CONDITIONERS AND HAIR FIBER HARDNESS 581 3. Conditioner treatment. The cleaned and "indented" hair fibers were moistened and exposed to a total of ten 2-minute treatments with 0.5 % of Polyquaternium-10 (infinite bath), each time rinsed for 1 minute in running DI water (gal/min) at 40øC, and dried with a hair dryer at moderate temperature. 4. Nano-indents aj%r ten conditioner applications of the same hair fibers. A total of 42 nano- indentations were placed again onto the 10x conditioner treated, "clean-appearing," flat regions of surface cuticle cells of the adjacent regions of the same hair fibers, which had been indented prior to the conditioner treatment (not onto the same scale face indented prior to the conditioner applications). AFM techniques used Fiber surface properties were obtained on a nanometer scale with a NanoScope © Mul- timode TM scanning probe microscope from Digital Instruments, equipped with nano- indentation capabilities (3-6). Specific AFM scanning techniques used to characterize conditioner-induced changes in the surface properties of hair fibers were: 1. Height, phase contrast, and amplitude signals: These techniques characterize topog- raphy/morphology, surface adhesion, and viscoelasticity, and can be used to measure microroughness and total surface area. 2. Topographical 3-D height profile (in the "tapping mode"): This measures topography by "tapping" the fiber surface with an oscillating diamond probe tip. 3. Nano-indenting: This measures the microhardness of the fiber surface by indenting the surface with a diamond probe tip mounted on a metal-foil cantilever. The surface indent is imaged, recorded, and measured in real time. A schematic of the technique is shown in Figure 1, which shows the various positions of the indenter and the corresponding force curve giving the force at each position of the indenter. Nano-indentation is a relatively new technique, which has been adapted by the AFM protocol to determine or compare the microhardness of surfaces of materials (3-6). In this study, nano-indentation measures the microhardness by indenting the hair fibers (prej•rably the same hair fibers) before and after treatment with a conditioner. In this study, the indents were made on the scale faces with a defined maximum force of 30.1 laN and separated from each other by several micrometers. The cantilever constant of the indenter was 405 N/m. The maximum surface indentation depth was approximately 60 nm. Profile analysis measured the depth of the images of the indents, which were saved in real time. 4. Profile scanning analysis: (also called "cross section analysis"): This measures the depth of the saved images of the indentations (which were recorded in real time) after the experiment. From these images, features related to the relaxation behavior of the material can be derived. RESULTS AND DISCUSSION PRELIMINARY STUDY In the preliminary study, untreated and conditioner-treated hair fibers were indented on the scale faces and the scale edges. Compared to untreated fibers, indents were deeper on the scale faces and shallower at the scale edges of conditioner-treated hair fibers. The

582 JOURNAL OF COSMETIC SCIENCE Figure 1. "AFM force measurement" of cantilever-sample interaction (left) shown schematically at several points along the force curve (right), including: (A) approach, (B) jump to contact, (C) contact and inden- tation, (D) adhesion, and (E) pull-off or separation. (From Digital Instruments Brochure.) outcome of this study suggested that conditioners may have a softening effect on the surface of the harder scale faces and a hardening effect on the originally softer scale edges. Although these results can be explained on the basis of interactions and polymer prop- erties, the fact that the work was carried out on different hair fibers before and after conditioner application, disregarding differences in pre-existing variations in hardness, questioned the reliability of the data. The presence of significant amounts of conditioner residues further confounded the outcome of this study. DETAILED STUDY In this in-depth study, multiple indentations were made on the scale faces of a consid-