The Opposite of Bubbles

Image: A giant antibubble (from antibubble.com)

Most people associate bubbles with being shiny spheres of infantile joy that float off on an autumn breeze. Either that or having to do the dishes. If only they could see beneath the surface when filling the sink maybe they would notice that there exists bubbles of a different nature, a different disposition. There exists a type of bubble enshrouded by a dark ring, a type of bubble that can sink.

Antibubbles, as the name suggests, are the opposite of common, garden variety bubbles. So instead of having gas inside them they are actually filled with liquid. However they do rely on gas to form their shell. Somewhat ironically, this is what the water inside floats on. The last feature of antibubbles is that the medium they pass through is liquid and not gas. Essentially they are a shell of gas with liquid on the inside and liquid on the outside. As such, when viewing antibubbles they appear to have a dark ring around the outside due to total internal reflection (1).

Image: Ordinary garden variety bubble

Over the past decade a group of scientists from Université de Liège have published several papers with theories explaining several interesting observations made. According to one of their papers, “this unusual object was first reported by Hughes and Hughes” in 1932 (2).

One of the researchers, Stéphane Dorbolo explains, “The antibubble is fascinating because it is not a (sic) stable object as "normal" bubble can be. Moreover it is a very smart manner to separate two miscible liquids using only air. I do think that there is still much (sic) things to discover about this beautiful and so easy to do (sic) object.”

Despite being somewhat intriguing, very little research has been done into the mechanics of antibubbles (1). This is probably a result of their discrete public profile and hitherto lack of practical application.

“The first time I have heard about antibubble is through a colleague who found it on internet. After that, we notice that no serious works have been performed about these particular object (except a Nature paper in 1934),” elaborates Mr. Dorbolo.

Image: creating antibubbles with a beaker (2)

So how are these instable creatures formed? It is not precisely know what combinations of fluids and gases are eligible but a common way to make them involves water with a little bit of detergent (which from now on shall be referred to as just water) as the liquid and air as the gas in the shell. The process itself is very simple yet due to the relatively unstable nature of antibubbles it is highly dependant on certain environmental conditions. The basic method is to simply pour water into a body of water, it is that simple! The catch is that for the antibubbles to form the water has to have the right amount detergent in it, it has to be poured at the right height and at the right rate. Furthermore it apparently helps if the body of water has a ‘clean’ surface and any voltage differential between the body of water and the water being poured should be avoided (2). The reason for this will be discussed later.

Image: Antibubbles forming underwater. Notice the wire connecting the body water to the bubble water (1).

Whilst intact, antibubbles will rise slowly in similar water due to the buoyancy of the air shell. However, if the water used in the bubble is made slightly denser (by adding salt for example) than the body water, then bubbles can be made to more or less ‘float’ in the water or even sink. Floating or hovering antibubbles are interesting because, since they go with the flow of the fluid around them and have no other forces acting on them, they can be used as a rough visualisation of the strain of a fluid element in a flow. In the photos across, antibubbles are observed interacting with a whirlpool in two different ways. Depending on where the antibubble is formed it may either form a horizontal coil following the whirl (b & c) or become vertically elongated in the eye of the whirl (d) (3). Given antibubbles are naturally spheres, and unstable ones at that, it is interesting that they can form these shapes.

Image: Antibubbles interacting with a whirlpool (3).

When sinking however, it has been shown that antibubbles will quickly meet their demise (2). Irrespective of their size or the velocity at which they are falling, antibubbles will reach a critical depth where a combination of the hydrostatic pressure acting on the bubble and the atmospheric pressure at the surface will cause the shell to rupture. The result of this can be seen in the figure below. The small air bubble that was the shell rises to the surface whilst the bubble water mixes with the body water and forms a couple of vortices (1).

This is similar to the way a normal bubble would burst. Notice that the strength of the shell is compromised by air being pushed to the top where the pressure is less. As such, antibubbles will always self destruct from the bottom. Unlike normal bubbles, the shell of an antibubble is quite compressible and once it is thin enough the tails of the surfactants in the water get close enough for Van Der Waals attraction force to become significant and then the whole structure collapses.

Image: Antibubbles popping due to hyrdostatic pressure (1).

Since this drainage of air also happens over time regardless of depth, it limits the lifespan of all antibubbles (4). An antibubble spontaneously popping due to age is shown below.

Image: An antibubble bursting due to aging (4).

However, there are many other factors that can burst your bubble before it gets too old. As mentioned previously, antibubbles are susceptible to potential voltage since it results in the air shell becoming unstable. This is because it acts like a capacitor with concentric electrodes (1).

Another, more obvious way to pop a bubble is with a pin. The figure above right shows how the shell is forced away from the point of rupture (like when a water balloon bursts) whilst the water inside goes in the opposite direction, back towards the pin (1). This results in vortices similar to the result of a depth related pressure failure.

Image: An antibubble that was resting just under the surface being popped with a pin (1).

So what’s the point of all this? Like a lot of scientific investigation, there isn’t one yet although it does represent a new way of mixing liquids so it could have an application with drugs for example. Another example of where knowledge of antibubbles may be applied is in calculating void fractions. A void fraction represents the amount of air in water, and since antibubbles are filled with water, anyone visually calculating a void fraction needs to make sure they aren’t counting antibubbles as bubbles.

On the web –
www.antibubble.org
www.antibubble.com
www.youtube.com/stephanedorbo

Note: For better quality images see original sources.

Reference:
1. S Dorbolo, H Caps & N Vandewalle. Fluid instabilities in the birth and death of anitbubbles. New Journal of Physics 5 (2003) 161.1–161.9
2. S Dorbolo & N Vandewalle. Antibubbles: evidences of a critical pressure. www. arXiv.org. 2003.
3. S Dorbolo, H Caps, N Vandewalle, G Delon & D Terwagne. Antibubbles in a cyclone eyewall. www.arXiv.org. 2009.
4. S Dorbolo, N Vandewalle, E Reyssat & D Quéré. Aging of an antibubble. Europhys. Lett. 69 (2005) 966

Skip to the end: A brief investigation into antibubble research.