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Recently, I found an interesting phenomenon. I observe that there are some air bubbles formed on my bottle wall, as shown in the image. The air bubble on the bottle wall

Then, I tilt the water bottle gently such that the water level is just above the air bubble, shown in the next image. Tilting the water bottle

Then, by slowly tilting the bottle even further (just make sure the bubble will not burst), we can see that the air bubble will also move down. At the end, we can use this method to "push" down the air bubble by quite a large distance, shown in the next image. enter image description here

So, my question is what causes the air bubble to move downwards? My guess is that it is because of the surface tension of water. As the air bubble gets closer to the water level, the upper part will have a smaller surface tension compared to the lower part.

However, I am not entirely sure about this and I am not very familiar with the concept of surface tension. Is it caused by some other reasons or mechanisms I failed to realise?

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  • $\begingroup$ @SG8 Completely different phenomenon. $\endgroup$
    – noah
    Commented May 28, 2021 at 10:44
  • $\begingroup$ @noah - Since you are sure I deleted the links. $\endgroup$
    – SG8
    Commented May 28, 2021 at 10:45
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    $\begingroup$ your pictures are not clear, as they do not show any tilt, at least the last one, but a horizontal water level . . Also it is nog clear If the bubble is stuck on the wall of the bottle, I would repeat the experiment with a grid of line stuck on the glass outside the bottle to show whether the bubble has moved from the wall, and take pictures where the tilt is known. $\endgroup$
    – anna v
    Commented May 28, 2021 at 10:51
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    $\begingroup$ @annav actually I only used the second picture to show the tilt. I wanted to use the last photo to show that the bubble was actually pushed downwards as you can see the air bubble is now further away from the horizontal water level, compared to the first photo. But your suggestion is quite helpful, I will change the picture later. $\endgroup$
    – Abl grp
    Commented May 29, 2021 at 4:11

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My guess: the bubble is initially pinned in place by interaction with the wall (on some impurity?). Then the surface of the liquid, when the bottle is tilted, goes down, meets the bubble and pushes it down, as a function of the tilt. Surface tension is probably responsible here to not burst the bubble in close proximity with the liquid surface. In this new position the bubble finds a new pinning point on the wall, and remains there when the surface of the liquid goes again up, as you put the bottle straight.

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