How does a self-cleaning surface work? There are two basic ways to make something self-cleaning. You can make it superhydrophobic or superhydrophilic. Both of these methods will use water to clean the object.
Self-cleaning surfaces are very useful for places that are difficult to get to and manually clean, such as the windows on the side of a very high building. They are also used in places that can be touched by a lot of people, such as the buttons on an elevator. One place where self-cleaning surfaces are useful is in space. You don’t need them on the outside of the spaceship or space station because there is nothing that will stick, but you do need them inside. A lot of microbes are carried aboard the spacecraft from Earth, and they can stick to surfaces and multiply. While they are in space, astronauts’ immune systems get weaker, and they can be more susceptible to these bugs than on Earth. Self-cleaning surfaces can help to keep microbes at bay.
Hydrophobic and hydrophilic refer to water hating or water loving. Oil, for example, is hydrophobic and sugar is hydrophilic. You cannot mix oil and water because the oil hates the water. Sugar readily dissolves into water because the sugar loves the water. These two methods can be used to make something self-cleaning.
Let’s look at a superhydrophobic self-cleaning item first. This method comes from nature and is often called the lotus method because the lotus flower uses it to keep its leaves clean. The lotus can often be found in pretty dirty and muddy water, but its leaves are always clean. That is because its leaves are not smooth but have microscopic bumps on them.
Water forms a sphere because all of the molecules attract each other and pull in towards the center. This is why water has a strong surface tension. When water drops fall onto a surface, the bottom of the water drop hits the surface, but it is not strong enough to support the weight of the water drop, so it collapses and spreads out. If the surface is perfectly flat, every part of the water is connected to the surface and it has high friction preventing it from moving. On the other hand, if a surface is rough and has a lot of bumps and ridges, the water will sink into those ridges and the friction will still be high, preventing the water from sliding off. The lotus leaf has a remarkable adaptation that makes the water slide off. The leaves are not flat. They have microscopic bumps all over the leaves, but these bumps are 10 to 15 micrometers apart. That is just enough distance to stop the water droplets from spreading out and sticking to the leaf, but not enough distance for the surface tension of the water droplet to break and the water to get stuck between the bumps. That means the water slides off the leaf and takes all of the dirt with it. This effect can be used to make self-cleaning items.
The second method is the superhydrophilic method. The surface of the item that needs to be self-cleaning is coated with a highly hydrophilic material. When a water drop hits that surface, it is attracted to the surface and spreads out very thinly over the surface. If the surface being cleaned is a window, being hydrophilic will make the water droplets, probably the rain, spread out, and then gravity will make all of the water slide down the window, pulling all of the dirt with it. On a normal window, the rain will stay in droplets and run down the window in these droplet forms. That means some parts of the windows will be cleaned and some won’t, causing those streaky marks. A hydrophilic window will be evenly cleaned.
Some windows use a third method called photocatalysis, which makes use of sunlight. Photocatalytic self-cleaning is often used on windows and it is accomplished with titanium dioxide. When titanium dioxide is hit by light photons from the sun it uses some of the energy to burn the bottom part of dirt particles that are stuck to the window. This loosens them. Titanium dioxide is also hydrophilic so the water spreads out in a flat, thin sheet. The windows are vertical and gravity pulls the water straight down the window and the water washes off all of the dirt that has been loosed from the window because it was burned by the titanium dioxide.
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Sources
https://www.biolinscientific.com/blog/what-does-self-cleaning-mean
https://pmc.ncbi.nlm.nih.gov/articles/PMC6968945
https://en.wikipedia.org/wiki/Self-cleaning_surfaces
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