Imagine a high-tech composite material that exists as fibres that are just 50 µm wide and up to 1 m long. The same material has a tensile modulus in excess of 5 x 109 Pa and can spring back to its original length when it has been stretched by over 25%. It also fails to rupture under loads exceeding 3 x 108 Pa, and remains exceptionally flexible. You’re probably thinking of a man-made fibre. Even if you are told that this is a natural material, you might think of the silk spun by spiders. But you’d be wrong – the material is human hair. This combination of superior mechanical properties results from the complex and unique organization of the different elements that make up hair.
In the May issue of Physics World Frédéric Leroy of L’Oréal Recherche, Aulnay sous Bois, France, explains how health and beauty companies like L’Oréal are using ever more sophisticated characterization and imaging techniques – many of them physics based – to understand the structure of hair, and to measure the effectiveness of shampoos, conditioners, colourants and permanent-wave treatments on the hair’s strength, body and shine.
