When the temperatures outside drop, and especially in the evening and at night, the car windshield gets covered with a layer of steam - or at least that's what most of us call the moisture that accumulates there and prevents us from seeing. What exactly is that layer and what can be done to treat it? We went out to check and demonstrate
By Dr. Avi Sayeg, courtesy of the Davidson Institute for Science Education, the educational arm of the Weizmann Institute of Science
When the temperatures outside drop, and especially in the evening and at night, the car windshield gets covered with a layer of steam - or at least that's what most of us call the moisture that accumulates there and prevents us from seeing. What exactly is that layer and what can be done to treat it? We set out to test and demonstrate (watch the video):
Note: It is recommended to watch the video on the mobile phone, in length, in full screen.
It's not steam
In everyday language we call what coats the car windshield, as well as the lenses of the glasses or the mirror in the bathroom - steam. Due to its widespread use, this is also the name we gave to this article, but actually, it's not true! This is because steam is water in a gaseous state, while the glass is not covered by any gas. If there really was steam there, it wouldn't bother us so much, because water vapor is transparent and does not block the view. What we call steam is the result of water vapor in the air: the condensation of the water vapor, i.e. turning it into liquid water, due to cooling. Therefore what coats the windshield is liquid water.
The problem is that the water condenses on the glass in the form of tiny droplets scattered on it, as can be seen under the microscope:
What we call steam is the condensation of water vapor, i.e. turning it into liquid water, due to cooling. A car windshield with condensation through a microscope Photo: Shirley Bash and Avi Sayegh
Every drop of water serves as a kind Focusing lens, as can be found in a magnifying glass. The lens 'refracts' the light rays, i.e. causes them to change direction, and the general result is that visibility through the windshield is blocked.
The "steam" on the windshield, then, can be more accurately described as moisture from the air that condenses on the cold windshield as droplets. In the video we review four different approaches to treating the problem, each of which deals with a different aspect of the phenomenon:
First approach: if the water condenses on the windshield because it is cold, we will simply heat it and thus prevent condensation on it | Shirley Bash, from the video
Heating
The first approach to treating the problem is heating. The logic is clear: if the water condenses on the windshield because it is cold, And low temperature causes water vapor to change state of aggregation to liquid - We will simply heat it up and thus prevent condensation on it. In many vehicles, the car manufacturers have built a special system for this purpose: the windshield has ventilation openings directed at it, and hot air can be flowed through them. In addition to heating, the wind blowing on the windshield also helps to evaporate water droplets that have already condensed on the windshield (see the extension below).
The rear windshield has another mechanism: a kind of 'wire lattice', which are actually tiny heating elements that heat the windshield. By the way, these wires glued to the windshield are very thin and delicate, and you have to be very careful about tearing them - a tear may cause them to disconnect, preventing them from working.
Second approach: if there is no moisture in the air, there will be nothing to condense on the windshield Shirley Bash, from the video
Drying
Another possibility, and perhaps a bit surprising, is actually to turn on the air conditioning in the car when it is running on cooling mode. This action will help dry the air, because when the air passes through the air conditioning systems to cool down, the moisture in it condenses already inside the air conditioner itself. You must have seen air conditioners on the street that 'drip' water during their cooling operation in the summer - this is the moisture in the air that has condensed on the air conditioner.
Here, too, the logic for treating the phenomenon is clear: if there is no moisture in the air, there will be nothing to condense on the windshield. In addition to this, if you direct the stream of dry air on the windshield, it accelerates the evaporation process of the drops that have already condensed, as any wind does. An explanation of the phenomenon can be seen in the following video segment:
Changing the surface
The third method came to us, like many other developments, from NASA - the American space agency. In 1966 he discovered Astronaut Eugene Cernan (Cernan) because when going into space the vitreous of his space suit was completely covered by the condensation layer, to the extent that he could not see almost at all! Of course, this kind of thing also happens to us on Earth, for example on the lenses of our glasses - in which case we simply wipe them. But an astronaut in space can't do that: he can't even touch the inside of his helmet lens. Blowing hot or cold air towards the helmet is also impractical, and it is impossible to dry the air - after all, every breath of the astronaut takes out more moist air, right on the glass.
