The porous polymer coatings, which switch from white to transparent when wetted, can be put into plastic enclosures to make panels that control light and temperatures of buildings. Credit: Jyotirmoy Mandal/Columbia Engineering
Buildings devote more than 30% of their energy use to heating, cooling, and lighting systems. Passive designs such as cool roof paints have gone a long way toward reducing this usage, and its impact on the environment and climate, but they have one key limitation—they are usually static, and thus not responsive to daily or seasonal changes.
Columbia Engineering researchers have developed porous polymer coatings (PPCs) that enable inexpensive and scalable ways to control light and heat in buildings. They took advantage of the optical switchability of PPCs in the solar wavelengths to regulate solar heating and daylighting, and extended the concept to thermal infrared wavelengths to modulate heat radiated by objects. Their work is published on October 21, 2019 by Joule.
“Our work shows that by wetting PPCs with common liquids like alcohols or water, we can reversibly switch their optical transmittance in the solar and thermal wavelengths,” says Jyotirmoy Mandal, lead author of the study and a former Ph.D. student in the lab of Yuan Yang, assistant professor of materials science and engineering. “By putting such PPCs in hollow plastic or glass panels, we can make building envelopes that can regulate indoor temperatures and light.”
The team’s design is similar to smart windows, but with a higher optical switchability, and is built using simpler, inexpensive materials that could make it implementable at large scales. It builds upon earlier work that demonstrated a paint-like fluoropolymer coating with nano-to-microscale air voids that can cool down buildings. That coating was static, however. “In places like New York, which sees warm summers and harsh winters, designs that can switch between heating and cooling modes can be more useful,” says Yang.
The team began their work on optically switching PPCs serendipitously, when Mandal noticed that a few drops of alcohol spilled on a white fluoropolymer PPC turned it transparent. “What we saw was the same mechanism that causes paper to turn translucent when wetted, but at a near-optimal level,” says Mandal. “The physics of this has been previously explored, but the drastic switching we saw led us to explore this particular case, and how it can be used.”
A porous material like paper appears white because the air in the pores has a different refractive index (~1) to that of the porous material (~1.5), causing them to scatter and reflect light. When wetted by water, which has a refractive index (~1.33) closer to the material, scattering is reduced and more light goes through, making it translucent. Transmission increases when the refractive indices are closely matched. The researchers discovered that their fluoropolymer (~1.4) and typical alcohols (~1.38) have very close refractive indices.