April 15, 2026
Description
You can find an up-to-date list of Flexify designs here: https://www.printables.com/@FlyingGyroscope/collections/3017828.
Explore buoyancy and density with a swimming jellyfish! This project is a creative take on the classic Cartesian Diver. Out of water, the flexible arms make a fun fidget, and underwater they make a great counterweight. They also have a hidden secret. The ends are hollow and sink at a slightly slower rate than the rest of the arm. This creates a life-like swimming motion as the jellyfish rises and falls!
Every now and then, I ran into inconsistent bouyancy after the first couple of dives. I suspect that some amount of water was stuck on the flat bottom in the internal cavity, unable to drain. I modeled a tapered bottom (while keeping the overall volume the same) for more reliable draining.
Inspired by a model of an extra long flexible cat, I thought it would be fun to make fidgets with longer arms. These do not have the correct density for a Cartesian diver – they are just for fun.
Water bottle with tight fitting lid
20mm diameter (or larger) opening at the top
Water
3D printed jellyfish
Fill the bottle with water.
Gently drop the jellyfish into the bottle. If it tips over from a rough landing, it may not swim properly. If lots of large air bubbles collect on the jellyfish surface, tap the bottle and jellyfish to dislodge them.
Put the cap on tightly.
Now squeeze the bottle and watch the jellyfish sink! Let go and watch it float back up.
A complete explanation can be found here: https://en.wikipedia.org/wiki/Cartesian_diver.
The jellyfish is hollow and a hole on the bottom equalizes pressure between the surrounding water and a pocket of trapped air. The density (mass per volume) of the jellyfish and the air pocket is less than the density of water, so it floats. (In more technical terms, the jellyfish displaces a weight of water that is greater than its own weight.)
When you squeeze the bottle, pressure forces water into the jellyfish. The air inside is compressed and takes up a smaller space, creating room for more water. The compressed air and extra water increase the density of the diver. With enough pressure, the density becomes greater than the density of water and it sinks. When you stop squeezing, pressure and density return to normal and the jellyfish floats again.
The perfect Cartesian diver needs to barely float in water. If it is too light, you will not be able to generate enough pressure to sink it. If it is too heavy, it will never float.
The jellyfish was designed for PLA filament with a density of 1.22 g/cm^3. Do not worry if your filament is not an exact match. I describe how to make adjustments in the next section. Start by copying my print settings:
0.4 mm diameter nozzle
Arachne perimeter generator
0.2 mm layer height
4 perimeters
6 top and bottom layers
50% rectilinear infill
For fully functioning print-in-place arms, you need a good first layer and good printing tolerances. The arms on mine loosened up after playing with them for a while.
If your jellyfish will not sink when you squeeze the bottle, you need to increase density:
Add more infill
Use a shorter jellyfish with a smaller air pocket
Glue extra weight onto the jellyfish
If your jellyfish will not float, you need to decrease density:
Reduce infill
Use a taller jellyfish with a larger air pocket
Over time, water and air can slowly move through the tiny gaps between extrusion lines. With my test prints, I observed a noticeable change in density after a day soaking in water. You should see consistent results by only using it for a few hours, then letting it dry out.
To help the jellyfish stay airtight, consider adjusting some settings:
Increase extrusion multiplier
Be mindful of print-in-place tolerances
Increase infill/perimeter overlap
Decrease seam gap
Thanks for visiting, and enjoy!
License:
Creative Commons — Attribution — Noncommercial — Share Alike
Flying Gyroscope
