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Hair Dryers: The Physics of Rapid Evaporation

The invention of the hair dryer has given hairstylists a powerful tool of the trade and the rest of us a quick way to dry soggy fireworks. Check out Groupon’s explanation of the science behind dry hair.

Given time, everything wet—even hair after a long shower—dries due to evaporation. When left to “air dry,” hair slowly loses its moisture as the air scoops it up and converts it to water vapor. Hair dryers speed up the drying process by blowing hot air over the wet hair—a process that works wonders thanks to two key principles of evaporation:

• Evaporation requires energy, and more energy—in this case, heat—helps break the bonds holding water molecules together more quickly; when those bonds are loose, the water changes states.
• A pocket of air can only hold a certain amount of vapor before it becomes saturated. Thus, blowing air over the hair increases the amount of air that can potentially pick up the vapor.

All hair dryers consist of an electric fan and a heating element. As the fan pushes air down the barrel, it passes over the heat and emerges as hot air. Though some models heat coils of wire with an electrical current, others have ceramic heating elements, which can dry hair more quickly by producing the more radiant, safer far-infrared heat. Lately, many stylists have begun to adopt ionic hair dryers, which expel hot air with a surplus of negatively charged ions, reducing static electricity and resulting in smoother hair.