Scientists have been able to understand the unusual phenomenon of antibubbles - the rare, reverse form of normal bubbles - by forming them in beer.
Antibubbles are the reverse of normal bubbles
Typical bubbles are thin films of liquid enclosing pockets of air, but antibubbles are a thin film of air which encloses pockets of liquid.
Belgian researchers said on Monday they could make antibubbles appear in a many different liquids - not just alcohol.
Dr Stephane Dorbolo details his team's work in the New Journal of Physics.
Beer or dishwater
The scientists observed that antibubbles could be created by pouring a liquid containing a surfactant (a substance such as soap that alters the nature of a surface) on to an identical liquid.
They form because a thin film of air is sometimes pulled down along with the liquid itself.
This air film then separates the two liquids with the same composition and is therefore called an antibubble - since a real bubble is a liquid film separating two regions of air.
Out of curiosity, the researchers also attempted to create antibubbles in Belgium's most famous export - beer.
Dr Stephane Dorbolo, from the University of Liege, said: "We tried to create them in beer for fun, and didn't think it would be possible, but were amazed when we managed to create giant antibubbles which lasted for almost two minutes.
"You can't create antibubbles in pure water, alcohol or oil. But beer is a special case because it is very similar to dishwashing liquid and contains surfactants which are needed to produce antibubbles."
The scientists found that when antibubbles burst they took on a shape they nicknamed "the jellyfish", which moved slowly and then faded until they disappeared altogether.
Antibubbles have been recognised for a century but the physics that underlies their existence is still being investigated.
- Take some water in a clear bowl or tank
- Fill it to the very brim with dishwashing liquid (water with a few squeezes of washing-up liquid in it)
- Keep some of this liquid back in a beaker or jug
Very gently pour the liquid from the beaker onto the surface of the bowl
- Watch beneath the surface as you pour and vary the speed at which you pour
- If you find the right speed, you might see antibubbles form as the stream of water descends beneath the surface
- You can now watch these antibubbles move and sink downwards and they will eventually burst.