Agulhas and Somali Current Large Marine Ecosytems Project

If you'd like to make your own Cartesian Diver, read on.

But first, some background science!

Density is the mass of an object per unit volume.
Buoyancy is the force that is applied by the water displaced by the object;
- positively buoyant objects float - they displace as much water as they weigh when floating at the surface (or on their way there);
- negatively buoyant objects sink - they displace less water than they weigh;
- neutrally buoyant objects can “hover” when they have the same density as the fluid surrounding them.

An object which can vary its density (which a cartesian diver does) is able to rise, fall and even hover at a particular depth in water. Argo floats use variable density to travel through the water column, as do submarines. Most fish and even SCUBA divers find variable density helpful to get around under water too!

Squeezing the bottle will result in the straws sinking, once enough water enters through the bottom of the straw to make the straw more dense than the surrounding water. Essentially, the increasing pressure inside the bottle as you squeeze it compresses the air inside the straw, allowing water to enter. The straw displaces less water, becomes more dense, and sinks.

As you probably know, gases (like air) are very compressible (can be squeezed into a smaller space), whereas liquids (like water) are relatively incompressible. In the cartesian diver experiment, squeezing the bottle can't really compress the water, but the air spaces inside the straws are compressible, resulting in the air spaces obeying Boyle's Law and becoming smaller through the increased pressure you apply on the bottle. (Remember from Boyle's Law that increasing pressure will reduce the volume of a gas?). If your straw is fairly clear, you'll probably be able to see the bubble inside decreasing in volume as you squeeze.

Stop for a moment and think how a very very heavy object can be made to float on water - something like an enormous ship.

It's quite simple - if the object is able to displace as much/more water than it weighs, then it will float. A solid block of a metal (like steel) will sink, but if you make it less dense (by making it hollow, for instance, and full of air, which is much less dense than water (or steel!), you can make the object effectively less dense. This is why boats have a fairly thin hull with a lot of air inside (boats are mainly empty space) and tall sides - they push as much water out of the way as they weigh before water is able to come over the side - but, of course, if you get a hole in the boat or enough water comes over the side, it will eventually sink! So, even though boats can weigh many thousands of tonnes, they push just as many thousands of tonnes of water out of the way to ensure they float.

How to make a Cartesian Diver

You'll need:

• Squeezable bottle (2l fizzy drinks bottles are excellent)
• Straws
• Plasticine, Prestik or any modelling clay
• Sticky tape
• Scissors
• Take away Ketchup packets (optional)
• Water

Instructions:

1. Fill the bottle with water right to the top with water
2. Cut the straw to about a third of its normal length
3. Bend the top over a bit and secure it with sticky tape (to stop the air leaking out of the top)
4. Put some Plasticine or Prestik around the bottom of the straw, taking care not to cover the hole
5. Test the completed straw in a bowl of water; if it sinks to the bottom, remove plasticine/prestik until it floats with the top just above water level; if it floats too high in the water, add more plasticine/prestik

Other things to try:

• Try different lengths of straw. Which ones sink and rise the fastest?
• Try half filling the staw with water while you're building it (at the 3rd step above, just before you bend the top over and tape it - make sure the straw is dry before you try to tape it!
• Try to get your straw to hover somewhere in the middle of the water, motionless. You'll need to vary the pressure; if it sinks to the bottom, squeeze a bit less hard (slowly release the pressure and watch the straw rise; squeeze slightly harder to get it to stop where you want. If you achieve this, you straw is neutrally buoyant at the depth it is hovering.
• Bend a straw in half and connect the two halves with a paperclip with some plasticine/prestik as a weight hanging off the paperclip. Click here for diagram.
• Many ketchup or other condiments in small plastic packets act as cartesian divers too - you'll have to experiment with some until you find ones that float just right and can sink. Try different sauces like vinegar and barbeque or peri peri if you can find them. Not all will work because they don't all have the right density and air content to work - a fun investigation!
  Simply stuff one through the mouth of a bottle, put the cap on and squeeze!
• Try using an eye dropper as a Cartesian Diver