What do you call a bubble that's been inverted? An antibubble! Unlike a regular bubble, which is a thin film of liquid wrapped around air, an antibubble is a thin film of air surrounding liquid. Regular bubbles rise to the surface fairly quickly in liquid; antibubbles are a bit lighter than the surrounding liquid, which means they rise much more slowly. If the fluid inside is heavier than the water, the antibubble will actually sink.
It's fairly simple to make your own antibubbles. Fill a glass and a squeezable bottle with a soapy liquid, then use the squeezable container to inject a stream of liquid into the glass. That downward stream drags down air, which can envelope drops of the soapy solution. Antibubbles don't last very long, but it's possible to see their inverted structure a little bit better by adding a bit of food coloring to the liquid. Then it becomes obvious that it is liquid surrounded by air.
Making them is one thing. Studying antibubbles is quite another. They disappear so quickly that physicists are still unsure of the underlying fluid physics that govern them.
Back in 2003, Belgian physicist Stephane Dorbolo of the University of Liege and his colleagues were able to watch antibubbles in motion for the first time using a high-speed video camera. They didn't just make the antibubbles in soapy liquid, either — they also managed to create antibubbles in Belgian beer. This can't be done with pure water or alcohol, but beer has the same surfactant properties as dishwashing liquid.
Another way to study antibubbles is to make them in a microgravity environment, such as the one found aboard the International Space Station. Here is astronaut Don Pettit demonstrating just how much fun you can have with antibubbles in space, courtesy of NASA and Physics Central:
Video: Don Pettit's antibubble experiment on the space station. Credit: NASA.