Tech It Out: How to make a water drop dance?
Updated 21:41, 21-Nov-2019
By Yang Zhao, Meng Tao

Introducing a magic dancer – a drop of water.

There is no force around to contain the water drop, which is just falling freely.

Chinese scientists proved that a drop of water can dance; all it needs is the right stage.

You would think a drop of water should really make a rebound like this. That's because, according to Newton's law of impact, if an object hits a solid surface vertically, it should bounce back.

Professor Song Yanlin from the Institute of Chemistry, Chinese Academy of Sciences (CAS), said that they discovered it by chance. They used to make printing plates, and there were patterns on the surface of the printed plate. They found that when the printing ink dropped on a surface with patterns, it would move in a way that was obviously different from the traditional impact law of Newton.

Inspired by the discovery, Professor Song's group designed a special surface, which consisted of a water-repellent area and a water-adhesive area.

When a water droplet touches the surface like that, the part that touches the water-adhesive area will rebound much slower than the part that touches the water-repellent area.

To put it simply, some of the drops will be pushed away, while the rest of it will be pulled in, generating a powerful spinning effect.

So powerful it could rotate at 7,300 revolutions per minute.

Song mentioned that then they designed different patterns to achieve different moving behaviors of water droplets. By doing so, they established a relationship between the patterns and the droplet movements.

This is how it works:

They have found that when the hydrophobic pattern on the surface is rotationally symmetric, like a pinwheel, the droplet will bounce upwards, spinning violently. However, the droplet rolls and deflects if the pattern has mirror symmetry, like this half-moon.

The droplets may experience two or more different effects simultaneously – including deflections, gyrations or rolling – when the pattern is asymmetric.
When the surface pattern meets both symmetries, forces from different directions will counteract each other, and the water drop will bounce linearly upward.
Scientists also show how the energy produced from a dancing droplet can be utilized. They set up a magnetically suspended surface. When the water landed on the surface and rebounded, it causes the board to rotate.

With a great many of these water drops falling, hydropower is possible.

Currently, this technology is not so perfect, the real breakthrough is the fundamental theory and concept.

Song concluded that the concept could be applied beyond the water droplet. All materials have surfaces. Different surfaces have different surface energies. It used to be impossible for people to take advantage of surface energies. But now, they think that surface energy can be utilized by constructing this asymmetrical structure.