HOW TO MAKE THE VORTEX

two un-crushed, 2-liter soda bottles
Try to get bottles that have not been really crushed, because the dents that remain can interfere with the spinning water.
1/2" diameter PVC plastic pipe.
It is our good fortune that very cheap and easy to find 1/2" PVC pipe fits snuggly inside 2-liter bottles. When used to connect two bottles, the pipe keeps them from flopping around.
Some hardware stores might sell it by the foot (you only need 2 inches per project). From other places, like building centers, you will have to buy 5 or 10 feet at a time. It is still little more than a dollar for 10 feet. PVC pipe is white. Do not get "CPVC" pipe, which is more expensive and a tanish color. One more confusing thing: 1/2" diameter pipe is actually bigger than 1/2". Go by what it says on the pipe, not by actually measuring the diameter.
clear plastic food wrap
The only really challenging part of this project is making the bottles completely sealed--no leaks. The best way to do it is with plastic wrap. By stretching the plastic tightly as it is wrapped around the bottles, it seals very well.
electrical tape or duct tape or masking tape; scissors; hacksaw (or just a hacksaw blade)
Duct tape is best. The hacksaw or hacksaw blade is for cutting the plastic pipe.
cold water, or refrigeration or ice or snow or cold weather to make it cold.
I know it seems a bit strange to be concerned about air and water temperature as we seal the two bottles together. The reason behind it is quite interesting. I go into it more in the Activities and Explanations page.
For building this project, you should know this much: It is important that the air and water in the bottles be cold when they are being sealed. As the air warms up, it will expand a little. Because it's trapped inside, the air pressure inside the bottles becomes a little higher than the air pressure outside. That's good, because it keeps the bottles firm, just like the higher air pressure in a bicycle tire keeps the tire from going flat.
Without the higher pressure inside, the bottles will dent a little. Because they are no longer perfectly round, it's harder to make a good vortex. Fortunately, this is easy to prevent by keeping things cool when sealing the bottles.
Step 1
Cut a 2" long piece of pipe.
PVC pipe is easy to cut and it only takes a few seconds. Even if you don't have a hacksaw frame, you can cut with just a blade if you wrap a little tape around as a handle. Hack saws are much safer than any other kind of saw.
I am a believer in letting even young kids participate as much as possible, so I offer this illustration as a proven way kids (green hands) can do some sawing, while you (blue hands) maintain control of the saw. Because of the way the pipe is supported on both ends, it doesn't slide around.
Here you can see a fast and safe jig for cutting, made of a scrap of wood and 5 nails. The hacksaw slides against two nails that automatically cut a 2" piece. It's worth making if you are working with a group.
When you're done sawing, clean out the plastic "crud" that sticks to the end of the pipe where you made the cut. Use your fingernail.
Step 2
Prepare the bottles for sealing.
These sub-steps don't have to be done in any particular order:
*Rinse out bottles.
*Peel off labels. Save a colorful piece for the next part.
*Cut some confetti from the plastic labels you just peeled off. They become tracers in the vortex (they remind me of the house spinning around in the Wizard of Oz). No piece should be larger than the finger nail on your pinky. Put the pieces into one of the bottles.
*Cut a piece of electrical tape 3" long, or cut duct tape into a a 3/4" slice, 3" long. Wrap it tightly around the middle of the pipe. This tape will keep the pipe from falling into one bottle or the other.
*Fill one bottle about 2/3 full of the coldest water you can get. For example, if there is a drinking fountain with chilled water, it would be worth it to catch it in a cup and pour it into the bottle. Alternately, you could but half a handful of crushed ice or snow in both bottles, or you could put the bottles in the refrigerator or outside (if it's cold) for half an hour.
If the kids making the vortex are very young (kindergarten or first grade), only fill the bottle half full. It will be easier to carry and easier to start the vortex.
Step 3
Just before you wrap.
Push the pipe into the filled bottle until it gets hard to push (because of the tape). Turn the other bottle upside down and push it on the other side of the pipe. Push and twist it pretty hard so it squishes some of the tape.
Just before sealing the bottles, turn them upside down so the empty bottle fills (a few drops of water might leak out). This will cool the air inside. The top bottle will probably dent in a little, which means the air in the system is contracting. Pull the bottles apart for an instant to let more air in, so the bottle is not dented anymore. Do this just before sealing so the air doesn't have a chance to warm up much.
Step 4
Seal the bottles together.
The key to sealing the bottles is to keep the plastic wrap tight as you wrap it on.
Take the roll out of the box and hold it in one hand. Start wrapping the end around the bottles with the other hand. At some point, the wrapped part won't slip off even when you pull very hard. You should notice the plastic stretching over the handles. That is where the seal will be made. Wrap at least 10 tight wraps around the bottles. I know I'm repeating myself, but if you don't apply the plastic under stretching tension, it won't seal.
Although the bottles should now be sealed, wrap some duct tape or electrical tape a few of times around the plastic. This keeps it from unraveling, and it keeps the bottles from separating when you lift by the top bottle.
Step 5
Use it!
Turn the bottles over. Grab the very top an swish it in a circular motion two or three times, then stop suddenly. This will give the water enough circular momentum to create the vortex.
If the bottle dents, don't worry. It will fill out in a hour or so, as the water warms--if you sealed it well. Until then, at the same time you swishing the top end in a circle to get the water moving, squeeze the the bottom bottle. When the dent is on the bottom, it doesn't interfere with the vortex.