The basic idea: You take a large plastic bottle filled with water, then add a “diver” who floats. When you squeeze the bottle, he dives to the bottom. When you release pressure, he returns to the surface.
The science behind it: Why does it work? The diver has an air bubble trapped inside, making the density of the diver a little less than the water, so it floats (it’s positively buoyant). When you squeeze the bottle, it increases the pressure, water is forced up into the diver, and the air bubble trapped in the diver compresses (gets smaller). Water is denser than air, so this increases the mass and density of the diver, so it sinks. When you stop squeezing, the air bubble expands, forcing water out of the diver, and the diver rises. (Sources: Steve Spangler, Science Bob, Physics.org.)
The history: In 250 BC, Archimedes described buoyancy
Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.
In the 16th century, scientist and mathematician Rene Descartes is said to have invented this experiment which demonstrates buoyancy.
Options for the diver
Criteria: You need something that can contain some air or catch an air bubble underneath it. You need to be able to adjust the weight till you get it just right. It needs to fit through the bottle opening. It needs to just barely float.
- Use an eyedropper. Fill it partway with water until you’ve achieved the right buoyancy. http://www.stevespanglerscience.com/lab/experiments/eye-dropper-cartesian-diver/
- Use a pipette. Add weight to the bottom to weigh it down and to keep it upright.
- Use a pen cap and modeling clay: https://sciencebob.com/make-a-cartesian-diver/
- Use a straw and blue tack: www.physics.org/interact/physics-to-go/cartesian-diver/
- Cut a diver out of foil or heavy plastic, create an “oxygen tank” with a straw and a paper clip. Weight feet with putty as needed: http://coolscienceexperimentshq.com/bottle-diver-science-experiment/
- Use a ketchup packet: www.stevespanglerscience.com/lab/experiments/cartesian-diver-ketchup/ (or a mustard packet or soy sauce packet – all the same principle)
- One book recommends a wooden matchstick with a little putty on one end
- We used plastic straws, cut to shorter lengths, with silly putty to seal the top end, as our air tanks. Then we weighted them with paper clips. We had a variety of lengths of straw, various sizes and types of paper clips, and the putty is infinitely adjustable, so it was easy to fine-tune the variables.
Building, Testing and tinkering with the diver: Have a cup full of water to test in. Set the diver in. He should just barely float. If he sinks, remove some weight or add more air. If he tips over onto his side, add more weight to the bottom. Keep adjusting till you’ve got him just right. This is the hard part…
Assembling the bottle: Use a plastic bottle, like a one-liter water bottle. Remove the labels to get a good view of the dive. Fill it almost all the way. (If you fill it all the way, then when you add the diver, he’ll displace some water, and it will spill out.) Gently add the diver. Screw lid on tight. Squeeze to test. (You’ll have to squeeze hard.)
Have fun tinkering!