Vortex

1. Remove labels from bottles.
2. Fill one of the soda bottles about two-thirds to three-fourths full of water. For effect, you can add a little food coloring or some bits of paper or glitter to the water.
3. Apply a small ring of hot glue just inside the mouth of the water-filled bottle, then immediately insert one end of the PVC pipe into the opening, simultaneously pushing and twisting until half of the PVC pipe is in the bottle. (Note: the hot glue will dry very rapidly, so be sure to insert the PVC pipe as quickly as possible.) 
4. Apply a small ring of glue around the top of the mouth of the same bottle (some of the glue will get on the PVC pipe, but that's okay), then immediately push and rotate the empty bottle onto the end of the pipe until the mouths of the two bottles meet (or come as close as possible) to form a glue joint. 
5. Apply some additional hot glue to seal the joint where the two bottles meet.
6. After the glue dries, proceed to the To Do and Notice section. If the bottles leak when you are using them, use a paper towel to dry off the area where the mouths come together and apply more hot glue there to obtain a better seal.

Orient the connected bottles so the filled bottle is on top and upside down, then set the assembly on a table. Watch the water slowly drip down into the lower bottle as air simultaneously bubbles up into the top bottle. The flow of water may come to a complete stop.

With the filled bottle once again on top, rapidly rotate the bottles in a circle a few times. Observe the formation of a funnel-shaped vortex as the bottle drains.

Notice the shape of the vortex. Also, notice the flow of the water as it empties into the lower bottle.

If you only have one 2-liter bottle, you can still make a vortex by twirling the bottle and holding it over a water basin or the ground to drain, but you will have to refill the bottle each time you use it.

When the water is not rotating, surface tension creates a skin-like layer of water across the small hole in the center of the connector.

If the top bottle is full, the water can push out a bulge in this surface to form a bulbous drop, which then drips into the lower bottle. As water drops into the lower bottle, the pressure in the lower bottle builds until air bubbles are forced into the upper bottle. The pressure that the water exerts on the surface in the connector decreases as the water level in the upper bottle drops. When the water level and pressure drop low enough, the water surface can hold back the water and stop the flow completely.

If you spin the bottles around a few times, the water in the upper bottle starts rotating. As the water drains into the lower bottle, a vortex forms. The water is pulled down and forced toward the drain hole in the center by gravity. If we ignore the small friction forces, the angular momentum of the water stays the same as it moves inward. This means that the speed of the water around the center increases as it approaches the center of the bottle. (This is the same reason that the speed of rotating ice skaters increases when they pull in their arms.)

To make water move in a circle, forces called centripetal forces must act on the water. These “center-pulling” forces are created by a combination of air pressure, water pressure, and gravity.

You can tell where the centripetal forces are greater by looking at the slope of the water. Where the water is steeper, such as at the bottom of the vortex, the centripetal force on the water is greater. Water moving with higher speeds and in curves of smaller radius requires larger forces. The water at the bottom of the vortex is doing just this, and so the wall of the vortex is steepest at the bottom. (Think about race cars: Racetracks have steeper banks on high-speed, sharp corners to hold the cars in their circular paths around the track.)

The hole in the vortex allows air from the lower bottle to flow easily into the upper bottle. This enables the upper bottle to drain smoothly and completely.

Vortices occur in nature in many forms: tornadoes, whirlpools, weather systems, even galaxies. The essence of a vortex is that objects are drawn together toward the center, then miss!

Spiral waves form in the water surface of the vortex. These waves appear to move in slow motion as they travel upward through the downward-flowing water.