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.
Mechanical engineer Mouad Boudina and colleagues wanted to understand how the pouring conditions affected the volume of that enticing sound. The key, the researchers found, was how much the incoming stream of water rippled as it fell.
As a column of water falls, an effect called the Rayleigh-Plateau instability causes the smooth stream to form lumps and bumps before eventually breaking up into droplets. Those ripples impact the surface of the liquid, forming air bubbles that vibrate and produce sound.
In laboratory experiments, water poured from a tube close to the surface of a water vessel was inaudible, as the stream hadn’t fallen far enough to form ripples. For water poured from a greater height, the streams became bumpy, and the sound was louder, Boudina, of Seoul National University in South Korea, and colleagues report in the December Physical Review Fluids.
The width of the stream of water mattered, too. Thinner jets were louder than thicker jets poured from the same height. That’s because, as they fall, thin streams become wiggly more quickly, as compared to thicker ones.
Once the pouring height was large enough that the streams broke up into individual droplets, what mattered was the size of the drops. That means thicker jets, which pinch off into bigger drops, were louder than thinner ones.