Experts interpret the second lesson of the "Tiangong Classroom": the mystery behind space experiments

On March 23, the second lesson of the "Tiangong Classroom" began, and the astronauts of the Shenzhou 13 flight crew, Zhai Zhigang, Wang Yaping, and Ye Guangfu, gave space lectures on the China Space Station. This is the main classroom on the ground, the students in the China Science and Technology Museum listening to the lecture. Photo by Xinhua News Agency reporter Guo Zhongzheng

Beijing, 23 Mar (Xinhua) -- On the afternoon of 23 March, the "Tiangong Classroom" on the Chinese space station resumed classes, and "space teachers" Zhai Zhigang, Wang Yaping, and Ye Guangfu demonstrated the space "ice and snow" experiment, the liquid bridge demonstration experiment, the water and oil separation experiment, and the space parabolic experiment.

Why do these scientific experiments? What are the scientific mysteries behind it? What is the connection with human exploration of the vast universe? Xinhua News Agency reporter interviewed a number of experts to interpret one by one.

Experiment 1: Warm "Ice Hockey"

[Phenomenon Review] This scene seems to occur in the "magical world": the transparent liquid ball floats in mid-air, Wang Yaping points at the liquid ball with a small stick, and the ball instantly begins to "freeze", and in a few seconds it becomes a snow-white "ice ball". Wang Yaping said that this "ice ball" is warm to the touch.

[Expert interpretation] "The space 'ice and snow' experiment is actually a process of supersaturated sodium acetate solution-shaped nuclei and crystallization, in which heat is released. Zhang Lu, a researcher at the Space Application Engineering and Technology Center of the Chinese Academy of Sciences, introduced that the crystallization of supersaturated solutions usually requires external "disturbance", and the "mystery" of this experiment is that the small stick is stained with crystal powder, which provides a condensation nucleus for the supersaturated sodium acetate solution, and then precipitates the sodium acetate trihydrate crystal.

【Extended reading】When performing crystallization experiments on the ground, the appearance of the crystals may vary greatly depending on the shape of the container. In the microgravity environment, the crystal is not limited by the container, and can be suspended in mid-air to "grow freely", which echoes the containerless material experiment cabinet in the Chinese space station. At present, the containerless material experiment cabinet has two main purposes: one is to realize the process of materials from melting to cooling and solidification in a containerless state, for researchers to collect physical parameters for research; the other is for the growth of special materials in orbit, shortening the time for new materials to go from the laboratory to the assembly line and enter the public vision.

Experiment two: "Pulling continuously" liquid bridge

【Phenomenon Review】Ye Guangfu squeezed the water on two liquid bridge plates, and the water ball resembled an upside-down bowl. The liquid bridge plate is closed, and the two water balls "bowl bottom" are next to the "bowl bottom"; the liquid bridge plate is separated, and a thin middle and thick "bridge" at both ends connects the two plates; Wang Yaping then pulls the liquid bridge plate away, and the liquid bridge becomes thinner and longer, and still has not been broken.

【Expert interpretation】Zhang Lu introduced that the microgravity environment and the surface tension of the liquid are the main reasons for the formation of the liquid bridge. Liquid bridges in daily life are not easy to be detected, such as when washing hands, two fingertips accidentally form a few millimeters of liquid columns, and if you pull a little farther, you will collapse by gravity. On the space station, the astronauts easily demonstrated a liquid bridge hundreds of times larger than the ground, which is impossible to see on the ground.

【Extended reading】Liquid surface tension is a high-frequency word in the "Tiangong Classroom", and the water film and water polo experiments done in the space lecture of Tiangong-1 and the first space lecture of the Chinese space station have explained this principle. Kang Qi, a researcher at the Institute of Mechanics of the Chinese Academy of Sciences, introduced that the space station can get rid of the influence of ground gravity to the greatest extent, creating good conditions for fluid mechanics research, including liquid bridge experiments. On September 15, 2016, the Tiangong-2 space laboratory took off with the liquid bridge thermal capillary convection experiment project.

Experiment three: "Inseparable" water and oil

【Phenomenon review】Wang Yaping vigorously shakes a bottle containing water and oil, so that the water and oil are fully mixed, and the bottle is yellow. Time passed minute by minute, nothing changed in the bottle, and the oil droplets were still evenly distributed in the water. Ye Guangfu came to help, grabbing the string tied to the bottle and swinging the bottle. After a few laps, the water and oil are clearly separated, with the oil in the upper layer and the water in the lower layer.

[Expert interpretation] "We all know that oil on the ground is lighter than water, and we are accustomed to seeing oil flowers when we usually drink soup." Liang Wenjie, a researcher at the Institute of Physics of the Chinese Academy of Sciences, said that in the space station, the situation is very different, and the reason why water and oil are "inseparable" and remain mixed for a long time is due to the disappearance of density stratification in the microgravity environment, that is, the buoyancy has disappeared.

"The reason for the successful separation of water and oil in the sky is that the bottle is similar to a centrifuge when rotating at high speed, which can be understood as centrifugal action that makes buoyancy reappear." Zhang Lu said.

【Extended reading】Researchers can use the characteristics of the microgravity environment to carry out research, such as the use of density layering disappears, in the microgravity environment to inject gas into the fusion gold, can obtain an important material in the field of aerospace, energy and environmental protection - foam metal.

Related to this, the high microgravity science experiment cabinet can provide a high microgravity environment, and its internal microgravity level is a hundred to a thousand times that of the space station module, which is closer to the real universe space; the external design of air flotation and maglev two-stage suspension reduces the acceleration generated by the attitude and orbit control maneuver of the space station, the torque and vibration generated by the operation of various instruments, the changes and impacts of the centroid brought about by astronaut activities, the disturbance of the solar wind and the thin atmosphere, and other interference factors, which can support more sophisticated scientific experiments.

Experiment 4: The "Ice Pier" that Turns Over

【Phenomenon review】The mascot of the Beijing Winter Olympics, "Ice Pier", made its debut in the finale, ushering in the "highlight moment" of the space journey. Wang Yaping threw the "ice pier" ornament forward horizontally, and the "pier" posture, which has always been humble, was particularly light, and several successive "flips" drew a beautiful straight line, standing firmly in Ye Guangfu's hands.

[Expert interpretation] Space parabolic experiments show the phenomena described by Newton's first law. In the space station, the "ice pier" ornament is almost unaffected by external forces after being thrown out, maintaining a nearly uniform linear motion. Zhang Jian, the main classroom teacher on the ground of the "Tiangong Classroom" and a physics teacher at the Second Affiliated Middle School of Beijing Normal University, introduced that the ideal state of object motion in the eyes of the earth's people can now be explored in space.

【Extended reading】Why should we carry out on-orbit science experiments? Zhang Lu introduced that the experimental projects currently underway are, first, to reveal special phenomena in the microgravity environment, which belong to understanding the world from a scientific point of view; second, to help ground scientific research and improve the level of technology through on-orbit experiments; third, there is a high vacuum environment, irradiation, sub-magnetic field, etc. outside the cabin, and the impact of these special environmental factors on organisms, materials, components, etc. is also the content we want to study; the fourth is to further explore unknown areas, including dark matter detection, planetary origin exploration, etc. After the launch of the Q&T and Montien experimental modules, a large number of cutting-edge scientific experiments will be carried out on the China Space Station.

Edited by Linzhi Sun