Get a piece of chewing gum, chew it for a bit and then take it out of your mouth and stretch it. As it gets longer, the gum will get thinner and thinner. This happens with all kinds of materials from rubber to steel. But not all materials get thinner when you stretch them. There are strange substances, called auxetic materials, which actually get wider as you stretch them longer. Now mathematicians are helping chemists and engineers understand what makes these materials work.

Although auxetic materials seem really advanced, scientists have known about them for more than 100 years. A German scientist named Woldemar Voigt did some experiments and found that fool’s gold (iron pyrite) was probably auxetic. Other experiments have suggested that many other materials could be auxetic, including paper and even living bone. However, until recently it has been very hard to work out exactly how an auxetic material will respond to forces.

A team of scientists from the University of Malta decided to investigate. The researchers imagined an auxetic material as a collection of strong rectangles that are joined at the corners. When the material is stretched, the rectangles don’t bend, but they rotate against one another (see picture). This rotation causes the gaps between the rectangles to grow, which allows the material to expand in all directions at once.

The researchers used their idea to write a computer program that could predict how real auxetics would respond to forces. Their program (called a ‘model’) gave good results for a wide range of auxetic materials. Having a good model means that engineers and scientists can work out how auxetic materials are going to behave without having to build them. Making new products out of these strange materials should now be a lot easier.

What are auxetics used for?

Auxetic materials get wider when you stretch them, but the reverse is also true. If you compress an auxetic material, it gets thinner as well. This shrinking means that the material becomes more dense (tightly packed) in the part that is getting squeezed, which could help absorb impacts. Engineers think that auxetic materials might be good for protective equipment, from bike helmets to body armour.

http://central.ly/sensationalhair