# Science over the globe, part 1

In July 2012, the semi-annual watch on the ISS of the astronaut Donald Pettit ended. In orbit in his free time, Don recorded popular science videos with experiments in zero gravity called Science off the Sphere . The experiments were very unusual and beautiful, I remember, with what pleasure I watched them five years ago. Perhaps, because of the anniversary date, remembering them again, I was surprised to notice how few views on YouTube were collected by these videos. Well, then for a larger number of readers they will be a novelty, and it will be useful to recall them.

The clips came out as a popular science program, with a week or two intervals, and at the end of each video Don asked the audience a thematic question. Answers under the spoilers so you can calmly think (there are very difficult questions). Speech in the videos, of course, English, but you can read the automatic translation of subtitles, and I prefaced the video with comments / explanations.

## Episode One. Knitting Needles Experiment

On the ISS, you can take a small amount of personal belongings, and knitting needles, most likely, went into space for the first time. But not for knitting, but for experiments with electrostatics. If you rub a needle, it will acquire an electric charge. And the oppositely charged drop of water will be attracted to it, flying in circles. The force of attraction obeys the law of inverse squares, and the drop will move like a small satellite (the forces of gravity also obey this law). With one exception - the source of gravity in nature can be represented by a material point (stars, planets and other heavy objects have a spherical shape), but here the force field turned out to be cylindrical, and the drop moves not in the orbit plane, but in the three-dimensional region. The motion of a drop can also be compared with the behavior of charged particles of the solar wind that fall into the magnetic field of the Earth.

Question: At the end of the video dr. Pettit places a nylon needle around the syringe, which the water injects around the teflon needle. Why does Don have a nylon needle, and why should the second needle be Teflon?

Teflon takes electrons from those materials with which it is rubbed, acquiring a negative charge. Nylon, on the contrary, gives up electrons when it is rubbed, and acquires a positive charge. Water droplets flying around the nylon needles acquire a small positive charge from it. Different charges are attracted, and water droplets begin to strive for a negatively charged spoke.

## Episode Two. Bistronauts

This is not mentioned in the video, but the same Don Pettit invented the cup for drinking in zero gravity back in 2008 in his previous flight. Usually astronauts and astronauts drink from plastic bags with tubes. You can dilute powdered drinks in them, brew tea or coffee. But if we make a special cup with an angle on one side, the capillary effect will cause the liquid to rise in this place. And from the cup you can sip the liquid. In the video, astronauts and astronauts clatter for the first time in zero gravity. The design of Pettit is quite simple, then they developed beautiful figured cups , but the effect they use is the same. The same capillary effect is used in “serious” rocket production - such angles hold liquid fuel near the tank necks so that when starting up the engines do not hit the bubble gas of pressurization. Already then, when the engine begins to gain traction, the liquid fuel will be down under its own weight.

Question: Why can't I use a regular cup in weightlessness?

Due to the wetting forces, the water will tend to spread along the walls. And a small disturbance like a sip can tear water away from the surface. If the surface tension forces cannot hold water, it will scatter everywhere. In the space mug, the liquid remains at the walls, and the capillary forces do not allow water to break away from them when you drink.

## Episode 3. Physics of thin films

On Earth, you can get a film of water only if you seriously reduce its surface tension. This experiment is often unwittingly put children playing with soap bubbles. When there is gravity, a film cannot be obtained from distilled water, but surprising effects appear in weightlessness — water droplets thrown out of a syringe can enter the film, be reflected from it, or even fly through. And if we take a soldering iron and create a temperature gradient, then the Marangoni effect arises in the film - the movement of a substance due to the difference in surface tension. In this case, you can get the movement in the opposite direction. If the water film is convex, that is, thicker in the center, then the convection will be out of the way, and if the film is concave, that is, thinner in the center than at the edges, then the convection will be directed to the center.

Question: Why does the shape of a water film determine the direction of the Marangoni effect?

Heating reduces surface tension, and water begins to move away from the heat source. But in which direction will it go? Water heats up faster where its layer is thinner, so it tends to go along a more subtle path. In the case of a convex film, this is the edge, in the case of a concave - the center.

## Episode 4. Whirlwinds and lenses.

We continue experiments with thin films. The viscosity forces are comparatively weak in them, therefore, if we twist such a film, after tinting it beforehand, it will be seen that it is capable of rotating in minutes. A drop of dye, once on the film, forms a mushroom-shaped figure, in fact, which is a longitudinal section of a vortex ring. The same effect can be obtained if you blow through the tube on the film. And finally, the water film works like a lens - the convex will be collecting (positive), and the concave - dissipative (negative). Flat film will neither enlarge nor reduce the image.

Question: How does viscosity affect the vortex?

The greater the viscosity, the smaller the vortex, because the greater the viscosity, the greater the attraction of the molecules, therefore, for example, in honey, the vortices will be smaller than in water.

## Episode 5. Entertainment with antibubbles

Due to surface tension forces in zero gravity, it is possible to make a bubble from the air inside the bubble from water, and with a certain success for a while - a bubble from the air inside the bubble from water flying inside the air bubble inside the water bubble. And it all also rotates.

Question: Why, when the largest bubble rotates, are the bubbles inside it aligned to the center?

Without gravity, only centrifugal force can act on the bubbles. Water is denser than air, so it is thrown to the periphery (but the water has a stronger bond between molecules, so it does not crumble), and air collects in the center, since the rest of the space is occupied by water.

## Episode 6. Earth in the infrared

At the station there is a camera that shoots in the near infrared range. The presence of a similar camera, shooting in the visible range, allows you to take turns to look at the same area in different ranges. The infrared range makes vegetation very noticeable, which makes it possible not only to take beautiful photos, but also to use the data obtained in science and the national economy.

Question: Why are the plants in the infrared range red, and the city - gray?

Plants reflect infrared light. Photos were taken on the day side, so the plants reflect IR, and the concrete cities absorb. On the night side will be the opposite, because the city will emit accumulated heat.

## Episode 7. Sound waves in space.

In this video, Don found old speakers, dripped water on them, began to give clean tones in the region of 20-40 Hertz and watch what happened. It turned out very beautiful, and on Earth gravity will not allow this to be seen.

Question: Why are low frequencies used?