Whirlpool between two bottles

What they will learn

• What a vortex is: spinning water forms a funnel with a hollow of air in the centre.
• That water and air need to «take turns» to get through a narrow neck, and that the whirlpool opens a path for each of them: water round the outside, air up the centre.
• That as the water gets closer to the centre it spins faster and faster (like a skater pulling in her arms).
• How to measure and compare draining times by repeating the test.

Prepare beforehand

• Remove the labels from the bottles (hot water and soap) and dry the outsides: tape won't stick to wet plastic.
• Print template A, stick it onto the plastic lid and cut out the washer; make the 9 mm centre hole.
• Cut 4 strips of electrical tape 30 cm long (1.2 m in total) and leave them stuck by one end to the edge of the table.
• Work over a sink, a tray or outdoors in case the joint leaks on the first test.

Materials and tools

• Plastic bottles — 2 identical
  1.5 l, clear, smooth, label removed, with the standard fizzy drink/water mouth (≈ 27 mm thread)
• Plastic washer — 1 (piece A)
  Ø 25 mm with a 9 mm centre hole, cut from a plastic tub lid (ice cream, large yoghurt) or a plastic folder
• Electrical tape — 1–1.4 m
  PVC, 15–19 mm wide (stretchy and watertight)
• Tap water — 1.1 l (¾ of a bottle)
  at room temperature
• Food colouring — 3–4 drops
  blue or green
• Visible bits (optional) — 1 teaspoon
  aluminium foil rolled into 2–3 mm balls, large sequins or plastic confetti, to see the spin
• A few drops of washing-up liquid (optional) — 1 drop
  so the air funnel looks cleaner

Tools

• Strong scissors — cut the washer out of the plastic
• Hole punch or awl and nail scissors — make the washer's 9 mm hole adult
• Measuring jug and funnel — pour in 1.1 l without spilling
• Stopwatch — time how long it takes to drain

Pieces and templates

• A · Flow washer ×1 — plastic from a tub lid or folder (0.5–1 mm)

Step by step

1. Fill the first bottle

   Put the funnel in a 1.5 l bottle and fill it with 1.1 litres of water, up to about 4 cm below where the shoulder begins (where the bottle starts to narrow towards the neck): three quarters of the bottle.

   Check: There are about 400 ml of air left at the top: the whirlpool needs it to form the funnel.

2. Add colour

   Add 3–4 drops of food colouring and, if you like, a teaspoon of little aluminium foil balls or sequins. You can add 1 drop of washing-up liquid.

3. Cut out the washer adult

   Cut out washer A, Ø 25 mm, from tub-lid plastic and make the 9 mm centre hole with the awl and nail scissors.

   Check: The washer covers the rim of the bottle mouth exactly, sticking out no more than 1 mm.

4. Position the washer

   Dry the mouth of the full bottle thoroughly and rest the washer on top, centred. If you want it to stay put, hold it with a small piece of tape on two sides.

5. Add the second bottle

   Place the second bottle, empty and dry, upside down on the washer, with the two mouths lined up as if they were one long bottle.

   Check: Look from the side: the two bottles form a straight line.

6. First turns of tape adult

   While someone holds the bottles straight, wrap 4 turns of well-stretched electrical tape right over the joint between the mouths, pressing with your fingers so there are no wrinkles.

7. Strengthen the joint

   Carry on taping in a spiral upwards and downwards until you cover both threads and both neck rings, about 1–1.4 m of tape in total. Finish with 2 turns over the joint.

   Check: Hold the whole thing with the full bottle on top over the sink for 10 seconds: it must not drip. If it drips, add more tight turns over the joint.

8. Turn it over

   Turn everything over so the bottle with water is on top. Hold the bottom bottle with one hand and the joint with the other. You'll see the water glug down slowly.

9. Make the whirlpool

   With one hand on the top bottle and the other on the joint, make 5–6 quick horizontal circles about 10 cm across in 3 seconds, as if you were stirring a saucepan. Then stop and hold the whole thing still and upright.

   Tip: If the funnel doesn't appear, make smaller, faster circles.

10. Watch the tornado

    Within 2–3 seconds a funnel of air forms in the centre and reaches down to the neck, and the water spirals down. Look closely and follow a foil ball with your eyes.

    Check: The funnel reaches from the surface to the neck and the water drains without glugging.

11. Repeat the other way

    When all the water is at the bottom, turn the whole thing over and repeat, now swirling the opposite way. Time the draining both times.

Does it work? The test

With 1.1 l of water and the 9 mm washer, time the draining with the bottle held still (no swirl) and then with the whirlpool.

What should happen: Without a swirl the water glugs down and takes 40 s or more (sometimes it almost stops). With the whirlpool the funnel forms in 2–3 s and the bottle drains in about 15–25 s, continuously.

