In a first, researchers have shown that as materials age, they 'remember' prior stresses and external forces, which can then be used to create new materials with unique properties. The scientists, including those from the University of Pennsylvania (Penn) in the US, were able to use this new 'memory' to control how a material ages, and encoded specific properties that allowed it to perform new functions.
In the current study, published in the journal Science Advances, the researchers wanted to see if they could use a disordered material's "memory" of the prior stresses it had encountered to transform the material into something new. They ran computer simulations of normal materials under pressure, and selectively altered atomic bonds in these to see which changes could make the material shrink perpendicularly when compressed, and become thicker when stretched, also called an auxetic material.
They discovered that, by cutting the bonds along the areas with the most external stress, they could digitally create an auxetic material. According to the researchers, such auxetic materials are rare but are suspected to be better at absorbing energy, and be more fracture-resistant. To test this in the lab, the team took a Styrofoam-like material and added "memory" by allowing the material to age under specified stresses.
The researchers made this material auxetic by applying constant pressure to it, and letting it age naturally. "With the whole thing under pressure, it adjusted itself. It turned itself from a normal material into a mechanical metamaterial," said Andrea Liu, study co-author from Penn. According to Liu, this is a "totally different" way to think about making new materials.
"You start with a disordered system, and if you apply the right stresses you can make it come out with the properties you want," she said. Using simpler approaches as starting point the researchers could create complex human-made structures that take inspiration from the wide range of properties seen in biology, the scientists said in a statement.
In addition to making auxetic materials. we have also used a computer to design in precise mechanical control of distant parts of the material by applying local stresses. This too is inspired by biological activity. We now need to see if this, too, can be made to work by ageing a real material in the laboratory," study co-author Sidney Nagel said.