Summary: In recent years, the field of radiative cooling (see below) has seen a fair bit of interest and different designs for various applications. However, cooling roofs and walls of buildings remain its greatest application, and white paints, owing to their convenience and modest radiative cooling capability, remain the benchmark for radiative coolers. Curiously, they are seldom mentioned in prominent works that have come out of late, and the paint industry, in turn, has been somewhat distant from advances made in the field.
This article aims to draw research interest into paints as highly efficient radiative coolers. Specifically, simple material and morphological alterations that can greatly enhance the cooler performances of paints are shown, and interdisciplinary challenges associated with their usage, such as the effect of dust or the need for durability, are discussed.
Summary: This work shows that porous polymers, which normally scatter light and appear white due to the air voids in them, can be turned transparent or translucent by wetting them with suitable liquids. The idea itself has been known for as long as humans have noticed paper or cloth turn more translucent when wetted. What I try is to push it to a limit and use it for switchable cooling/heating applications.
The key to achieving a white to near-transparent switching is choosing the right liquid. In its porous form, the polymer contains air voids which have a different refractive index (n~1) from that of the polymer (n~1.4-1.5), causing light to scatter off the pores and yield the white colour. But if the pores are filled with a liquid that has the same refractive index as the polymer, then to light the whole system just behaves like one uniform material, so light transmits, as it would through glass.
We achieve this behaviour by using common materials and liquids, and also extend the switching behavior to thermal infrared wavelengths. Promisingly, the switching in the thermal is opposite to that in the solar wavelengths, meaning that porous polymers can switch from icehouse to greenhouse states – something that has not been observed with electrochromic or other switchable designs as far as I know.
With regard to applications, we show that this behavior can be used for switchable heating and cooling of buildings depending on the season, controlling daylight in buildings, thermal camouflage and other uses. Given the low cost and simplicity of the designs we use, they could potentially see large scale uses.