Recently, in some parts of the southern part of the mainland, there has been a unique phenomenon of "returning to the southern sky", and netizens have said that they "feel like living in a water curtain hole" and "feel that the walls of the home are crying". The reason for this phenomenon is because the outdoor temperature is high, the indoor temperature is low, and the water mist generated by the condensation of indoor water vapor covers the walls and floor surfaces, which is the same as the fogging of glasses in winter. For the general public, just wait for the weather to continue to warm, and the "back to the south" will naturally pass. However, for medical workers and some special operators, avoiding fogging on the surface of the equipment is crucial. The surface is anti-foggy, and the doorway here can be deep; and the water-loving soap and waterproof mosquitoes may be our secret weapon to defeat the "return to the south".
Written by | Wang Wei (Professor, Harbin Institute of Technology, Shenzhen)
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Back to the south of the day with fog
When I left the house on Saturday morning, I thought I had arrived at the bathhouse: the walls and windows in the corridor were covered with water droplets, and the ground was as wet as if it had been sprinkled with water. The downstairs of the community is more exaggerated: some shaded positions, the water on the tile floor can splash out.
The corridor of the author's house. No one dragged the floor or rubbed the wall.
It turned out that the annual "Back to the South Heaven" was here again.
Picture source: Shenzhen Health Commission public number
Students in Fujian, Guangdong Province, may be familiar with "returning to Nantian". In simple terms, the reason for this phenomenon is that the outdoor heating up suddenly, the humidity is large, but the indoor is still cool, and there will be water vapor condensing on the indoor surface.
Image source: China Weather Network public number
Based on the same reason, when we go to the bathhouse in winter or go outdoors from the air-conditioned room in the summer, our glasses are prone to fogging. In addition, sitting in a closed car in winter, the inside of the window will fog; in the summer, the window of the air-conditioned car is also prone to fog.
Image source: https://zhuanlan.zhihu.com/p/51680101
Returning to the southern sky, or fogging on the eyes and glass, may only bring some inconvenience to our lives. However, for medical staff, workers operating key instruments and equipment, sudden fog on goggles and glasses will blur the line of sight, thus bringing huge safety hazards; fogging of aircraft, car windshields, rearview mirrors and so on may also cause serious traffic accidents. Therefore, the development of anti-fog materials has great practical significance.
Image source: https://www.sohu.com/a/381267390_649720
Anti-fog is also of great significance in nature. For example, for smaller flying insects such as butterflies and mosquitoes, if water droplets condense on their wings, eyes, or other parts of their bodies, they can be devastated.
Image source: https://m.popao.cn/shuoshuo/3551.html
To avoid fogging, we must first understand that there are three elements of fogging on the surface of an object: the need for air with high humidity, the need for a surface with low temperatures, and the formation of small water droplets. Therefore, by dehumidifying the air, or heating the surface, fog can be avoided fundamentally.
Image source: Author Mapping
But no matter the air exhaled by people or the environment in which insects are located, water vapor is continuous; whether it is the doctor's goggles, or the eyes and wings of mosquitoes, they cannot be heated at will. So what else can be done to prevent fogging on the goggles and mosquitoes?
Let's look back at the three elements of fogging, since it is impossible to heat the surface and reduce the humidity of the air, it is only possible to make a fuss about water droplets. In other words, condensation of water can be allowed, but as long as water droplets are avoided. Starting from this line of thinking, there are two anti-fog strategies, namely the "hydrophilic" strategy of making water condense into a uniform water film, and the "hydrophobicization" strategy of making water condense into large water droplets rolling down.
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One of the anti-fog strategies: hydrophilic surfaces
Artificial anti-fog materials usually use the first method, that is, by chemically modifying the surface, covering some "hydrophilic" substances, so that the condensed water can be spread into a uniform water film. Such a water film is as transparent as glass, so it does not interfere with the line of sight.
This "hydrophilicization" can be achieved with special instruments. For example, bombarding the glass surface with "oxygen plasma" to produce a group containing oxygen atoms, thereby greatly improving the hydrophilicity of the glass surface, so that the water can be fully moistened and a water film is formed.
