The shape of water poured out of a glass

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Discussion Overview

The discussion explores the shape of water as it is poured from a glass, drawing parallels to physical phenomena such as standing waves. Participants consider the role of surface tension and fluid dynamics in explaining the observed behavior of water during pouring.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions why water takes on a specific shape when poured, suggesting it resembles nodes and antinodes of standing waves and speculating about the role of interatomic forces and surface tension.
  • Another participant asserts that surface tension is indeed responsible for the shape of water, describing how a drop of water in free fall maintains a spherical shape and oscillates between elongated and flattened forms.
  • A third participant introduces the concept of the "Rayleigh instability" as a relevant phenomenon for analyzing fluid jet dynamics, indicating that the behavior of the water can be understood through established fluid dynamics principles.
  • Some participants express skepticism about the simplicity of analyzing fluid jet dynamics, questioning the extent of research dedicated to the topic.

Areas of Agreement / Disagreement

Participants generally agree that surface tension plays a significant role in the shape of water when poured, but there is no consensus on the simplicity of analyzing fluid dynamics or the implications of the Rayleigh instability.

Contextual Notes

Some discussions involve assumptions about fluid behavior and the complexity of fluid dynamics, which may not be fully addressed or resolved in the conversation.

hale2bopp
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Why does water get that shape when poured out of a glass, almost like the nodes and antinodes of standing waves in a string?
Also, the 'parts' of water (For want of a better term) seem to be perpendicular to each other, alternatively.
I really think I'm doing a bad job of explaining this, if so, let me know?
But why does this happen? Is it the interatomic forces between the water particles that contributes a sort of tension? Surface tension, perhaps? And if so, how does it explaint that the 'parts' are alternatively perpendicular to each other?
 
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Yes, it is surface tension.

Imagine, if you will, a drop of water in free fall and ignore air resistance. The drop has an equilibrium shape which is spherical. If you elongate it, it will spring back to round. If you flatten it, it will spring back to round. There is mathematics that correlates pressure, surface tension and curvature on a small area of the surface of the drop to establish this behavior, but there is no need to go that deep. It is enough that the drop is "springy".

If the drop is elongated and springs back to the equilibrium shape, its parts are in motion. The shape will not stop at "round". The parts will keep on coasting past "round" and on toward "flat". The drop will end up oscillating between "flat" and "long" until it eventually damps out to just "round".

Now put this drop of water into a stream with a bunch of other drops. Imagine that "long" is oriented across the lip of the glass and "flat" is at right angles. The drop oscillates between "long" and "flat" as it flows down the stream.

The oscillations are regular and the flow velocity is predictable. So the places where the flow is "flat" or "long" will be stable.
 
hale2bopp said:
Why does water get that shape when poured out of a glass, almost like the nodes and antinodes of standing waves in a string?
<snip>

The dynamics of a fluid jet are, in many cases, straightforward to analyze. The phenomenon you refer to is the "Rayleigh instability":

http://en.wikipedia.org/wiki/Plateau–Rayleigh_instability
 
Andy Resnick said:
The dynamics of a fluid jet are, in many cases, straightforward to analyze.

:smile:
 
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Andy Resnick said:
The dynamics of a fluid jet are, in many cases, straightforward to analyze.

Dang, then what are all those researchers spending their entire careers working on? You should go help them.
 

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