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I recently noticed an interesting phenomenon while watching the tap running in my kitchen. At the time, there was bright sunlight coming in the kitchen window. As the stream was running out of the tap into the sink the sunlight was shining directly on to it.

This allowed me to observe that there were a constant appearance of water particles appearing to exit the stream for about three or four inches (approx. 10 cm) before falling into the sink. This appeared to be happening throughout the full length of the stream. What could be causing this to happen? Shouldn't the surface tension hold the stream together?

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    $\begingroup$ could you add a picture, please? $\endgroup$
    – basics
    Commented Oct 23, 2023 at 13:15
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    $\begingroup$ Something like this? youtube.com/watch?v=SAby8PFQQ1k $\endgroup$
    – anjama
    Commented Oct 24, 2023 at 1:15
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    $\begingroup$ That is exactly it. Is it air in the system that is causing this effect $\endgroup$ Commented Oct 24, 2023 at 10:57

4 Answers 4

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Did your faucet nozzle contain a bubbler (which mixes air into the water and produces a white, frothy stream down to the bottom of the sink)? Most kitchen faucets do, and when the bubbles closest to the surface of the stream pop, they spit out a tiny droplet of water with surprising speed.

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    $\begingroup$ At last an answer that makes some sense.. Thank you. The stream of water does not appear to be white or frothy but it possible that air somewhere in the system could be the cause of this effect. $\endgroup$ Commented Oct 23, 2023 at 18:02
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    $\begingroup$ The term for this part is an aerator. $\endgroup$
    – JimmyJames
    Commented Oct 24, 2023 at 14:39
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    $\begingroup$ @MichaelMcgarry You can try unscrewing it (might as well flush out any sediment while you are at it) and see if it alters the effect. It definitely changes the way the water comes out. $\endgroup$
    – JimmyJames
    Commented Oct 24, 2023 at 14:42
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The water is accelerating as it falls and so the diameter of the water column decreases.
This induces (Plateau–Rayleigh) instability in the column of water due to the increasing influence of surface tension trying to minimize surface area as the liquid column becomes thinner with the result that water droplets are produced.
As the surface area/volume ratio increases the influence of surface tension trying to reduce the surface area becomes more important.

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    $\begingroup$ This seems like a good answer however I am not sure it explains the phenomenon fully. The water droplets are tiny and only visible in bright sunlight. The real problem I have with this answer is the distance they are ejected from the column of water. Three to four inches seems a large distance. Also the water is under pressure until it leaves the tap so it is probably slowing down not accelerating. $\endgroup$ Commented Oct 23, 2023 at 14:45
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    $\begingroup$ @MichaelMcgarry, Farcher is saying that the water is accelerating under the influence of gravity after it leaves the tap. $\endgroup$ Commented Oct 23, 2023 at 16:44
  • $\begingroup$ Essentially you are saying that the increase in surface tension causes the water column to lose surface tension and so emit water droplets. $\endgroup$ Commented Oct 23, 2023 at 17:45
  • $\begingroup$ @MichaelMcgarry As the surface area/volume ratio increases the effect of surface tension trying to reduce the surface area becomes more important. $\endgroup$
    – Farcher
    Commented Oct 23, 2023 at 22:13
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    $\begingroup$ @Farcher I think you have maybe misunderstood the question. The effect you describe refers to the stream itself breaking up and becoming droplets. That is not what is happening. The droplets I am referring to are emerging from the stream horizontally for a distance of three to four inches. $\endgroup$ Commented Oct 24, 2023 at 10:52
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I have noted this phenomena in the past (easy to do if you just hold your hand a couple inches from the stream, it'll become damp) and have always assumed it was related to the transition from no surface tension inside a full pipe and the air/water surface tension interface. It's most easily seen with an old corroded faucet nozzle that's not round but oval, I have therefore presumed that it's due to the rough oval surface causing the surface tension interface to throw off droplets rather than a perfect stream as the surface of the water exiting the pipe is not perfectly smooth. This may or may not be the cause, but it's my thoughts on the matter, for what its worth.

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Possibly an example of the Armstrong effect - fluid coming out of an orifice can become electrically charged by friction, and when a stream of water becomes charged it can throw off droplets in a spray. (Anyone who has seen a Kelvin water drop generator has probably noticed how the droplets repel one another.) Just a guess. Another possibility is that a flow limiter in your faucet is creating turbulence that is scattering the droplets, but it's not clear why they would be repelled so far from the stream or continue to come out several inches down.

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