Water Drop Trampoline

Water you doing today? Add some bounce to your bubble and some dance to your drops with the water drop trampoline! Columns of air moving at different speeds let you float drops in a hovering boogie.

We set out to make this as a homemade version of one of our favorite exhibits at the Exploratorium, "Dancing Drops." Now you can have it in your own classroom or home for the cost of almost free. Just the relational cost of stealing your housemate's blowdryer. :)

This is a great introduction to fluid dynamics (which also involves gases), and dealing with pressure and flow. Let's go!

• What: Water Drop Trampoline
• Concepts: fluid dynamics, flow, pressure, volume, surface area
• Cost: $2 + blowdryer
• Time: ~ 30 minutes
• Materials:
  • Skinny straws (use a bunch, we used 26)
  • Fat Straws (use a bunch, we used 17, but want more)
  • Rubber Bands x 4
  • Tube (we used mailing)
  • Blow Dryer (more CFU air power the better)
  • Duct Tape
  • Screws
  • Wood for stand
• Tools:
  • Eyedropper
  • Drill press
  • Drill

H2Okay! Let's bounce!

Grab a bunch o' skinny straws and rubber band them together at the top and bottom. you'll find with some mushing you can assemble them into a fairly circular pattern. We used 26, but you can adjust depending on the size of your tube and power of your blowdryer.

Nerd note: This will be the interior where the water drops will be suspended. Air will move more slowly through the skinny straws than the fat ones due to greater a higher ratio of drag from the sides of the straw compared to the air column inside. This is interesting and sometimes confusing as it is kind of the opposite of the thumb-on-the-hose effect on pressure. Check out this fluid dynamics page to start learning more.

Attach a wad of fat straws onto your skinny straw bundle. We found that it was easier to mush them into even position after we rubber banded them on both ends. For an even more improved version, add an extra layer or two of fat straws. Of course, you'll have to make sure your blowdryer is powerful enough to suspend water at that volume.

The fat straws with faster moving air will create a wall effect, correcting the water drops toward the middle.

Stick your straw bundle in your tube, and if all's right, friction will hold it there. You can adjust the number of straws inside depending on your tube, until you get a good fit. Push the straws ends down to just below the surface of the tube.

We used a mailing tube, but grab any tube you got and test it out with your blowdryer. You'll find that a greater number of fat straws on the rim will really keep those drops more stable.

If you need to get them back out, poke from the other end. If you need to after we attach the blowdryer, try some pliers.

First, tape on your straw tube. Then after some experiments, choose the settings you want, and we taped those down for student use. We knew that we always want cool air (so there's no melting of stuff), and we chose to always have the "Turbo" on. Who doesn't want turbo?

We left the switch open for slow, fast, and off so that we could experiment with different flows rates.

Safety Note: Make sure that you understand your blowdryer. Keep the heat off, and you can even disconnect it if you feel savvy enough. You want to make sure that if any water should escape down into it (which is rare, but technically possible) that you will be okay. You can also add some things like mesh grates or a p-stop if you want to be extra certain. If any problems should occur, turn off your blowdryer, and unplug it. Yay safety!

There are many ways to make a base, and not all have to be out of wood. Here is one example of ours. Measure your blowdryer (we did this before taping), and choose a distance a little longer to be able to support the tube. We chose 10" in height with a 6" crossbeam. Cut your wood, and sand!

Drill a hole that's large enough for your tube to fit through. You will probably have to use forstner bits, or cut in with a jig saw to make a hole big enough.

Huzzah! Throw in a couple screws, and your base is ready to rock! Stick your blowdryer up through it so you get a nice vertical water drop trampoline. Add support if necessary depending on the shape of your blowdryer.

You did it! Angle your eyedropper at an angle, make some drops, and pull it away to watch them dance and bounce.

You can see the flat bottoms of the droplets as they move, and eventually get propelled outward, disperse, or drop in to the straws. Play around with the flow settings and straw set-up to get different results.

If you're looking around, this is a great first system of fluid dynamics and here is a great follow-up resource on aerodynamics.

Happy trampolining!