It's tempting to think that the "plink" sound produced by a falling water droplet on a liquid surface is caused by the droplet itself, but new research points to an unexpected source of this familiar, yet annoying, sound. Excitingly, the researchers have also identified a neat hack to stop it.
The sound produced when a water droplet hits a liquid surface is caused by the vibrating movements of a small air bubble trapped beneath the water's surface, according to new research published today in Science Reports.
Seems weird that we are only learning this now, but it's a mystery that's vexed scientists for over a hundred years. Back in 1908, American scientist Arthur Worthington conducted his "Study of Splashes", which produced the earliest known photos of droplet impacts.
His work generated considerable interest, and by the 1920s scientists were investigating the curious sounds produced by water droplets. Many theories were proposed over the years to explain the distinctive "plink" noise, but no experimental studies were able to confirm the various hypotheses.
"A lot of work has been done on the physical mechanics of a dripping tap, but not very much has been done on the sound," Anurag Agarwal, a researcher at the Department of Engineering at Cambridge University and the lead author of the new study, said in a statement.
"But thanks to modern video and audio technology, we can finally find out exactly where the sound is coming from, which may help us to stop it."
To solve this mystery, Agarwal and his colleagues set up an experiment to record droplets falling into a tank of water using an ultra-high-speed camera, a microphone and a hydrophone (a microphone that works underwater).
Based on previous work, the researchers already knew that when a water droplet hits a surface, its inertia creates both an air bubble and an air cavity. The air cavity quickly recoils owing to surface tension, and this produces a rising column of liquid. The speed of the recoil is what causes the small air bubble to get trapped underneath.
Over the years, scientists figured that the "plink" sound was either caused by the impact itself, the resonance of the vibrating cavity, or sound waves spreading along the water surface. But as noted, none of these ideas were ever proven experimentally.
However, the new observations made by Agarwal's team showed that none of these things, the initial splash, the formation of the cavity, or the jet of liquid, produced any sounds of consequence. Instead, the source of the "plink" comes from the trapped air bubble itself.
"Using high-speed cameras and high-sensitivity microphones, we were able to directly observe the oscillation [that is, vibrations] of the air bubble for the first time, showing that the air bubble is the key driver for both the underwater sound, and the distinctive airborne 'plink' sound," said Sam Phillips, a co-author of the new study.
"However, the airborne sound is not simply the underwater sound field spreading to the surface, as had been previously thought."
Diagram showing the bubble vibrations (red arrows) involved in producing a drip sound. Illustration: S. Phillips et al., 2018/Scientific Reports
Indeed, in order to produce a sound that's loud enough to drive us batty up at night, the trapped air bubble needs to be close to the bottom of the cavity. The bubble then vibrates against the surface like a piston, driving sounds waves into the air. And into your ear.
The researchers also found that changes to the surface tension of the liquid surface can stop, or diminish, the sound. So for those of you with a leaky tap, your best bet is to add a bit of dishwashing liquid to whatever container is catching the water drips. Simple. Who needs an expensive plumber when you have fluid dynamics at your disposal?