BOULDER, Colo. — Reminiscent of the 90’s film “Flubber,” scientists at the University of Colorado Boulder are developing an amazing new, rubber-like film that can leap high into the air like a grasshopper. What does it take to get this film hopping? A little bit of heat.
Study authors explain their invention could help pave the way for similar materials in the near future capable of helping to embody “soft robots” (bots that don’t need gears or other hard components to move) to leap or lift. According to co-researcher Timothy White, the film is somewhat similar to grasshoppers in the sense that it jumps 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,” says White, the Gallogly Professor of chemical and biological engineering at CU Boulder, in a university release. “We’re trying to create synthetic materials that emulate those natural properties.”
This project focuses on the unusual behavior of a class of materials known as liquid crystal elastomers. These materials are solid and stretchy polymer versions of the liquid crystals found throughout laptops or TV displays.
The research team fabricated small wafers of liquid crystal elastomers roughly the size of a contact lens, then placed them on a hot plate. As the films heated up, they began to warp, eventually forming a cone that rose up until, suddenly and explosively, it flipped inside out — causing the material to shoot upwards toward 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,” adds study lead author Tayler Hebner, who earned her doctorate degree in chemical and biological engineering at CU Boulder in 2022.
Incredibly, this leaping behavior was discovered almost by accident. Hebner, now a postdoctoral researcher at the University of Oregon, and her colleagues were experimenting with designing different kinds of liquid crystal elastomers in order to observe how they changed their shape under shifting temperatures.
“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 explains. “We both just looked at each other kind of confused but also excited.”
Thanks to extremely careful experimentation and help from collaborators at the California Institute of Technology, the research team uncovered what was making the film jump.
Prof. White explains that each of the films are made up of three layers of elastomer. Those layers tend to shrink as they get hot, but the top two layers shrink even faster than the bottom one. That incongruity, in combination with the orientation of the liquid crystal molecules within the layers, leads the film to contract and form a cone shape. The process is somewhat similar to how painted vinyl sidings can warp under the Sun’s rays.
As the cone forms, stress builds up in the film until, all at once the cone inverts, slapping the surface and knocking the material up. The same film can even hop several times without wearing itself out.
“When that inversion happens, the material snaps through, and just like a kid’s popper toy, it leaps off the surface,” Prof. White comments.
Unlike those poppers, however, the team’s liquid crystal elastomers are truly versatile. The researchers can tweak and adjust the films so that they hop when they get cold instead of hot. They can also give the films legs to facilitate jumping in a particular direction.
The majority of robots won’t be able to use this popping effect to spark movement. However, Prof. White says that this project shows what similar kinds of materials could be capable of — more specifically, storing a significant amount of elastic energy, then releasing it all at once.
“It’s a powerful example of how the fundamental concepts we study can transform into designs that perform in complex and amazing ways,” White concludes.
The study is published in the journal Science Advances.