QUANTIFYING HAIR MOTION 377 different conditions—i.e., the low and high frequency sides of the infl ection point. Both are shown in Table I—with both yielding comparable rankings of the different tresses, namely, higher frequency/energy is necessary to produce comparable motion in pro- gressively shorter hair. Similar experiments were performed to investigate the effect of hair weight. In these experi- ments, fi rst two and then three tresses were affi xed together in a back-to-back manner— again using plastic cable ties. Results showed a notable increase in amplitude when doubling up the hair—with a further slight increase from use of triple tresses. It is con- cluded that heavy, short tresses produce the highest amplitudes of oscillation—and would therefore have the least sensitivity in experiments intending to fi nd means of increasing hair motion. Therefore, all further experiments have been performed on 24-cm single tresses. PRODUCT AND INGREDIENT EFFECTS Silicone oil was applied to tresses at the lowest dosage considered possible. This was ac- complished by touching a fi nger to the surface of the oil—such that only the smallest amount was retained. This minute dosage was then massaged thoroughly through the tress to ensure uniform application. Later, after initial testing, a second dosage was similarly applied. Amplitude versus frequency curves for untreated hair and these two silicone treatments are shown in Figure 6. Results show the treated hair giving rise to notably lower ampli- tudes of motions, although higher frequencies (energy) are also needed to attain a given Figure 6. Effect of silicone oil on the shape of amplitude versus frequency plots.

JOURNAL OF COSMETIC SCIENCE 378 amplitude. Visually, these treated tresses exhibit considerably less motion. Therefore, lower amplitudes at higher frequencies is surmised to be an indication of diminished motion. Conversely, higher amplitudes in combination with lower frequency/energy would seem- ingly equate to improved motion. Silicones are widely thought of as lubricants and so ability to reduce interfi ber friction might have been speculated to enhance motion. This said, silicones are lubricants under high shear conditions. Here, fi bers rubbing against each other presumably constitutes low shear conditions—where the presence of silicone oil possibly produces a degree of interfi ber cohesion. The increased weight of the silicone on the hair may also be a negative contributor. Commercial hair conditioner. The lubricating ability of typical commercial conditioner prod- ucts is well recognized. To that end, amplitude versus frequency curves for conditioner- treated hair shift slightly to the left (lower frequency), although also decreasing slightly in amplitude. Per the previous argument, this might be interpreted as slightly less over- all motion but being attained under the application of less energy input. Heat straightening. Heat straightening appears to improve hair motion. Despite the known damaging effects of these treatments—short-term benefi ts involve a highly aligned, straight conformation that seemingly possesses visibly enhanced movement properties. In this instance, amplitude versus frequency curves became taller, slightly broader but without any notable shifting of the peak on the frequency axis. This is interpreted as enhanced motion without meaningfully more energy input. In contemplating the differing ways in which amplitude versus frequency curves can be altered by these treatments, it is perhaps useful to again think about the way the previ- ously mentioned fundamental fi ber properties are affected. For simplifi cation, we consider hair weight, hair stiffness, and fi ber–fi ber interactions. The application of silicones adds weight to the hair, seemingly provides interfi ber cohe- sion but presumably does not change fi ber stiffness. Commercial conditioner deposits (i.e., quats and fatty alcohols) might add lesser weight and provide interfi ber lubrica- tion, while also having no effect on fi ber stiffness. Meanwhile, heat straightening would decrease fi ber weight via water removal, with fi bers becoming stiffer concomitantly, whereas improved alignment and de-swelling result in lesser interfi ber friction. It is evident that each of these hair treatments impacts fundamental properties in different manners and so amplitude versus frequency curves might also be anticipated to change in different ways. HUMIDITY EFFECTS To this end, relative humidity might be expected to have considerable effect on hair motion. Hair adsorbs progressively higher levels of moisture with increasing relative humidity (5)—and, therefore, becomes heavier. Moreover, water is a plasticizer for hair and so fi bers also become progressively less stiff. Furthermore, elevated water content leads to a degree of radial swelling within fi bers, which raises overall tress volume and likely induces some increased friction. Figure 7 shows amplitude versus frequency curves for the same hair equilibrated at fi rst low humidity and then high. Although notable differences arise, interpretation is not