Physics explains why poured water burbles the way it does

Its volume also depends on the smoothness of the poured stream of water

Three images show water being poured from a teapot. The shape of the droplets is described by an effect known as Rayleigh-Plateau instability.

A stream of water poured from a teapot forms ridges as it falls. Then it breaks into droplets. This affects how air bubbles form in the liquid receiving the pour (bottom left). That, in turn, explains the different sounds of water poured from different heights.

Ho-Young Kim et al, APS 2023

Ah, the refreshing sound of a cool drink of water being poured. You might feel thirsty just thinking about it. Or, if you’re a scientist, you might feel curious.

Mouad Boudina certainly was. He’s a mechanical engineer at Seoul National University in South Korea. He and his colleagues wanted to know how pouring conditions affect the volume of cascading water. The key, they learned, was how much a stream of water rippled as it fell.

As a smooth stream of water falls, it tends to form lumps and bumps. Then, it breaks into droplets. (This happens due to a physics effect called the Rayleigh-Plateau instability.) As that rippling water hits the surface of another liquid — such as a cupful of water — it forms air bubbles. The vibrations of those air bubbles make sound.

Boudina was part of a team that poured water from a tube near the surface of a container full of water. In lab experiments, this pour could not be heard. That’s because the incoming stream of water hadn’t fallen far enough to form ripples.

When the team poured water from a greater height, things changed. The falling stream of water became bumpy. This increased the sound of the pour.

The researchers shared this finding in the December Physical Review Fluids

The width of a poured stream of water matters, too, it turns out. Thinner jets were louder than thicker ones poured from the same height. Why? As thin streams fall, they more quickly become wiggly than thicker ones do.

Once the pouring height was large enough that the streams broke up into droplets, what mattered was the size of those drops. Thicker jets pinch off into bigger drops. And they were louder than thinner ones.

Science News physics writer Emily Conover studied physics at the University of Chicago. She loves physics for its ability to reveal the secret rules about how stuff works, from tiny atoms to the vast cosmos.

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