Columbia Engineering built a pigment-free paint filled with microscopic air pores; it stayed 6°C below surrounding air during hot desert tests in Arizona

Imagine this scenario: It’s 110°F in Phoenix; the asphalt has apparently become molten lava, and somewhere up on a roof, a small piece of plain white paint is sitting cool as cucumbers at 10.8°F (6°C) below the ambient temperature, using no power source, no compressor, and relying on passive radiative cooling rather than mechanical refrigeration. This isn’t science fiction; it’s an actual achievement accomplished by a group of researchers at Columbia University.

According to the 2018 Columbia Engineering research paper titled “Hierarchically porous polymer coatings for highly efficient passive daytime radiative cooling,” published in Science, a team headed by Yuan Yang and Nanfang Yu, and doctoral candidate Jyotirmoy Mandal as the lead author, described a coating made of a polymer material filled with microscale air pockets which reflects more than 96 percent of solar energy and emits the collected heat back out to space while remaining cool at six degrees below the surrounding air temperature in Arizona’s harsh desert heat and three degrees in humid and misty Bangladesh. Additionally, the coating can be painted, dip-coated, or spray-coated onto substrates, giving the technique what the researchers described as a paint-like simplicity.

Paint that's colder than the air around it


And here comes the clever bit: this coating contains no white pigment. Conventional white paint gets its color by adding titanium dioxide pigment particles; TiO₂ is highly effective at reflecting visible light, which is why it is widely used in white paints. However, conventional pigment-based coatings can have lower reflectance in parts of the solar spectrum, particularly the near- and short-wave infrared, and the Columbia researchers sought to overcome those limitations. But researchers at Columbia University took a different approach: replaced the conventional pigment-based scattering mechanism with a hierarchically porous polymer structure. Using a phase-inversion process, they created numerous micro- and nanoscale air voids within the polymer matrix.

These microscale air bubbles scatter light because of the refractive-index contrast between air and polymer, in much the same broad physical principle by which porous structures such as snow scatter light. This results in a material that is visually white while strongly reflecting incoming sunlight and efficiently emitting thermal radiation in the atmospheric infrared window. However, the phenomenon of passive daytime radiative cooling is not entirely new.

According to the 2014 Nature article titled “Passive radiative cooling below ambient air temperature under direct sunlight,” by Stanford University scientists Aaswath Raman, Shanhui Fan et al., the group reported an experimental demonstration to nearly 5°C below ambient air temperature under direct sunlight. The device used a seven-layer photonic structure made from hafnium oxide and silicon dioxide that reflected 97 percent of incident sunlight and emitted strongly in the atmospheric transparency window. The approach was highly engineered, whereas the Columbia work sought a simpler, scalable coating that could be fabricated using a phase-inversion process and applied much like paint.

The Purdue plot twist

Purdue University’s Department of Mechanical Engineering made headlines in 2021 due to the fact that its scientists had developed a paint recognized by Guinness World Records as the whitest paint known to humankind. According to the research titled “Ultrawhite BaSO4 Paints and Films for Remarkable Daytime Subambient Radiative Cooling,” published by Purdue University in the journal ACS Applied Materials & Interfaces, the barium-sulfate-based paint achieved a solar reflectance of 98.1 percent and a sky-window emissivity of 0.95. In field tests, the BaSO₄ film remained more than 4.5°C below ambient, while the BaSO₄-acrylic paint showed similar cooling performance. Guinness World Records lists the barium-sulfate paint’s reflectivity as 98.1 percent and the record date as April 15, 2021.

The skeptical part

None of this is available on the Home Depot shelf just yet, and it’s been a few years since either paper was published. Columbia’s spray or coating is an experimental lab model with a provisional patent but not an actual product; even the university said the researchers had filed a provisional patent application for the work. The original Columbia paper also reported accelerated thermal-aging and moisture testing, but moving from a laboratory coating to a durable, mass-produced commercial product remains a separate challenge.

As for Purdue’s paint, its recognition by Guinness World Records was followed by considerable media and industry interest, but the 2021 research described a laboratory-developed paint rather than a mass-market product. The gap between a successful laboratory formulation and widespread commercial deployment can involve manufacturing, durability, cost, regulatory and supply-chain considerations.