In the same year as Anderson’s experiment demonstrated the existence of antimatter, another oft-overlooked discovery was made.
William and Alun Hughes published an article on Nature in which they stated that in pouring soap solution drops in water, they could observe in addition to floating drops some other small drops (about 1-4 mm of diameter), “which sink to a considerable depth and slowly rise again, coming to rest immediately underneath the surface”. That was the first published observation of antibubbles, first so-named by Connett Stong in 1974.
An antibubble is a spherical drop of liquid enclosed by a thin film of air, which in turn is immersed in the same liquid. In many structural aspects, an antibubble is the exact opposite of a soap bubble and, although some structural similarities, it has very different properties from those of a bubble.
If you have ever washed dishes, it is very likely that you have – purely accidentally – created some antibubbles. The way to create antibubbles is very simple and it consists of gently pouring a drop of detergent/soap solution into a liquid of the same solution , from a certain height. A globule of liquid first appears at the liquid surface and it does not coalescence immediately with the liquid surface, but it can grow under incoming liquid flow, finally forming different antibubbles. Typical dimensions of antibubbles are of the order of 1 cm or less, but by using large tanks of solutions equipped with valves and pumps, Terry W. Fritz, an amateur scientist from US, successfully created antibubbles of 5 cm in diameter.
Scientists initially considered the physics of antibubbles to be of mere intellectual interest or even a sort of fun-science for kids, and antibubbles started to be generated in a variety of liquids (including beer). However, in the last decade scientists have started to seriously investigate their formation mechanism, as well as their physical properties, and have started to investigate their potential practical applications.
First of all, scientists discovered other methods to produce antibubbles: in the late 1990s Stefan Hutzler and his colleagues at Trinity College in Dublin found that if several soap films are created in such a way that they meet all at a vertical seam, this can gulp down a spherical droplet of soap solution and transform it in an antibubble that descends while trapped in the seam.
In 2002, Tufaile and Jose C. Sarotrelli from the University of Sao Paulo found during their investigation of non linear and chaotic systems that bubbles often bumped into each other and coalesce. Occasionally, a droplet of the viscous liquid penetrates through the surface of the bubble and gets coated by a shell of air. An antibubble is formed.
As anticipated, antibubbles have different properties from bubbles. They are often much less stable and their stability critically depends on the air shell surrounding the inner liquid. Antibubbles typically live for seconds or minutes and the longest-lived antibubbles observed last a day. On the other hand, soap bubbles, when protected from evaporation and contact with surfaces, can remain intact for months, even though their liquid skin has a thickness of just 0,00001 mm.
Another difference between bubbles and antibubbles is the movement within the surrounding fluid. While bubbles, having air inside, tend to have negative buoyancy in air and sink towards the ground, antibubbles, which contain liquid, tend to have positive buoyancy in liquid and rise towards the surface. Moreover, due to the force of gravity acting on the internal liquid, the disruption of an antibubble starts from the bottom of its air shell. By contrast, in a bubble the top of the liquid shell is the starting point of the explosion, due to the action of the gravity force on it.
The use of high-speed videocameras and computers has allowed scientists to investigate more deeply the mechanism of formation and rupture of antibubbles. P. G. Kim and H. A. Stone from Harvard University found that two stages are critical for the formation of an antibubble. The first stage is the formation of an air pellet between a descendent drop into a liquid, and its vertical propagation in the surrounding liquid. The second stage is the pinching of the liquid column, which may or may not occur depending on the surface tension of the drop solution. They also searched for the optimal velocity of the drop poured into the liquid in order to form an antibubble and for a drop falling from a tab with aperture radius of 2 mm at a height of 11 mm, an antibubble can be formed only if the initial velocity of the drop is less than 24 cm/s. If the drop falls from a too elevated height the formation of antibubbles is hindered by instabilities which break up the liquid jet in multiple drops, and which disturb the surface of the liquid, breaking the formation of the air film.
S. Dorbolo and collegues from the University of Liege in Belgium, studied the death of an antibubble, which is related to the collapse of the air shell and they found that this occurs in 6 steps: the air film is first pierced and this generates a circular propagating front; then the inner liquid of the antibubble enters suddenly in contact with the surrounding liquid; at this point the air film shrinks around the point of the initial popping, but in opposite direction; hence, the air film starts to oscillate and ripples and finally the air film disintegrates into air pockets. This whole process occurs in less than 50 ms.
Scientists are learning to control antibubbles for their use in different technological applications. For instance, antibubbles have a great capacity for lubrification and if long-lasting foams of antibubble (antifoams) will one day be fabricated, they could be used as new types of lubrificant or another route to filter air or gases. When compared to ordinary gas bubbles, antibubbles provide twice the surface area through which molecular exchange and chemical reactions can occur, and they also rise relatively slowly towards the surface. These properties could confer many advantages for chemical processes, such as removing smokestack pollutants and manufacturing chemical and drugs.
Scientists are currently also attempting to replace the air of the shell with another liquid. Water globules in an oil shell surrounded by water have been generated and show a much greater stability so that stable antifoams might also be formed from them. Oil globules enclosed within alcohol shells inside oil have been also fabricated. If scientists are one day able to insert a light-activated liquid-polymer in the air shell and harden it by using ultraviolet light, then antibubbles could have a glorious future as drug delivery capsules, by filling the inner liquid with the solution of the active drug – not a bad prospect at all for a bit of ‘fun-science for kids’.