Engineers at the University of Colorado Boulder have designed a new type of rubber membrane that can leap high into the air like a grasshopper — all on its own, without outside intervention. Just heat it up and watch it jump!
The researchers describe their achievement in the Jan. 18 issue of the journal scientific progressSimilar materials could one day help enable “soft robots” (robots that don’t need gears or other hard parts to move) to jump or lift, they say.
Study co-author Timothy White said the composite reacted a bit like a grasshopper jumps by storing and releasing energy in its legs.
“In nature, many adaptations like grasshopper legs use stored energy, such as elastic instability,” said White, a professor of chemical and biological engineering at CU Boulder. “We’re trying to create synthetic materials that can mimic these natural properties.”
The new research takes advantage of the unusual behavior of a class of materials called liquid crystal elastomers. These materials are solid and elastic polymer versions of the liquid crystals found in laptop or TV monitors.
For the study, the team fabricated small wafers of liquid crystal elastomers about the size of a contact lens, then placed them on a hot plate. As these films heated up, they began to warp, forming a rising cone, until it suddenly and explosively flipped inside out — launching the material nearly 200 times its own thickness in just 6 milliseconds.
“This opens up opportunities to use polymeric materials in new ways, for applications such as soft robots that we often need to use these high-speed, high-force actuation mechanisms,” said study lead author Tayler Hebner, who has a PhD in chemical and biological engineering. 2022 At CU Boulder.
Hebner, now a postdoctoral researcher at the University of Oregon, and her colleagues discovered this jumping behavior almost by accident.
She is trying to engineer different kinds of liquid crystal elastomers to see how they change shape under changes in temperature. She was joined by Joselle McCracken, a senior research fellow in White’s lab.
“We just looked at the liquid crystal elastomer sitting on the hot plate and wondered why it wasn’t forming the shape we expected. It jumped from the test bench to the bench top,” Hebner said. “We were all just looking at each other, kind of confused, but also excited.”
With careful experimentation and the help of Caltech collaborators, the team discovered what makes their material jump high.
Each of these films consists of three layers of elastomer, White explained. The layers shrink when heated, but the top two shrink faster than the bottom, he said. This mismatch, combined with the orientation of the liquid crystal molecules within the layer, causes the film to shrink and form a cone. It’s a bit like painted vinyl siding warping in the sun.
As the cones form, strain is created in the film until suddenly—snap! The cone is inverted, tapping the surface and knocking the material up. The same membrane can also jump several times without wearing out.
“When this inversion occurs, the material bounces off, like a child’s popper, it bounces off the surface,” White said.
Unlike those poppers, however, the team’s liquid crystal elastomers are versatile. The researchers can tune their films so that they jump when they get cold, for example, when they don’t get hot. They can also give the films legs, making them jump in certain directions.
Most robots probably won’t be able to use this popping effect to move their parts. But White said the project demonstrated the ability of similar materials to store an impressive amount of elastic energy and then release it all at once. And, Hebner said, the project brought some fun to the lab.
“This is a powerful example of how the fundamental concepts of our research translate into designs that execute in complex and striking ways,” she said.
Grasshopper, meet your new competitor.