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Photo Source: one of the biggest troubles after wearing a mask at the Federal Institute of Technology in Zurich is that glasses get foggy in winter. No matter just walked outside, or just entered the room, the eyes are suddenly hazy. In response, scientists at the Federal Institute of Technology in Zurich have come up with a new way to heat the lenses spontaneously under a certain amount of light stimulation without wiping them to prevent some chemicals from damaging the original lens coating.

Ever since I put on the mask, I have made one more move on the road: press the metal strip on both sides of the nose of the mask with your hand. Especially in winter, because the hot air you exhale will climb into the glasses along the gap between the mask and your face, leaving a layer of water droplets on the mirror like a high tide, that is, creating fog; if you are lucky, the fog will soon dissipate. But if you have to walk into the subway station from the outside, you will find that the outside of your glasses is also covered with fog.

Glasses fog is actually due to the temperature difference between the glasses and the surrounding air, and the process of condensing water vapor into water. It is worth mentioning that there are several additional films on the lens to reduce reflection, waterproof, oil-proof, blue light, etc., which are often hydrophobic materials, making it difficult for water to stick to the mirror surface, thus forming water droplets. These small water droplets will scatter the incident light in all directions and make it impossible to see clearly.

At this point, if you passively wait for the droplets on your glasses to evaporate to dissipate the fog, you may have to keep walking for a few minutes with such a pair of hazy glasses. But what else can you do?

Demos Prikkos (Dimos Poulikakos) is the head of the Thermodynamics Department of the Federal Institute of Technology in Zurich and the founder of the emerging technology thermodynamics laboratory of the university, mainly engaged in the research of interface and thermodynamics. Prikakos and his colleagues were so interested in the fogging of glasses that they began thinking about how to develop a new eyeglass coating about six years ago. They hope that the coating can not only remove fog or prevent fog quickly, but also match with existing technology to achieve large-scale production. Recently, Prikakos's team published their latest results in the journal Nature Nanotechnology (Nature Nanotechnology).

Let the glasses heat themselves? What is special about this new technology is that it mimics the defroster in the rear window of the car. People who own a car should know that the horizontal lines on the rear windowpanes are actually electric wires, the purpose of which is to turn on electric heating to speed up evaporation and achieve the effect of defrosting / fog. However, instead of actively providing energy (such as electricity), Prikakos and his colleagues wondered whether part of the lens could be heated by itself in a daily environment (with a certain amount of light) to reduce the temperature difference. in order to prevent the formation of fog or speed up the speed of fog removal?

Image source: pixabay "people usually use dark surfaces to absorb light to convert it into heat." Said Efstratios Mitridis, a doctoral student on Prikakos's team. But we know that glasses have to be transparent enough, so they need to design a special transparent coating that not only has a high enough transmittance, but also achieves an effect similar to a dark surface: absorbing enough infrared to raise the temperature of the object.

Scientists have known that for metal nanoparticles (which can be regarded as a special kind of plasma), resonance occurs when the oscillation frequency of free electrons on the metal surface is equal to that of incident light. therefore, it will produce a strong local absorption of the corresponding wavelength of light, while allowing other wavelengths of light to pass through. The resonance frequency often appears in the visible band.

Nowadays, in some man-made optical metamaterials, scientists tend to embed specific forms of nanoparticles on different surfaces and stack them to form a kind of multi-layer structure. They can adjust the size, position and direction of nanoparticles, or change the thickness of each layer of nanoparticles to regulate the interaction between light and nanoparticles, so that the materials show different optical properties. For example, the plasmon resonance frequency is broadened to the near infrared band. This is also a strategy finally chosen by Prikakos's team.

The team's idea: to prepare a class of metamaterial coatings to transmit as much visible light as possible while absorbing as much near-infrared light as possible. Along this line of thinking, they first reported on their coating in 2019, which consists of a layer of titanium dioxide and a layer of gold nanoparticles stacked alternately. "our coating absorbs infrared light from the sun as well as some of the visible light," said Christopher Walker, lead author of the study. "the light absorbed is converted into heat." This increases the temperature of the lens by 3-4 ℃, reducing the temperature difference that causes the glasses to fog. The study was published in the journal Nano Letters.

However, the visible light transmittance of this version of the coating is only about 36%, and the absorptivity is about 30% to 40% (for example, visible light transmittance + absorptivity + reflectivity = 1). This is likely to affect the light transmittance of the lens, as well as the final clarity and distortion. So for the next three years, Prikakos and his colleagues have been trying to optimize the material structure to find a better optical coating for glasses. And where will the nature of the mutation occur?

Three more years have passed. Percolation threshold (percolation threshold) and percolation phenomena are often used in conductive composites. This material has a very important feature: their electrical conductivity increases nonlinearly with the increase of the volume fraction of conductive particles, and suddenly increases at a certain critical value, the range of change can reach more than 10 orders of magnitude, and then decreases nonlinearly.

