The physics of straightening curly hair with heat

Smithsonian : Human hair varies in curvature and texture. Because many people with curly hair want to straighten it, various methods have been developed that involve the use of heat. However, how much heat and how often it can be applied without damaging the hair had never been scientifically studied. Now Tahira Reid of Purdue University and her colleagues have classified hair into eight types based on its curvature and tested each one’s thermal conductivity and ability to dissipate heat. They found that many variables come into play, including the number of fibers being heated, how closely they’re packed, and the total surface area. Although it appears that curlier hair is more likely to have weak cross-sectional points and be vulnerable to damage, there is considerable variation between different hair types and between different fibers of the same hair type. The researchers say that a larger number of hair samples will be required for a more statistically reliable study.