If you make a Cartesean Diver, be sure to fill the dropper partly with water and adjust the amount of water so this the dropper just barely floats.

Why does the dropper sink when the capped bottle is squeezed?

The water, being a liquid, is incompressible, but the air, being a gas, is compressible. When pressing on the side of the bottle, the gas compresses which allows water to rise into the dropper making it more dense and it sinks.

Does the dropper rise when the pressure released?

The answer to this is that it depends on how delicately balanced the dropper is at the surface. If just barely floats, the pressure of the water above the dropper when it becomes submerged can keep the air compressed. If the air stays compressed the density will remain high and the dropper will stay submerged.

Scuba divers wear a buoyancy control device called a BCD. The BCD is an inflatable life vest. As the diver submerges, his density increases because the air bubbles in the wet suit become compressed. The diver becomes more dense the deeper the dives. To compensate, the BCD is inflated just enough to adjust the diver's density so the he or she neither floats nor sinks at the depth of the dive. When returning to the surface, the extra air in the BCD expands. During assent, the diver's density decreases and he tends to float faster and faster towards the surface. The extra air is slowly vented to prevent the assent from occurring too fast. Dive instructors are sometimes willing to visit a class to talk about diving.

Variations on the Cartesean Diver:

• Place a dropper that just barely sink into a sealed bottle with a rectangular cross section. Squeeze on the short side which causes the long sides to bulge making the diver rise instead of sink
• Seal the bottom of the dropper so that water can not get into the dropper and suspend like a conventional diver. Squeeze the bottle and the dropper still sinks. However note that it sink because the rubber bulb compresses under pressure. The density of the conventional diver increases because of increasing the mass of the dropper. The density of the modified diver increases because of decreasing the volume of the dropper.
• Seal the bottom of the dropper AND place a hard plastic sleeve into the rubber bulb to keep if from collapsing.

  Note: On both variations that are sealed, it will be necessary to tighten the seal of the bulb to the glass with a tightly wrapped wire to keep water from seeping into the dropper at the glass rubber seal when the bottle is pressed.

feedback: bill_switzer@ncsu.edu