One of the more amusing gifts that Jodi (errrr.... I mean Santa!) got for her younger son Joe was a drinking bird science toy. Of course, the amusement proved more long-lasting for Jodi and me (particularly me) as Joe, after a few minutes of enchantment, moved on to his Kung Fu Panda video game.
While I’d seen a drinking bird before, I’d never really pondered the device much. I knew it was a thermodynamic engine of some sort, but figured it was just powered by the difference between the ambient temperature in the room and the liquid and that after a period of time it would stop. That turns out not to be true. The bird is powered by the by the temperature difference between the evaporating water on the beak (evaporative cooling) and the temperature in the abdomen, using a difference in vapor pressure to move the internal fluid. Really quite sophisticated in its own way and has even been the subject of an article in the American Journal of Physics.
Eventually even I would have lost interest and moved on to my equivalent of a video game (meaning a crossword puzzle or two) but the other morning the bird got my attention again. I came down the stairs to find it on the dining room table where Jodi had left it the night before, but instead of bobbing up and down it was almost perfectly stationary with its beak submerged in the water. My first guess was that it had hung on the hinges, or the equilibrium had been broken and it was resting against the glass, but on closer inspection it was bobbing up and down ever so slightly and the fluid inside was moving slightly back and forth in a slow cycle. I realized that by chance we must have hit a near stable point, where the evaporation from the head — all of which was now saturated with liquid — was at just the right rate to create an equilibrium. Who says Christmas presents can’t be educational?
Left to my own choices, I would have waited a few days until the water level dropped enough that the equilibrium was broken, but we had guests coming over for dinner last night so, without asking and thinking only of our guests, Jodi moved everything out of the way — personally, as a major science nerd, I think it would have made a great conversation piece for our friends. Fortunately, I did think to take a picture first. So here you have it, the “stable” bobbing bird. If you look closely you can see the slight gap between the body and the glass.
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