Cartesian Diver Project
Description:
This project creates a miniature diver which can be placed within a plastic bottle that has been filled with water, and then this miniature diver can be made to sink and resurface by sealing the bottle with it inside and squeezing and releasing it's sides. As the pressure changes, you can easily observe the effect on the volume of air within the diver. Cartesian divers (as they are known) can be made with simple equipment, materials, and a bit of imagination.
Origin:
This assembly is formally known as a Cartesian diver -- after Rene Descartes, a 17th century French mathematician who used them to explain various principles of density and buoyancy.
One simple way to explain this phenomenon is this: If we consider the assembly (diver) to consist of the dropper, the copper wire and both the air and water inside, then as we squeeze the bottle, we are forcing more water up into the assembly, since the air pocket inside the dropper is compressible. This action adds to the assembly's mass without changing it's volume, thus increasing it's density.
So, as the diver's density increases, it soon surpasses that of the surrounding water, and the diver sinks. When the pressure is released, the compressed air pocket inside the diver's body cavity pushes the extra water back out, and the diver assumes its original density and floats back up to the surface.
Scientific Concepts:
Whether an object floats or sinks in a fluid depends on whether that object's density is less than or greater than the density of the fluid.
Density is equal to mass divided by Volume or D = m/V. Therefore, if you add to the mass of an object without changing its volume, the object's density increases. If you decrease the object's volume while keeping the mass constant, the density of the object will then increase as well.
Boyle's Law: as the pressure on a gas sample is increased, it gets compressed into a proportionately smaller volume.
Whereas gases are easily compressible, liquids and solids are not.
Purpose:
Home Experiment
These divers may be used as a demonstration tool or as an experiment to correlate with initial discussions of density, buoyancy, and or the compressibility of gases, liquids, and solids. The divers may be used to explore the gas laws.
Even though the diver presented is amusing and educational, the real challenge in creativity comes in trying some unique variations.
Materials:
2-Liter plastic soda bottle
large plastic cup
small disposable plastic dropper
thin gauge bare solid copper wire
scissors
small pliers
push pin
permanent marker
*optional - plain paper (for background)
*optional - colored pencils (for background)
Safety:
Adult supervision is recommended when using scissors for cutting the plastic materials and wire.
Build Time:
5 minutes to prepare materials.
20 to 30 minutes to assemble and detail divers.
2 minutes to calibrate.
5 to ?? minutes to dive, observe, and explore!
Assembly:
The tip is removed from the disposable dropper and all liquid contents are completely drained.
The excess plastic tab is removed from the top of the dropper using scissors, taking care not to puncture the cavity.
Two small holes are made in the sides of the lower portion of the dropper near the opening, using a push pin.
A 4 inch piece of copper wire is pushed through the first hole and out the second so that there is equal length remaining on each side of the dropper.(It's important that the holes are small enough to remain air tight with the wire inserted)
Each tail of copper wire is then folded to approximately 1/2 inch in length forming limbs.
A separate 3 inch piece of copper wire is wound around the length of each limb forming the two legs. (any excess copper is trimmed)
Two more holes are made in the sides of the center portion of the dropper, using a push pin.
Once again, a 4 inch of copper wire is pushed through the first hole and out the second so that there is equal length remaining on each side of the dropper.(It's important that the holes are small enough to remain air tight with the wire inserted)
Each tail of copper wire is then folded to approximately 1/4 inch in length forming limbs.
A separate 3 inch piece of copper wire is wound around the length of each limb forming the two arms. (any excess copper is trimmed)
Once all the limbs are complete, it's time to finish detailing the suit.
A permanent marker is used to detail a simulated diver's helmet window and deep sea pants.
Calibration & Dive Procedure:
Use a large plastic cup as a floatation calibration tank. Fill the cup with water, then place the completed diver assembly into the water and observe that it remains in an upright position with the copper wire acting as ballast. If the diver floats, squeeze out some of the air and draw some water up into the dropper. Now check the buoyancy. If you draw up too much water, the assembly sinks. If this happens, simply lift it out of the water, squeeze out a few drops of water, release the squeeze to allow air back inside. Using these techniques, adjust the amount of water in the assembly so that it just barely floats. This will fine-tune the assembly's density to make it slightly less than that of the water. (If the diver initially sinks to the bottom, trim a slight amount of the copper wire equally from the ends of both arms or both legs)
After adjusting the water level in the diver, be careful not to squeeze out any water while transferring the diver from the cup. Place the diver in a 2-L bottle, which is filled completely with water, and screw on the cap securely. As you squeeze the bottle, observe the diver sink to the bottom, and then rising back to the surface as you release the pressure.
Project tips:
Copper wire is recommended. Other metals may not bend and shape as easily or may end up oxidizing or rusting over time.
It's more convenient to adjust the density and to test for flotation in a cup of water, rather than in the bottle itself, since retrieving the diver from the bottom of the cup would be easier.
It is helpful to fill the 2-L bottle completely with water. That way, when you squeeze the bottle, all of the pressure goes into compressing the air pocket in the diver and isn't wasted in compressing a large air space at the top of the bottle.
Factoid:
Submarines also use the principles observed in this diver.
In order for submarines to submerge, pumps compress air and allow water into onboard tanks, essentially increasing the submarine's density. Then for the submarine to surface, air is allowed to expand back to its original volume and expel the water which makes it more buoyant.
Tags:
gases, submarine, pressure, volume, Cartesian divers, buoyancy, gases, liquids, compressibility