Water Drop Trampoline




Introduction: Water Drop Trampoline

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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!

Step 1: Skinny Straw Bundle

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.

Step 2: Fat Straw Wall

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.

Step 3: Tube Time

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.

Step 4: Blowdryer Taping

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!

Step 5: Make a Wood Base

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!

Step 6: Drill a Hole

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.

Step 7: Screw It All Together

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.

Step 8: Water Bounce Time!

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!

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    5 years ago

    Oh, I would definitely use a curved tube to direct the airflow into the tube without having the blower directly below. There isn't any danger from a drop or two getting into the electronics (detaching the heating element won't help in that regard, BTW -- if you spilled a large amount of water you'd still have a problem) but the moisture would increase the rate of corrosion of the metal parts. And an accidental spill could lead to a short, which even if not too dangerous could cause problems such as ruining the blower, popping a circuit breaker, etc. Also adding a dimmer switch for smoothly varying the speed would be great.

    Thoughts on experiments to try:

    Dropping with different sizes of eyedroppers or even syringes,

    from different heights,

    using (safe) fluids other than water, such as different weights of mineral oil

    using a non-newtonian fluid (water with cornstarch)

    THREE or more different sizes of straws for more graduated flow

    Varying the length of the straws -- straws go all the way down in the middle for slowest flow, the outside straws very short at the top. Might be able to use all the same diameter straws this way. and graduate each "circle" of straws.

    Oh, what am I doing? Part of the fun of science is figuring out new things to test and new ways to test them! Have fun, kids! And remember, measure, record, and take notes!

    Just wanted to say you're doing a super job with these fun and lively Instructables!


    7 years ago on Step 2

    It seems to me that if any of those drops managed to fall down into the hair dryer, you'd have a fire hazard at the least, and probably also an electrocution hazard.

    I'd judge this to be an unsafe project.

    You could fix it by adding a p-trap between the straws and the blower, though.

    The Oakland Toy Lab
    The Oakland Toy Lab

    Reply 7 years ago on Introduction

    Hi SumGie!

    It's great that you added a cautionary note. For us, we made it so that the heating element is off, and thus with our hair dryer, the air mover is purely mechanical if water should make it past the straws.

    So not a danger here, but we'll add a safety note for sure to make sure everyone's safe out there! Thank you much for the comment!


    7 years ago on Introduction

    Really cool idea! I wonder if you make the area of the straws bigger with a larger volume of air rushing by if you could get a water droplet to float for longer.

    The Oakland Toy Lab
    The Oakland Toy Lab

    Reply 7 years ago on Introduction

    Hey tomatoskins, yes! And please experiment with it. More air flow with a bigger volume could make a wonderful water trampoline. We'll do an experiment soon, but if you come up with something in the meantime, let us know!