Adsorption Properties of Hair 354 described as being “more-porous.” Thus, there are enhanced effects from treatments that involve actives penetrating (i.e. perms), while color fading after permanent dying occurs more rapidly. Approaches for measuring the extent of hair fiber swelling originally involved microscope-based methods often coupled with special holder attachments. However, laser micrometers are now preferred. Nonetheless, the historical literature teaches that non- homogeneity of hair necessitates measuring multiple points on many fibers. Interaction of Hair and Liquid Water (Wettability) It is common to hear about the hydrophobic nature of healthy hair, which would appear at odds with the earlier content of this chapter concerning the considerable adsorptive capacity for water. The reason for this apparent disconnect relates to properties of the fiber bulk versus those of the surface. Reviewing some of the concepts outlined in Chapter 1, cuticle scales themselves have a laminate sub-structure containing different keratin compositions (see Figure 16). The cross-link density of these keratins increases towards the outer surface and consequently the low-cystine endocuticle is thought to be the primary pathway for water (and water-soluble materials) to enter the hair. Conversely, the structures above have progressively less affinity for water, with the very outermost surface also possessing a lipid-membrane structure, often termed the f-layer. Therefore, water droplets will bead on healthy hair in an attempt to minimize contact with this hydrophobic surface. Conversely, Figure 16. Schematic of cuticle sub-structure