How does a Dippy Bird work?

by | Oct 24, 2010 08:00 PM ET

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Hi, I'm Marshall Brian with today's question: how does a Dippy Bird work? A Dippy Bird is a toy that you'll frequently find at toy shops or novelty shops. They cost about $5 and they look a little bizarre. A Dippy Bird has the following parts: first, two equal size hollow glass bulbs, a long glass tube that connects the bulbs, a fuzzy water absorbent material covering the head, and often some kind of little top hat on the head as well, two plastic legs with a pivot connection and then some methylene chloride in the abdomen. This is usually a reddish liquid. In real life, methylene chloride is an industrial paint stripper and a solvent.

One thing that dissolves easily in methylene chloride is caffeine. So you'll often see it used to decaffeinate things. Methylene chloride helps make a Dippy Bird work because it evaporates easily. It boils at just 100 degrees Fahrenheit or 40 degrees Celsius. To operate a Dippy Bird you get its head wet. As the water evaporates off the head, fluid moves up into the head causing the bird to become top heavy and dip forward. Once the bird dips forward, fluid moves back into the abdomen causing the bird to become bottom heavy and tip back up.

It'll tip up and back, up and back essentially forever as long as there's water that can cool its head and the room is at room temperature. Here's how a dippy works. When water evaporates from the fuzz on the Dippy Bird's head, the head is cooled. The temperature decrease in the head condenses the methylene chloride vapor that's floating around in the tube anyway decreasing the vapor pressure in the head relative to the vapor pressure in the abdomen. The greater vapor pressure in the abdomen forces fluid up through the neck and into the head.

As the fluid enters the head, it makes the Dippy Bird top heavy. The bird then tips. Liquid travels to the head and the bottom of the tube is no longer submerged in the liquid in the lower bulb. Vapor bubbles travel through the tube and into the head. Liquid drains from the head displaced by the bubbles, and the fluid drains back into the abdomen making the bird bottom heavy. Then the bird tips back up and starts this cycle over again. If the bird dips into a cup of water so its head stays wet, the fuzzy material absorbs water over and over again and the cycle just keeps repeating. Your warm hand can cause fluid in the Dippy Bird to rise as well. If you hold a Dippy Bird upright in your hand touching the abdomen, the following happens: your body heat warms the fluid in the abdomen; the heat increases the vapor pressure in the abdomen relative to the head, the reverse of what happens when you get the head wet; the fluid rises into the head in response to the pressure difference, moving from high pressure to low pressure; the bird becomes top heavy and it tips forward. Either way, you've created a little heat engine when you look at a Dippy Bird that's using the evaporation and condensation of a liquid to cause motion.


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