My answer to What are the dangers of using a flat iron on wet hair?
Answer by O. Franklin:
Using a flat iron on wet hair will cause bubble hair. Bubble hair is when the naturally occurring spaces that exist in hair become filled with water, and then heat is applied to the hair. The water in the hair vaporizes, and when it vaporizes it expands and causes hair damage.
The following study explains this very nicely:
“Bubble hair is a sign of thermal injury. Hair dryers operating at 175°C or more can cause bubble hair. The use of hair curling tongs operating at 125°C and applied to the hair for one minute can also induce bubbles in hair fiber. All hair fibers contain air-filled spaces called vacuoles. These spaces can also become filled with water when the hair is wet. Too much heat vaporizes the water in the hair fibre into steam. This vaporization of the water may force the spaces in the hair to expand, eventually turning the hair into a sponge-like structure. These damaged hairs are weak and brittle as the bubbles destroy the integrity of the fiber.[ ] Chemical treatment may also precipitate the onset of bubble hair and any already weak hair, whatever the cause, may be more susceptible to bubble development.[ ]”