How tornadoes form in bottles

By Jonathan Ram, Year 12

We’ve all seen the cool party trick: someone swirls their bottle to create a tornado of fluids inside. But what actually is the science behind this?

Turns out we can actually see little swirls/tornados in bottles with little to no effort. When you pour water outside of a bottle simply by holding it upside down, water comes out in sections, then air goes in, which creates the glug,glug,glug sound until the bottle is empty. The basic principles behind this is simple: air needs to go into the bottle and water needs to come out. The only problem with pouring a bottle out like that is that the air can’t come in at the same time as the water going out, which results in the pour being very time consuming and inefficient.

Now, try this. Fill up a transparent bottle and flip it upside down, but instead of just holding it straight, first spin the bottle around laterally. You will notice that the water comes out significantly faster. Another thing that can be noticed is the spiral or tiny tornado which forms in the centre of the bottle. What happens in this situation is that a vortex is formed inside the bottle due to the initial rotation that is given. As the bottle is spinned, the water at the top of the bottle (the part that is being held) begins to rotate due to friction between the water and the plastic. As gravity pulls that water down, the angular momentum of the water stays the same, only over a smaller area, which means it is moving faster. This effect is called a vortex and the benefit is that it allows air which needs to go into the bottle to pass through the middle of the bottle (the vortex), and the water to move around the vortex. This creates a balance between a steady air inflow and steady water ejection.

There are other methods of getting a bottle to pour a liquid out quickly, such as with a straw, which has the same concept as the vortex in the sense that water goes out at the same time as air going in. 

To conclude, just remember that whenever you need to pour liquids out of a bottle quickly, just add a little bit of spin — don’t be afraid, as this tornado is harmless!