Just over a year ago, Fang began collaborating with researchers at the University of Hong Kong, who were keen on finding ways to reduce the energy usage of buildings in the city, particularly in the summer months, when the region grows notoriously hot and air-conditioning usage is at its peak.
"Meeting this challenge is critical for a metropolitan area like Hong Kong, where they are under a strict deadline for energy savings," says Fang, referring to Hong Kong's commitment to reduce its energy use by 40 percent by the year 2025.
After some quick calculations, Fang's students found that a significant portion of a building's heat comes through windows, in the form of sunlight.
"It turns out that for every square meter, about 500 watts of energy in the form of heat are brought in by sunlight through a window," Fang says. "That's equivalent to about five light bulbs."
Fang, whose group studies the light-scattering properties of exotic, phase-changing materials, wondered whether such optical materials could be fashioned for windows, to passively reflect a significant portion of a building's incoming heat.
The researchers looked through the literature for "thermochromic" materials -- temperature-sensitive materials that temporarily change phase, or color, in response to heat. They eventually landed on a material made from poly (N-isopropylacrylamide)-2-Aminoethylmethacrylate hydrochloride microparticles. These microparticles resemble tiny, transparent, fiber-webbed spheres and are filled with water. At temperatures of 85 F or higher, the spheres essentially squeeze out all their water and shrink into tight bundles of fibers that reflect light in a different way, turning the material translucent.
"It's like a fishnet in water," Fang says. "Each of those fibers making the net, by themselves, reflects a certain amount of light. But because there's a lot of water embedded in the fishnet, each fiber is harder to see. But once you squeeze the water out, the fibers become visible."
In previous experiments, other groups had found that while the shrunken particles could reject light relatively well, they were less successful in shielding against heat. Fang and his colleagues realized that this limitation came down to the particle size: The particles used previously shrank to a diameter of about 100 nanometers - smaller than the wavelength of infrared light - making it easy for heat to pass right through.
Instead, Fang and his colleagues expanded the molecular chain of each microparticle, so that when it shrank in response to heat, the particle's diameter was about 500 nanometers, which Fang says is "more compatible to the infrared spectrum of solar light."