Scientists develop new material that can jump 200 times its thickness

The material was adapted by the likes of a grasshopper’s leg, which utilizes stored energy such as an elastic instability.
Deena Theresa
A grasshopper
A grasshopper

University of Colorado 

Engineers at CU Boulder have designed a new material that could pose strong competition to grasshoppers. This new, rubber-like film can jump high into the air like a grasshopper, entirely by itself, without any stimulation or outside intervention. According to scientists, such materials could manifest into soft robots.

The researchers published their findings on January 18 in the journal Science Advances

"In nature, a lot of adaptations like a grasshopper’s leg utilize stored energy, such as an elastic instability," Timothy White, Gallogly Professor of chemical and biological engineering at CU Boulder, said in a statement. "We’re trying to create synthetic materials that emulate those natural properties."

"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 Tayler Hebner, the study lead author, now a postdoctoral researcher at the University of Oregon.

Scientists develop new material that can jump 200 times its thickness
Graphic showing how a cone slowly builds up in an "elastomer" film, then inverts to launch the film high into the air over the span of less than a second.

An accidental discovery that leaped out of a plate

The material was not a planned design. If anything, it was almost an accident. Hebner and her colleagues discovered the material's leaping behavior. 

She was experimenting with designing different kinds of liquid crystal elastomers, a class of materials, to see how they changed their shape under shifting temperatures. These materials are solid and stretchy polymer versions of the liquid crystals found in laptops or TV displays. 

Small wafers of liquid crystal elastomers, about the contact lens size, were set on a hot plate. "As those films heated up, they began to warp, forming a cone that rose until, suddenly and explosively, it flipped inside out—shooting the material up to a height of nearly 200 times its thickness in just six milliseconds," according to the release.

"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."

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Liquid crystal elastomers are versatile and can store elastic energy

After careful experimentation and help from collaborators at the California Institute of Technology, the team discovered what was making their material do the high jump. 

According to White, each of these films is made of three elastomer layers. When they get hot, the layers shrink, and the top two layers shrink faster than the bottom. When combined with the orientation of the liquid crystal molecules within the layers, the film contracts and forms a cone shape. 

When the cone forms, strain builds up in the film until the cone inverts, slapping the surface and knocking the material up. 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. 

However, liquid crystal elastomers are much more versatile, unlike poppers. "The researchers can tweak their films so that they hop when they get cold, for example, not hot. They can also give the film, legs to make them jump in a particular direction," as per the release.

According to White, the project shows what similar kinds of materials could be capable of—"storing an impressive amount of elastic energy, then releasing it a single go."

Study Abstract:

Snap-through mechanisms are pervasive in everyday life in biological systems, engineered devices, and consumer products. Snap-through transitions can be realized in responsive materials via stimuli-induced mechanical instability. Here, we demonstrate a rapid and powerful snap-through response in liquid crystalline elastomers (LCEs). While LCEs have been extensively examined as material actuators, their deformation rate is limited by the second-order character of their phase transition. In this work, we locally pattern the director orientation of LCEs and fabricate mechanical elements with through-thickness (functionally graded) modulus gradients to realize stimuli-induced responses as fast as 6 ms. The rapid acceleration and associated force output of the LCE elements cause the elements to leap to heights over 200 times the material thickness. The experimental examination in functionally graded LCE elements is complemented by computational evaluation of the underlying mechanics. The experimentally validated model is then exercised as a design tool to guide functional implementation, visualized as directional leaping.

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