Drinking bird toy generates usable electricity

24 Mar 2024 Hamish Johnston

Dippy device: the team’s generator could power 20 LCDs. (Courtesy: Device/Wu Zheng Qin et al.)

Before we had Instagram and TikTok to amuse us, we had an array of quirky desktop toys that captivated our imaginations. Some of these even demonstrated physical principles – including Newton’s cradle with its suspended spheres that clacked back and forth.

For me, the most mysterious of these toys is the drinking bird. Often called a dippy bird, it pivots about an axle, continuously dipping its beak in and out of a container of liquid with no obvious source of power.

Rather than being a perpetual motion machine, a dippy bird is a heat engine. It comprises two glass bulbs that are connected by a glass tube. The tube is attached to the axle so the dumbbell-like configuration can rotate. A “beak” made of absorbent material such as felt is attached to the bulb that forms the head of the bird (see figure).

Pressure drop

The interior of bird is partially filled with a liquid that is highly volatile such as methylene chloride. If the beak is dipped into a glass of water and then removed, evaporation will cause the upper bulb to cool, causing some of the vapour in it to condense and the pressure to drop.

This mismatch in pressure will cause some liquid from the lower bulb to flow up to the top bulb. The now top heavy bird will dip down, putting its beak into the water and starting the process all over again.

So, lots of idle fun watching the bird drink – but how can the effect be used to generate electricity?

Practical use

When working as a postdoc at The Hong Kong Polytechnic University, Hao Wu was trying to boost the voltage created by an evaporation energy generator. She thought of the dippy bird and realized that it could have practical uses beyond demonstrating thermodynamics.READ MORE

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Now a professor at the South China University of Technology, Wu and her colleagues began with a commercial dippy bird toy, which they modified by adding two triboelectric nanogenerator modules. These generate energy from the transfer of electric charge between two surfaces that move across each other. In this case the surfaces were fixed to the pivoting bird and the stationary stand.

The big challenge for the researchers was to minimize the friction at the interface, while still generating electricity. When they got it right, their dippy bird achieved an output voltage exceeding 100 V and could power 20 liquid-crystal displays.

The team is now exploring practical applications for their dippy device. You can read more about the research in an open access paper in Device.

Hamish Johnston is an online editor of Physics World

from physicsworld.com   25/3/2024