Research at UIUC, Akron contributes to understanding of bird-nest metamaterial mechanics
When a bird selects a stick to add to a nest, it instinctively relates the properties of the stick to the overall nest that’s taking shape. As the bird adds various new materials, the nest gradually becomes much more than just the sum of its parts: it becomes a “metamaterial” – a material with unique mechanical properties that can’t be found in any one naturally occurring material by itself.
As explained by Hunter T. King of the University of Akron, “The [nest’s] long-term stability, despite rustling from the wind and the parents’ landing and leaving, could be attributed to the material’s hysteretic behavior” – in other words, the manner in which the metamaterial’s state depends on its history. “Without it, the applied energy could cause internal vibrations that gradually cause it to fall apart.”
Such metamaterials can have a range of valuable applications for humans, too. However, before any new metamaterial can be applied usefully, its emergent mechanical properties must be understood.
For that reason, King and other researchers at the University of Akron and the University of Illinois Urbana-Champaign – including Mattia Gazzola, an assistant professor of mechanical science and engineering at UIUC – have been studying what happens when metamaterials are compressed, akin to what happens when birds pack down the materials of a nest.
They leveraged granular physics – the study of extracting bulk behaviors from individual behaviors – to better understand the associated energy loss and hysteresis. Their goal was to learn how a material’s properties change when it is compressed versus when it is released from compression. The ability to tune that mechanism would be a crucial parameter for some applications, such as ones involving shock absorption and toughness.
“This study is very fundamental, and isn’t necessary targeting future applications,” said Gazzola. “But one imagines that it could lay the foundation for the development of new materials based on these engineering properties.”
Among other outcomes, the team’s findings could facilitate the development of metamaterials that can be taken apart and reassembled, in that no material damage would occur to the constituent elements when they are separated – much as a bird’s nest could be infinitely reformed and reassembled without harming the sticks and bits of fluff the bird had collected.
The researchers’ latest findings are described in a Physical Review Letters paper entitled, “Micromechanical Origin of Plasticity and Hysteresis in Nestlike Packings,” https://doi.org/10.1103/PhysRevLett.128.198003.