The physics behind the bubbling of poured water explained

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

Mouad Boudina certainly did. He is a mechanical engineer at Seoul National University in South Korea. He and his colleagues wanted to know how water injection conditions affect the amount of water that cascades. They learned that the key is how much the water ripples as it falls.

When a smooth stream of water falls, it tends to form lumps and irregularities. It then breaks into water droplets. (This is caused by a physical effect called Rayleigh plateau instability.) When that rippling water hits the surface of another liquid, such as a cup of water, bubbles form. Sound is produced by the vibration of the bubbles.

Boudina was part of a team that poured water from a tube near the surface of a container filled with water. In laboratory experiments, this sound was not audible. That’s because the incoming water flow didn’t fall far enough to form ripples.

Things changed when the team poured water from higher up. The flow of falling water became uneven. This has increased pouring sound.

Researchers shared this discovery in December physical review liquid.

It turns out that the width of the water flow is also important. Thin jets pouring from the same height were louder than thick jets. why? When a thin stream falls, it wiggles more quickly than a thick stream.

Once the pouring height is large enough and the stream breaks up into droplets, the problem is the size of the droplets. Thick jets are pinched off into larger droplets. And they were bigger than thin ones.