How does a vortex form between two bottles?

Water can only get out if air can get in

Turn a full bottle upside down, and the water comes out in fits and starts. That is because a bottle cannot empty without air. For every bit of water that comes out, some air has to go in.

With two bottles on top of each other, it works the same way. The water in the top bottle wants to go down. The air in the bottom bottle has to go up, to the place where that water was.

Without a swirl: glugging

Water and air have to pass through the same narrow opening. In Wurvl, that is the copper core. As long as the water is not swirling, water and air get in each other's way there.

A bubble of air pushes up, a splash of water falls down, and then another bubble. You hear that as glugging. It happens in jerks, and slowly.

With a swirl: an open channel

When the water does swirl, the picture changes. The water flows down along the wall in a spiral. In the middle, an open channel of air forms.

The air rises through that channel, while the water around it falls down. They no longer get in each other's way. So the bottle empties in one smooth movement, and a good deal faster too.

Why the water spins faster and faster

The closer the water gets to the opening, the smaller the circle it turns in. And the smaller the circle, the faster it spins.

You can see the same thing when a figure skater does a pirouette. When she pulls in her arms, she spins faster. That is why the water spins fastest right next to the opening.

A small twist to start

Sometimes the water starts to swirl by itself. Sometimes it helps to make a few small circular movements with the top bottle. That gives the water its first twist.

Source

The Institute of Physics describes this experiment on its website: Bottled tornado.

A daily moment

You can watch this movement again every day. You fill a bottle, turn the bottles over and watch the water find its way down. For a moment, nothing else. Just water in motion.

Want to try it yourself? Read how to use Wurvl.