Childhood games inspire new technology

　　Blowing bubbles, splashing water, folding paper cranes, did you play these interesting games when you were a child? Do you know that?/You know what? There are still some scientific puzzles hidden in these games, and a deep understanding of them can solve some big troubles for us.

　　How amazing it is to blow bubbles.

　　When you blow out round, big and colorful bubbles from a gourd-shaped or spherical stick, have you ever wondered why it can remain spherical and why it breaks so quickly?

　　These questions are not only curious to us, but also curious to scientists. In fact, the shape of bubbles does not come from round sticks. No matter what tools are used, when a layer of bubble water is lifted into the air, it will gather into a ball. The soap bubble experiment designed by British experimental physicist Chapois can prove this point. In 1889, Boyce showed a soap bubble experiment to teenagers: when a ring was copied in soapy water, a soap film was formed; If a line is tied in the ring (and one section is a double line), three films will be formed, and the film in the double line will be punctured, and the area wrapped by the double line will naturally form a circle.

　　At this time, if an external force is applied to a round soap bubble, its shape can also be changed. For example, two rings are used to stick the soap bubble, and then the two rings are pulled to both ends, and the soap bubble will be pulled into a cylindrical shape. However, this non-circular soap bubble is easy to break. After being pulled to the limit, the middle part of the cylinder will gradually become thinner until it finally breaks into two separate semi-circular bubbles, and these two hemispheres will quickly repair the gap and become two independent circular bubbles.

　　We can see that whether there is external interference or not, the "will" of the bubble is to become a ball. In this regard, Boyce's explanation is that there is some kind of force that restricts the liquid forming bubbles and makes them aggregate into balls. Now we know that this force is actually the inherent property of water and other liquids, that is, surface tension. When the liquid film of soap bubbles is disturbed and locally thinned, the surface tension in this part of the area will become larger, and it will produce greater pulling force on the surrounding liquid. As a result, the liquid will be naturally pulled to the thinning place, completing self-repair and maintaining a stable spherical shape.

　　However, Cheng also lost to Xiao He and Xiao He, and the "eccentricity" of surface tension was also the chief culprit of bubble bursting. In order to find out the truth of bubble bursting, in 2020, James Bird, a physicist at Boston University in the United States, set up a high-speed camera in silicone oil with different viscosities. In silicone oil, the bursting speed of bubbles is slow, and the stress changes in various parts of bubbles can be seen more clearly.

　　Bird saw that the direction and magnitude of gravity in different areas of bubbles are not the same, but their rupture process is basically the same, so it is not the decisive factor to determine their rupture behavior. Further experiments and calculations show that the surface tension of bubbles is balanced in the process of generation, but when it reaches the maximum, bubbles begin to shrink inward, and the gravity is greater where the water film is thicker than the surface tension. If the contraction speed is faster, wrinkles will be generated and will be crushed by the surface tension in other areas.

　　Bubbles are not only interesting toys, but also can be seen everywhere in industrial production: in food industry, adhesives and other fields, a large number of bubbles are needed to form cavities and fill them with fillers; In coating industry and other fields, defoamer is needed to eliminate bubbles to improve the quality of paint film. By understanding the characteristics of bubbles, we can better control their existence and serve industrial production.

　　The secret of wasting water

　　Drifting is a game that tests skills. The stones thrown by some people can jump on the water for dozens of times without sinking. The stones thrown by some people have sunk before jumping ... What is the secret?

　　In 2004, the experiment of French physicists Ledrick Bocquet and Christopher Claire revealed this mystery. They built an ejection device, shot an aluminum plate at a water tank, and then recorded the splashing water with a high-speed camera. They found that in order to keep a stable jump, stones must be thrown as quickly as possible, and the throwing speed should be balanced with the moving direction, because the rotating speed is directly proportional to the throwing speed, and the same direction can ensure a longer rotating time. In addition, throwing the stone at the water surface with an inclination of 10 ~ 20 can make it jump for a longer time.

　　Since then, more and more scientists have studied the waste of water, and some people have summarized a more detailed "strategy"; Others try to throw different shapes of "stones" on different media to find the "king of Shui Piao" ...

