Why do ripples appear in falling water?

Question

I was pouring water into a cup.

As the cup filled up, the height between the water in the cup and the spout decreased to about 3cm.

I was able to pour slowly such that the diameter of the falling water column was smaller than the diameter of a drop of water. Furthermore, when the water column contacted the cup's water, there was no splashing.

There were distinct bands that appeared in the water column, like the water column was varying in size.

How would you explain this phenomenon? Is it due to the slow pouring, so that the surface tension just manages to hold neighboring water droplets together?

Or is it that upon contact of the falling water with the water in the mug, waves are produced, which travel back up the water column, and upon equilibrium, appear as standing waves?

Jug of water with a narrowing spout. Water in both jug and cup was almost still. — Zhengqun Koo, Feb 06 '17 at 04:21
These are (one form of) instabilities on the water's surface, which if they grow will cause breakup of water stream into droplets. Read up Rayleigh-Plateau instability. — Deep, Feb 06 '17 at 05:37
@Deep that sounds like something that could be expanded into an interesting answer. — Selene Routley, Feb 06 '17 at 06:31
@WetSavannaAnimalakaRodVance I have nothing much to say apart from what is there in the comment. Here is a nice link: http://web.mit.edu/2.21/www/Lec-notes/Surfacetension/Lecture5.pdf , particularly see fig-5. — Deep, Feb 06 '17 at 07:10
Thanks so much for the links guys! Will read up on these intuitive answers, sure beats Wikipedia. If what Jake said is the same as Rayleigh-Plateau instability, I'll accept his answer. — Zhengqun Koo, Feb 06 '17 at 14:48

score 0 · Accepted Answer · answered Feb 06 '17 at 09:09

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If it's not the case that there are minute differences in flow rate, and your hand is completely steady as you pour, I'd venture that it could be an interplay between gravity and surface tension. A smaller stream has a greater surface-to-volume ratio, flows slower, and has considerably less mass than even a slightly larger stream. All of these factors seem like they would allow a small stream to be more affected by surface tension. Perhaps it's the case that the liquid is on the verge of becoming a series of drops?

answered Feb 06 '17 at 09:09

Jake Watrous

473

This is basically the same thing @Deep said in his Comment. – Chet Miller Feb 06 '17 at 12:43
You're right. Sorry, didn't notice that. – Jake Watrous Feb 06 '17 at 21:00

Why do ripples appear in falling water?

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