One of the many conundrums in the world of curly hair is that some people experience a relaxation of their curls as their hair gains length, while others experience the converse: their curl increases with the length of their hair.
The former trend makes sense without having to give it much thought. Longer hair has more weight and is pulled down by gravity, which lengthens and loosens the curl. However, the latter phenomenon seems counterintuitive.
This behavior can be so perplexing, causing curls to disappear with a haircut or to suddenly begin developing as someone grows their hair out for perhaps the first time. While this seeming contradiction may be baffling and even frustrating, it is possible to understand what is going on if one looks at what causes hair to curl and some mathematic principles that can be used to describe curly hair.
Morphology of Hair
Human hair is a marvelously complex biomaterial, comprised of many nanoscale substructures woven together into intricate patterns, both beautiful and functional. The building block of hair is the protein keratin, which is made up of long chains of amino acids. The amino acids in the keratin strands have very specific bond geometries that give the fiber an α-helical conformation. Individual keratin fibers bundle together with other keratin fibers to form aggregates called microfibrils. Clusters of microfibrils bundle together into macrofibrillar structures which occupy the central cortex of the hair. Fatty acids and keratin-based cuticles encapsulate the entire strand.
Human hair keratin is made up of 14 percent sulfur-containing amino acids (cysteine and cystine). It is from these amino acids that many of the properties of hair are developed, particularly curl. When two strands of keratin are adjacent to one another, the –SH bonds for nearby cystine groups can be oxidized to form a disulfide (S-S) bond between the two strands. This is a chemical crosslink that ties the adjacent keratin strands together. A high proportion of disulfide bonds twist the hair strand into a helical pattern. Adjacent hair strands tend to assume the same pattern, and then cluster together into multi-helical structures that form curls. In this manner, the nanoscopic structure is repeated at the macroscopic level. Nature loves patterns.
The permanent wave process exploits this by breaking disulfide bonds and then reforming them (and forming new ones) with hair locked into the desired helical shape.
There are a number of factors that contribute to degree of curliness. These include, but are not limited to:
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