Eye Dropper Cartesian Diver

Here's What You'll Need

A plastic soda bottle with a cap
A glass eye dropper
Water

Let's Try It

Fill the plastic soda bottle to the VERY top with water.
Fill the glass eyedropper about 1/4 full with water. You may need to experiment with the amount of water in the eye dropper, as it should be just enough to make it so the eye dropper is barely floating.
Place the eye dropper into the soda bottle. The eye dropper should float and the water in the bottle should be overflowing. Seal the bottle with the cap.
Gently squeeze the sides of the bottle and notice how the eye dropper (called a diver) sinks. Release your squeeze and it floats back up to the top.
Squeeze again and observe the water level in the eyedropper (it goes up).
Practice making the diver go up and down without making it look like you’re squeezing the bottle. Amaze your friends with your ability to make the eyedropper obey your commands!

How Does It Work

Squeezing the bottle causes the diver (the eye dropper) to sink because the increased pressure forces water up into the diver, compressing the air at the top of the eye dropper. This increases the mass, and density, of the diver causing it to sink. Releasing the squeeze decreases the pressure on the air at the top of the eyedropper, and the water is forced back out of the diver, lowering its density and allowing it to float back to the top of the bottle.

Science Fair Connection

Making an eye dropper move up and down is pretty cool, but it isn’t a science fair project. You can create a science fair project by identifying a variable, or something that changes, in this experiment. Let’s take a look at some of the variable options that might work.

Try changing the size of the plastic bottle or eye dropper. Does the size affect how hard you have to squeeze to make the diver move?

Try changing the water temperature. Do different temperatures affect the density of the diver?

That’s just a couple of ideas, but you aren’t limited to those! Try coming up with different ides of variables and give them a try. Remember, you can only change one thing at a time. If you are testing different bottle sizes, make sure that the other factors are remaining the same!