An antibubble is a droplet of water surrounded by a thin film of air, as opposed to an air bubble, which is a sphere of air surrounded by a thin film of water. Antibubbles are formed when water drops or flows turbulently into water. They can either skim across the surface of the liquid, in which case they are also called water globules, or they can be completely submerged in the liquid.
Antibubbles are a common but widely unrecognised phenomenon, in part because of their resemblance to air bubbles, and in part because of their typically transient nature. With certain (soapy) solutions, they can be made to last much longer.
The behavior of antibubbles differs from that of air bubbles in three primary ways, and provides a ready means of identification:
- Antibubbles are held in place by surface tension, and move rapidly across the surface of the water. They can also be seen to ricochet off other objects in the water (such as air bubbles) and off the sides of a container in a manner similar to that of billiard balls.
- Antibubbles can be short-lived. An air bubble with a soap skin may last a minute or two. Antibubbles often have lifetimes of a few seconds or less; however, if the electrical potential between the inner and outer fluid is equalized, antibubbles can last as long, or longer, than air bubbles.
- Antibubbles refract light in a different manner than air bubbles. Because they are water droplets, light entering them is refracted back toward its source in the same manner as rainbows are produced. Because of this refraction, antibubbles have a bright appearance.
Antibubbles are easily created by allowing a tap to drip into a container of water to which a drop or two of soap has been added. The soap reduces the water's surface tension and allows the skin of air surrounding the droplet to remain in place for more than just a fraction of a second.
Just as soap bubbles, with air inside and air outside, have negative buoyancy and tend to sink towards the ground, so antibubbles, with water inside and water outside, have positive buoyancy and tend to rise towards the water surface. But again, just as soap bubbles can be filled with a lighter gas to give them positive buoyancy, so antibubbles can be filled with a heavier liquid to give them negative buoyancy. Using a drinking straw to drop droplets of sugar solution onto soapy water will produce antibubbles that sink.
Antibubbles usually pop when they touch the bottom or the side of the vessel containing the liquid. This can be prevented by tipping a few teaspoons of sugar into the soapy water and giving it some time to dissolve (but without stirring it). This will produce a denser layer of sugary water at the bottom of the container. Antibubbles made from sugar solution will then sink through the water and rest on top of the denser layer at the bottom. Antibubbles made this way can last for several minutes.