Super hydrophobic material 30.01.2015

The lotus is considered a symbol of purity: growing in dirty swamp water, stunningly beautiful and purest flowers are born. It would seem that what can be common between the symbol of purity and physics? It turns out that there is a common thing, and this is called the scientific term - the hydrophobic properties of the surface. If the surface is hydrophilic, then water can easily spread over it and form a uniform film (an example of such a surface is glass). But if you spill a few drops of water on a Teflon pan, they will not stick to the surface, and if you tilt it at a certain angle, the drops will begin to roll - this is an example of a hydrophobic surface.

Lotus leaves are a natural superhydrophobic surface, it does not get wet in water, but drops roll off it, taking dust and dirt particles with them, cleaning the plants. Lotus leaves are a natural superhydrophobic surface, it does not get wet in water, but drops roll off it, taking dust and dirt particles with them, cleaning the plants.

And there is also such a property as superhydrophobicity. On such surfaces, water drops cannot fix at all, and begin to roll off it at very small angles of inclination. For example, dew drops on grass have a spherical shape, the surface seems to repel water, preventing it from getting wet. Lotus leaves are the most beautiful example of nature's superhydrophobic surface. The secret lies in the microstructure of the leaves, they not only repel water, but also contribute to the self-purification of the plant. Drops of water, rolling down from the surface of the leaf, carry away particles of dust and dirt, leaving the flower perfectly clean. By the way, the wings of butterflies and other insects have the same property, otherwise they simply would not be able to fly under the weight of adhering dust.

Man has learned to make artificial hydrophobic surfaces that are in no way wetted by water. But two physicists from the University of Rochester, Anatoly Vorobyov and Chan-Li Guo, went further and were able to create a material from which water drops bounce like a tennis ball off a wall. It would be more accurate to say that they came up with a method by which such a unique property can be given to different materials. How did they do it?

To treat the surface, the researchers needed a powerful sapphire laser that generated short, femtosecond pulses. After focusing the laser beam on the metal surface, parallel grooves about 100 µm wide and 75 µm deep were applied to its surface. This size is comparable to the thickness of a human hair. On the resulting ribbed surface, somewhat reminiscent of a bed with hilled potatoes, even smaller nanostructures formed, which made the grooves and furrows "rough". It took the experimenters about an hour to process a metal plate measuring 2,5 x 2,5 cm with a laser.

The result is amazing material. For their experiments, physicists took three metal plates made of platinum, titanium and brass. After processing, the shiny metal surface became completely black - it stopped reflecting light. But the most impressive discovery was made when water was dropped onto such a laser-modified plate - the falling drop simply bounced off it. The surface turned out to be so water-repellent that a drop that fell on it was able to save up to a third of its kinetic energy and bounce back. Naturally, the metal plate itself remained absolutely dry. To test the self-cleaning properties of the created material, the researchers applied a layer of dust taken from a conventional vacuum cleaner onto it. After that, ordinary water began to drip onto the surface - each drop "glued" dust fragments onto itself, taking them with it. And the material itself remained in its original purity, no worse than a beautiful lotus flower.

Drops jumping on a plane are, of course, beautiful and interesting, but how can such material be useful? The lotus flower turned out to be not only a symbol of purity, but also helped people of science figure out how to make our world a little better. First of all, self-cleaning surfaces will help save water. This is very important where every liter of water counts. In addition, such materials may have anti-icing properties - the formation of ice on the aircraft controls is fraught with very sad consequences. Superhydrophobic surfaces will be more resistant to mold and other micro-organisms. The property that the surface of the metal becomes absolutely black can be useful for the efficient absorption of solar energy.