For many people, rain is a very annoying thing. On rainy days, I can only hide at home and watch the water splash. But can the water drop only splash on the ground? Can it be used to do something interesting?

Recently, the research team of Song Yanlin, Institute of Chemistry, Chinese Academy of Sciences, and Feng Xiqiao and Li Qunyang of Tsinghua University collaborated to find that the falling water droplets can produce high-speed rotation after hitting a special plane. ". This research achieved precise control of droplet collision behavior through clever design, and for the first time realized the transformation of the movement form of objects before and after collision, which may have application value in water energy collection, self-cleaning and anti-freezing.

A series of evolutionary morphologies after the water droplets collide with the plane (Image source: Song Yanlin's group)

The study was published in "Nature Communications" on March 5, 2019 (DOI: 10.1038 / s41467-019-08919-2), and was selected as a research highlight by "Nature" magazine, with "Putting a spin on droplets" as The subject was reported.

Newton said that the collision between objects

The collision of two objects is a common process. As early as more than three hundred years ago, the great scientist Isaac Newton did a lot of experimental research on this process, and concluded the famous Newton collision law:

The ratio of the collision speed between two collision objects to the approach speed before collision is called the recovery coefficient. For fully elastic collisions, the recovery coefficient is 1, but this situation will only occur in ideal situations; for fully inelastic collisions, the recovery coefficient is 0. In the real collision process, the recovery coefficient is mostly between 0 and 1. Generally speaking, what changes before and after the collision is the speed and direction of the object, and it is difficult to change the movement form of the object.

In this work, for the first time, the motion of water droplets changed from translation before collision to rotation after collision. This phenomenon is obviously different from the classic "Newton's law of collision".

We can control how the water drops fly

The dynamic behavior of a water drop after falling on a solid surface is generally completed within a few milliseconds to a dozen milliseconds. The result of water droplets hitting the surface (water droplets bounce back or splash) depends on the structure and chemical properties of the solid surface. However, because the water droplets are deformable and the impact water droplets interact with the solid at a very fast speed, it is difficult to manipulate this behavior.

The researchers used a synchronous high-speed imaging system to record and analyze this behavior in detail. They found that by constructing a high-adhesion pattern on the surface of the substrate with low hydrophobic adhesion, the collision process of the water droplets was induced to enable the droplets to rotate at a high speed of more than 7300 rpm. The droplets formed a four-cornered split structure under the action of adhesion and jumped up "ballet" in the air.

Combining theoretical analysis and numerical simulation, they revealed the scientific principles behind the process. After the droplet collides with the surface, it first spreads to form a circular liquid film, and then the liquid film begins to retract under the action of surface tension. At this time, since different regions of the substrate surface have differential adhesion, the retraction speed of each part of the droplet is different, and a moment is formed inside the droplet. The effect of torque gradually accumulates as the liquid film retracts, forming angular momentum after the liquid film is retracted, giving the droplet the ability to rotate.

Image source: Song Yanlin's research group

In the future, the power generation model may change!

After the droplets collide, a high-speed rotation is generated to realize the conversion of the liquid's translational kinetic energy into rotational energy. This is similar to the kinetic energy of water converted into the kinetic energy of the generator rotor in the process of hydropower generation to generate electrical energy. The rotational energy of droplets during the collision of droplets can also be collected and used.

Based on this principle, the researchers developed a new type of droplet driver that uses a single droplet to drive the object. The patterned infiltrating substrate floats in the magnetic levitation system, and the droplets fall on the surface to produce rotational motion. During this process, the substrate can rotate in a specific direction under the drive of the droplet.

This research has opened up new ideas and directions for the use of liquid kinetic energy (such as hydropower). For example, it is possible to install this kind of micro-hydroelectric power plant in a suitable place without being limited to the hydroelectric power station on the big river. When the raindrop falls on the top, it can drive the rotor below to rotate, converting the kinetic energy into electrical energy for people to use.

This project was supported by the National Nano Key R & D Program, the National Natural Science Foundation of China, the Postdoctoral Innovative Talent Program and the Postdoctoral Fund.

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