- 2 plastic bottles, with screw caps
- thin plastic tubing, such as the ink tube from a ballpoint pen, or tip of a narrow plastic syringe
- drill (or hand-driven drill bits)
Rescue some plastic bottles with screw caps from the recycling bin and rinse them thoroughly.
Glue two screw caps back-to-back (we used hot glue, which is probably not the best choice, but has held up so far). Now you can poke a hole through both caps... which doesn't work terribly well. It turns out that if you only poke a single hole, you have to make it fairly large, because as the liquid wants to drip down, the air in the bottom bottle wants to go up, so if you poke a small-ish hole, surface tension will stop the whole thing from working!
We found out that it is much more effective to drill or poke two holes through the caps, and glue in place two little tubes, one pointing up, the other pointing down. That way there is a separate route for liquid to drop down, and one for air to bubble up.
You may need to slice the tubes at an angle to prevent surface tension from stopping the flow of drops. We used the ink tube from an empty Bic pen first, which worked but did not give a very reliable stream. Searching around for some other plastic tubes to use, we stumbled upon a box of very narrow plastic syringes in the lab. The tip of those syringes proved perfect for our purposes (see picture). Have a look around! You might stumble across something that works just as well - maybe the dispenser tip of a tube of superglue?
Fill up one of the bottles with your liquid of choice (for demonstration purposes, we used water with some green food coloring). Screw on the double cap, and screw the empty bottle on top of that. Invert and watch the show...
This prototype, with the syringe tips, took about 9 minutes to empty, and ran very reliably.