Chapter 10 355 degradation or removal of the f-layer (possibly by chemical treatments, heat, and/or abrasion) eliminates this repulsion and allows water to freely contact the surface with subsequent spreading or “wetting.” A practical demonstration of this occurrence is frequently employed by hair stylists in an attempt to assess whether a client’s hair is in suitable condition to receive a chemical treatment. A salon worker will clip a few strands of hair and drop them onto the surface of a glass of water. Healthy hair will float on the surface (due to the hydrophobic f-layer repelling water and preventing effective wetting), while damaged hair is quickly wetted and rapidly sinks. In the laboratory, the affinity of a liquid for a solid surface is quantified by evaluation of the contact angle between the liquid/ air interface and a solid surface. This concept is illustrated by the schematic in Figure 17. A beading water droplet on a hydrophobic surface demonstrates a high contact angle, while this value falls as the surface becomes more hydrophilic. In general, a liquid is said to “wet” a solid if the contact angle is below 90 degrees. Evaluation of this contact angle can be obtained by using high- resolution photography coupled with image analysis software, or by employing the so-called Wilhelmy balance approach.28 With this all said, it is unclear as to whether changes in the wetting properties of hair are perceived in any way by individuals. That is, the external stimuli that degrade the f-layer also affect many other hair properties. However, it is a relatively trivial task to re-hydrophobe the hair surface through the use of conventional conditioner products. Namely, as described in Chapter 3, the Figure 17. Schematic of contact angles for a liquid droplet on different surfaces