Bubble blowing is a familiar childhood game. As long as you use detergent to reduce the surface tension, you can blow a lot of soap bubbles.

what we are going to introduce today is the opposite of soap bubbles: anti-bubble (Anti-Bubbles). On the contrary, what happened? Look at their structural comparison: the air inside and outside the bubble is separated by a thin liquid film, while the anti-bubble is separated by a layer of air film in the liquid environment.

people are very familiar with ordinary bubbles, while far fewer people know about antibubbles, and there are fewer studies on antibubbles than bubbles. In fact, anti-bubbles are not that rare in life, but most of the time they go unnoticed. This is partly because underwater antibubbles (with a drop of water inside) may at first glance be indistinguishable from underwater bubbles (with only air inside); on the other hand, antibubbles are more difficult to stay stable than soap bubbles and may disappear quickly.

what does an antibubble look like? Let's first take a look at the demo video:

(recorder: special4k4)

at first glance, these bubbles do look a lot like underwater bubbles, but if you look closely, you can see the difference: anti-bubbles burst underwater, and only a small amount of air floats to the surface after the burst.

if you dye the water droplets that make the anti-bubble, you can see more clearly the structure of the anti-bubble air film enclosing the liquid:

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(an anti-bubble floating near the liquid surface. You can see the colored liquid flowing out after the destruction of the air film, and a vortex ring is formed. Physics Girl)

so, how do you "blow" anti-bubbles? First of all, to add a little detergent to the water, the surfactant can also stabilize the anti-bubble a little bit. Next, use an eyedropper to absorb the solution and quickly squeeze it close to the top of the liquid surface.

physical picture:

(recorder: FlinnScientific)

this process is more like an advanced version of suspended droplets. If the speed of the droplet is not great, it will float on the liquid surface for a while, separated by a layer of air, and if the liquid is dripping fast enough, it will have a chance to completely submerge into the water with the surrounding air film. such an anti-bubble is born.

schematic diagram:

because a layer of air is trapped, the density of the anti-bubble is slightly lower than that of the solution and tends to float up. Floating to the surface and touching the wall of the container will make the anti-bubble burst easily.

in order to keep the anti-bubble suspended in the liquid for a while, you can change the density of the experimental solution. First, add some corn syrup to the soap solution (I think honey should also work), and the sticky syrup will sink to the bottom; then stir it a little (don't stir it completely). This creates a density gradient in the cup (the density increases gradually from top to bottom); then, absorb the solution from about the middle to make anti-bubbles, so that the anti-bubbles should be able to suspend in the middle of the cup.

feel that the phenomenon of anti-bubble is very subtle and inconspicuous. Explain that it is very special, but people can't see any difference between anti-bubble and ordinary bubble without explanation. _ (: bubble "∠) _