Why is it that when I try to scoop bubbles out from the surface of a cup of tea, they always slip off the spoon even if some of the tea does not?

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    $\begingroup$ It's not only bubbles that do this; any object less dense than the liquid you are scooping will have this behavior because it will follow liquid motion at the top surface. More generally, this is a question of "Why does liquid pour out of the top when you scoop it?" The answer is gravity. $\endgroup$ – Drew May 24 '19 at 20:14
  • $\begingroup$ I did some experiments with a plastic and metal spoon and did not find the effect reported by @Physics. The bubbles cling to the spoon, even if I poured all the tea out of it. $\endgroup$ – Alex Trounev May 25 '19 at 15:35

For contrast, consider what happens when you scoop dirt with a shovel. As you lift the shovel, a pile of dirt stays on the shovel, and an empty space opens up below the shovel as you lift it. You can lift the pile of dirt all the way up out of the hole, above the level of the surrounding ground, and the dirt still stays piled up on top of the shovel, leaving a hole underneath. That's good, because otherwise trying to dig a hole with a shovel would be a very frustrating experience.

Tea is different. As you lift the spoon through the liquid, the liquid above the spoon does not remain in a pile on top of the spoon, and it does not leave an empty space underneath the spoon. Instead, the liquid above the spoon runs off to the sides to keep the surface of the liquid at a uniform height across the width of the teacup, and liquid from around the sides of the spoon flows underneath to fill in what would have been the empty space. Bubbles float on top of the liquid, so they follow the flow of the surface-liquid that would have been lifted up in a pile (if it were dirt) as it spills off the edges of the spoon to keep the level uniform.

Some tea remains in the spoon, but only enough to fill the spoon up to the height of the spoon's edges. By the time you've lifted the spoon from even a slightly lower depth, the liquid that was above that height has already flowed off to the side, carrying the bubbles with it... unless you're able to keep the bubble so perfectly centered in the spoon that it can't decide which way to go, like balancing a pencil on its tip. I haven't tried this, but I'll bet it's very difficult.

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  • $\begingroup$ Thank you @ChiralAnomaly, but why does the tea need to maintain a uniform height across the surface? $\endgroup$ – Physics May 25 '19 at 8:14
  • $\begingroup$ @Physics Because of gravity. That's why you can't have a pile of water. There are small surface-tension effects that allow the water level in the spoon to be slightly higher than the edge of the spoon (same reason you can have a drop of water on the table that doesn't completely flatten itself), and the same effect can allow the water level around the edges of the cup to be slightly higher than elsewhere, but that doesn't change the essence of the answer. $\endgroup$ – Chiral Anomaly May 25 '19 at 14:19

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