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I was recently introduced to the liquid rope coil effect by this video:

https://www.youtube.com/watch?v=zz5lGkDdk78

I noticed while the video mentioned the equations governing the motion, it didn't really talk about the reason for the effect. All it really said was that the honey is flowing too fast so it needs to get out of the way of itself.

I also looked at any papers I could find on the subject, and while many have a mathematical model for what's happening and the speed the coils are formed, I couldn't find any intuitive explanation for why/how the coils are formed in the first place.

So in particular, below are the things I don't understand about the effect. I would appreciate help with understanding either of these questions.

Why does it fall in a circle pattern? Is it that once the honey has been displaced a bit from the center and has some tangential velocity, it experiences a viscous force towards the center (and also upwards, but gravity counteracts that) from the honey above it which causes it to move into a circle?

Why does it start to coil? If the honey is dropping, obviously it starts out by moving straight down. I can imagine how once some honey falls in the middle, the next honey that falls will fall on the middle and then slide off the middle column and fall outwards. So at this point we have our stream slightly displaced from the middle but has no tangential velocity. What effect gives the stream a tangential velocity so it can start to move circularly?

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  • $\begingroup$ Perhaps start by asking whether you would expect the same thing from water. What if we heated up the honey to near boiling, would it still coil in the helical pattern shown in the video? $\endgroup$ – honeste_vivere Dec 3 '15 at 13:52
  • $\begingroup$ I tried it out with water and found no coil effect as I expected. If I swish the water bottle a little bit so the water is no longer pointing straight down, it then after a small amount of times goes back to straight down. Gravity wants the flow to be straight down after all. So is the viscous effect only needed to cause the initial tangential acceleration and then gravity does the rest? Although I'd imagine a viscous fluid would be needed to make the centripetal force large enough to be visible. $\endgroup$ – user35734 Dec 3 '15 at 14:22
  • $\begingroup$ I do not think centripetal acceleration is the issue here. I think the effect arises entirely from viscous stresses/strains... $\endgroup$ – honeste_vivere Dec 3 '15 at 14:55
  • $\begingroup$ don't those stresses/strains cause a centripetal acceleration? $\endgroup$ – user35734 Dec 3 '15 at 15:31
  • $\begingroup$ My point was to keep you from confusing different concepts. Centripetal acceleration occurs because the Ref. frame to which one is tied accelerates due to an unbalanced force. That is, centripetal acceleration is a consequence of the reference frame, not a force in and of itself. $\endgroup$ – honeste_vivere Dec 3 '15 at 15:52
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the coil effect only works when you have high viscous fluids like honey. It doesn't work with a low viscous fluid because the low viscous fluid flows faster than the honey, making the fast flowing fluid like water to not have the rope coiling effect

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  • $\begingroup$ also maybe the honey does slide and displace each other causing that helix to form when it falls to the bottom of the container $\endgroup$ – passwordisincorrect Feb 20 '17 at 17:28
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    $\begingroup$ This answer pretty much reduces to "it is so because it is so". I think k a bit more detail of the role of viscosity would be helpful. Water is viscous so why does it not behave this way? $\endgroup$ – Floris Feb 20 '17 at 17:45

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