If something goes wrong

• The joint leaks — Tape not tight enough, mouths wet or out of line. Solution: Dry the mouths, line up the bottles and wrap 4 more well-stretched turns right over the joint.
• The funnel doesn't form — Swirling too slowly or too widely, or the bottle moves at the end. Solution: 10 cm circles, fast, 5–6 turns, then hold the bottle still.
• The water rushes down in 3–4 s with no tornado — The washer is missing or its hole is too big. Solution: Fit the washer with an 8–10 mm hole.
• The water hardly goes down, even when swirled — The washer hole is too small (< 6 mm) or covered by tape. Solution: Widen the hole to 9 mm and check that the tape isn't blocking the opening.
• The joint bends — Not enough tape or bottles too heavy. Solution: Cover both threads with tape and, when swirling, always keep one hand on the joint and the other on the top bottle; use 1.5 l bottles.
• The funnel is hard to see — Uncoloured water or bottles with a pattern moulded in. Solution: Add colouring and foil balls; use smooth bottles.

Safety

• The full bottles weigh 1.2 kg: the youngest children should swirl them sitting down, resting the bottom one on the table or on their lap.
• Do the tests over a sink or a tray: if the joint comes apart, more than a litre spills and the floor gets slippery.
• The awl and the hole punch are used by an adult, with the lid resting on a piece of wood.
• Don't use loose glitter that ends up down the drain; keep small pieces (sequins, balls) away from children under 3.

The science behind it

For children

When the bottle is still, the water wants to go down and the air below wants to go up, but they both try to get through the little hole at the same time and push against each other: that's why it goes «glug-glug» and drains slowly. When you swirl the bottle, the water starts going round like a merry-go-round and presses against the sides, leaving an empty tunnel in the middle. The air goes up that tunnel, and the water goes down round the outside. Each one has its own path! That spinning funnel is called a vortex, and it's the same thing that happens in a tornado or when you empty the bath.

For the adult

With the bottle still, water can only leave if an equal volume of air enters through the same opening: this produces an unstable, glugging flow (bubbles rise one at a time) and a low flow rate. Swirling gives the water angular momentum. As it moves down towards the opening, its radius of rotation decreases and, because angular momentum is conserved (v·r ≈ constant, free vortex), the tangential speed increases sharply near the axis. Keeping to that curved path requires a centripetal force, provided by a pressure gradient: pressure is lower in the centre, the free surface dips and the funnel forms (roughly hyperbolic in shape). This air core connects the two bottles, air rises through the centre without interrupting the water, and draining becomes continuous and faster. Talking about «centrifugal force» is the way of describing it from the point of view of the spinning water (a non-inertial frame). The 9 mm washer limits the flow so the vortex has time to form and can be seen for 15–25 s. Atmospheric tornadoes intensify in a similar way, by concentrating rotation into a narrow column; the direction of spin in the bottle is decided by your hand, not the hemisphere (the Coriolis effect is negligible at this scale).

• vortex: a fluid spinning around an axis, forming a whirlpool.
• fluid: something that can flow and change shape, like water or air.
• centrifugal force: the feeling of being pushed outwards when something spins.
• pressure: how hard a fluid pushes; it is lower in the centre of the whirlpool.
• angular momentum: the «amount of spin»: if something spinning pulls itself in, it spins faster.

By age

• 5–7 years: The adult prepares the washer and the joint; the child fills with the funnel, adds colour and swirls. Key idea: «the air goes up the middle and the water goes down spinning round».
• 8–10 years: They build everything except the hole in the washer. Have them time it with and without a swirl and average 3 tests.
• 11–12 years: Experiment with one variable (hole or amount of water), a table and a graph. Challenge: explain the funnel using conservation of angular momentum.

In a group or in class

Pairs, one tornado per pair (ask each family for 2 identical 1.5 l bottles). Prepare the washers (1 per pair) and the cut tape in advance. Fill the bottles at a station with a washing-up bowl and jugs. 40-minute session: 10 min building, 15 min testing and timing, 15 min sharing results with a table on the board (everyone's times with and without a swirl). Set up tables with trays or do the activity in the playground.

Challenges

• Make the tornado last as long as possible by changing just one thing.
• Make a reusable connector with two bottle caps glued together and a 9 mm hole (with an adult's help).
• Add 2 tablespoons of cooking oil: what happens to the oil inside the whirlpool?
• Make a tornado in a 500 ml bottle and compare which is «faster» per litre of water.

More free projects

• Air vortex cannon
• Bottle diver
• Dry tissue under water
• Five-liquid column
• Flameproof water balloon
• Giant soap bubbles
• Layered water filter
• Paperclip floating on water
• Pierced water bag that doesn't leak
• Soap-powered boat
• Spiral that spins with heat
• Cloud in a jar
• The bottle that swallows an egg

See all projects