Researchers have created dynamic structures that leap into the air on a predetermined schedule without intervention from computers or external stimuli. Precisely when these "metashells" jump, and how high they jump, is engineered into the physical structure of the materials.
"There are structures that 'jump' immediately when loading is removed – such as when a coiled spring is released," says Jie Yin, corresponding author of a paper on the work and an associate professor of mechanical engineering at North Carolina State University. "We wanted to create a structure that does not rely on external stimuli and allows us to dictate the timing of the jump in advance. We have developed a technique that allows us to precisely schedule when the structure leaps into action, whether that is in seconds or hours."
For this work, the researchers created spherical structures made of strands of a material called polyethylene terephthalate (PET) which are connected into a complex lattice pattern. The design of the structure maximizes the material's ability to store energy. When a load is applied to the structure, it is deformed out of shape, but when that load is removed it returns to its original shape. However, PET has viscoelastic material properties, which means it doesn't snap back into its original shape right away.
Instead, once the load is removed, the viscoelastic metashell slowly starts "creeping" back to its original shape before reaching a critical point – at which point it snaps the rest of the way back to its original shape all at once. And the longer the load is applied to the structure, the longer it takes for the structure to reach that critical point once the load is removed.
"The end result is that if you apply a load to our spherical metashell, it is compressed into a shape like a flower bud," says Haitao Qing, first author of the paper and a Ph.D. student at NC State. "When the flower bud snaps back into the spherical shape, the release of energy hurls the structure into the air. And you can dictate exactly when that jump will happen by controlling how long the load is applied to the structure. This also controls how high the metashell jumps, because the longer the load is applied, the less high the metashell will jump."
"Material properties play a critical role, and the design of the structure we created amplifies those properties," says Yin. "The material is viscoelastic and the structure design is responsible for storing elastic energy, and both features are critical to the performance of this technology."
In testing, the researchers demonstrated that jumps could be scheduled from three seconds to 58 hours in advance. The metashells could jump up to nine times their height into the air, or as little as 0.5 times their height, depending on how far ahead the jumps were scheduled. The researchers also demonstrated that the metashells could jump effectively on a variety of surfaces – from solid surfaces to sand, snow or water – and at temperatures as low as minus 15 degrees Celsius.
The researchers also demonstrated that the metashells could be loaded with cargo, such as seeds, and would disperse those contents when it jumped. Video of the metashells in action can be found at https://www.youtube.com/watch?v=6LWB3MujBTc .
"For this test, we were inspired by explosive seed dispersal, which we see in nature, such as Impatiens balsamina," Qing says. "We showed that a 100-millimeter metashell was capable of dispersing seeds across an area of 1.5 meters."
"Moving forward, we are interested in exploring biodegradable materials that would work with this design and investigating various potential applications," Yin says. "We are also open to collaborating with other researchers or the private sector on ways to make use of this technology."
The paper, "Programmable seconds-to-days long delayed snapping in jumping metashell," will be published the week of June 2 in the Proceedings of the National Academy of Sciences. The paper was co-authored by Caizhi Zhou and Fangjie Qi, Ph.D. students at NC State.
This work was done with support from the National Science Foundation under grants 2126072 and 2329674. Qing and Yin are co-inventors on a pending patent invention disclosure filed by North Carolina State University related to this work.