"Hydrophilicization" can also be achieved by covering it with a special coating. For example, researchers at Xiangya Hospital of Central South University have developed a composite coating containing silicone oxygen compounds and acrylic acid, which has good hydrophilicity, light transmittance, and can be tightly combined with optical lenses without falling off. [1]
Many anti-fog goggles based on similar ideas are now available on the market. However, in the early days of the outbreak, due to price, supply and other problems, medical staff may not be able to use these anti-fog goggles in large quantities. Is there a cheaper, more accessible way to make the goggles "ready-to-use anti-fog"?
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Apply soap to prevent fog
In addition to plasma treatment and coating, "hydrophilization" can also be achieved by applying chemical molecules called "surfactants". Such molecules are divided into two ends: one end is the head that prefers water, usually containing an electric charge or some atoms and groups that can bind closely to water molecules; the other end is the tail that does not like water ("hydrophobic"), usually a long chain of organic molecules. Soap (dish soap, detergent, etc.) molecules are typical surfactants, which use hydrophobic ends to pull oil stains off the surfaces of clothes and dishes and dissolve them in water.
When surfactants are applied to the surface of an object, one of their hydrophobic ends attaches to the surface of the object, and the other end binds to the water molecules, helping the water molecules to spread out to form a water film.
This gave rise to a folk remedy to prevent the glasses from fogging in winter: "Dissolve products such as dishwashing detergent, soap, toothpaste or egg whites in water, and then soak the lenses in them... Isn't it amazing that the glasses don't fog up all day? [2] At the heart of this method is a layer of surfactant molecules applied to the surface of the glasses, allowing the condensed water to form a transparent film of water, rather than small droplets that obstruct the line of sight.
Don't look at this method as old-fashioned, but during the COVID-19 pandemic two years ago, this was the most urgently needed "card neck technique" for many medical workers. In a 2020 paper published in the Chinese Journal of Optometry and Vision Science[3], a team from the Affiliated Optometry Hospital of Wenzhou Medical University investigated seven ways to solve the problem of anti-fog in goggles, including hand sanitizer, soap detergent, anti-fog cream, iodine, car glass water, and swim goggle anti-fogging agent. They found that "the antibacterial hand sanitizer that can be found everywhere around healthcare workers can play an anti-fog role." Apply the hand sanitizer to the inside of the goggles, and then rinse with water and dry (Note: do not dry with gauze or paper towels), so that the goggles have an anti-fog function, which is convenient and practical, local materials, and the effect is long-lasting. The three ward medical teams led by Chief Nurse Zheng Xiuyun, one of the authors of this article, used this method to prevent fog, and the effect was stable and long-lasting. ”
In fact, whether it is low-cost soapy water, or the "swim goggle anti-fogging agent" that sounds tall, or even the "goggle anti-fog coating technology" developed by some scientific research teams, the principle is the same: surfactant molecules make water condense into a water film instead of water droplets.
It is precisely because these methods are the same in nature that they have similar drawbacks: these "anti-fog" molecules are only gently attached to the surface of the object, and it is easy to fall off when encountering mechanical forces (such as scratching), which is why the above paper specifically reminds "avoid drying with gauze or paper towels". On the other hand, these surfactant molecules are essentially soluble in water, so while the water condenses on the surface of the object, it will also take away these molecules, so most of this anti-fog means will fail after a day.
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Anti-fog strategy two: superhydrophobic surfaces
As we mentioned above, in order to avoid small water droplets when the water condenses on the surface, the surface can be physically treated, or covered with a hydrophilic coating to make the surface hydrophilic, so that the water condenses into a clear and transparent water film, without interfering with the line of sight. However, such coatings are easily polluted by oil, dust and other pollution in the air, becoming less hydrophilic, and easy to fall off and dissolve, so the goggles made by this "hydrophilic" strategy often do not have a long life.
For our daily use, maybe it is not a big deal to do anti-fog treatment once a day. But for organisms that have to fight water vapor in the air every day, these anti-fog methods are not only inconvenient, but if the hydrophilic coating fails, it becomes a big problem of life and death. Therefore, nature needs a stable, long-lasting, and reliable anti-fog strategy.
