Control by light alone, turning new, smart materials, bending and moving

Researchers at the Tufts University School of Engineering have created light-activated composite devices that are capable of performing precise, visible movements and forming complex three-dimensional shapes without the need for wires or other operating materials or energy sources. The design combines programmable photonic crystals with an elastomeric composition that can be designed on the macro- and nano-scale to respond to illumination.

The research offers new possibilities for the development of smart league-driven systems, such as high-efficiency, self-adjusting solar cells that automatically follow the sun’s direction and angle of light, micro-liquid valves powered by light, or soft robots that move with light on. application. . A ‘photonic sunflower’ whose petals curl to and away from the illumination and track the path and angle of the light demonstrates the technology in a paper published on March 12, 2021. Nature communication.

Color is the result of the absorption and reflection of light. Behind every flash of a gleaming butterfly wing or opal gemstone lie intricate interactions in which natural photonic crystals embedded in the wing or stone absorb light of specific frequencies and reflect others. The angle at which light meets the crystal surface can affect which wavelengths are absorbed and the heat generated from the absorbed energy.

The photonic material designed by the Tufts team joins two layers: an opaque film doped with syphibroin with gold nanoparticles (AuNPs), which form photonic crystals, and an underlying substrate of polydimethylsiloxane (PDMS), a polymer on silicone. In addition to remarkable flexibility, durability and optical properties, syphibroin is unusual with a negative coefficient of thermal expansion (CTE), which means that it contracts when heated and expands when cooled. PDMS, on the other hand, have a high CTE and expand rapidly when heated. As a result, when the new material is exposed to light, one layer heats up much faster than the other, so that the material bends as one side expands and the other side contracts or expands more slowly.

“With our approach, we can design these opaque films on different scales to design the way they absorb and reflect light. When light moves and the amount of energy absorbed changes, the material folds and moves differently as a function from his relative position to that light, ” said Fiorenzo Omenetto, co-author of the study and Frank C. Doble, professor of engineering at Tufts.

While most optomechanical devices that convert light into motion involve complex and energy-intensive fabrication or setup, “We are able to achieve excellent control of light energy conversion and generate ‘macro-motion’ of these materials without there is a need for electricity or wires, ‘Omenetto said.

The researchers programmed the photonic crystal films by applying stencils and then exposing them to water vapor to generate specific patterns. The pattern of surface water changed the wavelength of the absorbed and reflected light of the film, causing the material to bend, fold and rotate in different ways, depending on the geometry of the pattern, when exposed to laser light.

In their study, the authors showed a ‘photonic sunflower’, with integrated solar cells in the two-layer film, so that the cells detected the light source. The photonic sunflower kept the angle between the solar cells and the laser beam almost constant, which made the cells efficient as the light moved. The system works just as well with white light as with laser light. Such wireless, light-responsive, heliotropic (solar-tracking) systems could potentially increase the efficiency of the light-to-energy conversion for the solar power industry. The team’s demonstrations of the material also include a butterfly whose wings have opened and closed in response to light and a self-folding box.