Yanlin Song and colleagues from the Chinese Academy of Sciences, Beijing, mimicked the structure of the leaf’s lower surface using polymers spun into reflective films consisting of long, hollow uniform fibers.
The underside of the poplar leaf is better at reflecting light than the top. This is because of the ‘cool roof’ effect, in which a layer of hairs on the underside reflects the light, so that less heat penetrates the leaf. The leaf turns over in strong sunlight to reveal the underside and as the light is being reflected rather than absorbed, the leaf appears white. “Normally, the poplar tree looks green, but sometimes in the summer, the tree shows a white cast,” said Song.
The team discovered that controlling the film thickness and making the cross section of the fibers as similar to the leaf hair as possible is the key to high reflectivity. They tested their films by coating them onto the compound diarylethene, which changes from red to colorless in the presence of visible light—the structure changes from a closed ring to an open ring. They found that the coating stopped the diarylethene changing color, and had the additional benefit of being hydrophobic.
“The reflectance and waterproof nature of the coatings make them ideal candidates for a number of building situations,” said Robert Lamb, an expert on surface science. “Improving the
durability of such delicate interfaces with the environment will be the major hurdle.”
Song says that his team will continue to develop highly reflective materials, widening the wavelength at which they function, to eventually be used to improve the efficiency of lighting.