The lotus flower that "comes out of the mud without staining" makes us realize that there is also a "hydrophobic" anti-fog strategy that is very different from the human "hydrophilic" strategy. The reader may be familiar with the hydrophobic properties of the lotus leaf: not only will the water droplets not stick to the surface of the lotus leaf, but will become water droplets rolling down to the center of the lotus leaf as if they were on the surface of a non-stick pan. Since the German botanist Wilhelm Barthlott first studied the surface of lotus leaves under a microscope in the 1970s, people have basically recognized the hydrophobic principle of lotus leaves: in simple terms, the surface of lotus leaves has many convexities that are dozens or hundreds of times smaller than hair, and the surface is covered with hydrophobic wax. The water could not wet such a surface, but was forced to form large water droplets; when the lotus leaves tilted slightly, the water droplets rolled happily.
Image credit: Source: Ensikat, H. J.; Ditsche-Kuru, P.; Neinhuis, C.; Barthlott, W. Beilstein J. Nanotechnol. 2011, 2, 152–161
Insects that do not like to get wet also use this micro-nano structure to prevent fog. For example, the wings of a butterfly have special grooves, similar in size to the bulges on the surface of the lotus leaf, and are also at the micro-nano scale; the compound eyes of mosquitoes are composed of tiny hexagons arranged closely together, so that the water condensed on the surface cannot be spread, but form a droplet of water. What's even more amazing is that the falling water droplets can also take away the dirt on the surface of the wings and eyes, thus ensuring that the insect's body is dry and clean.
Image source: https://blog.scienceborealis.ca/superhydrophobicity-from-leaf-to-lab/
&https://www.quora.com/How-many-eyes-does-a-mosquito-have
The study of biomimetic anti-fog materials based on superhydrophobic surfaces is a very prosperous research field, and scientists on the mainland have also achieved world-leading research results in this field, and interested readers can read the references [4],[5] (Editor's note: See "From Nature to Biomimic: The Past and Present Lives of Superhydrophobic Materials"). You may be wondering, since nature has given us such a good strategy, can we use this superhydrophobic micro-nano structure for anti-fog goggles, windshields, glasses? Unfortunately, this method has not yet been used on a large scale in production and life. The reason is that on the one hand, to prepare such a fine structure at the micro and nano scale, and mass production, it may be costly and complex; on the other hand, our requirements for anti-fog surfaces are not as high as insects, and the demand for long-term, stable anti-fog materials is not so urgent.
In addition, this ultrahydrophobic surface based on micro-nano structure has a fatal flaw: it is not resistant to wear. Once such a delicate structure is destroyed, the "magic" of hydrophobicity is fundamentally lost. However, recently, Professor Deng Xu's team and collaborators at the University of Electronic Science and Technology of China have jointly developed a new strategy [6], using a microstructure "armor" with excellent mechanical stability to protect the superhydrophobic surface, thus greatly solving the problem of superhydrophobic surfaces afraid of wear. Perhaps soon we will see waterproof, self-cleaning walls, glass and even solar panels entering thousands of homes.
Who would have thought that mosquitoes and butterflies might be the key helpers in our victory over the return to the Southern Heavens?
bibliography
[1] Jian Li, Jiayi Liu, Yangde Zhang, Surface Modification of Optical Lenses by a Novel Nano-Reflection-Increasing Anti-Fog Film, China Tissue Engineering Research, 2011, 15(3): 445-449
[2] "What should I do if I wear a mask and my glasses fog in winter?" I'll teach you 5 tricks", NetEase
Huang Xiaoqiong, Qu Jia, Chen Yanyan, et al. Correct choice and anti-fog guidance for wearing goggles during the novel coronavirus pneumonia epidemic. Chinese Journal of Optometry and Vision Science,2020,22(04): 253-255,
[4] "Overview: Progress and Challenges of Super-Wetting Bionic Anti-Fog Materials https://www.materialsviewschina.com/2018/05/28848/"
[5] "Wang Pengwei, Liu Mingjie, Jiang Lei. Bionic multi-scale super-infiltrated interface material. Acta Physica Sinica, 2016, 65(18): 186801.
[6] Dehui Wang, Qiangqiang Sun, Matti J. Hokkanen, Chenglin Zhang, Fan-Yen Lin, Qiang Liu, Shun-Peng Zhu, Tianfeng Zhou, Qing Chang, Bo He, Quan Zhou, Longquan Chen, Zuankai Wang, Robin H. A. Ras, Xu Deng, Nature, 2020, 582, 55–59,
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