This image shows how a cone slowly builds up in a heated ‘elastomer’ film, before suddenly inverting to launch the film high into the air over the span of less than a second. Image: Hebner et al., 2023, Science Advances.
This image shows how a cone slowly builds up in a heated ‘elastomer’ film, before suddenly inverting to launch the film high into the air over the span of less than a second. Image: Hebner et al., 2023, Science Advances.

Engineers at the University of Colorado at Boulder (CU Boulder) have designed a new, rubber-like film that can leap high into the air like a grasshopper – all on its own and without needing outside intervention. Just heat it up and watch it jump!

The engineers report their achievement in a paper in Science Advances. They say that similar materials could one day help ’soft robots’ (those that don’t need gears or other hard components to move) to leap or lift.

According to co-author Timothy White, the material composite functions in a similar way to grasshoppers, which jump by storing and releasing energy in their legs. “In nature, a lot of adaptations like a grasshopper’s leg utilize stored energy, such as an elastic instability,” explained White, professor of chemical and biological engineering at CU Boulder. “We’re trying to create synthetic materials that emulate those natural properties.”

This study takes advantage of the unusual behavior of a class of materials called liquid crystal elastomers. These materials are solid and stretchy polymer versions of the liquid crystals found in laptops and TV displays.

The team fabricated small films of liquid crystal elastomers about the size of a contact lens, then set them on a hot plate. As the films heated up, they began to warp, forming a cone that rose up until, suddenly and explosively, it flipped inside out – shooting the material up to a height of nearly 200 times its own thickness in just six milliseconds.

“This presents opportunities for using polymer materials in new ways for applications like soft robotics where we often need access to these high-speed, high-force actuation mechanisms,” said lead author Tayler Hebner, who earned her doctorate degree in chemical and biological engineering at CU Boulder in 2022.

Hebner, now a postdoctoral researcher at the University of Oregon, and her colleagues discovered this leaping behavior almost by accident. She was experimenting with designing different kinds of liquid crystal elastomers to see how they changed their shape under shifting temperatures. Joselle McCracken, a senior research associate in White’s lab, joined her to observe.

“We were just watching the liquid crystal elastomer sit on the hot plate wondering why it wasn’t making the shape we expected. It suddenly jumped right off the testing stage onto the countertop,” Hebner said. “We both just looked at each other kind of confused but also excited.”

With careful experimentation and help from collaborators at the California Institute of Technology, the team discovered what was making their material do the high jump. Each of these films comprise three layers of elastomer, which shrink when they get hot, but the top two layers shrink faster than the bottom one. That incongruity, combined with the orientation of the liquid crystal molecules within the layers, causes the film to contract and form a cone shape. It’s a bit like how painted vinyl sidings can warp in the sun’s rays.

As the cone forms, strain builds up in the film until, all at once—snap! The cone inverts, slapping the surface and launching the material upwards. The same film can also hop several times without wearing out.

“When that inversion happens, the material snaps through, and just like a kid’s popper toy, it leaps off the surface,” White said.

Unlike those poppers, however, the team’s liquid crystal elastomers are versatile. The researchers can tweak the films so that they hop when they get cold, for example, not hot. They can also give the films legs to make them jump in a particular direction.

Most robots probably wouldn’t be able to use this kind of popping effect to make their parts move. But White said the project shows what similar kinds of materials could be capable of – storing an impressive amount of elastic energy, then releasing it in a single go. And, Hebner said, the project brought a bit of fun to the lab.

“It’s a powerful example of how the fundamental concepts we study can transform into designs that perform in complex and amazing ways,” she said.

This story is adapted from material from the University of Colorado at Boulder, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.