2008 TRI/PRINCETON CONFERENCE 137 to accommodate different hair assembly confi gurations. Prior to each set of pulls hair tresses were combed, treated with an anti-static device (Zerostat) and the weight of the tress zeroed on Instron prior to the measurement. The pulling was carried out at the speed of 1000 mm/min and the experiment repeated 3–4 times in both rotational and station- ary modes. Hair tresses were washed by wetting with 40°C running tap water, applying the treat- ment, lathering the tresses for 30 seconds and rinsing in running 40°C tap water for 30 seconds. This was followed by a fi nal rinse with room temperature DI water for 15 sec- onds. All hair tresses were blown dry using hand held hair dryer (Conair 1875) at warm heat and low air fl ow for ~3 minutes while combing, unless noted otherwise. AQUALON SLT MEASURED PARAMETERS The Aqualon SLT measures the rotational and stationary work of pulling. In the case of the rotational mode, the pins mounted on ball bearings are rotating freely. This elimi- nates the friction on the pins and the only contribution to the work of pulling (measured by Instron) is the resistance to deformation of an assembly of individual hair fi bers, hair- to-hair friction and friction in the bearings that is assumed to be negligible. We defi ne the combination of these two parameters as the apparent stiffness. In the stationary mode, the tress is subjected to the same deformation with pins immobi- lized and, in addition to the apparent stiffness, the friction on pins contributes to the work of pulling. Figure 4 shows force profi les produced by a Caucasian virgin hair tress in the two modes. The two profi les follow an identical pattern with the force profi le in stationary mode shifted upward by a proportionality factor related to the hair tress friction on the pins. Figure 2. The Aqualon SLT with the face plate dismantled, illustrating the operation in rotational and sta- tionary modes.

JOURNAL OF COSMETIC SCIENCE 138 Note that the pulling force decreases in magnitude at around 130 mm, as noted in the fi gure, this corresponds to the point where a hair tress clears the bottom pin. The indi- vidual profi le varies in from tress to tress. In order to eliminate the point to point vari- ability in data we base the analysis on the total work of pulling. The measured and calculated parameters are defi ned in equations 1 through 4. Equation 1 defi nes the work of pulling in the rotational mode as apparent stiffness. The description of the apparent stiffness excludes the contribution of hair assembly resistance to compression Figure 3. The Aqualon SLT with a hair tress threaded. Figure 4. Force profi le obtained by pulling a Caucasian virgin tress through the Aqualon SLT in stationary and rotational modes.