Soap and mosquitoes: the secret weapon to defeat the "return of the south wind"?

Recently, the unique phenomenon of "returning south wind" has appeared in some parts of southern my country. Many netizens have said that "it feels like living in a water curtain cave" and "it feels like the walls of the house are crying". The reason for this phenomenon is that the outdoor temperature is high and the indoor temperature is low. The water mist produced by the condensation of indoor water vapor covers the surface of the walls and floors, which is the same as the fogging of glasses in winter. For the general public, just wait for the weather to continue to warm up, and the "returning south wind" will naturally pass. However, for medical workers and some special operations operators, it is very important to avoid fogging on the surface of equipment. There are many tricks to prevent surface fogging; and hydrophilic soap and waterproof mosquitoes may be our secret weapons to defeat the "returning south wind".

Written by Wang Wei (Professor at Harbin Institute of Technology (Shenzhen))

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Return of the South Wind and Fog

When I left home on Saturday morning, I thought I was in a bathhouse: the walls and windows in the hallway were covered with water drops, and the ground was damp as if it had been sprinkled with water. Downstairs in the community was even more exaggerated: in some shady places, the water on the tiled floor could be seen splashing.

The corridor of the author's house has never been mopped or cleaned.

It turns out that the annual "return of the south wind" has come again.

Image source: Shenzhen Health Commission official account

Students in Guangdong and Fujian may be familiar with the "return of the south wind". Simply put, the reason for this phenomenon is that the temperature outside suddenly rises and the humidity is high, but the room is still cool, so water vapor condenses on the indoor surface.

Image source: China Weather Network Official Account

For the same reason, our glasses tend to fog up when we go to the bathhouse in winter or walk outside from the air-conditioned room in summer. In addition, the inside of the car window fogs up when we sit in a closed car in winter, and the outside of the air-conditioned car window fogs up in summer.

Whether it is the return of the south wind or the fogging of glasses and glass, it may only bring some inconvenience to our lives. But for medical staff, workers operating key instruments and equipment, the sudden fogging of goggles and glasses will blur their vision, thus posing a huge safety hazard; fogging of windshields and rearview mirrors of airplanes and cars may also cause serious traffic accidents. Therefore, the development of anti-fog materials has great practical significance.

Anti-fog also has important significance in nature. For example, for small flying insects such as butterflies and mosquitoes, if water droplets condense on their wings, eyes or other parts of their bodies, it may bring disaster.

To avoid fogging, we must first understand that there are three elements for fogging on the surface of an object: high humidity air, low temperature surface, and small water droplets. Therefore, by dehumidifying the air or heating the surface, fogging can be fundamentally avoided.

Image source: Author's drawing

But whether it is the air exhaled by people or the environment in which insects live, water vapor is constantly flowing; and whether it is the doctor's goggles or the eyes and wings of mosquitoes, they cannot be heated at will. So what other methods can be used to prevent goggles and mosquitoes from fogging up?

Let's review the three elements of fogging. Since it is impossible to heat the surface and reduce the humidity of the air, we can only work on the water droplets. In other words, water condensation can be allowed, but as long as the formation of water droplets is avoided, it will be fine. Based on this idea, there are two anti-fogging strategies, namely the "hydrophilic" strategy that allows water to condense into a uniform water film, and the "hydrophobic" strategy that allows water to condense into large water droplets that roll down.

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Anti-fog strategy 1: hydrophilic surface

Image source: Author's drawing

Artificial anti-fog materials usually adopt the first method, that is, by chemically modifying the surface and covering it with some "hydrophilic" substances, the condensed water can spread into a uniform water film. Such a water film is as transparent as glass, so it will not interfere with vision.

This "hydrophilicization" can be achieved through special instruments, such as bombarding the glass surface with "oxygen plasma" to produce groups containing oxygen atoms, thereby greatly improving the hydrophilicity of the glass surface, allowing water to fully wet and form a water film.

Image source: Author's drawing

It is also possible to achieve "hydrophilicity" by coating with a special coating. For example, researchers at Xiangya Hospital of Central South University have developed a composite coating containing organic silicon oxide compounds and acrylic acid, which has good hydrophilicity and light transmittance, and can be tightly bonded to optical lenses without falling off. [1]

Now, many anti-fog goggles made based on similar ideas are available on the market. However, in the early stage of the outbreak, due to price and supply issues, medical staff may not be able to use such anti-fog goggles in large quantities. Is there a cheaper and more affordable way to make goggles "anti-fog immediately after use"?

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Apply soap to prevent fogging

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 likes water, usually containing charges or some atoms and groups that can tightly bind to water molecules; the other end is the tail that does not like water ("hydrophobic"), usually a long organic molecule chain. Soap (dishwashing liquid, laundry detergent, etc.) molecules are typical surfactants. They use the hydrophobic end to pull oil stains from the surface of clothes and dishes and dissolve them in water.

Image source: Author's drawing

After applying surfactants to the surface of an object, their hydrophobic end will adhere to the surface of the object, and the other end will combine with water molecules to help the water molecules spread out to form a water film.

This led to a folk remedy for preventing glasses from fogging up in the winter: “Dissolve a product such as dishwashing liquid, soap, toothpaste, or egg white in water, then soak your lenses in that solution… your glasses will stay fog-free all day, isn’t that amazing?”[2] The core of this method is to apply a layer of surfactant molecules to the surface of the glasses, so that the condensed water forms a clear film rather than small droplets that obstruct vision.

