Glass walls are great for letting in natural light and brightening up living rooms and offices. But they are not the best for privacy, and can also overheat rooms and create too much glare on computer screens.

Researchers now report a material that lets through more light than a standard window but by diffusing most of the light much like a frosted glass window. What’s more, the thin film material cools ambient room temperatures by as much as 6°C. And its surface, which mimics a lotus leaf, cleans itself. The film could lead to smart windows for greener buildings that keep occupants comfortable and productive.

Frosted glass or light-diffusing films are a common solution for reducing glare and privacy concerns. These films are typically made of plastic embedded with tiny microscopic particles of transparent materials like silica.

Researchers have also developed many coatings with microscopic bumps and structures that repel water and prevent dirt accumulation. And there is a lot of ongoing work in the area of passive cooling materials that reflect sunlight and send heat into space.

The team from Karlsruhe Institute of Technology combined all these characteristics into one novel film. The new plastic-based material, reported in the journal Nature Communications, is covered with micro-pyramids, each one-tenth the diameter of a human hair.

While a standard glass pane window transmits 91 percent of light, glass covered with the new film is 95 percent transparent. But 73 percent of the light is diffused, which means the light is brighter, but the window obscures view and glare.

The material does not trap sunlight and heat. Instead, it sends heat in the form of long-wave infrared radiation that goes straight through the atmosphere into space. Indoor tests showed that the material stayed 6°C cooler than ambient air. The pyramids also create sharp angles that make water bead up and roll off, taking dust with it. This is much like the water-repelling surfaces of lotus leaves.

“The material can simultaneously optimize the use of sunlight indoors, provide passive cooling, and reduce reliance on air conditioning,” said lead author Gan Huang in a press release. “The solution is scalable and can be seamlessly integrated into plans for environmentally friendly building construction and urban development.”

Source: Gan Huang et al. Radiative cooling and indoor light management enabled by a transparent and self-cleaning polymer-based metamaterial. Nat. Commun. 2024.