The physics behind the classic thermodynamic drinking bird

For centuries, students and inventors alike have been intrigued by the idea of a perpetual motion machine. Alas, the second law of thermodynamics has held up to the test of time. It can be written in several forms but Rudolf Clausius may have said it best for our purposes: in an isolated system, a process will only occur if it increases the total entropy of the system. In other words, heat will not naturally flow from a body of lower temperature to one of higher. It will however, flow in the other direction. So what does all this have to do with our classic drinking bird? The answer: plenty. Couple this law of thermodynamics with Boyle’s law stating the inversely proportional relationship of temperature and pressure relating to volume and you can begin to understand how this magical little bird can seemingly bob up and down forever.

Our thermodynamic, entropy-loving, pressure-, temperature-, and volume-driven machine (the bird) is quite a fascinating creature. Most machines — refrigerators, cars, nuclear reactors — produce work by creating this temperature and pressure differential. Perhaps by igniting a combustible gas, using an electric motor to compress a gas, or by splitting an atom. The bird creates this differential by dipping its beak in a glass of water. Not as intellectually exciting as smashing electrons and protons, but a temperature differential nonetheless.

Just exactly how does our drinking bird do it? First, he is made of two glass bulbs connected with a glass tube. The top bulb (the bird’s head) is a simple reservoir with the tube extending from that bulb down most of the way into the lower bulb (the bird’s belly). The system is partially filled with a liquid of low boiling point. When in an upright equilibrium position, the fluid is in the lower bulb and the vapor between the lower and upper bulbs is separated. In this position there is no temperature differential between the two bulbs.

But who wants a bulbous glass bird at perfect equilibrium? The trick is to change the temperature differential between the head and belly. This could be done by either warming the lower bulb (the body heat from your hand would do the trick) or cooling the top bulb. We’ll get our feathered friend started by wetting his head (cooling the top bulb).

Since his head and beak are covered with a thin felt that wicks the water around the bulb when he takes a drink, the subsequent evaporation cools the bulb and creates a temperature (and thus a pressure) difference between the bulbs. With a lower pressure in his head, the fluid starts rising from the lower bulb – there’s that second law of thermodynamics again with the system naturally tending toward an increase in entropy.

When enough fluid has collected in the top reservoir, the center of gravity has changed enough that the bird starts leaning forward. Right about the time he becomes horizontal, the tube in the lower bulb is no longer obstructed by the liquid in the lower bulb and the two pressure chambers equalize allowing the fluid to drain back down to the lower bulb. Now the trick that keeps it going is that when the bird was horizontal, it dipped its beak into the glass of water, wicking more fluid around the top bulb, causing it to cool again, and thus start the cycle over.

Do you think when Robert Boyle published his gas law in 1662 he had any idea it would help create this intriguing little toy that has fascinated folks for generations? Probably so. He was a pretty sharp scientist after all. When you think about it, there are several physical principles at work in this system. I can think of at least six without even straining my brain:

-The capillary action of the wicking felt -The center of mass and torque around the pivot -The ideal gas law (the relationship between gas particles and pressure) -Boyle’s Law (the relationship between temperature and pressure) -Maxwell-Boltzmann equation (molecules at a given temperature can exist in different phases) -Latent heat of vaporization (heat transfers when a substance changes states

Can you think of any more principles at work here?

References:

Bowling, S.A. (1988). The Dunking Bird. Alaska Science Forum. Retrieved online from the Geophysical Institute of the University of Alaska: http://www.gi.alaska.edu/ScienceForum

Clausius, R. (1865). The Mechanical Theory of Heat – with its Applications to the Steam Engine and to Physical Properties of Bodies. London.

Watson, D.R. (2003). Further exploration of the Dippy Bird. Retrieved online from: http://sci.vu.edu.au/~drw/dippybird/moredippy.html