Although this method may seem old-fashioned, it was the most urgently needed “bottleneck technology” for many medical workers during the COVID-19 pandemic two years ago. In a paper published in the Chinese Journal of Optometry and Visual Science in 2020 [3], a team from the Eye and Optometry Hospital Affiliated to Wenzhou Medical University investigated seven methods to solve the problem of goggles’ anti-fogging, including hand sanitizer, soap-based detergents, anti-fogging cream, iodine tincture, car windshield washer fluid, and swimming goggles’ anti-fogging agent. They found that “the antibacterial hand sanitizer that is readily available to medical staff can play an anti-fogging role. Apply hand sanitizer to the inside of the goggles, then rinse with water and shake dry (note: do not wipe dry with gauze or paper towels). In this way, the goggles will have an anti-fogging function. This method is convenient and practical, uses local materials, and has a long-lasting effect. The medical and nursing teams in three wards led by one of the authors of this article, head nurse Zheng Xiuyun, who rushed to Wuhan to help, used this method to prevent fogging, and the effect was stable and lasted for a long time.”

In fact, whether it is low-cost soapy water, or the high-sounding "anti-fog agent for swimming goggles", or even the "anti-fog coating technology for goggles" developed by some scientific research teams, the principle is the same: surfactant molecules allow water to condense into a water film instead of water droplets.

Because these methods are essentially the same, they have similar drawbacks: these "anti-fog" molecules are only lightly attached to the surface of the object, and they are easily removed by mechanical forces (such as scratches), which is why the above paper specifically reminds "avoid wiping with gauze or paper towels". On the other hand, these surfactant molecules are essentially soluble in water, so when water condenses on the surface of the object, it will also take these molecules away, so most of these anti-fog methods will fail within a day.

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Anti-fog strategy 2: Super hydrophobic surface

As mentioned above, in order to avoid the formation of small water droplets when 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 vision. However, such coatings are easily contaminated by oil, dust, etc. in the air, becoming less hydrophilic and easily falling off and dissolving. Therefore, goggles made with this "hydrophilic" strategy often have a short lifespan.

For our daily use, perhaps it is no big deal to do anti-fogging once a day. But for organisms that have to fight water vapor in the air every day, the above anti-fogging methods are not only inconvenient, but also become a matter of life and death if the hydrophilic coating fails. Therefore, nature needs a stable, long-lasting and reliable anti-fogging strategy.

The lotus, which "emerges from the mud without getting stained", makes us realize that there is a "hydrophobic" anti-fog strategy that is completely different from the human "hydrophilic" strategy. Readers may be familiar with the hydrophobic properties of lotus leaves: water droplets not only do not stick to the surface of lotus leaves, but instead turn into water droplets and roll down to the center of the lotus leaves like 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 protrusions that are dozens or hundreds of times smaller than a hair, and the surface is covered with hydrophobic wax. Water cannot wet such a surface and is forced to form large water droplets; when the lotus leaf is slightly tilted, the water droplets roll happily.

Image credit: Source: Ensikat, HJ; Ditsche-Kuru, P.; Neinhuis, C.; Barthlott, W. Beilstein J. Nanotechnol. 2011, 2, 152–161

Insects that don't like getting wet also use this micro-nano structure to prevent fogging. For example, the wings of butterflies have special grooves that are similar in size to the bumps on the surface of lotus leaves, also at the micro-nano scale; and the compound eyes of mosquitoes are composed of closely arranged tiny hexagons, so that the water condensed on the surface cannot spread out, but forms droplets. What's even more amazing is that the rolling 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 research on bionic anti-fog materials based on super-hydrophobic surfaces is a very prosperous research field. Chinese scientists have also achieved world-leading research results in this field. Interested readers can read references [4], [5] (Editor's note: See "From Nature to Bionics: The Past and Present of Super-hydrophobic Materials"). You may be thinking, since nature has provided us with such a good strategy, can this super-hydrophobic micro-nano structure be used in anti-fog goggles, windshields, and 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, it may be very costly and complex to prepare such a fine structure at the micro-nano scale and mass-produce it; on the other hand, our requirements for anti-fog surfaces are not as high as those of insects, and the demand for long-lasting and stable anti-fog materials is not so urgent.

In addition, this super-hydrophobic surface based on micro-nano structures has a fatal flaw: it is not wear-resistant. Once such a fine structure is destroyed, it will fundamentally lose its hydrophobic "magic". However, recently, Professor Deng Xu's team at the University of Electronic Science and Technology of China and his collaborators have jointly developed a new strategy[6], using a microstructure "armor" with excellent mechanical stability to protect the super-hydrophobic surface, thereby greatly solving the problem of super-hydrophobic surfaces being afraid of wear. Perhaps we will soon see waterproof, self-cleaning walls, glass and even solar panels entering thousands of households.

Who would have thought that mosquitoes and butterflies might be our key helpers in defeating the return of the south wind?

References

[1] Li Jian, Liu Jiayi, Zhang Yangde, Surface modification of optical lenses using a new nano-antireflective and anti-fogging film, Chinese Journal of Tissue Engineering Research, 2011, 15(3): 445-449

[2] “What to do if your glasses fog up when wearing a mask in winter? Here are 5 tips for you”, NetEase

[3] Huang Xiaoqiong, Qu Jia, Chen Yanyan, et al. Correct selection and anti-fogging guidance for goggles during the COVID-19 epidemic. Chinese Journal of Optometry and Visual Science, 2020, 22 (04): 253-255.

[4] “Review: Progress and challenges of superwetting biomimetic anti-fog materials
https://www.materialsviewschina.com/2018/05/28848/”

[5] Wang Pengwei, Liu Mingjie, Jiang Lei. Bionic multi-scale super-wetting interface materials. Chinese Journal of Physics, 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 HA Ras, Xu Deng, Nature, 2020, 582, 55–59,

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