With widespread, pandemic-driven adaptation of face masks, glasses wearers have become all too familiar with fogged up lenses from improperly sealed masks. With a novel lens coating, that experience — along with glasses fogging from sudden temperature shifts — may soon be a thing of the past.
The sunlight-activated, photothermal coating is ultrathin, retains the optical properties of the lens, and provides strong fog resistance, according to research published in Nature Nanotechnology. The study authors say the research “validate[s] the real-world performance of our coating by applying it to prescription eyewear and proving its impressive antifogging effect under realistic outdoor conditions.”
At approximately 10 nm, the coating is thin enough to integrate with other lens coatings. A percolating gold layer, which is most absorptive in the near-infrared range, enables the lens to maintain visible transparency. Traditionally, coatings designed with photothermal effect have relied on broadband technology which compromises lens transparency. Other methods of enabling near infrared absorption involve producing a coating that is significantly thicker (1 µm) than the novel coating. The novel design, however, considers both thickness and transparency.
Researchers tested the antifogging and defogging performance of the coating both in the lab and in a real-world scenario. Applying the coating to a single lens, the antifogging test results show the treated lenses can “endure higher overpressure compared with the control sample, which implies a >4-fold improvement in the antifogging performance.”
In a defogging test, the researchers analyzed the ability of the treated lenses to recover visibility once the surface is completely fogged. The investigators cooled the sample lens until condensation formed and recorded the evolution of its fogging, finding that the coated lens completely defogs in less than 150 seconds. Compared with control samples, which took more than 340 seconds, this is a significant improvement.
In a real-world test, the researchers left test lenses — one coated and one uncoated lens — outside on a winter day. “After 5 min of sunlight exposure, we wore the glasses and exhaled, wearing a well-fitted face mask, mimicking a real-world condition,” the study authors report. The uncoated lens completely fogged over, but the coated lens retained full visibility, according to the researchers. They report similar findings experimenting with indoor, outdoor shifts.
The “ultrathin passive metamaterial coating [has] high transparency in the [visible spectrum] and strong absorption in the [near-infrared spectrum],” according to the researchers. “Due to its ultrathin nature and its standard fabrication process, the coating can be easily upscaled, and has the potential to be applied over large areas, or be integrated into existing multilayer coatings, adding an antifogging functionality.”
A failure to measure outside temperature may limit the findings of this research.
Haechler I, Ferru N, Schnoering G, Mitridis E, Schutzius TM, Poulikakos D. Transparent sunlight-activated antifogging metamaterials. Nat Nanotechnol. Published online December 12, 2022. doi:10.1038/s41565-022-01267-1