Vortex Rings Using Liquid Nitrogen

Materials: ★★★ Requires materials not commonly found in school laboratories
Difficulty: ★★☆ Can be done by science teachers
Safety: ★★★ Only to be attempted with adequate safety procedures and trained staff

Categories: Liquid Nitrogen

Alternative titles: Fog Ring Blaster

Summary

A vortex gun made from a bin or box can shoot doughnut-shaped rings of air called toroidal vortices. By filling the vortex gun with fog created by liquid nitrogen, the normally invisible air rings become visible as dramatic smoke rings.

Procedure

Cut a round hole in the bottom of a plastic garbage bin or in the side of a large cardboard box. The hole should be about one-third the diameter of the base.
Cover the open end of the bin with a sheet of plastic film (such as mattress wrapping) and secure it with tape, leaving it loose enough to push in and out.
Create fog by pouring in hot water then liquid nitrogen. This will generate a thick white vapor cloud.
Hold the vortex gun over the fog so it fills with the nitrogen fog.
Pull the plastic sheet back slightly and then push it inward to blast air through the hole.
Observe the visible toroidal vortex ring shooting out across the room.
Repeat, aiming at lightweight targets like paper cups or tissue balls.

Links

Smoke (fog) rings (vortices) with liquid nitrogen - MrGrodskiChemistry:

📄 Toroidal vortex gun - ABC Science: https://www.abc.net.au/science/articles/2012/07/17/3546850.htm?site=12science&

Variations

Use different size holes to compare vortex size and speed.
Shine a laser or flashlight through the fog rings to highlight their shape.
Try gentle taps for slow-moving rings or firm hits for powerful blasts.

Safety Precautions

Safety glasses required.
Wear cryogenic gloves when handling liquid nitrogen.
Use liquid nitrogen only in a well-ventilated space.
Keep hot water containers stable to prevent spills.
Do not place liquid nitrogen directly in the vortex gun - only use the fog it produces.
Ensure smoke detectors are isolated if doing this indoors, as nitrogen fog may trigger alarms.

Questions to Consider

Why does the fog make the vortex rings visible when air vortices are usually invisible?
What forces hold a toroidal vortex together as it travels?
How does this relate to the kinetic theory of gases and air pressure?
Where else in nature do toroidal vortices appear (e.g., volcanoes, underwater bubble rings, mushroom clouds)?
Why does the vortex stay stable for so long instead of dissipating immediately?