Imagine paint that adheres to a surface but releases on command, or road signs that change their reflectivity with changing weather conditions. These are two potential uses of a novel, responsive material designed by researchers in the University of Massachusetts Amherst polymer science and engineering department.
The research was published online in the journal Advanced Materials. Inspired by the way a Venus flytrap captures its pray, Alfred Crosby and his doctoral candidate Douglas Holmes created a polymer surface covered with small holes capped by thin lenses of the same material. The lenses can snap between convex and concave when triggered.
Venus flytrap leaflets work in a similar way. Through a combination of geometry and materials selection, the flytrap leaflets snap from concave to convex when an object triggers their hairs. The key to the flytrap’s ability to capture prey, and a key feature in Crosby and Holmes’ material, is the speed and sensitivity that accompany a “snap” transition. For the Venus flytrap, the transition occurs in roughly 100 milliseconds, and the “snapping surfaces” can snap at least as fast as 30 milliseconds. Even more important is the fact that this speed can be easily adapted for faster or slower transitions depending on the final use.
The new material consists of a number of lens-shaped patterns fabricated on a silicon substrate. Each individual element looks like a hemispherical shell, which can be in either a concave or convex orientation. Like the flytrap, the lenses in the material can switch between convex and concave orientations at speeds of around 30 ms. Such high-speed transitions are made possible by a phenomenon called “snap buckling instability”. When the pressure applied on a curved surface exceeds a certain threshold, it changes the orientation from a convex to a concave shape, or vice versa.
“It’s like taking a tennis ball and cutting it into two pieces,” says Crosby. “If you place one half between two vertical stands, and start pushing the bulge, the ball changes shape and begins to deform. Then at one critical point, it stops deforming and changes to a convex shape.” The curved surfaces in the new material react in the same way, but in this case the stimulus can come in the form of pressure, heat or an electrical current. In their first prototype, the team used a fluid-derived swelling to demonstrate the rapid transition. When the individual lenses change shape, the surface as a whole undergoes a transition that modifies its reflectivity or its focal length. This means that the material could be used in outdoor signs where the reflectivity of the surface keeps changing, and also as an adaptive lens with a variable focal length.