As a little kid, maybe five or six, one of my first remembered moments of heightened mechanical curiosity was over a Dippy Bird that somebody gave us. I have this very vivid memory of being utterly fascinated by it and wondering how it worked. The answer has been decades in coming. Here, MAKE Contributing Editor Bill Gurstelle, talks about Methylene Chloride, a plastic weld, and its use in powering Dippy Birds.
Methylene Chloride is the bonding agent I used to attach one piece of polycarbonate plastic to another piece when I was constructing the firepiston (see Feb 13 post in this blog.) MC works well because it’s thin and penetrates into seams well and does a good job of dissolving the plastic so it solvent welds together.
Coincidentally, I found out, while researching that methylene chloride is the same stuff used in the Dippy Birds to make them go up and down. The science of Dippy Birds, according to the How Stuff Works website, are this:
1. When water evaporates from the fuzz on the Dippy Bird’s head, the head is cooled.
2. The temperature decrease in the head condenses the methylene chloride vapor, decreasing the vapor pressure in the head relative to the vapor pressure in the abdomen.
3. The greater vapor pressure in the abdomen forces fluid up through the neck and into the head.
4. As fluid enters the head, it makes the Dippy Bird top-heavy.
5. The bird tips. Liquid travels to the head. The bottom of the tube is no longer submerged in liquid.
6. Vapor bubbles travel through the tube and into the head. Liquid drains from the head, displaced by the bubbles.
7. Fluid drains back into the abdomen, making the bird bottom-heavy.
8. The bird tips back up.
Methylene chloride is also used, apparently, in decaffeinating coffee. The MSDS says the stuff is somewhat dangerous, but apparently, not so much that it cannot be used in Dippy Bird toys – at least until someone complains.