The second lesson of "Tiangong Classroom" demonstrates 4 experiments on track

On the afternoon of March 23, the second lesson of "Tiangong Classroom" was officially held on the China Space Station. In the about 45-minute lecture, Shenzhou 13 astronauts Zhai Zhigang, Wang Yaping and Ye Guangfu cooperated with each other to teach, demonstrated the space "ice and snow" experiment, the liquid bridge demonstration experiment, the water and oil separation experiment, and the space parabolic experiment in orbit, explained the scientific principles behind the experimental phenomenon in a simple and simple way, and at the same time demonstrated some space science facilities and introduced the work and life on the space station. During the lecture, the astronauts interacted with the teachers and students of the ground classroom through video calls. In this space lecture, there are main ground classrooms in the China Science and Technology Museum, and two ground sub-classes in Lhasa, Tibet and Urumqi, Xinjiang.

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

● Space "ice and snow" experiments

Warm "ice hockey"

Removing the water bag containing the supersaturated sodium acetate solution and gently squeezing it, a blistered liquid ball slowly "ran" out of the nozzle and hovered in the space station cabin. Immediately after touching the liquid ball with a small stick stained with powder, the liquid ball with blisters begins to "freeze". In the space "ice and snow" experiment, Wang Yaping demonstrated this magical physical phenomenon for everyone.

Chen Zheng, deputy head of the science popularization lecturer group of the China Science and Technology Museum and a member of the space teaching science popularization expert group, told reporters that using white vinegar and baking soda can prepare this material called sodium acetate, which is characterized by its solubility in high temperature water, and it is easy to form a supersaturated solution. "But crystallization also needs a condition, a condensation nucleus, similar to a small disturbance or a small defect, to break its stability, at which time crystallization can begin."

The small stick stained with powder acts as a condensation nucleus, and the small crystals also begin to condense rapidly, and crystallize together with water molecules to form crystals of sodium acetate hydrate, and at the same time release a lot of heat. "Actually, if you heat it, it can turn back into liquid." Chen Zheng said that this is also a containerless experiment, and the crystallization process will not be disturbed by the container boundary.

●Water and oil separation experiment

"Inseparable" from water and oil

In the water-oil separation experiment, Wang Yaping held a bottle containing two liquids in his hand, and the bottom was transparent drinking water, and floating on top was a yellow cooking oil. In general, because the density of water is greater than that of cooking oil, when the two liquids are mixed on the ground, the oil will float on top of the water, forming a stable layering phenomenon. Even after shaking, after a few moments of standing, the water and oil liquids will still layer.

But in space it's different. After shaking, the water and oil in the vial in Wang Yaping's hand were mixed together, and the water and oil did not naturally stratify. Chen Zheng explained that because there is no gravity and buoyancy in space, when you shake the bottle, the oil becomes a small oil droplet, evenly dispersed in the water, and after dispersion, it will not be separated. So in space, you need to artificially create a gravity to shake it up, which is actually the working principle of the centrifuge.

Chen Zheng said that in space, the phenomenon of water and oil not separating will also bring advantages, such as making alloys. "On the ground, when we melt different metals and mix them, the density of various metals is not the same, and there will be a phenomenon of heavy running down and light running up. But in the space microgravity environment, the alloy can be mixed more evenly. ”

●Liquid bridge demonstration experiment

"Pulling constantly" the liquid bridge

A liquid bridge is a section of liquid that connects two solid surfaces. In the liquid bridge demonstration experiment, Wang Yaping held two transparent plastic plates, Ye Guangfu squeezed water on the surface of the plastic plate, and the two plastic plates gradually approached, and the water connected a "bridge" between the plates. Miraculously, Wang Yaping then gently pulled the plastic plate with both hands, and the "bridge" was still not broken.

"Liquid bridge is a basic experiment in the study of fluids, because there is no gravity in space, so you take two solids, and then put liquid on them, the liquid itself will attach to the solid surface, and then you put the two liquids close, the two liquids will be connected, under the action of surface tension, the liquid ends are attached to the solid, and then the middle will be pulled into a liquid column, which is the liquid bridge phenomenon." Chen Zheng said.

In space, the liquid bridge demonstration experiment can be used to study the properties of some liquid microflows. However, due to the weak surface tension of the liquid, it is not easy to achieve the liquid bridge phenomenon in the normal gravity environment. Because there is gravity on the ground, gravity will pull the liquid downward. Chen Zheng told reporters that in order to achieve this phenomenon on the ground, it is necessary to have some auxiliary means, such as by increasing the voltage.

● Space parabolic experiments

The "ice pier" of the flip heel bucket

In this space lecture, the top stream "ice pier" also appeared in the space classroom. In the space parabolic experiment, Wang Yaping and Ye Guangfu threw the "ice pier" out in turn, and it did not fall down with the trajectory of the parabola, but advanced at a uniform speed in the original direction. In fact, this is the most intuitive embodiment of the space station's microgravity environment.

Chen Zheng said that in the microgravity environment of the space station, an object will maintain a uniform linear motion when it is almost unaffected by external forces or zero external forces, but the physical images behind this experiment are very interesting. "If we look at the 'ice piers' on the ground, or at our astronauts, have they walked in a straight line? In fact, no, they are also doing a circular motion around the earth. Astronauts on the space station are in a rotating frame of reference that rotates at high speed around the Earth, which is different from the inertial frame of reference on the ground. The former has a centripetal acceleration, but the acceleration is directed at the center of the earth. ”

Chen Zheng said that when you understand why the same laws of physics see different phenomena in different places, you will really learn physics well.

Beijing News reporter Zhang Jianlin

Edited by Fan Yijing Proofreader Jia Ning