Thursday, April 09, 2009

Methylene Chloride and Dippy Bird Science



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 dippy bird physics, 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 decaffinating 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.

4 comments:

brain said...

also seen in those bubbler christmas tree lights!

Jim Senft said...

I was also fascinated by these "drinking" birds as a kid. My first sighting was of a whole flock of them bobbing away in a store window. I still retain a partial image of that sight. As a young adult, knowing then how they worked, I was driven to make one. And it was successful, even though my glassblowing skills were less than professional! I used ordinary acetone as the working fluid. I also made some "bubblers" that worked at room temperature using evaporative cooling at the top end.

Bill Gurstelle said...

Jim, if you have a picture of your home made dippy bird I would love to see it! Please contact me using the form at www.williamgurstelle.com

LikeAnIbis said...

While working at a print shop where we used MC to clean ink from presses, a 5 gallon container was left in the sun on a summer day. When we started to unscrew the cap, it started hissing, and so, rather than wait for the pressure to equalize, a co-worker ducked down and spun the cap off quickly. The resulting geyser blew the cap about 10 feet high, and vaporized instantly. Really. There was only about 2 gallons left in the container, and not one drop hit the ground. It's the mental picture I form when I hear the word 'volatile'.