A transparent metasurface developed at ETH Zurich contains embedded nanoparticles that plasmonically amplify light energy, heating the surface and driving off the condensation that leads to window fogging. [Image: ETH Zurich / Christopher Walker]
Eyeglasses, a motorcycle helmet faceplate, a car windshield, a home window in winter—all of them are subject to the vexing problem of surface fogging when they meet humid air. A team of engineers at ETH Zurich, Switzerland, has now come up with an elegantly simple solution: A clear metasurface, impregnated with gold nanoparticles that, when struck by visible light, plasmonically heat the window or lens surface to drive off the fogging moisture (Nano Lett., doi: 10.1021/acs.nanolett.8b04481).
The researchers report that their metasurface can clear away fogging as much as four times faster than state-of-the-art antifogging sprays and coatings, and can be applied to a range of materials including glass, polymers and flexible surfaces. The team is working with an industry partner to refine the technology and bring it to market.
Building a better passive defogger
The physics of surface fogging are quite straightforward: microsized water droplets in humid air meet a cool surface and condense there, scattering light and obscuring the view through the window. Automobile rear windows include a grid of thin electric “defoggers” to combat the problem by heating up the surface—a workable solution in cars, but less easily adapted to other surfaces like eyeglasses or ski goggles. Another solution is sprays or treatments that change the surface energy and cause the condensed mist to be distributed evenly across the surface in a thin film, reducing light scattering. Those coatings, however, quickly lose effectiveness and provide less-than-perfect visibility.
The ETH Zurich team took a different approach—one that combines the effectiveness of heating embodied in automobile window defoggers with the passive, self-contained nature of surface coatings. To create their metasurface, the researchers began with a 500-micron-thick wafer of clear fused-silica glass, and diced it into 5×5-mm squares. They then used sputter deposition to put down eight alternating, ultrathin layers of titanium dioxide (TiO2) film and gold nanoparticles, topped with a final TiO2 layer, to build up a clear metasurface approximately 60 nm thick.
In principle, the embedded nanoparticles, when struck by visible sunlight, should release heat through subwavelength plasmonic resonance, raising the surface temperature in a manner analogous to the electric heaters embedded in an automobile rear window—but without an additional external energy source. The passive heating, in turn, should evaporate and drive off the moisture that leads to fogging.
Keeping things clear
To test the surface, the team mounted the small squares in front of a broadband light source, and used an infrared camera to record the light-induced temperature changes at the surface. The researchers found that, in the course of a minute and a half, the plasmonic action of the embedded gold nanoparticles had heated the metasurface-treated glass by 3 to 4 °C above ambient temperatures, versus a temperature change of less than 1 °C for untreated controls.
The researchers also blew air at a controlled humidity across the surface to create a supersaturated fogging environment, and compared the results with those for untreated control samples and for samples treated with a state-of-the-art, super-hydrophobic coating. The untreated surface fogged up after less than a second, and visibility through the glass degraded over the course of the 60-second-long experiment. The super-hydrophobic surface remained clear for several seconds but had fogged up significantly after 15 seconds. By contrast, the samples that included the nanoparticle metasurface remained essentially condensate-free and clear throughout the experiment, as the 3 °C passive heating was enough to drive away any moisture that settled on the surface.
Durability advantage
In addition to the potential performance gains, the researchers believe that the nanoparticle metasurface could prove much more durable and long-lasting than typical antifogging coatings, which tend to wear off quickly, can degrade in the face of contamination and dirt, and must frequently be re-applied—in some cases daily.
The ETH team is now working to extend the lifetime of the metasurface “to ensure that it lasts for years,” according to lead author Christopher Walker, and to transition the technology “from lab scale to industry scale.” Once that exercise is accomplished, the team believes that the metasurfaces could represent “a viable solution for antifogging and defogging in commercial applications” ranging from windows and windshields to electronic displays, cameras, mirrors, goggle and eyewear.