Ordered systems have systematic and repeating patterns. Disordered systems are arranged randomly. When most materials are stretched in one direction, they shrink perpendicularly, and when compressed they expand perpendicularly. Materials that do the opposite (shrink perpendicularly when compressed and become thicker when stretched) are known as auxetics. Auxetics are rare. However, these materials are suspected to be better at absorbing energy and be more fracture resistant. Creating auxetic materials is, therefore, of interest to help improve the function of materials that could absorb shock.
Nidhi Pashine and Daniel Hexner, University of Chicago, Il, USA, and colleagues have investigated if they could use a disordered material’s “memory” of a prior stresses it had encountered to transform the material into something new. First, they ran computer simulations of normal materials under pressure. They selectively altered atomic bonds to see which changes could make the material auxetic. The team discovered that, by cutting the bonds along the areas with the most external stress, they could digitally create an auxetic material.
The team then applied a constant pressure to materials and let it age naturally. The materials adjusted itself under the pressure and turned itself from normal materials into mechanical metamaterials. Because of inhomogeneous local stresses, the material itself decides how to evolve by modifying stressed regions differently from those under less stress. This means, material evolution (referred to as aging of a material) occurs in response to stresses. Therefore, aging can be “directed” to produce sought-after responses and unusual functionalities that do not inherently exist. According to the researchers, this simple and effective process is a step closer to being able to create materials with specific atomic-level structures without the need for high-resolution equipment or atomic-level modifications.
The researchers also see their work in a strong connection to structures in biology. Organs, enzymes, and filament networks are natural examples of disordered systems that are difficult to copy synthetically because of their complexity. Their directed aging approach could be a starting point to create complex human-made structures that take inspiration from the wide range of properties seen in biology.