Why does hair curl?

The shape of hair is determined by the structure of the hair shaft, which is composed of a protein called keratin. Keratin is made up of long chains of amino acids that are twisted together to form a helical structure. The shape of this helix determines whether hair is straight, wavy, or curly.

Straight hair has a regular, cylindrical shape, while wavy hair has a slightly oval shape. Curly hair has a more pronounced oval shape, and the hair shaft is often twisted or bent.

The shape of the hair shaft is determined by the arrangement of the disulfide bonds between the amino acids in the keratin chains. These bonds create cross-links between the amino acid chains, which hold the helix in place. The more disulfide bonds there are, the stiffer the hair will be.

Curly hair has more disulfide bonds than straight hair, which is why it is more resistant to straightening. When curly hair is straightened, the disulfide bonds are broken and the hair shaft is able to relax and become straight. However, when the hair is exposed to moisture or heat, the disulfide bonds reform and the hair curls up again.

The exact shape of curly hair is also influenced by the diameter of the hair shaft and the amount of damage to the hair. Thicker hair tends to be curlier than thinner hair, and damaged hair is more likely to be frizzy and curly.