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Multimodal Sensing Solves the Water-Drop “Plink” Mystery

Modern sensing technologies are often used to investigate major questions; sometimes they address long-standing questions in science (see Photonics Spectra , LIGO Continues Making Waves“). But there are times when they are used to resolve apparently “minor” question which have been puzzling researchers (or those with too much time, graduate-student assistance, and grant money) for decades.

This is the case with the question of how that often-annoying, hard-to-ignore “plinking” sound is created when a drip of water (typically from a leaking faucet) hits the small pool it has created below itself. A recent scholarly paper in Nature Scientific Reports , The Sound Produced by a Dripping Tap is Driven by Resonant Oscillations of an Entrapped Air Bubble,” plus a feature in Physics Today , The annoying dripping tap” and similar one at physics.org , What causes the sound of a dripping tap—and how do you stop it?” discussed the result of some definitive research into the problem and resulting conclusion.

Right to the conclusion: a team based at Cambridge University (UK) concluded that the sound is produced by a multistep process, Figure 1 :

  1. the drop hits the pool’s surface, and carves out a “cavity” that recoils due to surface tension
  2. this, in turn, creates a small bubble under the surface (called bubble entrapment)
  3. the bubble shoots out a small jet of water which is invisible to the naked eye
  4. finally, the sound is produced at the moment the air bubble becomes trapped under the surface. The oscillation of this bubble forces the water surface to vibrate, which in turns acts like a piston to create the sound we hear. Somewhat counterintuitively, that initial splash, formation of the cavity, and resulting water jet are all silent; the sound is due to that trapped air bubble. (The references show the various waveforms versus time.)
Figure 1

A simplified schematic shows the entrained bubble oscillations driving oscillations of the water surface; the oscillations are indicated by double-headed red arrows. (Image source: Nature/Springer Nature Publishing AG)

A simplified schematic shows the entrained bubble oscillations driving oscillations of the water surface; the oscillations are indicated by double-headed red arrows. (Image source: Nature/Springer Nature Publishing AG)

Whether this news excites or bores you, what is interesting is how they set up the instrumentation to determine this. The arrangement used two high-speed video cameras (100,000 frames/sec) with one aimed below the surface and one above, plus a complementary pair of audio pickups via a subsurface hydrophone and an above-surface microphone, Figure 2 . Analysis of the video images and audio data, and their relative timing with respect to each other, provided the answers. They also had to minimize acoustic reflections (echoes) by lining the tank with wood and removing other artifacts from the acquired data set.

Figure 2

The research setup used a fish tank with ultrahigh-speed video cameras focused above and below surface, a hydrophone, a microphone, and a controlled 'drip' mechanism. (Image source: Physics Today/AIP Publishing LLC)

The research setup used a fish tank with ultrahigh-speed video cameras focused above and below surface, a hydrophone, a microphone, and a controlled “drip” mechanism. (Image source: Physics Today/AIP Publishing LLC)

An experimental arrangement such as this one with two cameras and two audio channels would certainly have been possible decades ago. But the ultrahigh-speed camera was yet developed, while the ability to analyze and correlate the multiple streams of resultant data would have posed a major challenge.

This isn’t the first time that this problem has been addressed and analyses. The Physics Today piece cites previous theoretical and experimental efforts to investigate this seemingly unimportant problem. In 1933, the Belgium/Dutch astronomer Marcel Minnaert turned his focus 180o from the heavens and developed a model and formula showing that the natural frequency of the pulsating underwater bubble is inversely proportional to its diameter. (I do wonder what inspired him to do so? Was it some possible relationship between water bubbles and astronomical events? Scientific curiosity? Or perhaps as a physics challenge to himself?)

Did this research provide us with new insight? Of course it did. Was it worth it? I don’t know; no one does, at least not yet. One comment at the phsyics.org site noted, ““This has to be nominated for ignoble” referring to the annual “Ig Nobel prizes” awarded by Improbable Research (and co-sponsored by the Harvard-Radcliffe Society of Physics Students and the Harvard-Radcliffe Science Fiction Association).

Still, there have been many cases where research into seemingly trivial questions with few immediate implications has, in fact, led to significant and important understanding and progress. Reality is that the creation and collapse of bubbles is a major concern as it closely relates to cavitation in ship propellers and associated audible noise, vibration, and inefficiency.

If you could use modern instrumentation to investigate a “trivial” question, what would it be? Have you ever wished you had the time and resources to resolve something “small” about which you repeatedly wondered?

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2 comments on “Multimodal Sensing Solves the Water-Drop “Plink” Mystery

  1. Rama Murthy
    January 7, 2019

    Nice summary. Sphagetti string breaking study is also as interesting.Some interesting insights in the sequence of events could be observed by recording in microsecond intervals as has been reported by MIT recently. That simple dropping (not even explosive bubble) water and Fracture mechanics events need microseconds resolution is a bit surprising.An expert at Philips said a few decades ago that as technology develops exponentially, applications devour them logarithmically. I wonder if such high speed cameras are available at sub $500 cost?

            As for the trivial question to be investigated, how about the same water droplet falling on a hot(100+ DEG C) dry plate? The sound of evaporating water is a plink with a bang.Is that because of the wave front being steeper or travelling faster (due to rapid evaporation) or both or something else? 

     Also how about evaluating the transient response of a tweeter usng high speed camera? Probably we can explain the phenomena of settling time in control systems better if we introduce this in engineering curriculum? Many physics and engineering experiments can be redone using this.

     

  2. DavidHamilton
    January 16, 2019

    And this guys, is why science is so important. Haha! I'm just kidding. Honestly, I love the way that you've been able to explore and examine this, just because we can! Now to take the experiment one step further and see if we're able to recreate the “plink” in different volumes and magnitude. That would be something I would honestly want to see being tested!

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