The Dunking Bird

He's rather a silly looking creature, this bobbing bird with the fuzzy head and bulbous glass rear. In apparent contradiction to all the laws of physics, he ducks over to put his head into a glass of water, then immediately flips back upright, barely rocking. But the rocking, instead of dying away, becomes steadily wilder until he again tips forward into the waiting glass. Generations of children have been amused by his antics, and generations of their parents have speculated on perpetual motion machines and similar violations of the laws of physics.

Perpetual motion machines (which don't exist) are those that try to violate the second law of thermodynamics, the one that is sometimes translated as "You can't even break even". (The first law is "You can't win.") What the second law is really saying is that in order to change heat (random molecular movement) to work (organized large-scale movement), you have to transfer heat from a warmer object to a colder one. No temperature differences, no work. Usually, we produce the temperature difference by heating something - the mixture of fuel and air inside the cylinder of a car, for instance. The dunking bird produces the necessary temperature difference by evaporative cooling of his head, but this does not violate the second law of thermodynamics.

But what makes the dunking bird keep dunking? The bird's body is made up of a glass tube with a bulb on one end (the bird's head), with the tube extending into another glass bulb at the lower end. The whole thing is about half filled with a liquid that has a low boiling point, with the rest of the bird being filled with the vapor. When the bird is upright, the vapor in his head does not connect with that in his tail.

Start with the bird horizontal and sticking his beak into his water dish. The fuzz on his head rapidly wicks water over the entire surface of the glass head. At the same time, the horizontal position of the bird's body allows the two vapor pockets to connect, so that the liquid in the body can flow freely. The bird is designed so that most of the liquid flows into the lower half of the bird, making him tail-heavy so he sits up.

The head, however, is now wet, and cools by evaporation. The pressure exerted by the vapor of a liquid near boiling is very sensitive to temperature, so the pressure in the cool head of the bird decreases, and the higher pressure in the tail forces the liquid higher and higher in the bird.

The bird was rocking a little before the cooling really got started, just as a reaction to flipping back upright. He now acts like a pendulum that is being shortened -- both the speed of his rocking and the angle of his swing increase. Eventually, so much liquid is sucked up into the head that the bird actually becomes top-heavy, and tips over for another drink. Of course once he is horizontal, the two vapor chambers are able to equalize pressure, and the liquid again flows back into the bird's bottom.

What keeps the bird from falling over on his back instead of flipping forward? The first one we tried to photograph did exactly that! The solution is to adjust the pivot points so that the bird is swinging around a center that is very slightly behind the center of his tube body. This gives a neutral (head dry) position with the head tipped slightly forward, and when the bird overbalances he falls forward.

The dunking bird is just one of many toys based on the principles of physics and chemistry. Have fun figuring them out!