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Vortex rings with a bottle : Fizzics Education

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Vortex rings with a bottle

Vortex rings with a bottle

Follow FizzicsEd 150 Science Experiments:

You will need:

  • A fog machine
  • A soft drink bottle (try different sizes!)

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A fog machine next to a plastic soft drink bottle on a desk
1

Warm-up your fog machine and then fill your soft drink bottle with stage fog. That’s it, you’re ready to go!

2

Gently squeeze the soft drink bottle in short bursts… you should see a vortex fog ring shoot out of the opening!

3 Two kids and Fizzics presenter shootig fog rings our of a bin

What is going on?

We love toroidal vortices! The way these rings form is due to the air rushing out of the bottle opening at different speeds. Think about what the air is doing as it travels out of the bottle; the air is being forced out of the bottle and some of this air is hitting the sides of the bottle opening. The air hitting the side of the bottle slows down due to friction, meanwhile, the air in the middle of the opening continues to leave the bottle at speed. The different air speed forces the ring-shape to form!

The underlying reason for the ring shape is due to a pressure differential. The mathematician Bernoulli found that moving air has less pressure than air that is still; this means that the fast air coming out of the bottle has a lower pressure than the slower air near the bottle sides. As high-pressure air moves towards low-pressure air, squeezing the bottle forces air to quickly form a ring as it leaves the bottle due to the low air pressure of the fast air rushing outwards.

Low air pressure occurs when air is sped up. Then the energy of the air molecules is being used mostly to move them faster, and less to bounce them off of each other and the balloons. Without as many collisions between the air molecules, the pressure between them is lower, and surrounding, slower-moving air molecules have more energy to bounce into the space, moving the balloons towards each other as they move in. The way that moving gases and fluids work is that slower air will move towards faster air.

Want to make larger fog rings?

Learn how to make large ones with a bin

Variables to try

  • Try different sizes of plastic bottles.
  • Does placing a triangle shape in the bottle opening create a triangle-shaped ring?

Learn more!

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