A slightly humble but interesting phenomenon!
physics girl, a well-known physics popular science blogger, recently mentioned in her video that she found an interesting phenomenon related to straws. Although she has expressed concern on social media that this phenomenon is unremarkable and not interesting, I think it is very interesting, and everyone can try it at home, so let's introduce it here today.
to put it simply: when you insert an ordinary plastic straw into the water, look down through the hole of the straw, and move the straw slightly down or up, you will find that the liquid surface in the straw becomes a temporary lens.
when observing this phenomenon, I recommend that you find a thick straw for drinking pearl milk tea. This phenomenon can be observed with both thick and thin straws, but the field of vision of the thick straw is a little wider. You can pour water into a transparent glass, press a piece of paper with a print or pattern under the glass, and then move the straw inserted into the water to observe the zoom changes in the pattern (be careful not to poke your eyes). The video of
physics girl says that pressing down the straw will see the pattern suddenly enlarge, while lifting up will shrink, which is very obvious, so it can certainly not be explained by the change in the distance from the eye to the pattern. The same is true of my own observation.
here is an example of how I posted a straw on a mobile phone lens.
this effect is temporary, and the liquid level will return to nearly flat after a short while (it will be easier to wobble if held by hand). In the
video, it is believed that this phenomenon should be seen as the bending of the liquid surface driven by water molecules attached to the wall of the straw when moving the straw (as shown in the following figure).
its effect should be related to the affinity of the pipette wall and liquid molecules, as well as the surface tension of the liquid itself. The effect of replacing water with ethanol (the surface tension of alcohol is smaller than that of water) is also tested in the video, and the scaling effect is found to be weaker.
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if there are conditions, I think it would be better to test all kinds of tubes with different affinity to water, such as hydrophilic glass straws and Teflon tubes which are more hydrophobic than ordinary plastics. Maybe even send a paper. For the original video of
physics girl, please see: https://www.youtube.com/watch?v=Iu_7V2gaYZk