents the concentration of the sebum solution rather than the sebum add-on on the fiber. Once again, considerable scatter is observed in the data. The regression line extrapolates to a value of 45 mg at zero concentration, slightly higher than the value for untreated fibers, which, in view of the scatter of the data, seems reasonable. The swatch treated with a 3% sebum solution was again treated with Vaseline © hair tonic by combing a few drops into the swatch until the swatch looked somewhat oily. Although this led to a pronounced compaction of the swatch, the fiber withdrawal force was significantly lower than that for the same swatch treated with sebum only, indi- cating that Vaseline hair tonic acts as a lubricant. Although no extensive work was done on Vaseline hair tonic, this observation was made for a number of treatments, supporting the assumption that it acts much more as a lubricant than sebum. The difference in behavior may be associated with the nature and especially the continuity of the film deposited on the fiber surface.

INTERFIBER ADHESION 365 Table VII Withdrawal Forces for Swatches Treated With Sebum Solutions of Various Concentrations concentration (%) Withdrawal force (mg) Untreated Sebum-treated 0.25 29.8 + 3.6 50.2 + 11.4 0.50 33.2 + 5.0 42.4 + 11.9 1.50 29.8 + 2.7 88.8 + 20.2 2.00 34.3 + 4.0 75.6 + 18.0 3.00 33.7 + 4.8 102.0 + 28.3 4.00 45.0 _+ 6.5 107.7 _+ 25.3 Packing density: 0.4-0.5 g/cm 3. All entries are averages of 15-20 measurements at 95% confidence level. WITHDRAWAL FORCE (rng } {20 - {00 - 80- 6O 4O 2_0- $e b.••um •reoted tonic Untreated 0 1 2 5 4 5 SEBUM SOLN. CONC. ( % w/v } Figure 15. Withdrawal forces as a function of concentration of sebum solution. Fiber packing density of 0.4-0.5 g/cm 3. CONCLUSIONS While the measurement of adhesive forces between human hair fibers by the point contact method discussed here is probably quite accurate, its repeatability along the length of the same fiber is relatively poor. This is thought to be mainly due to an irreproducibility of the conditions at the point of contact between the two fibers, which can be attributed to the nonuniform scale structure of the fibers and their elliptical cross section, which produces an uncertainty of the actual area of contact. When measuring the effects of topical treatments, another source of errors is introduced, which is the nonuniformity in the distribution and the thickness of the deposited surface film. Some of these problems disappear when liquids are deposited on the fiber surface. In this case, coalescence of the liquid films on the contacting fiber surfaces produces an attrac-