A short test tube submerged in water (open end down) and is mostly flooded. It floats or sinks in a large acrylic cylinder depending on the amount of pressure applied by the syringe that is connected through an airtight rubber stopper at the top of the cylinder.

Materials:

• Clear container (shown above) filled with water
• Secondary container for water
• Air-tight cap for container with rubber stopper
• Screw driver for removing and tightening the cap
• Short test tube with red tape (Cartesian diver)
• Syringe

Demo:

1. Fill the cylindrical container with water.
2. Float the test tube such that there is an air bubble but the diver still remains upright.
3. Once the diver is floating, place the cap on the container tightly and tighten the metal band using a screwdriver
4. Insert a syringe into the rubber stopper in the cap and use it to push air in or out, causing the diver to sink or float.

Explanation:

Before the syringe is compressed, the diver is buoyant in water because the air bubble in the test tube makes the diver’s overall density less than water, as shown in Figure 1. As the syringe is compressed, it increases the pressure inside of the water-filled container. Water is an incompressible fluid, so the water must transfer the force applied on it by the increase in pressure to the bubble inside of the test tube. This forces water into the test tube, compressing the air bubble, and hence increasing the overall density of the diver, as shown in Figure 2.

If the pressure applied on the system is high enough, the diver’s density will drop below the density of water. The buoyant force will no longer be strong enough to keep the diver afloat and the diver will sink to the bottom of the pressurized container. If the pressure is released by sucking air out with the syringe, the diver’s density again be less than that of water, as the water leaves the diver and allows the air bubble to expand, allowing the buoyant force will thus push the diver back to the top of the water level.

Notes:

• If the diver flips over and sinks to the bottom, use a secondary container to pour out the water and retrieve the diver.
• Be careful not to jostle the apparatus too much while the diver is in the tube, as it might flip over.