# The shape of water poured out of a glass

1. May 21, 2014

### hale2bopp

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?

2. May 21, 2014

### jbriggs444

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.

3. May 21, 2014

### Andy Resnick

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

4. May 22, 2014

### cjl

:rofl:

5. May 22, 2014