Making Antibubbles

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ANTIBUBBLES

Skins of air which float around underwater,
and vanish when touched!
1997 William J. Beaty

Everyone is familiar with bubbles! There are three basic types:

fig. 1 Pockets of air under water

fig 2. Air trapped under a water film

fig. 3 Bubbles which float in the air

Besides the above bubbles, there are three other kinds which are not as familiar. These are the ANTIBUBBLES. An Antibubble is similar to a bubble, but the roles of the water and the air are reversed.

The first type of antibubble is familiar. it is simply a drop of water falling through the air. It is the opposite of an underwater bubble: rather than being a blob of air in the water, it is a blob of water in the air.

Fig. 4 Falling droplet of water

When a drop of water falls into water, we expect it to vanish. But sometimes it does not. If the water is very clean, then a thin skin of air will become trapped between the water droplet and the rest of the water. This effect is sometimes called a "Water Globule" or "floating drops." If you've spent any time sitting in a car on a rainy day, you will have seen water globules skittering across the hood. When raindrops splash on the car hood, you'll see the splashing droplets roll across the wet surface without melding into the rest of the water.

Fig. 5 "Water Globule" on the surface

A water globule is the opposite of an air bubble on the surface of water. Rather than being a pocket of air with a thin skin of water separating it from the air, it is a blob of water with a thin skin of air separating it from the water.

The last type of Antibubble is usually called by the name... "Antibubble." It is a very thin skin of air which floats around under the water. Antibubbles don't form easily, and they usually pop quickly, so most people have never seen them. However, it is possible with a little practice to create them yourself.

Fig. 6 "Antibubble" floats underwater

Antibubbles have many of the characteristics of soap bubbles. They float underwater weightlessly, and will only rise to the surface of the water very slowly. When poked, they will pop. When a soap bubble pops it leaves behind a tiny drop of water, but when an antibubble pops it leaves behind a tiny normal bubble. Antibubbles also display rainbow colors. Normal soap bubbles have rings of color at the top of the bubble where the soap film is thinnest, while antibubbles have their colors at their bottom.

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BLOW YOUR OWN UNDERWATER ANTIBUBBLES

Needed:

• Kitchen sink
• Large clean jar
• Elmer's (tm) glue bottle, emptied and cleaned
• Dishwashing detergent

The key to creating antibubbles is to make a very clean water surface. The tiniest bit of surface dirt will prevent antibubbles from forming, or will make them quickly pop. To create a clean surface, allow a container of water to continuously overflow . The overflow causes the surface of the water to stretch and be pulled sideways, and any dirt on the water surface will be skimmed off. Fill your large jar, place it in the sink, and adjust your faucet to allow a continuous stream of water to pour in and overflow the jar.
Fig. 7 Constant overflow cleans the surface

Add some soap to the jar of water and stir well. Fill your glue bottle with soapy water from the jar.

To get familiar with how antibubbles behave, first try making some "globules." Spray some droplets up from below, so the droplets land on the surface of the water. If you can build up lots of globules, you'll see them bounce off each other, or "pop" and join the rest of the water. Sometimes they'll join together into larger and larger ones.

Fig. 8 Squirt some globules onto the water

You can demonstrate that the thin film of air is disrupted by electrostatic forces. Comb your hair to give the comb an electric charge imbalance. Spray some globules on the surface of the water, then wave the charged comb near them. They will abruptly vanish! They all "pop" and rejoin the water. The electrified comb causes the water in the globule to split into areas of positive and negative charge. This imbalanced charge attracts the water below the globule, and the globule crashes into the water below. On a dry day, your body can become electrified from walking on the floor, and this can disrupt antibubbles and globules. If your globules and antibubbles refuse to form, try touching the metal faucet to remove any charge imbalance from yourself. "Static electric" page

OK, let's make some real antibubbles. Follow the three steps below. First place the tip of your water-filled squirt bottle very near the water surface. Give it a gentle puff and create a single water globule. Immediately give a longer squeeze. This will send a jet of water through the globule and down into the jar. If your squeeze is gentle and brief, the water jet will take the air layer along. A long silvery worm will extend into the water. This "worm" is water which is coated with air. Do this several times, and sometimes the worm will break up into antibubbles of different sizes.

Fig. 9 Make a globule.

Fig. 10 Squirt through the globule.

Fig. 11 Watch for antibubbles!

Your first underwater antibubbles will probably be small, under 1/8 inch across. With practice it is possible to blow 1/4" antibubbles, and occasionally 1/2" antibubbles.

To prove that this is no ordinary bubble, poke it with a pencil or fork. It will instantly vanish.

Antibubbles will slowly rise to the surface, where they will often pop. To extend their life time, stick the bottle tip in the water and squirt at them to drive them deeper into the jar. Vibration is supposed to extend their lifetimes, so squirting them with underwater jets may keep them alive longer. Temperature difference is also said to lengthen their lifetimes. Try filling your glue bottle with hot water, while putting cold water in your overflowing jar.

Antibubbles display rings of color, but these colors appear at the bottom of the bubbles rather than at the top. Look closely at your antibubbles under bright light, and you'll see that each one has "soap film colors." But in this case we should call them "air film colors." The color of soap films (and of oil on water) are caused by light reflections from a very thin, transparent layer. Light reflects from the front of the thin layer and from the back. If the layer is almost as thin as light waves, the certain colors of light waves from the two reflections cancel out, producing a "subtractive rainbow" with cyan/magenta/yellow instead of red/green/blue. In a normal bubble the thin water layer creates the colors. In an antibubble, the colors are created by the thin layer of air.

Conventional bubbles can be filled with cigarette smoke, and they release a little cloud when they pop. Antibubbles can also be filled with colors. Just put some food coloring in your squirt bottle. If several kids have squirt bottles with differing colors of water, everyone can keep track of their own antibubbles in the same jar.

OIL BOTTLE:

To demonstrate all the various bubbles (antibubbles, bubbles, globules, antibubbles inside bubbles, anti-foams, etc.) make yourself an oil/alcohol bottle. Clean out a small jar and fill it half with salad oil, then fill the rest with rubbing alcohol. Try to fill it perfectly, right up to the top, so no air is trapped. Screw on the cap, then slosh it gently to create waves, or a bit harder to create all sorts of bubbles-within- bubbles. (Don't shake it hard, or it will take hours for the misty mixture to settle out.)

MORE INFO:

For more information about all of this, find a local library with back issues of Scientific American magazine. ( Or, order them through your library's Interlibrary Loan.) The "Amateur Scientist" columns for April 1974 and for August 1976 discuss antibubbles. On WWW, search for keyword "coalescence."

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• Terry's advanced antibubbles
• Chris N's antibubbles
• "Inverse Bubbles" at Nicolet HS
• Bubble Page at Exploratorium

Science Papers:

• Electric field-enhanced coalescence of spherical drops
• C.L. Stong, "Curious Bubbles in Which a Gas Encloses a Liquid Instead of the Other Way Around", Scientific American Magazine, THE AMATEUR SCIENTIST, April 1974

Note: a CDROM of THE AMATEUR SCIENTIST is sold by tinkersguild.com

http://www.amasci.com/amateur/antibub/antibub1.html
Created and maintained by Bill Beaty. Mail me at: billb@eskimo.com.
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