Published on Feb 21, 2013
The drinking duck, or drinking bird, sipping bird, insatiable birdie and dipping birds, are toy heat engines that mimic the motions of a bird drinking from a water source.
*This video was made in a cooler area of the building in order to emphasise the difference that temperature makes to its operation. The next video was made in a warmer part of the building and the gap between sips of water is significantly reduced as a result.
It was said that when Einstein first saw one of these in China in 1922, he was fascinated by it and could not figure out how it operated.
A drinking bird consists of two glass bulbs joined by a glass tube (the bird's neck). The tube extends nearly all the way into the bottom bulb, and attaches to the top bulb but does not extend into it. The space inside the bird contains a fluid, usually coloured. The fluid is typically dichloromethane, also known as methylene chloride.
Air is removed from the apparatus during manufacture, so the space inside the body is filled by vapour evaporated from the fluid. The upper bulb has a "beak" attached which, along with the head, is covered in a felt-like material. The bird is typically decorated with paper eyes, a plastic top hat, and one or more tail feathers. The whole setup pivots on an adjustable crosspiece attached to the neck.
The drinking bird is a heat engine that exploits a temperature differential to convert heat energy to a pressure differential within the device, and perform mechanical work. Like all heat engines, the drinking bird works through a thermodynamic cycle. The initial state of the system is a bird with a wet head oriented vertically with an initial oscillation on its pivot.
The process operates as follows:
1. The water evaporates from the felt on the head.
2. Evaporation lowers the temperature of the glass head (heat of vaporization).
3. The temperature decrease causes some of the dichloromethane vapour in the head to condense.
4. The lower temperature and condensation together cause the pressure to drop in the head (by the ideal gas law).
5. The higher vapour pressure in the warmer base pushes the liquid up the neck.
6. As the liquid rises, the bird becomes top heavy and tips over.
7. When the bird tips over, the bottom end of the neck tube rises above the surface of the liquid.
8. A bubble of warm vapour rises up the tube through this gap, displacing liquid as it goes.
9. Liquid flows back to the bottom bulb (the toy is designed so that when it has tipped over the neck's tilt allows this) and pressure equalises between the top and bottom bulbs
10. The weight of the liquid in the bottom bulb restores the bird to its vertical position
11. The liquid in the bottom bulb is heated by ambient air, which is at a temperature slightly higher than the temperature of the bird's head.
If a glass of water is placed so that the beak dips into it on its descent, the bird will continue to absorb water and the cycle will continue as long as there is enough water in the glass to keep the head wet. However, the bird will continue to dip even without a source of water, as long as the head is wet, or as long as a temperature differential is maintained between the head and body. This differential can be generated without evaporative cooling in the head; for instance, a heat source directed at the bottom bulb will create a pressure differential between top and bottom that will drive the engine. The ultimate source of energy is the temperature gradient between the toy's head and base; the toy is not a perpetual motion machine.
A 'dunking bird of the second kind' was introduced which, while similar to the original drinking bird, will operate without a temperature difference. Instead it utilizes a combination of capillary action, gravitational potential difference and the evaporation of water to power the device.
Such a bird works as follows: It is balanced such that, when dry, it tips into a head-down position. The bird is placed next to a water source such that this position brings its beak into contact with water. Water is then lifted into the beak by capillary action and carried past the fulcrum. When enough water has been absorbed by the device, the now-heavy bottom causes the bird to tip into a head-up position. With the beak out of the water, eventually enough water evaporates from the sponge that the original balance is restored and the head tips down again.