NASA scientists approached the problem and solved it using a different approach: if what interferes with seeing is the water that accumulates as drops, we simply find a way to prevent them from doing so. The method that was found, and is still used today in most commercial 'anti-vapour' materials, is to make the surface "love" the water more. Really - to love is the literal meaning of the scientific term: to make the surface more hydrophilic (from the Greek: hydro = water, philia = love. Obviously, this is a metaphor - no one thinks that the substance develops feelings). Substances can form strong bonds with water molecules - then they are called hydrophilic, and can not do so - then they are called hydrophobic, oil is a good example of a hydrophobic substance, which does not form bonds with water. When water is on a hydrophobic surface, it will try to reduce as much as possible its contact area with the surface, because good bonds are not formed between it and it. On the other hand, when water is on a hydrophilic, "water-loving" surface, it will tend to make contact with it in as large an area as possible:
When water is on a hydrophobic surface, they will try to reduce their contact area with it, while when they are on a hydrophilic surface, they will try to make contact with it on as large an area as possible. A drop of water on a hydrophilic (right) and hydrophobic (left) surface Illustration: Avi Sayeg
If there are many water droplets on a hydrophilic surface, they will connect to each other and a thin and uniform water surface will be formed. Such a surface does not block vision, because it does not have the spherical drops that are used as lenses and break the light rays. And there you go, we solved the problem!
Following this logic, cleaning the windshield with your hands is not recommended, because the hand is often lubricated with oils that our body produces naturally, that is, it is coated with a hydrophobic material. This substance encourages the accumulation of water precisely in the form of small drops, which block vision.
It should be noted that there are also a few materials on the market.Super hydrophobic' to treat the problem, when the logic is to completely reject the tendency of the water to accumulate on the surface. But since no material is completely hydrophobic, there will always be water droplets that will somehow manage to stick to it.
changing the water
When we put detergent on the windshield, the surface tension is broken and the drop does not remain spherical but becomes a thin and flat surface | Shirley Bash, from the video
The last approach demonstrated in the video comes from a tip that has been circulating on the Internet. The idea is to smear the windshield with toothpaste, and wipe it (without washing), so that a thin layer of toothpaste remains on the glass. The ointment of the two contains, among other things, substances that belong to the 'detergent' family, to which many cleaning agents also belong, and act similarly to soap. One of the actions these materials do is Weakening the surface tension of water.
To understand what it means we first need to understand what it is the surface tension of water: This is a kind of transparent and invisible membrane that "holds" the outer surface area of the water. In fact, it consists of the water molecules that are in the outermost layer of the drop, and the force that attracts them to each other. Think about a drop of water on a surface: it behaves similar to a water balloon - as if an invisible rubber balloon keeps it spherical and not flat. The surface tension is the same "balloon".
When we put detergent on the windshield it acts like a lubricant on a balloon full of water. The surface tension is broken and the drop does not remain spherical but becomes a thin and flat surface that does not interfere with vision - just like in the previous method. We don't have to use toothpaste: any application of liquid soap would achieve a similar effect. This 'flattening' action of the droplets can be seen if water is sprayed onto the windscreen afterwards. In the area where we applied the detergent, the water does not remain as drops:
Splashing water on a windshield covered with a layer of toothpaste does not form drops on it (the right part of the photo) | Photo: Shirley Bash
Good trip and good visibility everyone!
More of the topic in Hayadan:
2 תגובות
The scientists?! Boil a potato, and when it boils and steam comes out, put a towel or a rag on it, to absorb the steam in the cloth,,, wipe the window with the rag and nothing will accumulate any moisture! How simple,, simplicity!
I was hoping that I would finally know exactly why this was happening. But you didn't give the most practical and convenient advice. Turn on the air conditioner - it is important to turn on the air conditioner, not just the heating of the vehicle, for heating.
I haven't checked why but it works.
It is assumed that the air somehow condenses inside the system before it is heated, therefore you get hot and dry air. But before you apply toothpaste or steal the windows of the spaceship from the Nasa museum or travel with a double layer of coats and cooling or... - just turn on the air conditioner on heating. 22 23 degrees is also pleasant and there is no steam