Bing Dwen Dwen did a backflip in space? Can liquid be used to build a bridge? Here comes the precious footage of humans giving a lesson in space

Expert of this article: Liu Yan, Space Application Engineering and Technology Center, Chinese Academy of Sciences

The “online course ceiling” has been updated again!

At around 15:40 pm on March 23, 2022, our "space business trip trio" brought the second lesson from Tiangong to the people on Earth!

Source: CCTV News

What? What? I heard that on the Chinese space station:

“

Teacher Wang Yaping made an “ice hockey”?

Is the top trending song Bing Dwen Dwen too empty?

Can we build a bridge in space using liquid?

Is there interaction between heaven and earth?

”

Move the small stool, here comes the point~

Space "Ice and Snow" Experiment

What happens when a saturated liquid crystallizes in a weightless state?

On the ground, crystallization from saturated liquids is a method of separating and purifying solid substances.

Source: China Digital Science and Technology Museum, Science Popularization China

A supersaturated solution refers to a solution whose concentration exceeds the theoretical equilibrium concentration. The excess solute will precipitate into crystals, and higher purity solid substances can be extracted through filtering, washing, drying and other steps.

Supersaturated solution is an unstable state. When disturbed by external factors, it is easy to crystallize. Crystallization is affected by many factors, such as temperature, pressure, shear force, ultrasound, external magnetic field, etc. The crystallization process is that the crystal nucleus is first formed under the influence of external factors, and the crystal nucleus gradually grows to complete the crystallization process.

In the Tiangong classroom, when teacher Wang Yaping squeezed the supersaturated solution for the first two times, crystallization occurred at the tube mouth, which also illustrates the instability of the supersaturated solution. The pressure caused by the extrusion induced crystallization.

Under weightless conditions, the sodium acetate solution can be separated from the container and become a stable liquid ball. Professor Wang Yaping induced the sodium acetate supersaturated liquid ball to quickly complete crystallization by dipping the crystal nucleus. Crystallization is an exothermic process, so the ice ball feels warm to the touch.

Source: CCTV News

Source: China Digital Science and Technology Museum, Science Popularization China

On the ground, crystallization experiments of supersaturated solutions can only be carried out in containers. The shape of the ice crystals generated during crystallization is the same as the container, and cannot become ice balls like in space.

Liquid bridge demonstration experiment

Does the surface tension of water still exist in a weightless environment?

A liquid bridge is a small column of liquid between two solid bodies.

Liquid surface tension is the force acting on the surface of a liquid, reducing its surface area. All liquids have surface tension, and especially under the effect of weightlessness, surface tension becomes even more powerful.

Surface tension makes the surface of the liquid equivalent to something like an "elastic film" that restricts the movement of the liquid. Its essence is that there is a thin layer on the surface where the liquid contacts the gas, called the surface layer. The molecules in the surface layer are sparser than those in the liquid, and the distance between molecules is larger than that in the liquid. The interaction between molecules manifests as gravity. For example, when a spring is stretched, the spring tends to shrink.

Under the action of ground gravity, since the surface tension is very weak, the liquid bridge formed under normal gravity environment is very small (usually only a few millimeters). If the size exceeds this, the liquid bridge will not be able to balance its gravity and will collapse.

Source: China Digital Science and Technology Museum, Science Popularization China

In space, due to the effect of weightlessness, a liquid bridge with a very large diameter can be formed. A liquid bridge with a diameter of 50 mm was once built on the ISS (International Space Station).

Liquid bridge between fingers, Image source:

http://www.csu.cas.cn/kxcb/kpdt/201609/t20160907_4659506.html

Although the size of the liquid bridge on the ground is small, its role cannot be underestimated! If you observe carefully, there are many examples of liquid bridges in life:

On the beach, dry fine sand can be molded into various shapes after mixing with water; (This is because water forms liquid bridges between tiny sand particles, allowing the loose sand to gather together)

On a smooth table, a white paper dipped in water is stuck because a liquid bridge is formed between the paper and the table.

…

Water-oil separation experiment

Will the water-oil separation phenomenon disappear in a weightless environment?

On the ground, water and oil are layered, with water always at the bottom and oil at the top. This is because water and oil are immiscible, and the density of oil is lower than that of water, so it floats on top of water.

Source: CCTV News

However, in the space station, since there is almost no gravity, the denser the water is, the less dense the water is. The water and oil are mixed together, and the boundary between them cannot be seen. By rotating, the water and oil can be separated into layers.

So why is this?

Source: CCTV News

In the case of the same volume, the density of water is greater than that of oil, and the mass of water is greater. According to the centripetal force formula F=mrw2, when the angular velocity w and radius r are the same, the greater the mass m, the greater the centripetal force required by the water droplet, that is, the greater the equivalent centrifugal force, so the water with a larger mass is thrown to the bottom of the bottle. Water and oil are separated into layers.

Space parabolic experiment

What is the difference between parabolic trajectories in the sky and underground?

When an object is thrown from the ground, it is always subject to the vertical downward force of gravity. To analyze its trajectory, we mainly look at the direction of the thrown speed:

When the throwing speed is in the vertical direction, whether throwing up or throwing down, it is a linear variable speed motion;

When the speed of the thrown object is no longer in the vertical direction, whether it is thrown obliquely or horizontally, the object's motion is curved variable speed motion.

In space, since objects are inside the space station, they are almost unaffected by gravity. No matter which direction they are thrown, they can almost be regarded as uniform linear motion, because the objects are only affected by weak air resistance, and the changes in the direction and magnitude of the speed are very small.

Source: CCTV News

Therefore, astronauts must be cautious when throwing objects in the space station cabin, and they must transform into "Harry Potter" and grab flying objects steadily, accurately and ruthlessly to prevent them from hitting other instruments and equipment.

Some people say that the space station is very close to the earth, about 400 kilometers away. If calculated according to g=GM/r2, the gravitational acceleration is about 0.9 of that on the ground. The gravitational acceleration is so large, why is it microgravity?

The principle of microgravity in the space station,

Source: http://www.cmse.gov.cn/xwzx/zhxw/202203/t20220315_49392.html

This is because as the space station orbits the earth, there is a centrifugal force acting on it, and the gravity it is subjected to exactly offsets the centrifugal force. Therefore, all objects in the cabin are no longer affected by gravity and will float up.

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