“Transparency is an enabling feature because you can use this gel in windows, and you could use it in extraterrestrial habitats,” said Ivan Smalyukh, a professor in the Department of Physics. “You could harvest sunlight through that thermally insulating material and store the energy inside, protecting yourself from those big oscillations in temperature that you have on Mars or on the moon.”

The defining feature of aerogels, as their name suggests, is air, Smalyukh explained. By weight, these thin films are 90 percent gas. Engineers achieve this feather weight by generating crisscrossing patterns of solid material that trap air inside billions of tiny pores, similar to the bubbles in bubble wrap. It’s that trapping capacity that makes them such good insulators.

“You create a very tortuous network of these nanoparticles that link together in the aerogel, preventing the heat from going through,” Smalyukh said.

Beer to windows

That same network, however, tends to scatter light, making aerogels look cloudy and explaining why some engineers call them “frozen smoke.”

To make a more translucent gel, Smalyukh and his colleagues begin with the common plant sugar cellulose. By carefully controlling how cellulose molecules link up, the team is able to orient them into a lattice-like pattern.

That pattern is so uniform, he said, that it allows light to pass through unbothered, giving the gel its transparent appearance.

Problem solved. In order to find a ready supply of cellulose for their space-age material, the researchers turned to a substance with humble beginnings: a refreshing IPA.

Unused beer wort, or waste liquid produced during the brewing process, can make cellulose when scientists add in specialized bacteria. The researchers began driving to breweries across the Boulder area to collect tubs of unwanted liquid from beer-makers.

“So not only are we recycling and saving this valuable material from entering the landfill, but we’re also producing this raw material cheaply,” said Andrew Hess, a Ph.D. student in physics at CU Boulder. 

Currently, it takes the team about two weeks to culture the cellulose, but the rest of the process of making the aerogel moves quickly. The final product of the team’s efforts is a thin, flexible film that is roughly 100 times lighter than glass. This gel is so resistant to heat that you could put a strip of it on your hand and light a fire on top—without feeling a thing. 

Joshua De La Cruz, a Ph.D. student in the CU Boulder Materials Science and Engineering Program, pours beer wort into a tray before adding cellulose-producing bacteria. (Credit: CU Boulder)

 

Mars to Antarctica

While the researchers have their eye on putting this material on space habitats, more immediate applications are already available on Earth.

Most windows are poor insulators. According to the Department of Energy, roughly one-quarter of the energy that is expended to heat and cool buildings in the United States goes toward offsetting the loss of heat through windows, potentially costing building operators billions of dollars per year. 

Covering glass in sheets of the aerogel, however, could dramatically slow down the loss of heat, said Hess, who also leads the project’s tech-to-market transfer work. And you wouldn’t have to replace the windows in the process.

“Windows are incredibly expensive to replace,” Hess said. “We’re envisioning a retrofitting product that would basically be a peel-and-stick film that a consumer would buy at Home Depot.”

On a larger scale, cities could use the gel to retrofit windows on skyscrapers, dramatically increasing energy efficiency. 

To get to that point, the researchers say that they would need to learn how to produce more aerogel faster. But they’re already moving forward on that goal. Smalyukh’s team has been exploring partnerships with window manufacturing companies.

Earlier this summer, they won NASA’s 2018 iTech competition, a national contest that seeks out Earth-bound technologies that might one day help people travel to space. The team’s research has also been supported by the National Science Foundation and the Department of Energy’s Advanced Research Projects Agency-Energy (ARPA-E).

“Our approach so far has been around windows,” Hess said. “However, we also see our technology being enabling for so many other applications, including smart clothes, for insulating cars and protecting firefighters.”

And there are more fanciful uses, too. Smalyukh said that one afternoon, his lab got a visit from the young daughter of one of his team members. For fun, the researchers coated her hand in the non-toxic aerogel, giving her a crystal-clear and flexible glove. That inspired Smalyukh to think about using aerogels to make clothing that would be both ultra-warm and transparent.

“This opens your imagination,” he said. “We were joking about going to the South Pole and doing a group outing with penguins. You could sun tan on top of the ice.”