Gold gifts glasses wearers fogging respite
The current cold snap will mean many glasses wearers will be facing a familiar problem: fogging. Especially common in the era of masks, it’s when warm damp air - like breath for instance - encounters the cold surface of your lenses. The water condenses into millions of tiny droplets that scatter the light coming through and obscure your view. One way to deal with the problem - other than giving your glasses a wipe down - has been to add a water-attracting coating to lenses, which pulls the water droplets into a continuous thin film, preventing the scattering problem. But these coatings aren’t very robust and often need frequent reapplication. Now, Tom Schutzius and his team at ETH Zurich have come up with a new way to solve the problem: a coating made from tiny particles of gold that are transparent to visible light but strongly absorb infrared light, or heat, warming up the lens surface to prevent condensation forming in the first place…
Tom - Fog is only really annoying because you want to see, so light is already present. So could you, for example, take advantage of some ambient sunlight, maybe light in a room. What we first looked at was then what is really available to us. So if we stick with sunlight, there's quite a lot of light that of course comes to us in what's called the visible spectrum. And that actually accounts for about 50% of the energy, but there's actually another 50% which is what we would feel as heat in the infrared wavelengths. And so that's what we played with. We focused then on trying to develop a coating which could absorb in this infrared regime while then being transparent in the visible.
James - That makes sense for this layer to be any use on glasses the coating of course has gotta be transparent. So what did you arrive at? How did you overcome this problem?
Tom - The one that we ultimately settled on was using what's called a plasmonic effect. It's an effect where you have a metal particle - metals are useful because they're electrically conductive, they've got a lot of these free electrons that are available to take light which has an electric field to it. And you impinge that on such a particle to cause these electrons to jostle, oscillate and move so they can absorb some of that energy. There's different metals that are good for this, and one really nice one is gold. Gold has a good property that when it's a small particle and you hit it with visible light, this electron cloud that's around the particle can oscillate at resonance. So it becomes a very, very strong absorber at a specific light wave length. We settled on that and we said, "okay, how could we tune that?" So if you change the size, you can start to get two different wavelengths where it becomes a strong absorber. And then if you go with maybe two particles or three and you start to pack them close together, you can get not just one wavelength where you absorb but a more broadband one. And so we tuned that packing and that structure in a way where it was transparent in the visible spectrum, but then for us it was more absorbing in the infrared. So we had this nice balance of properties.
James - I've got you - harnessing the power of the sun. But I don't think of gold as a transparent material?
Tom - So when it's a bulk material, like a film, it can look like a mirror just like silver. But then, when you start to break it up and you make it much, much smaller, so no longer a continuous film but tiny particles, it no longer possesses that bulk reflective behaviour. And also, just to give you some context, these little structures and particles that we use are much, much smaller than the wavelength of light that they interact with. So it's sort of a nanoscale property that emerges there.
James - I think that kind of answers my next question, but people will hear that there's a new coating containing gold and that might make them worry that when they next go to the opticians, this anti-fog coating's going to be an expensive add-on to their new pair of glasses. Is that going to be the case? Or are we dealing with such small amounts of gold?
Tom - I just bought some glasses recently and they already do of course many layers there and, I don't even remember, but each layer I had the impression was adding on quite a bit of cost to what I had. But the point is that because it's such a minute amount of material - it's extremely, extremely thin layers that we deal with -the cost in terms of the materials is actually not a significant aspect to it. Although I'm not going to tell you if you had it on there, you could still tell your friends and family of course that you got gold on it and you're a bit fancier with it, but it won't be adding a significant cost.
James - And could this ultra thin layer with gold as a component be used for other purposes? Or is it limited to the use on glasses?
Tom - No, of course it can be used for other things. I think here we talk about it because it is very annoying with the visibility, and there are also safety issues as well for us. But you can also think about things like sensors. A lot of things nowadays, especially cars and all of that, rely on sensors and they see a lot of times similarly to how we do. So having fog there also can become a big issue. And so having such coatings can also bring benefits not just for us and this rather annoying problem, but also to these other more significant safety issues and sensing and so on and so forth.