According to the cause of percolation, we may speculate that with the increase of the concentration of gold nanoparticles, when these gold nanoparticles form some kind of continuous network, the optical properties of the materials suddenly increase rapidly. That's what the research team thinks and does. They want to use this percolation concept to find the optical performance threshold of the glasses coating.

Compared with the gold nanoparticles used in 2019, they chose to use a traditional thermal evaporation process to prepare gold nanoparticles. According to them, islands are formed when gold is deposited on the substrate, but the islands are small and independent of each other, making it difficult for them to resonate collectively; as more gold deposits, the islands will be electrically connected to each other. to form a larger, interconnected network.

In fact, this nano-gold film is sandwiched between two layers of titanium dioxide. when the thickness of the nano-gold film is 4.75 nm, the material with this structure has the highest near-infrared light absorptivity of about 36.9%. It also has a high visible light transmittance (67.1%) and hardly absorbs visible light-for example. Absorption of near-infrared light = absorbed near-infrared light / absorbed incident light.

More importantly, the new coating can even warm glasses by 8.3 ℃ under 1 solar radiation (after calculation, physicists have confirmed that the power density of solar radiation on Earth is about 1000 watts per square meter, so it is referred to as 1 solar radiation for short).

One of the lenses was coated with their newly developed metamaterial, and the other was unprocessed. The results show that the metamaterial coating plays a role in preventing the lens from fogging, while the untreated lens fogs completely. Picture source: original paper

Of course, the actual application may encounter very weak solar radiation, then this kind of coating can still play the role of heating?

Fortunately, the glasses have two lenses, so it is easy to perform a controlled trial: one lens is coated with their newly developed metamaterial and the other is untreated. First, Prikakos's team placed the glasses outdoors and exposed them to sunlight (0.2 to 0.3 solar radiation) for five minutes. Then the researchers wearing masks put on the glasses and exhaled. It is obvious that untreated lenses are completely foggy, while lenses coated with metamaterials can maintain complete visibility even under such non-ideal conditions.

The researchers took the material coated with the coating to the mountains of Switzerland for testing. Picture source: original paper

Prikkos and his colleagues even brought polyester / silicon dioxide wafers coated with this coating to the Swiss mountains to demonstrate its anti-fog / defog ability in harsh outdoor conditions.

To the market, they submitted a patent application for this and expected to be able to go to the market. But when you think of the gold in the coating, you may be deterred from it. But scientists have calculated an account for you in advance and found that it is not so expensive: "because the thickness of the nano-gold film is less than 5 nanometers, the overall coating requires very little gold." specifically, the coating contains 100mg of gold per square meter, which means that a pair of glasses requires about 0.3mg of gold, which translates to 0.017 US dollars. " Said Ivan Heichler (Iwan Haechler), lead author of the new study. Of course, the overall coating and other prices are not included.

Speaking of products, at present, the main product on the market anti-fog wipes is to wipe, leaving a film on the lens to prevent fog. They contain nothing more than hydrophilic or hydrophobic materials, but the original lens coating may be damaged during wiping. As for the principle, they mainly change the contact angle between the water droplets and the lens surface by adjusting the surface wettability, which is completely different from the strategy of the Prikakos team.

For example, super-hydrophilic materials can spread water droplets to form a water film so that light can pass through. Therefore, the lens appears to be transparent to the naked eye, but it does not actually prevent the condensation of water vapor. However, the surface energy of the hydrophilic surface is higher, so it is easy to absorb impurities such as dirt, dust and oil droplets, and only a few impurities can weaken the performance of the super-hydrophilic surface.

When it comes to super-hydrophobic surfaces, we can imagine the dewdrops on the lotus leaves-the super-hydrophobic surfaces mimic the structure of the lotus leaves so that the water droplets do not stick to the wet surface and leave. But the preparation of such a perfect bionic structure is a very challenging task. So not all the water droplets clustered on the surface will slip down in time as you want, and they are likely to continue to "encroach" on the surface, thus eventually losing its function.

As for Prikkos's newly developed nano-gold-based lens coatings, there is no doubt that they will play an anti-fog / anti-fog role, but whether they can meet the needs of long-term use, and the current products remain to be tested.

Holding your breath properly is also a good skill.

Original paper:

Https://www.nature.com/articles/s41565-022-01267-1#Sec15

Reference link:

Https://pubs.acs.org/doi/10.1021/acs.nanolett.8b04481

Https://www.nature.com/articles/s41565-022-01269-z

Https://doi.org/10.1016/j.pmatsci.2018.09.001

Https://doi.org/10.1016/0275-5408(95)00023-2

Https://phys.org/news/2019-03-nanotechnology-sunlight-visibility.html

Https://www.zmescience.com/science/news-science/new-ultrathin-gold-coating-can-make-your-glasses-defog-themselves/

Https://www.zmescience.com/other/feature-post/how-to-prevent-your-glasses-from-fogging-when-wearing-a-face-mask/

This article comes from the official account of Wechat: global Science (ID:huanqiukexue). Article: Wang Yibo, revision: chestnut

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