　　Why are these scientists so "doing nothing"? In fact, there are many important applications in life. For example, during the Second World War, British engineer Barnes Neville Wallis put forward the infamous "bouncing bomb" design, which made the bomb bounce on the water before hitting the target, then sank and exploded underwater, making it impossible for the enemy to prevent. In 1943, the British Air Force used a bouncing bomb against Germany in the war, and the effect was remarkable. The use of peace is embodied in seaplanes, which can take off, land and park on the water, and are mainly used for maritime patrol, anti-submarine, rescue and sports. In 1929, Theodore von Kalman, a fluid dynamicist, conducted many experiments to determine the maximum pressure when a seaplane landed on water. In 1932, the research of space engineer Herbert Wagner showed that the take-off and landing of seaplanes were essentially a collision and sliding process on the liquid surface. However, due to the limitation of time and funds, it is impossible for scientists to build planes to really carry out water experiments. At this point, the wasted stones can perfectly simulate the motion process of seaplane.

　　Scientists from China Southwest Jiaotong University and Beijing Institute of Mechanical and Electrical Engineering have summarized the new seaplane design guidelines through experiments. The researchers used a flat aluminum plate as a "stone", threw the aluminum plate on the water surface mechanically, and used a compressor to inject air to control the speed of the aluminum plate.

　　By collecting data during launch, "flight" and landing, the researchers found that when a stone hits the water surface, it not only reflects the gyro effect (the effect of the gravity of the gyro rotating at high speed to keep it rotating stably), but also affects its trajectory. How to balance the two effects is the key to keep the stone (seaplane) in balance and stable motion on the water.

　　Fold everything in the world with paper

　　What can you fold with a piece of tissue paper, a paper airplane, a paper crane or a boomerang? There is a scientist who can stack all the objects designed by computer, which is his housekeeping skill, and he has solved many practical problems with origami.

　　The scientist's name is Robert Lang, and his origami stunt actually comes from the help of a computer. In 1990, Robert Lang, an American mathematician, had a whim. Can computers fold origami better than human design? To prove this point, he spent several months writing a program, which can design different components and combine them seamlessly, thus folding origami of various objects. After eight years of improvement, this program has become more and more powerful. So far, Lang has collected thousands of origami ornaments "stacked" by computers.

　　Lang's paper folding technology has also helped NASA a lot. With the development of science and technology, Hubble Space Telescope can't meet the needs of astronomers, so it is urgent to design and launch a more powerful telescope.

　　Roderick Hyde, a scientist at Lawrence Livermore National Laboratory, put forward the idea of building a space telescope 40 times larger than the Hubble Space Telescope. However, the Hubble Space Telescope itself is not small, with a length of 13 meters and an aperture of 2.4 meters. Hyde's proposed space telescope has an aperture of nearly 100 meters and a length of several hundred meters. If this kind of thing is really designed, how can we put it into orbit?

　　The researchers came up with a brilliant idea: design a foldable lens to be put into orbit and then unfold it when it is installed in orbit. Is this idea feasible? The researchers asked Lang for help and asked him to design a folding lens model for them with origami.

　　In the following year, Lang built a prototype with a diameter of 5 meters for the laboratory team, which verified the feasibility of this idea. This idea has been successfully applied to the James Webb Space Telescope, the next generation of Hubble Space Telescope, which was launched in 2013. Its huge lens can be folded like origami and put into a rocket to be carried to the sky. After it is in orbit, it will be unfolded and placed at a suitable angle, and finally the design requirements will be realized.

　　In daily life, Lang's origami has also contributed to the improvement of automobile airbags. At ordinary times, the airbag is tightly folded into the compartment in the steering wheel or instrument panel. In case of a collision accident, a large amount of gas is released, which will support the airbag from a flat state and turn it into a ball to protect the head and chest of the human body from injury. So, how to fold it to ensure that the airbag is instantly deformed and not damaged? In order to do this, engineers need to "actually fold" the airbag.

　　Using his own computer program, Lang designed a scheme of folding airbags for EASi Engineering Company in Germany. In essence, the airbag is represented as a series of polygons, and its edges should be aligned during and after folding-this task can be achieved through a detailed folding mode, just as Lang is used for origami models.

　　You see, there is so much scientific knowledge hidden in the game that we used to be used to. Next time you pick up your favorite toy, you might as well think deeply about the principle. Maybe it can also help you solve some long-suffering problems.