New rubbery film rockets into the air like a grasshopper
In a serendipitous discovery, engineers at the University of Colorado Boulder (CU Boulder) have developed a material that first deforms and then shoots into the air when it is heated. The researchers say the material could one day be used to help soft robots jump or lift objects.
As part of a study, researcher Tayler Hebner, who earned her doctorate degree in chemical and biological engineering at CU Boulder in 2022, and her colleagues were looking at the different ways in which a class of materials known as liquid crystal elastomers acted when exposed to heat. These materials are the solidified and elastic version of the liquid crystals used inside displays such as those in laptop screens or TVs, and may even one day be used in windshields to protect pilots from getting temporarily blinded by laser pointers.
“We were just watching the liquid crystal elastomer sit on the hot plate wondering why it wasn’t making the shape we expected," Hebner said. "It suddenly jumped right off the testing stage onto the countertop. We both just looked at each other kind of confused but also excited.”
Further investigation with colleagues from the California Institute of Technology revealed that the cause of the leaping behavior was due to the fact that the film, which was about the size of a contact lens, consisted of three layers. When heated, the top two layers shrink faster than the bottom layer, causing it to form a cone shape. Eventually due to a buildup of tension, the cone inverts, striking the surface on which it is being heated and snapping up into the air. In tests, the researchers found that in just six milliseconds, the film could soar to almost 200 times its thickness.
“When that inversion happens, the material snaps through, and just like a kid’s popper toy, it leaps off the surface,” said study co-author Timothy White, professor of chemical and biological engineering at CU Boulder. In this regard, the research echoes that carried out at Harvard University in 2020 in which the design of a popper toy inspired an actuator that might one day help soft robots get across tough terrain.
The researchers compare the action of the film to the way in which grasshoppers achieve their impressive displays of leaping and believe it too could have a place in the development of soft robots – those machines that operate without metallic or other hard materials such as those that walk as fast as humans using light and magnetic fields.
“In nature, a lot of adaptations like a grasshopper’s leg utilize stored energy, such as an elastic instability,” said White, “We’re trying to create synthetic materials that emulate those natural properties.”
As part of uncovering the structure of the films being used, the research team discovered that their material could also be made to jump by cooling instead of heating it. They also say that by adding legs to the film, the material can gain directional control. While the team admits that the material may not become a primary locomotion source in soft robots, they feel that the discovery helps add to the knowledge of how to make these innovative machines function better, as it shows the potential of materials to store and then release energy under certain conditions.
“It’s a powerful example of how the fundamental concepts we study can transform into designs that perform in complex and amazing ways,” said Hebner.
The research has been published in the journal Science Advances. You can see the material in action in the video below.
Source: CU Boulder