"Tiangong Class·Hong Kong and Macau Special" space lecture held, Shenzhou 23 crew started lecture

The "Tiangong Classroom·Hong Kong and Macau Special" space teaching event was held today. The Shenzhou 23 crew - commander Zhu Yangzhu, astronaut pilot Zhang Zhiyuan and Hong Kong's first payload expert Li Jiaying - who are performing missions on the China Space Station, gave explanations to Hong Kong and Macao students. Today's Tiangong class did two water-related experiments: a wet towel that couldn't be wrung out, and a water polo and table tennis competition.

Why can’t the space wet towel be wrung out?

In the weightless environment of space, why can't a wet towel be wrung out no matter how hard it is twisted? Zhang Xiaotian, associate professor at the School of Aeronautics and Astronautics at Beihang University, explains the scientific principles behind the experiment.

Water film tension experiment: invisible surface tension

Zhang Xiaotian: Astronauts have also done interesting experiments related to water on the space station before. In the water film tension experiment, we created a circular water film and continuously poured water into it. We found that the thickness of the water film became thicker and thicker, but it could still maintain its shape.

Liquid Bridge Demonstration Experiment: Surface Tension Against Gravity

Zhang Xiaotian: The liquid bridge experiment is to build a small bridge with water between two plates. As the distance between the plates increases, the bridge becomes longer and longer, but it will not break. Both phenomena are derived from the surface tension of water. It is different from the phenomenon in the gravity environment on the ground. The surface tension of water is the mutual attraction of water molecules. It tends to minimize the surface area when the volume remains unchanged. This is a physical law. Under the action of this law, water will take on a specific shape. If it is in a gravity environment, the surface tension and gravity of water will "tug of war" with each other. If we construct a water film on the ground, and the water film is slightly larger and thicker, the gravity tug-of-war will win. It will exceed the degree of surface tension and the water film will not be maintained. The same is true for liquid bridges. On the ground, if we put a drop of water between two fingers and leave a small gap between the two fingers, there will be a liquid bridge at this time, but as long as the distance is a little farther, the liquid bridge will break immediately. This is because under the action of gravity, the liquid bridge can no longer maintain its shape, because the effect of gravity exceeds the effect of surface tension.

Wet towels that won’t dry out: tiny liquid bridges lock in moisture

Zhang Xiaotian: The experiment of twisting towels also comes from surface tension. There are many tiny fluff structures on the towel, which can be regarded as some very tiny liquid bridges. A lot of liquid bridges are established between a series of villi. In a microgravity environment, without the pull of gravity, it can better maintain the shape and integrity of the liquid, and after solidification, it will adhere to the surroundings of the towel. At this time, when we twist the towel, we actually just twist the water from the inside of the towel to the outside, so we see that the liquid still collects on the surface of the towel and it will not drip down. This is also due to a unique phenomenon that only occurs in a microgravity environment.

Why water polo and table tennis games cannot be replicated on the ground

In today's Tiangong classroom, the astronauts also played a water polo and table tennis game. Why can't such a game be replicated in the ground environment?

Water polo and table tennis: demonstrating hydrophilic and hydrophobic properties

Zhang Xiaotian: The water table tennis experiment is also a very interesting experiment. Compared with the towel twisting experiment, it reflects another characteristic of water. Different object surfaces are hydrophilic or hydrophobic, which is a characteristic brought by the molecular structure of the object surface. Some materials are hydrophilic, making it easier for water to adsorb onto them. Some materials are hydrophobic, making it difficult for water to stick to the surface. There is an example in life. It is very easy for dew to roll down on the top of a lotus leaf in a pond. That is because the microstructure of the surface of the lotus leaf is a typical hydrophobic structure.

Hydrophobic towel racket meets space water polo

Zhang Xiaotian: In this table tennis experiment, a layer of towel was wrapped on the racket. This towel is a hydrophobic sports towel made of hydrophobic material. When the liquid ball comes into contact with the towel on the surface of the table tennis racket, because it is a hydrophobic material, it will not absorb the ball, so the ball bounces back. But this phenomenon is difficult to occur on the ground. This goes back to the "tug-of-war" relationship between the surface tension of the liquid and gravity mentioned above. When there is no gravity, water will appear in a spherical shape. It cannot maintain this state when it is on the ground. Under the pull of gravity, the water ball will burst or even be elongated, and there is no way to maintain the spherical shape.

Water polo and table tennis: Surface tension wins the 'tug of war'