This illustration shows the ratcheting building block that could be embedded in new materials. After vertical compression, the ratcheting block can keep the materials collapsed, releasing its energy in response to a sideways pull. Image: Stoyan Smoukov.What do a flea and an eagle have in common? They can both store energy in their feet so that they don’t have to continuously contract their muscles to jump high or hold on to prey. Now scientists at Queen Mary University of London and the University of Cambridge, both in the UK, have created materials that can store energy this way, allowing them to be squeezed repeatedly without damage and even change shape if necessary.
These kinds of materials are called auxetics and behave quite differently from regular materials. Instead of bulging out when squeezed, they collapse in all directions, storing the energy inside.
Current auxetic material designs have sharp corners that allow them to fold onto themselves, achieving a higher density. This is a property that has been incorporated recently in lightweight armor designs, where the material can collapse in front of a bullet upon impact. This is important because the mass in front of a bullet is the biggest factor in armor effectiveness.
The sharp corners also concentrate forces and cause the material to fracture if squeezed multiple times, but this is not a problem for armor as it is only designed to be used once. In this study, reported in a paper in Frontiers in Materials, the scientists redesigned auxetic materials with smooth curves for distributing the forces, making repeated deformations possible for applications where energy storage and shape-changing material properties are required. The work establishes the basis for designs of lightweight three-dimensional (3D) supports, which can also fold in specific ways and store energy that could be released on demand.
"The exciting future of new materials designs is that they can start replacing devices and robots," said principal investigator Stoyan Smoukov from Queen Mary University of London. "All the smart functionality is embedded in the material, for example the repeated ability to latch onto objects the way eagles latch onto prey and keep a vice-like grip without spending any more force or effort."
The team expects its nature-inspired designs could be used in energy-efficient gripping tools required by industry, re-configurable shape-on-demand materials and even lattices with unique thermal expansion behavior.
"A major problem for materials exposed to harsh conditions, such as high temperature, is their expansion," added Eesha Khare, a visiting undergraduate student from Harvard University who was instrumental in defining the project. "A material could now be designed so its expansion properties continuously vary to match a gradient of temperature farther and closer to a heat source. This way, it will be able to adjust itself naturally to repeated and severe changes."
The flexible auxetic material designs, which were not possible before, were adapted specifically to be easily 3D-printed, a feature the authors consider essential. "By growing things layer-by-layer from the bottom up, the possible material structures are mostly limited by imagination, and we can easily take advantage of inspirations we get from nature," said Smoukov.
This story is adapted from material from Queen Mary University of London, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.